THE SEQUENCE STRATIGRAPHY OF THE
COMMANCHEAN - GULFIAN INTERVAL,
BIG BEND NATIONAL PARK,
WEST TEXAS
A THESIS SUBMITTED TO THE GRADUATE SCHOOL
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE
MASTER OF SCIENCE
BY
NICHOLAS S. TIEDEMANN
DR. RICHARD FLUEGEMAN
BALL STATE UNIVERSITY
MUNCIE, INDIANA
MAY 2010
Chapter
AKNOWLEDGEMENTS
ABSTRACT
LIST OF TABLES
LIST OF FIGURES
1. INTRODUCTION
2. SYSTEMATIC PALEONTOLOGY
3. BIOSTRATIGRAPHY OF THE BUDA LIMESTONE
4. BIOSTRATIGRAPHY OF THE LOWERMOST BOQUILLAS FORMATION
5. STABLE ISOTOPE GEOCHEMISTRY
6. DISCUSSION AND CONCLUSION
PLATES
REFERENCES
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AKNOWLEDGEMENTS
I would like to thank the National Park Service for encouraging research in such a beautiful and
fascinating place as Big Bend National Park. Their work preserving America's National Parks is
deserving of much recognition. I am grateful for my advisor, Dr. Richard Fluegeman, whose
wealth of knowledge and guidance are seemingly limitless. This research was made possible by
internal grants from the Ball State Department of Geological Sciences and from Dr. Richard
Fluegeman. I would like to thank Brenda Rathel and Mike Kutis for assisting me with everything
from filling out grants and making copies to making thin sections and using assorted pieces of
software. I owe thanks to Drs. Scott Rice-Snow, Kirsten Nicholson, and Jeff Grigsby for their
valuable guidance and support. I would like to extend my wholehearted gratitude to Dr. Brian
Lock and Drs. Roger and Dee-Anne Cooper for providing me with valuable field information,
supplementary documentation, advice, and genuine goodwill. Recognition is also due to
National Petrographic Services, Inc. for their expedient thin sectioning services. My thanks go
out to Dr. Greg Ludvigson, Gregory Cane, and the entire Keck Paleoenvironmental &
Environmental Stable Isotope Laboratory at the University of Kansas for their services and
assistance with stable isotope data collection. A special thanks goes out to Bob Liska for
guidance with foraminifera resources. I owe so much to my loving parents, Craig and Lisa
Tiedemann, and my grandfather, Jack Tiedemann, without whose moral and financial support I
would never have accomplished this work. Finally, this research would not have been possible
without my field assistant, proof-reader, lunch-packer, and companion, Erica Evans.
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ABSTRACT
Within Big Bend National Park, the unconformable contact between the Buda Limestone and
the overlying Boquillas Formation represents the Commanchean-Gulfian boundary. Previous
studies of the geochronology of this interval have relied primarily on provincial ammonite
faunas rather than foraminifera, and place the Buda and basal Boquillas in the Lower
Cenomanian. Because of its indurated nature, a comprehensive foraminiferal biozonation has
not been acquired for the Buda Limestone. Recent revisions to Cretaceous foraminiferal
biozonations and taxonomies necessitates a new biostratigraphic study of the Buda - Boquillas
interval. The overlapping ranges of F. washitensis, G. bentonensis, G. caseyi, P. appenninica, P.
delrioensis, P. stephani, and R. montsalvensis place the Buda within the upper portion of the
Early to Middle Cenomanian Th. globotruncanoides Zone. Microkarst found on the surface of
the Buda Limestone has been interpreted as representing a subaerial exposure and sequence
boundary. However, microkarst-like features can result from subaqueous or intrastratal
processes. Carbon and oxygen stable isotope analysis of the lower and middle Buda has
indicated a mean δ13C value of 1.73‰ VPDB, which is in line with other values reported from the
Lower Cenomanian. The top 2.6m of Buda contains a 0.62‰ negative δ13C shift from 1.88‰
VPDB to 1.26‰ VDPB in a 40 cm interval, expected if subaerial exposure occurred. Higher
variation in measured carbon isotope values beneath the contact also lend evidence for
meteoric alteration. The standard deviation in δ13C values from the top 2.8 m of the Buda is
0.207, which is 2.16 times larger than the rest of the studied section at 0.096. The Buda
contains a shallow pelagic-dominated fauna of heterohelicids (45-90%), globigerinellids (3-37%),
and hedbergellids (4-22%). Intermediate-depth globigerinellids display an initial increase
followed by a marked decrease in abundance upsection, interpreted as sea level transgression
and regression, respectively. The lower contact of the Buda with the Del Rio Clay has been
previously interpreted as a subaerial exposure, and a P:B break from ~0% planktonics in the
upper Del Rio to ~80% in the Buda supports this claim. This study therefore interprets both the
upper and lower contacts of the Buda as sequence boundaries. The overlying 1.2 m Boquillas is
nearly devoid of benthics and represents a deeper assemblage including the double-keeled
Dicarinella sp., as well as several Upper Cenomanian (D. algeriana Subzone) species. Based on
foraminiferal data, the duration of the Buda - Boquillas unconformity is roughly equivalent to
the missing Th. reicheli and Th. greenhornensis Biozones, or a sizable portion of the Middle
Cenomanian.
iv
LIST OF TABLES
Table
1. Distribution of identifiable foraminifera at Dog Canyon locality
2. Distribution of identifiable foraminifera at Highway localities
3. Distribution of identifiable foraminifera at Dagger Flat DS1 locality
4. Absolute and relative abundances of main genera at Dagger Flat DS1
5. Foraminiferal distribution and abundances at Dagger Flat DS2
6. Stable isotope data at Dog Canyon locality
7. Stable isotope data at Dagger Flat DS1 locality
8. Stable isotope data at Dagger Flat DS2 locality
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LIST OF FIGURES
Figure
1. Stratigraphic column of Big Bend National Park
2. Dog Canyon sampling locality map
3. Photograph of Buda Limestone outcrop at Dog Canyon locality
4. Photograph of Buda - Boquillas contact at Dog Canyon locality
5. Dagger Flat sampling localities map
6. Photograph of Buda Limestone outcrop at Dagger Flat DS1 locality
7. Photograph of Buda - Boquillas outcrop at Dagger Flat DS2 locality
8. Highway section sampling location map
9. Photograph of Buda Limestone exposure at Highway locality
10. Geochronologic distribution of selected foraminifera from the Buda
Limestone
11. Relative abundances of main genera from the Dagger Flat DS1
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19
20
21
22
23
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68
12. Absolute abundances of heterohelicids and globigerinellids per thin
section from Dagger Flat DS1
69
13. Geochronologic distribution of selected foraminifera from the Boquillas
Formation
81
14. Geochronologic positions of the Buda Limestone and Boquillas
Formation according to different authors
82
15. Idealized C-isotope profile of a meteorically-altered paleoexposure
surface
16. Geochronologic distribution of Oceanic Anoxia Events
17. Stable isotope profile for the Dog Canyon
18. Stable isotope profile for the Dagger Flat DS1
19. Stable isotope profile for Dagger Flat DS2
20. Dog Canyon δ13C and δ18O cross-plot (PLCT)
21. Dagger Flat DS1 δ13C and δ18O cross-plot (PLCT)
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CHAPTER 1 - INTRODUCTION
General Overview
Big Bend National Park (BBNP)
Big Bend National Park is located in Brewster County, TX, on the US – Mexico border, and
represents an area larger than 800,000 acres. The park was founded in 1935 and was the first
National Park in Texas. It occupies the southern tip of "Big Bend Country," the area of West
Texas bounded on three sides by the Rio Grande as it dips south toward the Coahuila Desert.
Big Bend Country contains thick sequences of mixed clastic and carbonate strata from the
southernmost portion of the Cretaceous Western Interior Seaway unconformably overlying
older Paleozoic strata (Frush and Eicher, 1975).
Stratigraphy
The Cretaceous of Big Bend National Park is represented, from youngest to oldest, by the Glen
Rose Limestone, Telephone Canyon Formation, Del Carmen Limestone, Sue Peaks Formation,
Santa Elena Limestone, Del Rio Clay, Buda Limestone, Boquillas Formation, Pen Formation,
Aguja Formation, and Javelina Formation (Fig. 1). These formations record two large-scale
transgressive-regressive cycles, the Commanchean Series and Gulfian Series, separated from
each other by the unconformity between the Buda Limestone and the Boquillas Formation.
The Commanchean Series of BBNP represents about 25 m.y. of earth history and
records mostly carbonate deposition with periodic clastic influxes. It begins with deposition of
the Glen Rose Limestone in approximately the middle Aptian and continues through the Albian
to the Lower Cenomanian deposition of the Del Rio and Buda Formations (Scott and Kidson,
1977; Scott and Mancini, 2005). The Commanchean Series contains several higher-order
sequences as outlined by Scott and Kidson (1977): The Trinity sequence is Glen Rose; the
Fredricksburg sequence is Telephone Canyon - Del Carmen; the lower Washita sequence is Sue
Peaks - Santa Elena; and the upper Washita sequence is Del Rio - Buda. More recent studies in
West Texas have shown that the stratigraphic relationships between the Commanchean
Formations may be more complex than the interpretation given by Scott and Kidson (1977),
especially within the Washita Group. Mancini and Scott (2006) interpret the Buda and Del Rio as
a single transgressive - regressive sequence, while Lock et al. (2007) interpret each unit as
unconformity bound within their own depositional sequences.
The Gulfian Series of BBNP begins with deposition of the Ernst Member of the Boquillas
Formation in the Middle to Upper Cenomanian. The Boquillas is a pelagic marine limestone and
shale unit representing deposition in deep water (Lock and Peschier, 2006). Progressively
younger strata records a generally regressive package and a shift from a mostly carbonate to a
mostly clastic regime. The Santonian to early Campanian Pen Formation represents a marine
shale, while the middle to late Campanian Aguja and Maastrichtian Javelina Formations
represent paralic and fluvial deposition, respectively (Ashmore, 2003).
2
Figure 1 - BBNP stratigraphic column. Stratigraphy from USGS.gov modified after: Scheubel, R.F. and D.H. Mruk,
eds., 1994, Road Log, Day 2, Laramide and late Tertiary tectonics and structures: Structure and Tectonics of the Big
Bend and Southern Permian Basin, Texas: West Texas Geologcial Society 1994 Field Trip Guidebook, p. 23-69. URL:
http://3dparks.wr.usgs.gov/bibe/html/bb_strat.htm.
3
Del Rio Clay (Washita Group)
Originally called the Exogyra arietina Marl (Shumard, 1860), the name Del Rio Clay was coined
by Hill and Vaughan (1898). Its type locality is 2 mi south of Del Rio, Val Verde Co., TX (Adkins,
1933). The Del Rio Clay marks a period in the Lower Cenomanian of relatively low sea level and
terrigenous influx between the deposition of the Albian Santa Elena Limestone and late Lower
Cenomanian Buda Limestone. The Del Rio's thickness in Big Bend National Park is variable,
ranging from around 30 m in the Dagger Flat area to less than 3 m near Rio Grande Village. This
variability in the thickness of the Del Rio Clay is likely due to deposition on a subaerial exposure
surface atop the Santa Elena Limestone followed by erosion of Del Rio sediment before the
Buda Limestone was deposited. The Del Rio consists of laminated, hummocky, and crossbedded claystones, carbonate muds, and very fine sandstones. It contains oyster packstones
with abundant Ilymatogyra arietina that are interpreted as resulting from storm-winnowing
events. Other fossils include echinoids, bivalves, and planktonic and benthic foraminifera,
including the large arenaceous Cribratina texana (Lock et al., 2006).
Lock et al. (2006) interpret the Del Rio Clay as a shallow deposit above storm wave base
that is bounded by subaerial unconformities. This interpretation is supported by a foraminiferal
P:B curve created by Mauldin and Cornell (1986). Their data show an initial increase in
planktonics during transgression from about 10-20% to over 60%, then a decrease to 0% during
regression. According to their research, the planktonic foraminifera Heterohelix moremani
represents 0-49% of total foraminiferal assemblage, but H. delrioensis, H. planispira, and F.
washitensis are all present in small quantities of <1% each.
4
The Del Rio Clay correlates to the Grayson Formation of north-central Texas and
Oklahoma (Scott and Kidson, 1977; Young, 1977) and the Cuesta del Cura Formation of northern
Mexico (Young, 1977; Lehmann et al., 2000). The Santa Elena Limestone - Del Rio Clay
unconformity of BBNP likely correlates to the unconformity beneath the Grayson Formation (the
base of the WA 6 cycle) assigned by Scott et al. (2002) in northern Texas. The base of the WA 6
cycle in turn may correlate to the CSB 5 unconformity at the top of the Aurora Formation (top of
sequence Co 5) in northern Mexico (Lehmann et al., 2000), and the SB4 sequence boundary
(base of sequence 4) in the Cretaceous Western Interior Seaway (Scott et al., 2001).
Buda Limestone (Washita Group)
Vaughan (1900) named the Buda Limestone after the town of Buda, Hays Co., TX. It is a dense,
porcelaneous foraminifer-calcisphere (?Calcisphaerula innominata Bonet) biomicritic
wackestone with little to no clastic material. Small (±2 cm) gastropods are by far the most
common macrofossils, but ammonites, oysters, and echinoids are also relatively common. The
unit weathers white to buff in color, but fresh exposures are generally light gray in color. The
lower members are generally nodular and the upper member consists of massive beds
occasionally separated by both nodular and thin fissile layers. Distinct bedding planes are
absent in outcrop, presumably due to extensive bioturbation. Pore spaces from freshly-cut
hand samples often contain a sticky, black petroleum-like substance that is presumably "dead
oil." Within Big Bend National Park, the thickness of observed Buda outcrops are roughly 7-9 m,
but the thickness most likely varies due to erosion.
In thin section, the Buda Limestone is a wackestone throughout the studied sections.
Micrite forms the matrix, but spar is present in primary void spaces and fractures. Microfossils
5
include both benthic and planktonic foraminifera, calcispheres, dinoflagellate cysts, and
echinoid spines. Small (0.5 mm) veins with sparry calcite infill are common throughout the unit.
Dolomitization varies from 0% to as high as 10-20% locally and normally consists of patchy
concentrations of dolomite rhombs. However, the unit is not generally dolomitic. Several (~2-3)
possible grainflows were observed in thin section, comprised of thin layers of brecciated
foraminifer, calcisphere and gastropod test material. Ooids also occur very rarely, and were
probably transported from shallower shelfal environments.
The Buda Limestone is an extensive formation spanning much of Texas, through Big
Bend National Park, and southward through Coahuila, Mexico. The middle of the Buda
Limestone of Travis County in central Texas reportedly contains a disconformity (Martin, 1967).
Documentation of a disconformity within the Buda Limestone in Big Bend region is, however,
unknown to the author. The Buda Limestone correlates with a portion of the upper Grayson
Formation in the far north of Texas (Hancock et al., 1993). It may also correlate
geochronologically with portions of the upper Graneros Shale of New Mexico, Colorado, and
Kansas (Scott et al., 2001), and with the uppermost portion of the Cuesta del Cura Formation of
northern Mexico (Young, 1977; Ice and McNulty, 1980). The Buda Limestone of the southcentral Texas Cretaceous trend, east of Big Bend Country, has represented a proven reservoir
rock for over 80 years. Hydrocarbon migration into fractures at the top of the Buda Limestone,
likely from the Eagle Ford Group, has historically produced substantial amounts of oil (Dawson,
1986).
6
Boquillas Formation (Terlingua Group)
The Boquillas Formation does not have an official type section. It was named in the vicinity of
Boquillas Post Office, along the west flank of the Sierra del Carmen Range, Brewster Co., TX by
Udden (1907). Directly overlying the Buda Limestone, the Ernst Member of the Boquillas
Formation is a 40 to 100 m thick reddish brown to tan colored flaggy argillaceous limestone and
shale. Decimeter thick calcareous foraminiferal-calcisphere wackestone and packstone beds are
often laterally discontinuous, and are vertically separated from each other by thin, shaley layers.
Slump folding, debris flows, and even possible turbidites have been reported in the lowest
member of the Boquillas in road cuts on Highway 90, north of BBNP (Lock and Peschier, 2006).
Above the Ernst, the San Vicente Member of the Boquillas Formation consists of resistant thinly
bedded gray argillaceous limestone (Ashmore, 2003). Fossils of the Boquillas Formation include
planktonic foraminfera, calcispheres, echinoderm fragments, fish teeth, gastropods, bivalves,
and ammonites. Benthic foraminifera are virtually absent from the basal Boquillas, indicating
deep water or dysoxia, or both (Frush and Eicher, 1975).
The Ernst Member of the Boquillas Formation is equivalent to the Eagle Ford Group of
eastern Texas (Pessagno, 1969; Lock and Peschier, 2006). North of Texas, this member also
correlates with the Greenhorn Formation of the Cretaceous Western Interior Seaway (Eicher
and Worstell, 1970). The San Vicente Member of the Boquillas Formation and the Pen
Formation of BBNP correlate with the Austin Chalk and Taylor Marl of the Gulf Coast region
farther east, and with Niobrara Chalk and Pierre Shale farther north in the Cretaceous Interior
Seaway (Ashmore, 2003). To the west of BBNP, the Boquillas thickens basinward to over 700 m
and becomes the Ojinaga Formation. Northwest of BBNP, Boquillas-equivalent rock is named
after nearby Chispa Summit, Texas, becoming the Chispa Summit Formation.
7
Geochronologically correlative sections also include the Morelos and Mexcala Formations of
southern Mexico (Aguilera-Franco et al., 2001), and the Calera Limestone of California (Sliter,
1999).
The Boquillas Formation, Eagle Ford Group, and correlative strata have attracted
attention recently as self-sourced reservoir rocks. Organic-rich black shales are present
throughout the lower and middle Boquillas, with TOC contents in surface exposures around 1%
on average (Trevino, 1988). Subsurface TOC values, however, are likely significantly higher (Lock
and Peschier, 2006). Modern shale fracturing techniques may allow for further exploitation of
these unconventional plays.
Purpose
Foraminiferal biostratigraphy and paleoenvironmental interpretation
The only foraminiferal biostratigraphic study of the Ernst Member of the Boquillas Formation in
BBNP was performed by Frush and Eicher (1975). Revisions to the Cretaceous foraminiferal
geochronology since 1975 (as outlined in the Systematic Paleontology) warrant a revisit to the
foraminiferal biozonation of the basal Boquillas Formation. While this biostratigraphic work in
the Cenomanian of BBNP has focused on the Boquillas Formation, a foraminiferal biozone for
the Buda Limestone in the region has not previously been acquired. Until the foraminiferal
biozonation of the Buda Limestone is established completely, the Commanchean - Gulfian
interval in BBNP remains poorly bracketed by foraminiferal data. This lack of data is likely a
result of the highly indurated nature of the Buda Limestone and the necessity of studying
foraminifera in thin section.
8
Additionally, phylogenetic trends of planktonic foraminifera within the Buda Limestone
can lend insight to the Cenomanian sequence stratigraphy of the region. For instance, recent
research has interpreted the Del Rio Clay and Buda Limestone occupying multiple sequences
(Lock et al., 2006), as opposed to a single transgressive-regressive sequence (Scott and Kidson,
1977; Scott et al., 2002; Mancini and Scott, 2006). This study will use foraminiferal phylogentic
trends to help answer whether the Buda Limestone occupies its own depositional sequence.
Paleoexposure of the Buda Limestone
The contact between the Buda and the overlying Boquillas Formation (Commanchean - Gulfian)
is interpreted as a subaerially exposed unconformity based on the presence of a pitted
microkarst surface at the top of the Buda Limestone (Lock and Peschier, 2006; Lock et al., 2007).
However, submarine processes can sometimes create sedimentary features resembling surface
karst. If there was no subaerial exposure during the Buda - Boquillas hiatus, then the Buda
Limestone and Boquillas Formation are related in terms of sequence stratigraphy, since eustatic
sea level need not have produced the unconformity surface. If there was subaerial exposure,
then carbon and oxygen isotope evidence for meteoric alteration should be present at the top
of the Buda Limestone (Allen, and Matthews, 1982).
This study seeks to determine whether the Buda - Boquillas unconformity arose from
subaerial or submarine processes, and how long the depositional hiatus lasted. A foraminiferal
biostratigraphic framework of strata adjacent to the unconformity will allow for precise dating
of time lost during the Commanchean - Gulfian hiatus and will assist with future regional
stratigraphic interpretations. Planktonic foraminifera are particularly useful for this kind of
study, because they are less provincial than faunas such as ammonites, and have a more
9
complete deep-sea sedimentary record than other fossils. Furthermore, regional-scale
paleoceanographic understanding of the Commanchean - Gulfian boundary, and of Cenomanian
and Turonian anoxia, depends upon local sequence- and biostratigraphic interpretations.
Oceanic Anoxia Events (OAEs)
Several Oceanic Anoxia Events have been recorded in Cretaceous sediments worldwide (Fig. 16).
During these periods, global oceanic oxygen levels are reduced, and large global shifts in the
isotopic composition of seawater can be produced (Schlanger and Jenkyns, 1976). These events
lead to the widespread deposition of organic carbon, and the subsequent potential for
hydrocarbon generation (Irving et al., 1974; Jenkyns, 1980). This study will attempt to place
these events in the stratigraphic context of West Texas.
Synopsis
The purpose of this study is to evaluate the Buda - Boquillas stratigraphic interval by: 1)
Identification of the planktonic foraminiferal biozones present within the Buda Limestone and
lowermost Boquillas Formation, 2) Correlation of these units with the global geochronologic
scale of Gradstein et al. (2004); 3) Determination of phylogenetic trends and the establishment
of paleoenvironmental and sequence-stratigraphic interpretations; 4) Isotopic evaluation of the
Buda - Boquillas to determine whether the unconformity originated from subaerial or submarine
processes; 5) Isotopic evaluation of the Buda - Boquillas to determine evidence for Oceanic
Anoxia Events (OAEs).
10
Localities
Dog Canyon (DC-DS1)
This site is located roughly 3 km east of the Main Park Road towards Dog Canyon, on the left
side of the canyon when facing east (Fig. 2). The structural geology of the Dog Canyon area is
complex, with thrusted and overturned Glen Rose and Buda Formations forming the crest of the
ridge. These overturned strata rest upon non-overturned Buda Limestone and Boquillas
Formation. This lower outcrop of cliff-forming Buda Limestone was sampled at Dog Canyon,
about halfway up the ridge. The sampling site consists of a 2.5 m exposure of the massive
member at the top of the Buda Limestone, and an exposure of overlying Boquillas Formation
(Fig. 3, 4). The Del Rio Clay and the lower nodular facies of the Buda Limestone are completely
covered with talus at this location. A total of 15 samples were taken from the top of the Buda
Limestone. Additionally, a large grapefruit-sized gastropod float was recovered sitting on top of
the Buda Limestone (on the Buda - Boquillas contact) at this locality.
Dagger Flat Dataset 1 (DC-DS1)
This sampling site is located on Dagger Flat Auto Trail approximately 7.9 km from the Main Park
Road and 3.8 km before the loop at the end of Dagger Flat Auto Trail, on the northwest side of
the road (Fig. 5). An arroyo crosses Dagger Flat Auto Trail in a small valley at this location,
flowing west-southwest (from left to right across the road as one faces toward the loop at the
end of Dagger Flat Auto Trail). If one continues farther northeast up Dagger Flat Auto Trail
toward the loop, exposures of Buda Limestone and Del Rio Clay can be seen to the on the high
cliff to the left.
11
A 6.5m section near the base of the Buda Limestone was sampled at this locality (Fig. 6).
The base of the outcrop consists of the nodular lower member of the Buda Limestone. A wellpreserved echinoid was recovered in talus that likely derived from the lower portion of the
outcrop. Other macrofossils observed include only 1-3 cm long gastropod shells. This portion of
the section consists of nodules of gray porcelaneous limestone that weathers white to tan in
color. Two-thirds of the way up on the sampled outcrop, bedding becomes massive, with dm to
m thick beds separated by thin nodule and fissile layers, much like the section exposed at Dog
Canyon. This portion of the section weathers slightly darker than the nodular portion beneath,
but fresh exposures are lighter gray-buff in color. The lower nodular member tends to form a
steep slope, as opposed to the massive upper member, which tends to form vertical cliffs. This
difference in erosion must stem from the difference in nodular vs. massive characteristics
between upper and lower members, since all hand samples taken are well indurated. The Buda
- Boquillas contact was not observed at the top of the outcrop, presumably due to erosion. This
section likely represents very nearly the entire section of Buda Limestone, however the bottom
±0.5-1 m and top of the section were inaccessible. A total of 31 samples were taken at this
location.
Dagger Flat Dataset 2 (DC-DS2)
This sampling site is located along Dagger Flat Auto Trail approximately 7.3 km from the Main
Park Road and 4.5 km from the loop at the end of the trail (Fig. 5). Looking south across about
200 m of flat plain, one can see hills consisting of westward dipping Boquillas Formation. An
arroyo runs westward along the top of the Buda Limestone at the base of these hills, and 7
samples of indurated carbonate were taken there in a 1.5 m outcrop where the Buda and
Boquillas contact is exposed (Fig. 7).
12
An anomalous dm-thick layer of very dark gray-black foraminiferal porcelaneous
limestone at the contact between the Buda and Boquillas was observed at this locality. The bed
weathers light gray and is not noticeable unless broken with a rock hammer, where after it
displays its black coloration on conchoidal broken surfaces. This layer has been found only in
one other location south of Dog Canyon (Cooper, 2008). Thin section analysis reveals that this
layer contains a foraminiferal assemblage more akin to the Boquillas Formation than the Buda
Limestone, since it contains abundant keeled species. Black opaque material is present in the
matrix surrounding sparry foraminifer and calcisphere tests, but does not cross-cut them. This
bed may be a local Corg-rich layer, may represent an outlier of some unit deposited during the
Buda - Boquillas hiatus, or may have resulted from some kind of hydrothermal alteration of the
lowermost bed of the Boquillas Formation. This location is relatively close to several igneous
intrusions, so the alteration hypothesis seems likely. It is noteworthy that similar outcrop
evidence for alteration was not observed in either the Buda Limestone directly below, or the
Boquillas Formation directly above, the black limestone.
The Buda Limestone at this locality is buff-light gray in color and well indurated. It forms
a resistant "spillway" for discharge to flow through the arroyo at this location. The Boquillas
here consists of resistant flaggy brown-gray carbonate beds 1-10 cm in thickness separated by
thinner, more friable shaley layers. Within 30 cm of the top of the Buda Limestone, and 20 cm
of the anomalous black carbonate bed, a 2 cm thick black shale layer was observed. A total of 7
samples were taken at this location.
13
Highway Section Datasets 1 and 2 (HW-DS1/2)
The highway section is located on Park Route 12 near the southern entrance to the Old Ore
Road, west of the Park Route 12 tunnel (Fig. 8). A weathered exposure of Buda Formation was
sampled on a hill on the south side of Park Route 12 (Fig. 9). A series of normal faults run
roughly north-south here, and the sampled outcrop is a topographic high formed by a horst.
The Del Rio Clay at this locality was not observed, and is only about 5 ft thick this far south in
BBNP (Lock et al., 2007). Boquillas Formation crops out to the west of the sampled hill, but the
Buda - Boquillas contact was not observed at this locality. The Buda Limestone weathers
brownish tan at this location, and exposed surfaces are light gray to buff in color. Two vertical
transects of Buda Limestone hand samples were taken at this locality, comprising datasets 1 and
2. Dataset 1 is the lower transect, and dataset 2 is a vertical continuation of dataset 1 in a
slightly different horizontal location on the hill. A total of 16 samples were taken, representing
approximately 3.2 m of section.
Methods
Field Methods
All research was conducted during March, 2008 with a permit granted from the National Park
Service. Field work was conducted on foot using the geologic map of Maxwell (1968), Garmin
handheld GPS device, and Brunton compass. At each outcrop, hand samples were collected
using a rock hammer at a 20 cm interval and notes were recorded in a Rite in the Rain field
book. Any macrofossils were noted and/or collected. Samples were then placed in labeled
plastic bags and transported back to the vehicle.
14
Laboratory Methods
All rough hand samples were cut into billets at Ball State University Department of Geological
Sciences, and approximately half were thin sectioned to 25µm in-house. The rest of the billets
were thin sectioned to 25µm by Applied Petrographic Services, Inc. Thin sections were then
examined under a petrographic microscope for identification of foraminifera. Foraminifera
recognizable to at least genus level were counted, photographed, and charted in Microsoft
Excel. Pessagno (1969), Silva and Sliter (2002), and others as outlined in the Systematic
Paleontology section were used to aid in identifications. A carbon and oxygen stable isotope
analysis for Dog Canyon and Dagger Flat samples was performed at the University of Kansas W.
M. Keck Paleoenvironmental and Environmental Stable Isotope Laboratory. Billets were
polished to 1000 grit and bulk-rock micrite was drilled with a dental drill to obtain 20-80 µg of
calcite powder for isotope analysis. Areas particularaly high in dolomite and/or secondary
calcite (veins) were avoided or removed from analysis completely. Samples were baked at
200:C for one hour to release volatile compounds, then transferred to glass vials and reacted
under vacuum with 3 drops of 100% prepared phosphoric acid (ρ=1.8913 g/cm3) at 75:C. CO2
was released and trapped cryogenically, then transfered online to an IRMS instrument where it
was measured 8 times versus a calibrated CO2 reference tank for δ ratio analysis and reported
versus the VPDB scale. Analysis was performed using a Kiel Carbonate Device III + Finnigan
MAT253 isotope ratio mass spectrometer. NBS-18 Carbonatite (NIST Ref. Mat. 8543) & NBS-19
Limestone (NIST Ref. Mat. 8544) were used as quality control standards. Analytical precision
can be reported as better than 0.02‰ for δ13C, and as better than 0.05‰ for δ18O. Microsoft
Excel was used for isotope data analysis.
15
Figure 2 - Dog Canyon sampling locality. Image courtesy of the USGS and Microsoft Research Maps. URL:
http://www.MSRMaps.com.
16
Figure 3 - Upper portion of the Buda Limestone exposed at the Dog Canyon locality. Looking E. Note flags of
Boquillas Formation above. Pencil provides scale.
17
Figure 4 - Buda Limestone - Boquillas Formation contact at the Dog Canyon locality. Looking NE. Rock hammer
provides scale.
18
Figure 5 - Dagger Flat sampling localities. DF-DS1 is the eastern location and DF-DS2 is the western location. Image
courtesy of the USGS and Microsoft Research Maps. URL: http://www.MSRMaps.com.
19
Figure 6 - Buda Limestone outcrop at Dagger Flat DS1 locality. Looking NW. Author Nicholas Tiedemann provides
scale.
20
Figure 7 - Boquillas Formation at Dagger Flat DS2 locality. Looking W. Rock hammer provides scale and is resting
upon anomalous black limestone layer.
21
Figure 8 - Highway sampling locality. Image courtesy of the USGS and Microsoft Research Maps. URL:
http://www.MSRMaps.com.
22
Figure 9 - Buda Limestone at Highway locality. Looking W. Scale card is 10 cm.
23
CHAPTER 2 - SYSTEMATIC PALEONTOLOGY
Genus: DICARINELLA (Porthault, 1970)
Remarks: Members of the genus Dicarinella are the first planktonic foraminifera to evolve
double keels (Premoli Silva and Sliter, 2002). The genus is easily distinguished by the recognition
of double keels in cross-sectional view.
Paleoceanographic significance: Isotopic evidence suggests that dicarinellids inhabited deep,
sub-thermocline waters, similar to the rotaliporids (Gebhardt et al, 2004); however the genus
might have inhabited more intermediate depths at the thermocline along with the
globigerinellids and praeglobotruncanids (Keller et al., 2001). In either case, the genus is here
interpreted as inhabiting open marine environments.
Dicarinella algeriana (Caron)
Praeglobotruncana algeriana Caron, 1966, Revue de Micropaléontologie, v. 9, p. 74-75.
Remarks: D. algeriana is biconvex with a low trochopire, and is of small to medium size. This
species possesses a shallow, wide umbilicus and double keel (Premoli Silva and Sliter, 2002;
CHRONOS, 2009).
Range: Dicarinella algeriana, the first member of its genus, first appears in the R. cushmani
Zone in the lower part of the Upper Cenomanian (Premoli Silva and Sliter, 2002). The FAD of
this species defines the beginning of the Upper Cenomanian D. algeriana Subzone of the R.
cushmani Zone. Multiple specimens of the genus Dicarinella (including D. algeriana) were
recovered from the basal Boquillas Formation in this study, based on the recognition of a double
keel.
CHRONOS Range:
FO stage: Cenomanian
FO zone: R. cushmani
FO age (Ma): 93.9
25
LO stage: Turonian
LO zone: M. schneegansi
LO age (Ma): 90.3
Genus: FAVUSELLA (Michael, 1972)
Paleoceanographic significance: Favusellids are interpreted as inhabiting shallow inner to
middle neritic waters, however large favusellids (350-500 µm), as observed in the Buda
Limestone, are restricted to water greater than 30-50m deep (Koutsoukos et al., 1989). Leckie
(1987) tentatively places F. washitensis in his Epicontinental Sea Fauna, and notes that the
species is characteristic of relatively warm, shallow epeiric or marginal seas.
Favusella washitensis (Carsey)
Hedbergella washitensis (Carsey) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 278, pl. 4,
fig. 9, 10a-c, 11a-c; Pessagno, 1967, Palaeontographica Americana, v. 5, p. 245-445.
Globigerina washitensis Carsey, 1926, Univ. Texas Bulletin 2612, p. 44, pl. 7, fig. 10.
Favusella washitensis (Carsey) Michael, 1972, Journ. Foram. Research, v. 2(4), p. 215, pl. 5, fig. 13.
Remarks: Favusella washitensis is problematic for thin section biostratigraphy, since it has been
divided into several species with similar taxonomic ranges, with some disagreement over which
is the genotypic species (Michael, 1972; Longoria and Gamper, 1977; Ross and McNulty, 1981).
High morphological variation in style of coiling, number of chambers, degree of inflation, and
the degree of test ornamentation further complicate identification. However, the genus does
have extensive global distribution (Hart, 1983; Koutsoukos et al., 1989). Favusella is distinctive
for ornamental ridges that form a polygonal, honeycomb-like pattern on the test surface, which
usually display as “spikes” in cross section, although in certain cross sections the polygonal
pattern is visible. Recognition of Favusella washitensis in thin section is made possible due to
26
the species’ relatively large size, trochospiral and umbilicate form, and thick double layered wall.
Favusella occurs commonly throughout the Buda Limestone as a predominately large and
globular form. The genus Favusella has enigmatic morphological relationship with Hedbergella
and Whiteinella, and may be regarded as a subgenus of Hedbergella (Hart, 1983; Koutsoukos et
al., 1989). Some specimens in this study bear resemblance to reference photomicrographs of
Hedbergella and Whiteinella, especially W. brittonensis, W. baltica, and W. paradubia. The
difficult task of differentiating these species in thin section was based upon some morphological
assertions: 1) Favusella differs from Hedbergella by the presence of the ornamental ridges on
the test, and by having a higher trochospire. 2) Whiteinella differs from Favusella by having
more offset chambers and by having a pustulose wall not ornamented in a polygonal fashion.
The latter category is difficult to distinguish in thin section; however some thin section
specimens of Favusella display the polygonal ornamentation. Whiteinella brittonensis and
Whiteinella paradubia may also have a larger size and higher trochospire than Favusella
washitensis, although size and trochospire height is not considered an absolute means of
differentiation, given the morphological variability within Favusella.
Range: F. washitensis was identified by Pessagno (1969) in the Buda Limestone only in the
vicinity of Austin, TX. The exact stratigraphic location within the Buda is not specified, nor is
whether F. washitensis was recovered in any other locations. Pessagno (1969) reports that no
specimens of F. washitensis have been positively identified in sediments attributable to the
Rotalipora cushmani Zone, yet specifies in a range chart that F. washitensis may extend into the
Rotalipora cushmani Zone in the Trans-Pecos region. Silva and Sliter (2002) indicate the range of
F. washitensis from the late Aptian Ticinella bejaouaensis Zone through the early Cenomanian
Rotalipora brotzeni Zone. Stewart and Pearson (2002) place the first appearance of F.
27
washitensis at 116.6 Ma (Late Aptian) and the last appearance at 96.2 Ma (Early to Middle
Cenomanian). Hart (1983) reports Favusella washitensis occurring within the Early to Middle
Cenomanian Thalmanninella reicheli Zone alongside Praeglobotruncana stephani,
Praeglobotruncana delrioensis, Parathalmanninella appenninica, Thalmanninella reicheli, and
Whiteinella brittonensis. Similarly, favusellids have been recovered from the top of the
Thalmanninella reicheli Zone in the Anglo-Paris Basin (Koutsoukos et al., 1989). While many
workers consider Favusella extinct by the Middle Cenomanian, there are scattered reports of
the genus occurring within the Early to Middle Cenomanian Rotalipora greenhornensis Zone and
even into the Upper Cenomanian (McNeely, 1973; Koch, 1977; Rösler et al., 1979).
CHRONOS Range:
FO stage: Aptian
FO zone: T. bejaouaensis
FO age (Ma): 116.6
28
LO stage: Cenomanian
LO zone: R. globotruncanoides
LO age (Ma): 96.2
Genus: GLOBIGERINELLOIDES (Cushman and Ten Dam, 1948)
Paleoceanographic significance: The genus Globigerinelloides inhabited intermediate depths
with normal salinity, below the surface mixed layer, and above keeled forms such as rotaliporids
(Leckie, 1987; Keller et al., 2001; Stewart and Pearson, 2002; Keller and Pardo, 2003).
Globigerinelloides and Hedbergella characterize the Shallow Water Fauna (>100m) of Leckie
(1987), and are also present in his shallower Epicontinental Sea Fauna (>100m), which is
dominated by genera such as Heterohelix, Guebelitria, and Gubkinella. Leckie (1987) reports
that members of the Epicontinental Sea Fauna increase relative to members of the open marine
Shallow Water Fauna with decreasing water depth. The abundance trends of Globigerinelloides
may be a proxy for water depth, with a decrease in Globigerinelloides shoreward (Sliter, 1972).
The number of Globigerinelloides sp. per thin section was counted for the Buda, which shows a
significant decrease up section. The data are interpreted as showing a lower deepening trend,
followed by an upper shoaling trend that persists to the top of the section.
Globigerinelloides bentonensis (Morrow)
Anomalina bentonensis Morrow, 1934, Jour. Paleont., v. 8, p. 201, pl. 30, figs. a-b.
Planomalina caseyi Bolli, Loeblich and Tappan, 1957, p. 24. pl. 1, figs. 4-5.
Globigerinelloides bentonensis (Morrow), Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 267278, pl. 2, figs. 8-10.
Remarks: Globigerinellids are identified in thin section by their distinctive planispiral test. G.
bentonensis is identified in thin section by its thin, smooth wall, rapidly increasing globular
chambers, and small umbilicus (Pessagno, 1967; Premoli Silva and Sliter, 2002). G. bentonensis is
29
very similar to G. caseyi, but is distinguished from G. caseyi by being less compressed in cross
section (Ice and McNulty, 1980), by being smaller and more evolute, and by having less rapidly
expanding and less inflated chambers (Eicher and Worstell, 1970). It is distinguished from G.
ultramicrus by having a larger size,and by lacking a pustulose wall (Petrizzo and Huber, 2006).
While characterizing the three species in thin section can be difficult, their complete taxonomic
range overlap in the Cenomanian mitigates problems caused by misidentification.
Range: G. bentonensis ranges from the Albian T. primula Zone through the Upper Cenomanian
D. algeriana Zone (Premoli Silva and Sliter, 2002). Ice and McNulty (1980) consider the species
an Early and Middle Cenomanian form. G. bentonensis is also reported from the Upper
Cenomanian Greenhorn Formation of Kansas and the Lower Cenomanian Grayson (Del Rio)
Formation of Texas (Pessagno, 1967).
CHRONOS Range:
FO stage: Albian
FO zone: T. primula
FO age (Ma): 107.3
30
LO stage: Cenomanian
LO zone: R. cushmani
LO age (Ma): 94
Globigerinelloides caseyi (Bolli, Loeblich and Tappan)
Planomalina caseyi Bolli, Loeblich and Tappan, 1957, U.S. National Museum Bulletin, v. 215, p.
24, pl. 1, figs. 4-5.
Globigerinelloides eaglefordensis (Moreman), Loeblich and Tappan, 1961, p. 268, pl. 2, figs. 3-7.
Globigerinelloides caseyi (Bolli, Loeblich and Tappan), Low, 1964, Contr. Cushman Found. Foram.
Res., v. 15, p. 122-123.
Globigerinelloides escheri (Kaufman), Eicher, 1969b, p. 167 (list only).
Remarks: Globigerinelloides caseyi is distinguished by its planispiral test and smooth wall. It has
less inflated chambers that G. bentonensis, and further distinguishing features are discussed in
the entry for G. bentonensis.
Range: G. caseyi ranges from the Early Cenomanian Th. globotruncanoides Zone through the
Santonian Dicarinella asymetrica Zone (Premoli Silva and Sliter, 2002). This species has been
reported from the Del Rio Clay and from the Eagle Ford Group, including the Boquillas Formation
(Pessagno, 1967).
CHRONOS Range:
FO stage: Albian
FO zone: P. appenninica
FO age (Ma): 100.6 (Approximation)
31
LO stage: Cenomanian
LO zone: R. cushmani
LO age (Ma): 94 (Approximation)
Globigerinelloides ultramicrus (Subbotina)
Globigerinella ultramicra Subbotina, 1949, Leningrad, Vsesoyuznogo Neftyanogo
Nauchnoissledovatel'skogo Geologorazvedochnogo Instituta (VNIGRI), Vypusk, v. 34, p. 33.
Remarks: Globigerinelloides ultramicrus is characterized by its small size, spinose wall, and
slowly enlarging chambers (Premoli Silva and Sliter, 2002). It is the smallest species of
globigerinellid observed in the Buda Limestone.
Range: This species ranges from the Albian Rotalipora subticinensis Zone through the
Campanian-Maastrichtian Gansserina gansseri Zone (Premoli Silva and Sliter, 2002).
CHRONOS Range:
FO stage: Albian
FO zone: R. subticinensis
FO age (Ma): 101.6
32
LO stage: Maastrichtian
LO zone: G. gansseri
LO age (Ma): 70.4
Genus: HEDBERGELLA (Bronnimann and Brown, 1958)
Paleoceanographic significance: Hedbergellids are interpreted as inhabiting near-surface
waters based on stable isotope rankings and paleogeographic distribution (Sliter, 1971; Leckie,
1987; Price et al., 1998; Keller et al., 2001; Keller and Pardo, 2004; Bornemann and Norris,
2007). The δ13C values of Hedbergella delrioensis are heavier than keeled morphotypes,
indicating a shallower depth ecology, and δ18O values are highly variable, possibly indicating a
relatively wide range of depth habitats and an opportunistic strategy (Price et al., 1998;
Bornemann and Norris, 2007).
Hedbergella delrioensis (Carsey)
Globigerina cretacea d’Orbigny var. delrioensis Carsey, 1926, Texas Univ. Bulletin, p. 43.
Hedbergella delrioensis (Carsey) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 275, fig. 1113.
Remarks: Hedbergella delrioensis is distinguished by its small size, low trochospiral test,
macroperforate wall, coarsely rugose first chambers, deep and small umbilicus, and smooth final
chambers (Pessagno 1967). It differs from H. planispira by having a deeper umbilicus and thicker
test, and by having more rapidly increasing chambers (Pessagno, 1967; Petrizzo and Huber,
2006).
Range: Hedbergella delrioensis was originally described from the Grayson Formation (Del Rio
Clay equivalent). This species has a large biostratigraphic range and makes a poor biomarker. It
33
is reported from the Early Cretaceous through the Coniacian (Silva and Sliter, 2002). This
species occurs sporadically throughout the Buda Limestone.
CHRONOS Range:
FO stage: Albian
FO zone: P. appenninica
FO age (Ma): 100.6
34
LO stage: Santonian
LO zone: D. concavata
LO age (Ma): 86 (Approximation)
Hedbergella planispira (Tappan)
Globigerina planispira, Tappan, 1940, Journal of Paleontology, v. 14, p. 12, pl. 19, fig. 12.
Remarks: Hedbergella planispira differs from other members of the genus be having a low
trochospiral to planispiral test (Pessagno, 1967). The species is recognized in thin sections of the
Buda Limestone by its relatively small size, comparatively smooth wall, and by its flat or slightly
depressed trochospire (Eicher and Worstell, 1970). This species is differentiated from the genus
Globigerinelloides by lacking a true planispiral test, the early chambers being slightly offset
above the horizontal axis.
Range: H. planispira has a seemingly large biostratigraphic range and is reported from the
Graneros Shale, Mowry Shale, and overlying Greenhorn-Fairport sequence (Eicher and Worstell,
1970). Its total range extends from the Aptian through the Campanian (Silva and Sliter, 2002),
but its early origins may be problematic (Petrizzo and Huber, 2006). This species is, however,
known from the Cenomanian (Petrizzo and Huber, 2006). It occurs frequently yet in low
abundance throughout the Buda Limestone.
Paleoceanographic significance: Keller and Pardo (2004) observed that low H. planispira
percentages correlate with 2-4‰ negative δ18O excursions. They suggest that H. planispira had
a lower tolerance for normal marine salinity than H. delrioensis and C. simplex, and report H.
planispira abundances are a good proxy for surface salinity.
CHRONOS Range:
FO stage: Aptian
FO zone: G. blowi
FO age (Ma): 118.1
35
LO stage: Coniacian
LO zone: D. concavata
LO age (Ma): 88.6
Clavihedbergella simplex (Morrow)
Hastergerinella simplex Morrow, 1934, Journal of Paleontology, v. 8, p.198, pl. 30, fig. 6 .
Clavihedbergella simplex (Morrow) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 279, 280,
pl. 3, fig. 12a-14b.
Remarks: Clavihedbergella simplex is a medium sized species with a low trochospire. It is
characterized in thin section by its large globular and elongate final chamber (Silva and Sliter,
2002).
Range: Clavihedbergella simplex has been reported from the Cenomanian and Turonian
portions of the Eagle Ford Group from the Rio Grande area to Dallas (Pessagno, 1967). The
species has a long biostratigraphic range, from the Albian Ticinella primula Zone through the
Coniacian Dicarinella concavata Zone (Silva and Sliter, 2002). It occurs very rarely and
sporadically in the Buda Limestone.
CHRONOS Range:
FO stage: Albian
FO zone: H. planispira
FO age (Ma): 112.2
36
LO stage: Coniacian
LO zone: D. concavata
LO age (Ma): 85.8
Genus: HETEROHELIX (Ehrenberg, 1843)
Paleoceanographic significance: Heterohelicids are interpreted as being opportunistic surface
dwellers, able to tolerate adverse environments (Sliter, 1971; Leckie, 1987; Keller et al., 2001;
Gebhardt et al., 2004). Leckie (1987) uses the genus Heterohelix, along with Guebelitria, and
Gubkinella, to define his shallow Epicontinental Sea Fauna (<100m), reporting that these genera
occur in drastically reduced abundances in open marine sections. Isotope work done by
Bornemann and Norris (2007) supports a shallow depth ecology. High abundances of
Heterohelix have been attributed to expansion of the oxygen minimum zone, oxygen
fluctuation, salinity fluctuation, and high nutrient supply (Eicher and Worstell, 1970; Sliter, 1971;
Leckie, 1987; Nederbragt et al., 1998; Keller et al., 2001; Gebhardt et al., 2004). Nederbragt et
al. (1998) showed that increased relative abundances of heterohelicids may correlate with
positive δ13C excursions due to global anoxia events, and demonstrated strong correlations
across the Cenomanian/Turonian boundary. Because heterohelicids were opportunists capable
of living in a variety of unfavorable conditions, an exact cause for high H. moremani abundance
in the Buda Limestone is difficult to determine. However, because benthic foraminifera and
gastropods are consistently present throughout the entire Buda Limestone, it is unlikely that the
abundance of H. moremani indicates low oxygen conditions or oxygen minimum zone
expansion. It may be that the Buda was deposited in shallow enough water to have created a
hostile planktonic environment with significant elimination of open ocean planktonic habitats,
yet still deep enough to permit a mostly planktonic assemblage. Variable or increased nutrient
supply and/or upwelling may also have played a role (Nederbragt, 1998; Gebhartd et al., 2004).
Further analysis is given in the Biostratigraphy and Stable Isotopes sections of this work.
37
Heterohelix moremani (Cushman)
Guembelina moremani Cushman, 1938, Contr. Cushman Lab. Foram. Res., v. 14, p. 10, pl. 2, figs.
1-3.
Guembelina washitensis Tappan, 1940, Jour. Paleont., v. 14, p. 115, pl. 19, fig. 1.
Heterohelix sp. Ayala, 1962, Soc. Geol. Mexicana, Bol., v. 25, p. 11, pl. 1, figs. 1a-c; pl. 6, figs. 1a-c
Remarks: Heterohelix moremani is distinguished in thin section by its small size, biserial test,
and smooth wall. The globular chambers enlarge slowly until they become parallel-sided and
equi-dimensional (Premoli Silva and Sliter, 2002). This species is distinguished from H. globulosa
by having more slowly enlarging chambers, lacking well-developed striae, and by being generally
smaller, although a range of transitions between the two species has been observed
(Nederbragt, 1991; Premoli Silva and Sliter, 2002). Heterohelix moremani dominates the
planktonic foraminiferal assemblage of the Buda Limestone, and in most samples represents
approximately half of all planktonic forms.
Range: Heterohelix moremani ranges from the Albian Pseudothalmanninella ticinensis Zone
through the Santonian Dicarinella asymetrica Zone (Premoli silva and Sliter, 2002). It is the
earliest known species from the Heterohelix genus and is ancestral to all later species
(Nederbragt, 1991). This species is known from Texas in the Grayson Formation (Del Rio Clay
equivalent) and Eagle Ford Group of Texas, and from the San Felipe Formation of Mexico
(Pessagno, 1967). It has also been recognized in the Morelos, Cuautla, and Mexcala Formations
of southern Mexico (Aquilera-Franco, 2003). Heterohelix moremani occurs ubiquitously and
consistently throughout the Buda Limestone, each thin section containing over 20 recognizable
specimens. Heterohelix moremani dominates the planktonic assemblage except in the lower-
38
middle portion of the Buda Limestone where globigerinellids reach their very greatest
abundance of about 30 – 40 recognizable specimens per thin section.
CHRONOS Range:
FO stage: Albian
FO zone: R. ticinensis
FO age (Ma): 101.6
39
LO stage: Turonian
LO zone: M. schneegansi
LO age (Ma): 90.7
Heterohelix globulosa (Cushman)
Textularia globulosa Ehrenberg, 1840, p. 135, pl. 4, figs. 2b, 4b, 5b, 7b, 8b.
Textularia striata Ehrenberg 1840, p. 135, pl. 4, figs. 1a, 2a, 3a, (? 9a).
Guembelina reussi Cushman, 1938, Cushman Laboratory for Foraminiferal Research,
Contributions, v. 14, p. 11., pl. 2, figs. 6-9.
Remarks: H. globulosa differs from H. moremani by having more rapidly increasing chambers,
and by possessing better-developed striae (Premoli Silva and Sliter, 2002). This species evolved
from H. moremani, and is thus very similar (Pessagno 1967, Nederbragt, 1991). This study
follows the conclusions drawn by Nederbragt (1991), which considers the species H. reussi and
H. striata as morphotypes of H. globulosa. The morphotype H. reussi is characterized by
triangular depressions along its median sutures, whereas the morphotype H. striata possesses
coarser costae.
Range: This species ranges from the Late Cenomanian D. algeriana Subzone of the R. cushmani
Zone through the end of the Cretaceous (Nederbragt, 1991). Variable cross sectional
orientation in thin section complicates identification of striae and chamber enlargement, making
H. globulosa difficult to distinguish from H. moremani (Nederbragt et al., 1998). Nevertheless,
certain specimens from the basal Boquillas are likely H. globulosa, since they display more
rapidly increasing chambers than the heterohelicids observed in the Buda. This species has
been reported from the basal 10-20 feet of the Boquillas in BBNP (Frush and Eicher, 1975).
CHRONOS Range:
FO stage: Cenomanian
FO zone: R. cushmani
FO age (Ma): 93.6
40
LO stage: Maastrichtian
LO zone: A. mayaroensis
LO age (Ma): 65
Genus: PARATHALMANNINELLA Lipson-Benitah, 2008
Paleoceanographic significance: (See genus ROTALIPORA).
Parathalmanninella appenninica (Renz)
Globotruncana appenninica Renz, 1936, Eclogae Geologicae Helvetiae, v. 29, p. 20.
Rotalipora appenninica (Renz) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 299-301, figs.
11a-c, 12a-c.
Rotalipora appenninica appenninica (Renz), Luterbacher and Premoli Silva, 1966, p. 266–268, pl.
19, figs. 1, 2.
Rotalipora appenninica (Renz), Robaszynski, Caron, and others, 1979, p. 60, pl. 4, figs. 1a-c.
Thalmanninella appenninica (Renz) González-Donoso, Linares, and Robaszynski, 2007, Journal of
Foraminiferal Research, v. 37, p. 183.
Parathalmanninella appenninica (Renz) Lipson-Benitah, S., 2008, Journal of Foraminiferal
Research, v. 38, p. 185, fig. 2.
Remarks: Parathalmanninella appenninica is characterized in thin section by its small to
medium size, low trochospiral test, smooth and macroperforate wall, single-keeled angular
peripheral margin, rapidly enlarging chambers, and oblate outline (Premoli Silva and Sliter,
2002; Petrizzo and Huber, 2006). The species is known for considerable taxonomic variability
(Lipson-Benitah, 2008).
Range: P. appenninica ranges from the base of the Upper Albian P. appenninica Zone to the
lower R. cushmani Zone in the Middle Cenomanian (González-Donoso, 2007; Lipson-Benitah
41
2008). P. appenninica is reported from the Maness Formation overlying the Buda Limestone in
Tyler County, TX (Barret and Goodson, 2006). Rare specimens have been identified from the
Buda Limestone by this study.
CHRONOS Range:
FO stage: Albian
FO zone: P. appenninica
FO age (Ma): 101.3
42
LO stage: Cenomanian
LO zone: R. cushmani
LO age (Ma): 94.3
Parathalmanninella micheli (Sacal and Debourle)
Globotruncana (Rotalipora) micheli Sacal and Debourle, 1957, Eclogae Geologicae Helvetiae, v.
78, p. 58.
Thalmanninella micheli (Sacal and Debourle) González-Donoso, J.M., Linares, D., and
Robaszynski, F., 2007, Journal of Foraminiferal Research, v. 37, p. 183.
Parathalmanninella micheli (Sacal and Debourle) Lipson-Benitah, S., 2008, Journal of
Foraminiferal Research, v. 38, p. 185, fig. 2.
Remarks: P. micheli is characterized in thin section by its medium size, asymmetrical profile,
and compact test. It appears similar to Th. brotzeni in cross-section, but is distinguished by its
curved final chamber (Premoli Silva and Sliter, 2002). Lipson-Benitah (2008) contends that this
species descended from P. appenninica rather than P. balernaensis, as proposed by GonzálezDonoso et al. (2007).
Range: P. micheli ranges from the upper portion of the Th. globotruncanoides Zone in the late
Lower Cenomanian to the base of the Th. reicheli Zone in the Middle Cenomanian (GonzálezDonoso et al., 2007; Lipson-Benitah, 2008). A single specimen was identified from the Buda
Limestone in this study.
CHRONOS Range:
FO stage: Albian
FO zone: P. appenninica
FO age (Ma): 99
43
LO stage: Cenomanian
LO zone: Th. reicheli
LO age (Ma): 96.8
Genus: PRAEGLOBOTRUNCA (Bermudez, 1952)
Paleoceanographic significance: Praeglobotruncanids are interpreted as inhabiting
intermediate depths, within the lower photic zone (Keller et al., 2001; Gebhardt et al., 2004;
Keller and Pardo, 2004). Leckie (1987) places the genus in his Deep Water Fauna (>100m), but
indicates that praeglobotruncanids likely lived above rotaliporids in the water column.
Praeglobotruncana delrioensis (Plummer)
Globorotalia delrioensis Plummer, 1931, Univ. Texas Bulletin, p. 199, pl. 13, figs. 2a-c.
Praeglobotruncana delrioensis (Plummer) Robaszynski, Caron, and others, 1979, p.29, pl. 43,
figs. 1a-c.
Remarks: Praeglobotruncana delrioensis is distinguishable in thin section by its small to
medium size, trochospiral test, biconvex and lobulate outline, truncate peripheral band, and
pustulose umbilical surface (Premoli Silva and Sliter, 2002; Petrizzo and Huber, 2006). The
species exhibits a large range of morphological variability in the number and shape of the
chambers, and in the degree of keel development (Petrizzo and Huber, 2006).
Range: Premoli Silva and Sliter (2002) indicate the first appearance of P. delrioensis in the
Albian Rotalipora subticinensis Zone and ranges into the lowermost portion of the Upper
Cenomanian Dicarinella algeriana Subzone of the Rotalipora cushmani Zone. Petrizzo and
Huber (2006) indicate that P. delrioensis first appears in the Albian Parathalmanninella
appenninica Zone and ranges into the upper part of the Middle-Upper Cenomanian Rotalipora
cushmani Zone. P. delrioensis is known from the Grayson Formation (Del Rio Clay) of Texas, and
from the upper Cuesta del Cura Formation of Mexico (Pessagno, 1967; Ice and McNulty, 1980).
44
CHRONOS Range:
FO stage: Albian
FO zone: P. appenninica
FO age (Ma): 99.9
45
LO stage: Cenomanian
LO zone: H. helvetica
LO age (Ma): 92 (Approximation)
Praeglobotruncana stephani (Gandolfi)
Globotruncana stephani Gandolfi, 1942, Rivista Italiana di Paleontologia, v. 4, p. 130, pl. 3, figs.
4a-c.
Praeglobotruncana stephani (Gandolfi) Robaszynski, Caron, and others, 1979, p. 47, pl. 48, figs.
1a-c.
Remarks: Praeglobotruncana stephani is characterized in thin section by its medium size,
moderately high trochospire, biconvex outline, umbilical-extraumbilical aperture (Premoli Silva
and Sliter, 2002; Petrizzo and Huber, 2006). The species differs from P. derioensis by having a
more convex spiral side, a higher trochospire, and by possesing a more highly developed line of
pustules on the peripheral margin forming a keel (Petrizzo and Huber, 2006).
Range: P. stephani has a moderately long biostratigraphic range and first appears in the Albian
R. appenninica Zone and ranges into the upper part of the Turonian H. helvetica Zone (Premoli
Silva and Sliter, 2002). Rare specimens are observed throughout the Buda Limestone and in the
basal Boquillas Formation in this study.
CHRONOS Range:
FO stage: Albian
FO zone: P. appenninica
FO age (Ma): 101.3
46
LO stage: Turonian
LO zone: H. helvetica
LO age (Ma): 91
Praeglobotruncana gibba Klaus
Praeglobotruncana stephani Gandolfi var. gibba Klaus, 1960, Eclogae Geologicae Helvetiae, v.
53, p. 304-305.
Praeglobotruncana stephani (Gandolfi) Loeblich and Tappan, 1961, p. 280-284, pl. 6, figs. 4a-b,
5a-c, 6, 7a-c.
Praeglobotruncana gibba Klaus, Robaszynski and Caron, 1979, p. 33-38, pl. 44, figs. 1a-c, 2a-c; pl.
45, figs. 1a-c, 2a-c.
Remarks: Praeglobotruncana gibba is distinguished in thin section from other members of the
genus by its medium to large size and very distinct high trochospire. The species is also marked
by its pustulose umbilical surface (Hasegawa, 1999; Premoli Silva and Sliter, 2002).
Range: P. gibba first appears just below the FAD of Th. greenhornensis in the Early Cenomanian
(Hasegawa, 1999) and ranges through the Turonian H. helvetica Zone (Premoli Silva and Sliter,
2002). A single specimen was identified with high confidence from the lower Boquillas in this
study. Possible fragments of P. gibba were also observed.
CHRONOS Range:
FO stage: Cenomanian
FO zone: P. appenninica
FO age (Ma): 96.5 (Approximation)
47
LO stage: Turonian
LO zone: H. helvetica
LO age (Ma): 90.3
Genus: ROTALIPORA Brotzen, 1942
Paleoceanographic significance: Rotaliporids are interpreted as inhabiting deep waters below
the thermocline, with stable isotope values close to those of benthic foraminifera (Price et al.,
1998; Keller et al., 2001; Gebhardt et al., 2004; Keller and Pardo, 2004). Leckie (1987) uses
rotaliporids, along with the genera Planomalina and Praeglobotruncana, to define his Deep
Water Fauna (>100m). Paleogeographic faunal distributions indicate that rotaliporids were
adapted for stable and well-stratified water masses, and were especially sensitive to
environmental changes (Leckie, 1987; Keller and Pardo, 2004).
Rotalipora montsalvensis (Mornod)
Globotruncana (Rotalipora) montsalvensis Mornod, 1949, Eclogae Geologicae Helvetiae, v. 42, p.
584-585, fig. 4 (Ia-c).
Rotalipora montsalvensis Mornod; Caron, 1976, p. 329-330, figs. 1a-c.
Remarks: Rotalipora montsalvensis is distinguished in thin section by its medium size,
biconvexity, weakly developed peripheral keel, and slightly inflated chambers (Caron and
Spezzaferi, 2005; González-Donoso et al., 2007). The species differs from R. cushmani by lacking
a spiral depression and fully-developed keel (Keller et al., 2001).
Range: R. montsalvensis ranges from the middle of the Lower to Middle Cenomanian Th.
globotruncanoides Zone through the Middle to Upper Cenomanian R. cushmani Zone (Caron and
Spezzaferri, 2006; González-Donoso et al., 2007). Rare specimens of R. montsalvensis were
48
recognized from the Buda Limestone, and represent important biostratigraphic indices. A single
specimen of R. montsalvensis was recognized from the basal Boquillas Formation.
CHRONOS Range:
FO stage: Cenomanian
FO zone: Th. globotruncanoides
FO age (Ma): 97.2
49
LO stage: Cenomanian
LO zone: R. cushmani
LO age (Ma): 93.8
Rotalipora cushmani (Morrow)
Globorotalia cushmani Morrow, 1934, Jour. Paleont., v. 8, p. 199, pl. 31, figs. 2, 4.
Rotalipora turonica Brotzen, 1942, Sver. Geol. Unders., Ser. C, No. 451, Ars. 36, n. 8, p. 32, figs.
10, 11.
Globotruncana alpina Bolli, 1945, Eclogae Geol. Helv., v. 37, p. 224, 225, pl. 9, figs. 3, 4.
Rotalipora montsalvensis Mornod, Hagn, and Zeil, 1954, Eclogae Geol. Helv., v. 47, p. 29, pl. 1,
fig. 4; pl. 5, fig. 2.
Rotalipora cushmani (Morrow) Loeblich and Tappan, 1964, Treatise on Invert. Paleont., v. 2, p.
659-661 fig. 528, 1a-c, 2a-c.
Rotalipora cushmani (Morrow) Desmares, Grosheny, and Beaudoin, 2008, Marine
Micropaleont., v. 69, p. 96, pl. 1, figs. 3a-c.
Remarks: Rotalipora cushmani is characterized by its medium to large size, both spirally and
umbilically inflated angular chambers, and strongly developed single keel (Premoli Silva and
Sliter, 2002; González-Donoso et al., 2007). It is differentiated in thin section from the similar R.
montsalvensis by the presence of a spiral depression and fully-developed keel (Keller et al.,
2001). Rotalipora montsalvensis may be the ancestor of Rotalipora cushmani; however the two
species may also have derived from a common ancestor (Desmares et al., 2008). In thin section,
R. cushmani is distinguished from Th. greenhornensis by having more inflated and less sharply
angular chambers, and by posessing a spiral depression.
Range: The Middle-Upper Cenomanian R. cushmani Zone is defined by the total range of the
nominate species. The extinction of Rotalipora cushmani at the onset of the OAE2 event
50
corresponds to the extinction of all rotaliporids except Anaticinella multiloculata, which likely
persists for a short time into the Upper Cenomanian W. archaeocretacea Zone (Caron et al.,
2006; González-Donoso et al., 2007). Rotalipora cushmani was not identified in the Buda
Limestone in this study. However, a small number of relatively poorly preserved specimens
were identified from the basal Boquillas Formation.
CHRONOS Range:
FO stage: Cenomanian
FO zone: R. cushmani
FO age (Ma): 96.2
51
LO stage: Cenomanian
LO zone: R. cushmani
LO age (Ma): 93.5
Genus: THALMANNINELLA Sigal, 1948
Paleoceanographic significance: (See genus ROTALIPORA).
Thalmanninella globotruncanoides (Sigal)
Thalmanninella brotzeni Sigal, 1948, Revue de l’Institut Francais du Pêtrole et Annales des
Combustile Liquides, v. 3, p. 102.
Thalmanninella globotruncanoides Sigal, 1948, Revue de l’Institut Francais du Pêtrole et Annales
des Combustile Liquides, pl. 1, fig. 4.
Thalmanninella brotzeni (Sigal) Premoli-Silva and Sliter, 2002, Practical Manual of Cretaceous
Planktonic Foraminifera, p. 406, pl. 163, figs. 9, 12.
Thalmanninella globotruncanoides (Sigal) González-Donoso, Linares, and Robaszynski, 2007,
Journal of Foraminiferal Research, v. 37, p. 184.
Remarks: There is some dispute whether Thalmanninella globotruncanoides and
Thalmanninella brotzeni represent a single species. The species were originally distinguished by
the presence of umbilical secondary apertures in Th. brotzeni, and the presence of umbilical
(first chambers of the last whorl) and sutural (last chambers) supplementary apertures in Th.
globotruncanoides (González-Donoso et al., 2007). Th. brotzeni is also smaller and more
biconvex. González-Donoso et al. (2007) consider Th. brotzeni a junior synonym of Th.
globotruncanoides, while Lipson-Benitah (2008) considers the two species separate. LipsonBenitah (2008) differentiates the two species based on their sizes, growth rates, chamber
morphologies, and position of the last-formed supplementary apertures, and states that Th.
brotzeni appears before Th. globotruncanoides in the Cenomanian of Israel. This study does not
52
differentiate between the two species, since the most distinguishing characteristics are not
observable in thin section.
Range: Thalmanninella globotruncanoides ranges from the base of the Lower Cenomanian to
the upper part of the Middle-Late Cenomanian R. cushmani Zone (González-Donoso et al., 2007;
Lipson-Benitah, 2008). The first appearance of Th. globotruncanoides through the first
appearance of Th. reicheli defines the Lower-Middle Cenomanian Th. globotruncanoides Zone.
A single specimen of this species was tentatively identified from the Buda Limestone. This
species is included here due to its importance for Lower Cenomanian biostratigraphy.
CHRONOS Range:
FO stage: Cenomanian
FO zone: Th. globotruncanoides
FO age (Ma): 98.9
53
LO stage: Cenomanian
LO zone: R. cushmani
LO age (Ma): 93
Thalmanninella greenhornensis (Morrow)
Globorotalia greenhornensis Morrow, 1934, Journal of Paleontology, v. 8, p. 199.
Rotalipora greenhornensis (Morrow) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 299-301,
pl. 7, figs. 7a-9c.
Thalmanninella greenhornensis (Morrow) González-Donoso, Linares, and Robaszynski, 2007,
Journal of Foraminiferal Research, v. 37, p. 184.
Remarks: Thalmanninella greenhornensis is distinguished in thin section by being nearly planoconvex and by its large size. The species is marked by its smooth spiral surface, broad umbilicus,
and angular chambers on the umbilical side (Premoli Silva and Sliter, 2002). Th. greenhornensis
displays reversed features with respect to the evolutionary trends between other members of
the genus. The Th. praebalernaensis to Th. globotruncanoides lineage is marked by further
development of the keel, decreased inflation of the chambers, and an increase of the umbilical
ornamentation (González-Donoso et al., 2007). In thin section, Th. greenhornensis may have a
weaker keel and slightly more inflated chambers compared to other members of the genus. A
tentatively recognized form is that of Anaticinella sp. cf. A. multiloculata. This species evolved
from Th. greenhornensis by the reduction of the keel and by the inflation of the chambers, likely
an adaptation to living higher in the water column as a response to expanded global oxygen
minimum zones in deeper water (Caron et al., 2006; Keller and Pardo, 2004). Differentiation of
A. multiloculata and Th. greenhornensis is difficult due to transitional morphotypes between the
two (Keller and Pardo, 2004), however several unidentified specimens in this study likely
represent A. multiloculata.
54
Range: Th. greenhornensis first appears in the Th. globotruncanoides Zone in the Lower
Cenomanian and ranges through the end of the R. cushmani Zone in the Upper Cenomanian
(González-Donoso et al., 2007; Lipson-Benitah, 2008). The species was reported from the basal
110 feet of the Boquillas Formation at the Hot Springs locality in Big Bend National Park by Frush
and Eicher (1975). Numerous specimens were identified in the lower 1.5 meters of the Boquillas
Formation by this study.
CHRONOS Range:
FO stage: Cenomanian
FO zone: R. cushmani
FO age (Ma): 96.2
55
LO stage: Cenomanian
LO zone: R. cushmani
LO age (Ma): 93.7
Genus: WHITEINELLA (Pessagno, 1967)
Remarks: The globular whiteinellids are distinguished from each other in thin section primarily
by trochospire height, width of umbilicus, and symmetry (Premoli Silva and Sliter, 2002). This
genus is likely present from samples taken from the lower Boquillas Formation; however all
specimens are poorly preserved and infilled with calcite spar. Sparry infill complicates the
recognition of the cross-sectional orientation of globular foraminifera, therefore making
identification to species level difficult. Pessagno (1967) reports the presence of whiteinellids
from the Eagle Ford Group in Texas, including W. brittonensis (Loeblich and Tappan) from
Cenomanian and Turonian portions of the sections.
Paleoceanographic significance: Whiteinellids likely occupied the lower part of the surface
mixed layer and preferred normal marine salinity (Keller and Pardo, 2004). They share
ecological affinities with hedbergellids and heterohelicids, and are interpreted as shallow to
intermediate depth opportunists. However, the genus is also commonly found in open ocean
deposits (Keller et al., 2001; Gebhardt et al., 2004).
56
Whiteinella aprica (Loeblich and Tappan)
Ticinella aprica Loeblich and Tappan, 1961, Micropaleontology, v. 7, p. 292, pl. 4, figs. 14-16.
Remarks: Whiteinella aprica possesses a particularly low trochospire, wide umbilicus, and
pustulose surface. Chambers are globular and enlarge slowly, making the species is nearly
symmetrical in cross-section (Premoli Silva and Sliter, 2002). Well-preserved specimens may
display thin, delicate umbilical plates (Eicher and Worstell, 1970).
Range: This species was identified by Frush and Eicher (1975) in the lowest sampled intervals of
Boquillas Formation at Chispa Summit and in the Jeff Davis Mountains. Whiteinella aprica
ranges from the Upper Cenomanian D. algeriana Subzone of the R. cushmani Zone to nearly the
end of the Turonian Helvetoglobotruncana helvetica Zone (Premoli Silva and Sliter, 2002).
CHRONOS Range:
FO stage: Cenomanian
FO zone: W. archaeocretacea
FO age (Ma): 94
57
LO stage: Coniacian
LO zone: M. schneegansi
LO age (Ma): 88.8
Whiteinella baltica Douglas and Rankin, 1969, Lethaia, v. 2, p. 197.
Remarks: This species is characterized by its medium to large size, by posessesing a low
trochospire, and by having a nearly symmetrical profile and narrow umbilicus (Premoli Silva and
Sliter, 2002).
Range: Whiteinella baltica ranges from the Middle Cenomanian R. cushmani Zone to the base
of the Santonian D. asymetrica Zone (Premoli Silva and Sliter, 2002). Two sepcimens were
identified from the basal Boquillas Formation in this study. Additional specimens of Whiteinella
sp. may also represent Whiteinella baltica.
CHRONOS Range:
FO stage: Cenomanian
FO zone: R. cushmani
FO age (Ma): 94.7
58
LO stage: Santonian
LO zone: D. concavata
LO age (Ma): 85.2
CHAPTER 3 - BIOSTRATIGRAPHY OF THE BUDA LIMESTONE
Previous work
Ammonites
The ammonite Neophlycticeras (Neophlycticeras) texanum (Shattuck 1903) is reported from the
lowermost Buda Limestone, while Neophlyicticeras (Budaiceras) hyatti is abundant in the middle
and upper portions of the Buda Limestone (Cobban et al., 2008). These species form individual
ammonite Zones in the late Lower Cenomanian that likely correlate to the Lower Cenomanian
Mantelliceras saxbii Subzone through the late Lower Cenomanian Mantelliceras dixoni Zone of
Europe (Hancock, 2004). Other ammonites from the Buda Limestone in this region include:
Budaiceras elegantior (Lasswitz 1904), B. alticarinatum sp., Faraudiella franciscoensis (Kellum
and Mintz 1962), F. roemeri (Lasswitz 1904), F. barachoensis sp., F. archerae sp., Mariella
wysogorskii, and Sharpeiceras tlahualilense (Kellum and Mintz 1962) reported by Young (1979);
as well as various species of the genus Mantelliceras Hyatt 1903 reported by several workers. In
addition, Cobban et al. (2008) produce a detailed list of ammonite species and references.
Foraminifera
A previous comprehensive thin section biostratigraphic survey of the foraminifera in the Buda
Limestone in the Big Bend region is unknown to the author. However, Pessagno (1969)
examined thin sections of the Buda from Chispa Summit, Jeff Davis County and Austin, Travis
County. At Chispa Summit, he reports Rotalipora appenninica (Renz), Hedbergella delrioensis
(Carsey), Praeglobotruncana delrioensis (Plummer), and Heterohelix sp. At Austin, he reports
Rotalipora appenninica (Renz), Hedbergella washitensis (Carsey), and Praeglobotruncana
delrioensis (Plummer). All of the species noted by Pessagno (1969) were also recovered from
the Buda Limestone in this study.
Other taxa
Cornell (1997) examined the Buda Limestone at Cerro de Cristo Rey, New Mexico for
dinoflagellate cysts. He reports an assemblage dominated by Spiniferites, with smaller numbers
of Tanyosphaeridium, Hystrichodinium, and others. A single genus of achritarch, Michrystridium,
was also reported. Cornell (1997) notes that the dinoflagellate cyst assemblage agrees with the
established Cenomanian age of the Buda Limestone.
Other age dating
In northern and central Texas, Denison et al. (2003) analyzed strontium isotopes from 10 oyster
shells collected from the base of the Buda at three locations. They report a Δsw mean of -174.2
± 1.1., and dated their youngest sample of Buda at approximately 97 Ma (latter half of the Early
Cenomanian).
60
Species used in the foraminiferal biozonation
Key species of foraminifera used in the biozonation of the Buda Limestone include Favusella
washitensis (Carsey), Globigerinelloides bentonensis (Morrow), G. caseyi (Bolli), Loeblich and
Tappan), Parathalmanninella appenninica (Renz), Praeglobotruncana delrioensis (Plummer), P.
stephani (Gandolfi), and Rotalipora montsalvensis (Mornod) (Tables. 1, 2, 3; Fig. 10). A single
noteworthy specimen of Parathalmanninella micheli (Sacal and Debourle) was also recovered
from the Buda Limestone. This species occupies a narrow biostratigraphic range in the upper
Th. globotruncanoides Zone to the lowermost Th. reicheli Zone.
The upper boundary of the Th. globotruncanoides Zone with the overlying Th. reicheli
Zone is defined by the first appearance of Th. reicheli. Similarly, the upper boundary of the Th.
reicheli Zone with the overlying R. cushmani Zone is defined by the first appearance of R.
cushmani. Neither Th. reicheli nor R. cushmani was recovered from the Buda Limestone,
constraining the unit below the Th. reicheli Zone. The Buda is constrained above the base of the
Th. globotruncanoides Zone by the first appearances of Globigerinelloides caseyi, Rotalipora
montsalvensis, and Thalmanninella micheli.
Foraminiferal biozonation discussion
The overlapping ranges of all species allow for placement of the Buda within the upper portion
of the Early to Middle Cenomanian Th. globotruncanoides Zone (Fig. 10). No definitive first or
last appearances of any species of foraminifera occur within the studied sections. Species
common in the Buda are present throughout, and species uncommon in the Buda occur too
sporadically to infer FAD or LAD occurrences. The scarcity of biostratigraphically useful
61
rotaliporid foraminifera in the Buda is likely a function of the water depth during the time of
deposition. However, enough rotaliporid specimens were recovered to infer a biostratigraphic
zonation. The precise placement of the Buda Limestone in the late Early Cenomanian using
foraminiferal data is in agreement with the ammonite zonation of Cobban et al. (2007). Finally,
the strontium isotope age assignment of 97 Ma given by Denison et al. (2003) falls within the Th.
globotruncanoides Zone and is in agreement with this study.
62
DC-DS1-1
DC-DS1-2
DC-DS1-3
DC-DS1-4
DC-DS1-5
DC-DS1-6
DC-DS1-7
DC-DS1-8
DC-DS1-9
DC-DS1-10
DC-DS1-11
DC-DS1-12
DC-DS1-13
DC-DS1-14
DC-DS1-15
63
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
1
1
1
1
2
Hedbergella sp.
Globigerellinoides caseyi
Globigerinelloides ultramicrus
Globigerinelloides bentonensis
Globigerinelloides sp.
Heterohelix reussi
Heterohelix sp. (H. moremani)
1
1
1
1
1
1
1
2
2
1
1
1
5
6
5
1
1
6
7
3
3
2
1
3
1
2
1
1
1
1
1
1
1
1
Total
A. planoconvexa
Loeblichella coarctata
Planomalina sp.
Shackoina cenomana
Shakoina sp.
Dicarinella algeriana
Dicarinella sp.
Whiteinella praehelvetica
Whiteinella aprica
Whiteinella baltica
Whiteinella sp.
Praeglobotruncana gibba
Praeglobotruncana stephani
Praeglobotruncana delrioensis
Praeglobotruncana sp.
Parathalmanninella micheli
Parathalmanninella appeninnica
Thalmaninnella brotzeni/globotruncanoides
Thalmanninella greenhornensis
Rotalipora cushmani
Rotalipora montsavensis
Rotalipora sp.
Favusella washitensis
Costellagerina libyca
Hedbergella simplex
Hedbergella planispira
1
Hedbergella delrioensis
1
4
7
7
7
3
3
0
9
10
6
6
6
2
0
6
Table 1 - This chart shows the distribution of recognizable foraminifera per thin section in the Dog Canyon section
(Buda Limestone). Shaded areas represent an uncertain identification. Areas marked with an 'x' indicate present,
but not counted.
HW-DS2-1
HW-DS2-2
HW-DS2-3
HW-DS2-4
HW-DS2-5
HW-DS2-6
64
1
1
1
1
5
1
1
4
3
2
1
1
5
6
Table 2 - These charts show the distribution of identified foraminifera from the Highway Datasets (Buda
Limestone). Shaded areas represent an uncertain identification.
1
1
1
2
1
1
4
Total
A. planoconvexa
Loeblichella coarctata
Planomalina sp.
Shackoina cenomana
Shakoina sp.
Dicarinella algeriana
Dicarinella sp.
Whiteinella praehelvetica
Whiteinella aprica
Whiteinella baltica
Whiteinella sp.
Praeglobotruncana gibba
Praeglobotruncana stephani
Praeglobotruncana delrioensis
1
Praeglobotruncana sp.
Parathalmanninella micheli
Parathalmanninella appeninnica
Thalmaninnella brotzeni/globotruncanoides
Thalmanninella greenhornensis
Rotalipora cushmani
Rotalipora montsavensis
1
Rotalipora sp.
1
Favusella washitensis
Costellagerina libyca
Hedbergella simplex
Hedbergella planispira
1
1
2
1
Hedbergella delrioensis
Hedbergella sp.
Globigerellinoides caseyi
Globigerinelloides ultramicrus
Globigerinelloides bentonensis
Globigerinelloides sp.
Heterohelix reussi
Heterohelix sp. (H. moremani)
HW-DS1-1
HW-DS1-2
HW-DS1-3
HW-DS1-4
HW-DS1-5/6
HW-DS1-7
HW-DS1-8
HW-DS1-9
HW-DS1-10
3
2
3
4
0
0
1
2
4
6
5
4
0
6
12
Total
A. planoconvexa
Loeblichella coarctata
Planomalina sp.
Shackoina cenomana
Shakoina sp.
Dicarinella algeriana
Dicarinella sp.
Whiteinella praehelvetica
Whiteinella aprica
Whiteinella baltica
Whiteinella sp.
Praeglobotruncana gibba
Praeglobotruncana stephani
Praeglobotruncana delrioensis
Praeglobotruncana sp.
Parathalmanninella micheli
Parathalmanninella appeninnica
Thalmaninnella brotzeni/globotruncanoides
Thalmanninella greenhornensis
Rotalipora cushmani
Rotalipora montsavensis
Rotalipora sp.
Favusella washitensis
Costellagerina libyca
Hedbergella simplex
Hedbergella planispira
Hedbergella delrioensis
Hedbergella sp.
Globigerellinoides caseyi
Globigerinelloides ultramicrus
Globigerinelloides bentonensis
Globigerinelloides sp.
Heterohelix reussi
Heterohelix sp. (H. moremani)
DF-DS1-31
DF-DS1-30
DF-DS1-29
DF-DS1-28
DF-DS1-27
DF-DS1-26
DF-DS1-25
DF-DS1-24
DF-DS1-23
DF-DS1-22
DF-DS1-21
DF-DS1-20
DF-DS1-19
DF-DS1-18
DF-DS1-17
DF-DS1-16
DF-DS1-15
DF-DS1-14
DF-DS1-13
DF-DS1-12
DF-DS1-11
DF-DS1-10
DF-DS1-9
DF-DS1-8
DF-DS1-7
DF-DS1-6
DF-DS1-5
DF-DS1-4
DF-DS1-3
DF-DS1-2
DF-DS1-1
65
66
x
70
x
40
x
33
x
56
51
x
76
x
77
x
55
x
62
x
52
x
66
x
42
x
47
x
32
38
31
27
2
3
2
3
2
2
5
5
3
5
6
6
3
10
3
2
11
4
3
7
15
11
28
28
42
27
19
15
10
17
16
1
1
2
2
2
3
1
1
3
2
4
2
1
1
1
1
2
3
1
2
1
1
2
4
4
3
1
2
1
3
3
1
1
3
1
2
5
4
1
2
1
1
2
1
2
4
1
1
2
1
1
1
2
1
1
1
3
8
3
5
4
7
7
3
1
2
2
3
3
1
1
1
1
1
4
2
5
1
2
4
2
3
3
2
2
1
3
1
3
2
3
7
5
7
2
1
2
3
6
12
74
1
1
1
1
1
1
1
80
50
1
1
1
1
1
1
1
1
1
1
1
1
1
1
48
1
62
59
1
1
1
16
87
1
91
62
1
77
1
1
67
1
84
1
23
81
1
94
54
54
57
59
Table 3 - This chart shows the distribution of identified foraminifera from the Dagger Flat Dataset 1 section (Buda
Limestone). Shaded areas represent an uncertain identification. Areas marked with an 'x' indicate present, but not
counted.
Total all planktonics
Benthics
A. planoconvexa
Loeblichella coarctata
Planomalina sp.
Shackoina cenomana
Shakoina sp.
Dicarinella algeriana
Dicarinella sp.
Whiteinella praehelvetica
Whiteinella aprica
Whiteinella baltica
Whiteinella sp.
Praeglobotruncana gibba
Praeglobotruncana stephani
Praeglobotruncana delrioensis
Praeglobotruncana sp.
Parathalmanninella micheli
Parathalmanninella appeninnica
Thalmaninnella brotzeni/globotruncanoides
Thalmanninella greenhornensis
Rotalipora cushmani
Rotalipora montsavensis
Rotalipora sp.
Favusella washitensis
Costellagerina libyca
Hedbergella simplex
Hedbergella planispira
Hedbergella delrioensis
Hedbergella sp.
Globigerellinoides caseyi
Globigerinelloides ultramicrus
Globigerinelloides bentonensis
Globigerinelloides sp.
Heterohelix reussi
Heterohelix sp. (H. moremani)
Figure 10 - This graph shows the geochronologic distribution of selected foraminifera from the Buda Limestone.
Note that the taxon range overlap indicates deposition in the Th. globotruncanoides Zone. Please refer to the
Systematic Paleontology for detailed references. Chart created with TimeScale Creator.
66
0
0
0
0
0
0
1
0
0
0
0
0
0
0
2
0
0
1
0
0
1
0
0
0
0
1
0
0
0
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
0
0
0
0
1
0
0
0
0
0
1
0
1
1
1
0
1
0
0
0
0
0
0
0
1
0
0
0
0
0
1
0
1
0
0
1
1
2
0
1
0
0
1
1
0
0
0
0
1
4.1%
89.2%
6.8%
0.0%
0.0%
2.5%
87.5%
8.8%
0.0%
1.3%
4.0%
80.0%
14.0%
0.0%
2.0%
10.4%
68.8%
16.7%
2.1%
0.0%
4.8%
8.5%
90.3%
86.4%
4.8%
5.1%
0.0%
0.0%
0.0%
0.0%
6.9%
87.4%
4.6%
0.0%
1.1%
11.0%
84.6%
4.4%
0.0%
0.0%
4.8%
88.7%
6.5%
0.0%
0.0%
5.2%
80.5%
10.4%
1.3%
1.3%
11.9%
77.6%
7.5%
0.0%
0.0%
14.3%
78.6%
6.0%
0.0%
0.0%
37.0%
51.9%
9.9%
0.0%
1.2%
36.2%
50.0%
10.6%
1.1%
1.1%
29.6%
22.2%
29.8%
30.5%
59.3%
70.4%
54.4%
45.8%
9.3%
7.4%
14.0%
22.0%
0.0%
0.0%
0.0%
0.0%
1.9%
0.0%
1.8%
0.0%
0.0% DF-DS1-31
DF-DS1-30
0.0% DF-DS1-29
DF-DS1-28
0.0% DF-DS1-27
DF-DS1-26
2.1% DF-DS1-25
DF-DS1-24
0.0% DF-DS1-23
0.0% DF-DS1-22
DF-DS1-21
0.0% DF-DS1-20
DF-DS1-19
0.0% DF-DS1-18
DF-DS1-17
0.0% DF-DS1-16
DF-DS1-15
1.3% DF-DS1-14
DF-DS1-13
3.0% DF-DS1-12
DF-DS1-11
1.2% DF-DS1-10
DF-DS1-9
0.0% DF-DS1-8
DF-DS1-7
1.1% DF-DS1-6
DF-DS1-5
0.0% DF-DS1-4
0.0% DF-DS1-3
0.0% DF-DS1-2
1.7% DF-DS1-1
Table 4 - This chart shows the absolute and relative abundances of main genera in the Dagger Flat data set 1
section (Buda Limestone). Note Favusella is included as a hedbergellid.
67
Height (m)
Sample
% others
% praeglobotruncanids
% rotaliporids
% hedbergellids
% heterohelicids
% globigerinellids
5
13
7
10
7
10
8
6
3
3
5
4
7
4
5
4
4
8
4
5
4
5
6
8
9
10
3
5
4
8
13
Total, all others
Total hedbergellids
66
x
70
x
40
x
33
x
56
51
x
76
x
77
x
55
x
62
x
52
x
66
x
42
x
47
x
32
38
31
27
Total praeglobotruncanids
Total heterohelicids
3
3
2
3
2
2
5
6
3
5
7
6
5
10
5
3
13
4
6
8
15
12
32
30
44
34
25
16
12
17
18
Total rotaliporids
Total globigerinellids
Sample
DF-DS1-31
DF-DS1-30
DF-DS1-29
DF-DS1-28
DF-DS1-27
DF-DS1-26
DF-DS1-25
DF-DS1-24
DF-DS1-23
DF-DS1-22
DF-DS1-21
DF-DS1-20
DF-DS1-19
DF-DS1-18
DF-DS1-17
DF-DS1-16
DF-DS1-15
DF-DS1-14
DF-DS1-13
DF-DS1-12
DF-DS1-11
DF-DS1-10
DF-DS1-9
DF-DS1-8
DF-DS1-7
DF-DS1-6
DF-DS1-5
DF-DS1-4
DF-DS1-3
DF-DS1-2
DF-DS1-1
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
m
m
m
Figure 11 - Relative abundances of the main genera from the Dagger Flat Dataset 1 section. Upsection is from right
to left. Note the decline in globigerinellids and increase in heterohelicids upsection.
68
# individuals per thin section
m
Figure 12 - Absolute abundances of heterohelicids and globigerinellids per thin section from the Dagger Flat
Dataset 1 section. The deepest water (Maximum Flooding Surface) is interpreted as the maximum in the
globigerinellid : heterohelicid ratio at approximately 1.2 m.
69
Foraminiferal abundances at Dagger Flat locality
The Dagger Flat DS-1 locality (DF-DS1) represents the most complete section of Buda sampled in
this study. Every recognizable foraminifer was counted in thin section from each of the 31 DFDS1 samples, except for the ubiquitous heterohelicids, which were counted in every other
sample. Some problems arise when attempting to analyze relative abundance trends of small
numbers of individual species of foraminifera studied in thin section. Because the absolute
numbers of foraminifera per thin section are relatively low in this study (~50-80 specimens),
abundance data for individual species is unreliable. However, analysis of larger-scale trends on
the genus level mitigates sources of error in low abundance assemblages, without eliminating
significant paleoceanographic data. The Buda Limestone contains a shallow water pelagic
foraminiferal assemblage roughly consisting of 46-90% heterohelicids, 3-37% globigerinellids,
and 4-22% hedbergellids (incl. favusellids) (Table 4; Fig. 11). Deeper water forms such as
praeglobotruncanids and rotaliporids are present in low quantities of 0-3 specimens per thin
section throughout the section, but are not abundant enough to warrant statistical analyses.
Quantitative analysis of samples DF-DS1-1, -7, -19, and -30 yielded 27, 23, 16, and 12
recognizable benthic specimens, respectively. Benthic foraminifera are consistently present in
every thin section, and make up roughly 45-15% of the total foraminiferal assemblage. Low
absolute numbers of benthic specimens per thin section make statistical analysis problematic.
The number of benthic specimens per thin section is highly dependent upon the thin section’s
orientation, as well as the presence or absence of larger bioclasts in the particular thin section.
Due to their thick, high-Mg calcite test walls, benthic foraminifera are more easily identified in
70
thin section than planktonic foraminifera. Hence, benthic counts in thin section may be inflated
due to preservational bias.
A more reliable method of thin section analysis uses shallow heterohelicid and deep
dwelling globigerinellid planktonic foraminifera. An increase in the numbers of globigerinellids
from 20-30% to 37% is recorded upsection from the base of the outcrop, followed by a decrease
to 14% at about 2m above the base of the measured section (Fig. 11). A declining trend in the
relative abundance of globigerinellids continues slightly until the top of the section. Absolute
numbers of globigerinellids per thin section undergo an order of magnitude decrease from the
bottom of the section to the top of the section (Fig. 12). Heterohelicids are abundant
throughout the section and show an overall increase in relative abundance from 46% at the
base of the section to 90% near the top. Two notable troughs in relative abundance at 1.6m and
5m above the base of the section correspond to relative peaks in the globigerinellid and
hedbergellid populations, respectively.
Foraminiferal abundances discussion
Foraminiferal assemblages dominated by relatively shallow-dwelling pelagic faunas imply water
depths between about 50 to 100 m (Baker, 1976). This interpretation agrees with Scott and
Kidson (1977) and Lock et al. (2007), who assigned an open shelf paleodepth for the Buda
Limestone in Big Bend National Park. Intermediate-depth, open marine globigerinellids show an
initial increase followed by overall decrease in abundance upsection. The accompanied increase
in opportunistic shallow-depth heterohelicids upsection indicates the development of
conditions more favorable to opportunistic species than to other species (Nederbragt, 1991).
71
Furthermore, open marine assemblages are expected to decrease in abundance shoreward
(Sliter, 1972; Leckie, 1987). Hence, these abundance data are interpreted as showing initial sea
level transgression followed by regression. The maximum flooding surface (MFS) likely occurs
near the first 1.2 m of Buda. Other effects of eustacy are apparently small, however, since the
entire Buda Limestone remains deep enough for a pelagic-dominated facies even in the
uppermost sections studied. It is likely that the extant Buda Limestone was deposited entirely
within the 50-100 m depth interval discussed above. Further paleoceanographic implications of
heterohelicid dominance are discussed in the Systematic Paleontology section of this work.
72
CHAPTER 4 - BIOSTRATIGRAPHY OF THE LOWERMOST BOQUILLAS FORMATION
Previous work
Ammonites
The basal ±1 m of the Boquillas Formation in Big Bend National Park is placed within the
Acompsoceras inconstans Zone based on the presence of Moremanoceras bravoense (Cobban
and Kennedy, 1989) and Euhystrichoceras adkinsi (Powell, 1963), and is assigned a late Early
Cenomanian age (Cobban et al., 2008; Cooper, 2008). Acompsoceras inconstans (Schlüchter)
and two possible specimens of A. renevieri (Sharpe 1857) were reported from east of Big Bend
National Park by Daugherty and Powell (1963). Acompsoceras inconstans is reported only from
the late Lower Cenomanian Mantelliceras dixoni Zone in northwest Europe (Cobban and
Kennedy, 1989; Hancock et al., 1993). All specimens assigned an Early Cenomanian age are
restricted to the lowest ±1 m of the Boquillas in Big Bend National Park (Cooper, 2008). Several
ammonites of the genus Inoceramus are reported from the within first ±11 m of the Boquillas,
and are assigned a Middle Cenomanian age (Cooper, 2008). The Middle Cenomanian ammonite
Zones Conlinoceras tarantense - Plesiacanthoceras wyomingsense of the Western Interior
Seaway, however, are not recognized in Trans-Pecos Texas (Cobban et al., 2008).
Foraminifera
Pessagno (1967, 1969) assigned the lowermost Boquillas to the Rotalipora cushmani-Rotalipora
greenhornensis Zone in the upper half of the Cenomanian. He specifies a significant amount of
time missing during the Buda - Boquillas unconformity during the middle portion of the
Cenomanian. Frush and Eicher (1975) sampled the first ±10-20 ft of Boquillas at the Hot Springs
locality in Big Bend National Park, and report Hedbergella delrioensis (Carsey), Heterohelix
globulosa (Cushman), Hedbergella amabilis (Loeblich and Tappan), Hedbergella planispira
(Tappan), Globigerinelloides bentonensis (Morrow), and Rotalipora greenhornensis (Morrow).
They report Rotalipora cushmani (Morrow) and Praeglobotruncana stephani (Gandolfi) ±40 ft
above the basal contact of the Boquillas with the Buda. According to their study, Whiteinella
aprica (Loeblich and Tappan) first occurs at ±60 ft, while Whiteinella archaeocretacea (Pessagno)
first occurs at about 100 ft above the basal contact. Frush and Eicher (1975) assign the base of
the Boquillas to the Rotalipora greenhornensis-Rotalipora cushmani Zone based on this
assemblage.
Other age dating
Denison et al. (2003) estimates the age of the base of the Gulfian Series (Woodbine Formation)
immediately above the Buda Limestone in northern and central Texas at 94.4 Ma, based on
strontium isotopes. This age determination corresponds to the Upper Cenomanian portion of
the R. cushmani Zone.
74
Species used in the foraminiferal biozonation
Key species recovered from the lowermost 1.2 m Boquillas Formation include Dicarinella sp.,
Dicarinella algeriana (Caron), Rotalipora sp., Rotalipora cushmani (Morrow), Thalmanninella sp.
cf. Th. greenhornensis (Morrow ), and Whiteinella baltica Douglas and Rankin (Table 5a).
Additionally, single (or very rare) specimens of Helvetoglobotruncana sp. cf. H. praehelvetica
(Trujillo), Heterohelix globulosa (Cushman), Praeglobotruncana gibba Klaus, Praeglobotruncana
stephani (Gandolfi), Shackoina cenomana (Shacko), Rotalipora montsalvensis (Mornod), and
Whiteinella sp. cf. W. aprica (Loeblich and Tappan) were also recovered.
Another tentatively recognized and potentially problematic form is that of Anaticinella
sp. cf. A. multiloculata, as outlined in the Systematic Paleontology section of this work. High
abundances of this genus are associated with the Late Cenomanian in the Hartland Shale at
Pueblo, CO. Though likely derived from rotaliporid stock, the genus Anaticinella apparently
survived the extinction of all other rotaliporids and persisted into the W. archaeocretacea Zone,
vanishing at the Cenomanian-Turonian boundary (Caron et al., 2006). Differentiation of
Anaticinella sp. and Th. greenhornensis is difficult due to transitional morphotypes between the
two (Keller and Pardo, 2004), and also morphological similarities with R. cushmani (Leckie,
1985). Given these potential morphological similarities, it must be noted that mis-identification
of Anaticinella as a “classic” rotaliporid may lead to the inclusion of the bottom of the W.
archaeocretacea Zone into the underlying D. algeriana Subzone, since the bottom of the W.
archaeocretacea Zone is defined by the disappearance of rotaliporids (specifically Rotalipora
cushmani).
75
Double-keeled dicarinellids are an especially important group of foraminifera for the
biozonation of the basal Boquillas Formation. The genus is first represented by the species D.
algeriana, which appears in the latter portion of the R. cushmani Zone, in the lower part of the
Upper Cenomanian. D. algeriana is the first species of foraminifera to evolve tests with a
double keel (Premoli Silva and Sliter, 2002). While the author is confident in the presence of
double-keeled dicarinellids in the sampled sections, it must be noted that Frush and Eicher
(1975) reported no members of the genus Dicarinella in the entire Boquillas Formation in any of
their localities. Most foraminifera recovered from the basal Boquillas by this study are in-filled
with calcite spar, and are not perfectly preserved. It may be that the recognition of a double
keel is not possible in washed samples, as prepared by Frush and Eicher (1975), but may instead
only be identified in thin section.
Foraminiferal biozonation discussion
Based on the overlapping ranges of recovered foraminifera, this study places the basal Boquillas
Formation in the Upper Cenomanian D. algeriana Subzone of the R. cushmani Zone (Fig. 13).
The lower constraint is given by the presence of Dicarinella algeriana, the first species of
foraminifera to evolve a double keel. The upper constraint (the base of the overlying W.
archaeocretacea Zone) is defined by the extinction of rotaliporids (Premoli Silva and Sliter, 2002;
González-Donoso et al., 2007). The presence of rotaliporids in all studied samples of Boquillas
therefore restricts the zonation below the R. cushmani Zone - W. archeocretacea Zone border.
This age designation is in agreement with the strontium isotope age given by Denison et al.
(2003) discussed above.
76
The original zonation given by Frush and Eicher (1975) is herein considered obsolete
(Fig. 14). For example, it is now known that the first appearance of Heterohelix globulosa
(Cushman) occurs in the Upper Cenomanian D. algeriana Subzone of the R. cushmani Zone
(Nederbragt, 1991). The first appearance of Whiteinella aprica (Loeblich and Tappan) also
occurs in the D. algeriana Subzone (Premoli Silva and Sliter, 2002). Therefore it now appears
that the lower 0-60 feet of the Boquillas is constrained to the D. algeriana Subzone given both
the data of Frush and Eicher (1975) and of this study. This re-zonation is supported by samples
gathered by Frush and Eicher (1975) at Chispa Summit and in the Jeff Davis Mountains, where
both Heterohelix globulosa (Cushman) and Whiteinella aprica (Loeblich and Tappan) are
reported in the first sample from the lowermost 5 ft of Chispa Summit Fm. (Boquillas Fm.) above
the Buda Limestone.
The late Early Cenomanian - Middle Cenomanian age designation for the basal Boquillas
given by ammonites is in disagreement with the Upper Cenomanian D. algeriana Subzone
designation given by this study (Fig. 14). There are likely several reasons for this discrepancy.
First, it must be pointed out that the duration of the Middle Cenomanian in comparison with the
Lower Cenomanian is relatively short, at approximately 1.2 million years. The duration of the
Middle Cenomanian means that an age discrepancy between ammonites and foraminifera is not
overly egregious, since the latest Lower Cenomanian and earliest Upper Cenomanian are
relatively close in absolute age. Also, ammonite faunas from the Western Interior and Europe
are generally not found in the Trans-Pecos region which limits correlation. A single ammonite
species found in the immediate vicinity of Big Bend National Park, A. inconstans, has been
correlated to European sections (Hancock et al., 1993). Reworking of faunal assemblages that
lived during times of non-deposition must also be considered, since the oldest ammonites are
77
found only in a limited stratigraphic interval at the very basal ±1 m of the Boquillas (Cooper,
2008). Young (1958) reports a silicified and abraded Lower Cenomanian fauna occurring along
with an Upper Cenomanian fauna in the basal Boquillas in the Jeff Davis Mountains, suggesting
that faunal reworking has occurred in this region. Finally, paleoenvironmental changes and/or
paraconformities in the rock record may artificially truncate the ranges of Trans-Pecos
ammonites. Ergo, these faunas may extend for longer intervals of time than their known
occurrences in the rock record might suggest.
Slump folding and signs of soft-sediment deformation have been reported in the park by
Maxwell (1969) and along US-90 and Lozier Canyon by Lock and Peschier (2006). It may be
suggested that contamination from sediment deposited later in time may have transported
younger foraminifera to a lower horizon. However, no clear signs of large-scale soft-sediment
deformation were observed at the Dagger Flat DS-2 locality, and laminar beds of Boquillas rest
upon the Buda Limestone. Since ammonite data suggests that the Upper Cenomanian occurs
approximately 11 meters above the contact with the Buda (Cooper, 2008), any model invoking
soft sediment deformation to explain transported foraminifera must account for a full 11 m of
section. Hypothetically, a structural process such as a detachment fault or a recent mass
wasting process such as slumping could have transported higher Boquillas Formation atop Buda
Limestone at this locality, but these explanations seem unlikely given that Frush and Eicher
(1975) also reported Heterohelix globulosa (Cushman) and Whiteinella aprica (Loeblich and
Tappan) in the lowermost Boquillas at their Hot Springs locality. These species, when
interpreted using the modern zonation, independently support placement of the basal Boquillas
Formation in the D. algeriana Subzone.
78
Foraminiferal abundances
The total number of foraminifera identified from the Boquillas Formation is too small to allow a
detailed statistical analysis. A composite count of all thin sections made from the lowermost 1.2
m of Boquillas yields 22% dicarinellids, 44% rotaliporids, 16% whiteinellids, 13%
praeglobotruncanids, and 22% all others (hedbergellids, etc.) (Table 5b). Only a single, broken
benthic foraminifer was recognized out of all thin sections analyzed from the Boquillas. While
these abundance data represent approximations, the Boquillas Formation is clearly dominated
by keeled forma. These forms include deep-dwelling rotaliporids, and intermediate-dwelling
dicarinellids and praeglobotruncanids (see Systematic Paleontology section for more detail).
Hence, data from this study supports a deep, open marine depositional setting for the
lowermost Boquillas.
79
2
1
1
4
4
1
1
1
1
1
1
Total
A. planoconvexa
Loeblichella coarctata
Planomalina sp.
Shakoina sp.
Shackoina cenomana
2
2
2
1
Dicarinella algeriana
2
1
1
1
Dicarinella sp.
1
Whiteinella praehelvetica
Whiteinella sp.
1
Whiteinella aprica
Praeglobotruncana gibba
1
Whiteinella baltica
Praeglobotruncana stephani
Praeglobotruncana delrioensis
Praeglobotruncana sp.
Parathalmanninella micheli
2
Parathalmanninella appeninnica
1
Thalmaninnella brotzeni/globotruncanoides
Rotalipora cushmani
8
Thalmanninella greenhornensis
Rotalipora montsavensis
1
Rotalipora sp.
Favusella washitensis
Costellagerina libyca
Hedbergella simplex
Hedbergella planispira
Hedbergella delrioensis
Hedbergella sp.
Globigerellinoides caseyi
Globigerinelloides ultramicrus
Globigerinelloides bentonensis
Globigerinelloides sp.
Heterohelix reussi
Heterohelix sp. (H. moremani)
DF-DS2-p100
DF-DS2-p80
DF-DS2-p60
DF-DS2-p40
Black shale
DF-DS2-m20
DF-DS2-m30
19
5
2
6
9
4
Rotaliporids
Dicarinellids
Whiteinellids
Praeglobotruncanids
All others
Gross percents
Genus
Table 5a - This chart shows the distribution of identified foraminifera from the Dagger Flat Dataset 2 section in the
Boquillas Formation and topmost Buda Limestone (DF-DS2-m30). Samples were measured in centimeters plus (p)
or minus (m) relative to a black shale horizon. Sample DF-DS2-m20 represents the anomalous black limestone
layer. Shaded areas on the graph represent an uncertain identification. Areas marked with an 'x' indicate present,
but not counted.
44%
22%
16%
13%
22%
Table 5b - This graph shows the relative abundances of genera from all samples taken from the Boquillas Formation
at the Dagger Flat Dataset 2 location.
80
Figure 13 - This graph shows the geochronologic distribution of selected foraminifera from the Boquillas Formation.
Note that the taxon range overlap indicates deposition in the D. algeriana Subzone. Please refer to the Systematic
Paleontology for detailed references. Chart created with TimeScale Creator.
81
Figure 14 - This graph shows the geochronologic positions of the Buda Limestone and Boquillas Formation
according to different authors. Frush and Eicher (1975) report W. aprica and H. globulosa, both with FADs in the
Upper Cenomanian, but place the Boquillas in the Middle Cenomanian. Their work with washed samples likely
prevented the recognition of double keels (Dicarinella). The oldest ammonites are found only in a limited
stratigraphic interval at the very basal ±1 m of the Boquillas (Cooper, 2008).
82
CHAPTER 5 - STABLE ISOTOPE GEOCHEMISTRY
Previous work
Paleoexposure
Carbon isotope δ13C values in a carbonate are generally unaffected by many diagenetic
processes, except when a carbonate is subjected to subaerial exposure (Heydari et al., 2001).
During times of subaerial exposure, meteoric waters percolating through Corg-rich soils become
enriched in 12C. Organic respiration and decomposition can increase soil PCO2 levels at least two
orders of magnitude higher than atmospheric levels (James and Choquette, 1983). Elevated soil
PCO2 can further enhance the dissolution of the rock in the meteoric environment. Dissolution of
metastable carbonates in this meteoric environment results in the re-precipitation of calcite
with comparatively low δ13C values beneath the exposure surface (Fig. 15). Oxygen isotope
values are subject to a more diverse range of diagenetic processes than carbon isotopes
(Heydari et al., 2001), so whether they represent primary values, or values retained from
subaerial exposure, or values from some other diagenetic event is difficult to determine.
Nevertheless, where δ18O values altered by meteoric diagenesis are preserved, a positive δ18O
shit at a subaerial exposure may be present, due to preferential depletion of 16O through
evaporation (Allen and Matthews, 1982; James and Choquette, 1983). Furthermore, if sea level
drop is caused by the growth of continental glaciers, rainwater isotopic compositions will shift
positive during lowstand and subaerial exposure. Although unlikely under Cretaceous
greenhouse earth conditions, this process may also account for observed positive δ18O shifts at
surfaces of subaerial exposure (Allen and Matthews, 1982). Hence, recognition of
paleoexposure surfaces in a wide range of carbonates is possible using both C- and O-isotope
profile data. For this study, the minimum magnitude of negative δ13C shift required for
definitive recognition of a subaerial paleoexposure surface is approximately 0.5‰ over a 1-2
dm-scale vertical transect (Goldstein, 1991; Railsback et al., 2003). Since the zone of
supersaturation with respect to CaCO3 may lie beneath the zone of dissolution, precipitation of
δ13C-poor cements may occur in a zone displaced approximately 0-3 m lower than the
paleoexposure surface (Allen and Matthews, 1982; Railsback et al., 2003).
Identification of subaerial exposure surfaces by isotopic analysis of bulk carbonate is not
without caveats. Isotopic expression of subaerial exposure is dependent on the duration of the
exposure, levels of CO2, amount of soil and vegetation cover, and climate. Evidence for
subaerial exposure is most pronounced in warm, humid regions but can often be completely
absent, especially in arid regions (James and Choquette, 1983). Exposure surfaces that did
devolop under hot, arid conditions may display both surface karst and caliche, but such features
often form slowly and in a thinner profile than wetter climates. Likewise, paleoexposures which
were devoid of substantial soil or vegetation cover may not generate an isotopic signature.
Furthermore, isotopic signatures associated with meteoric diagenesis can occur with substantial
heterogeneity in bulk rock samples due to localized abundance of meteorically altered phases.
Local variations in solubility, porosity, permeability, soil cover, and fracture pattern can all affect
the distribution of altered cements (Theiling et al, 2007). Some subaerially exposed marine
84
limestones may not show a δ13C shift by using bulk rock analysis in a single vertical transect, due
to initially scattered distributions of meteorically altered calcite cements (Goldstein 1991,
Theiling et al., 2007), or because the altered material was removed during subsequent
transgressive ravinement (Heydari et al., 2001). Furthermore, a positive shit in δ18O data may or
may not be present at the exposure surface. If similar diagenetic processes affected the rocks
above and below the exposure surface, δ18O values may not vary significantly between them
(Goldstein, 1991).
According to Theiling et al. (2007) the maximum shift observed between single adjacent
samples may or may not accurately represent the average shift over the given vertical transect.
In order to overcome lateral variation in the geochemical signature and the "patchiness" of
diagenesis, multiple vertical transects across the sequence boundary should be evaluated.
Comparison of the mean δ13C and δ18O values from each sampled horizon above and below the
unconformity has a much greater chance of showing any isotopic signatures. Because only a
single vertical transect of rock was sampled in this study, a mean isotopic value per horizon
cannot be established. However, averaging the isotopic values from both above and below the
maximum observed δ13C shift in the top of the Buda Limestone may dampen any effects of
statistical anomaly.
Theiling et al. (2007) showed that minima in mean δ13C values may either lie beneath
the exposure surface or above the exposure surface, presumably if transgressive lag (including
micritic components) was reworked into sediment overlying the exposure surface. Theiling et
al. (2007) also noted that the range of measured δ13C and δ18O values increases markedly in
sediments both immediately above and below a surface of subaerial exposure. Conversely, as
one traces away from this zone, variation between measured values decreases (Fig. 15). They
85
propose that the identification of increased variation in isotope values can be another way to
indentify subaerial exposure surfaces if the negative δ13C excursion is not present. A detailed
statistical analysis of isotopic data for the Buda - Boquillas contact is not within the scope of this
study, however the conclusions drawn by Theiling et al. (2007) are important when considering
data presented herein.
Hardgrounds, submarine unconformities, and other surfaces
Distinguishing between surfaces of subaerial exposure and other surfaces in carbonates can be
extremely difficult. Intrastratal or subjacent corrosion between lithological units in the
subsurface can create features that superficially resemble surface karst, complicating
stratigraphic surface identification (James and Choquette, 1983). Moreover, breakdown of
organic matter in sediments immediately overlying a carbonate can increase acidity enough to
cause subaqueous karstification, without the unit ever seeing meteoric processes (Froede and
Reed, 2007). Several diagnostic attributes for hardgrounds have been recognized. Because the
surface becomes partially to fully lithified beneath the surface of the water, hardgrounds are
often bored into or encrusted upon by organisms during their formation. Other features
associated with hardgound surfaces are pyritization, Fe-oxidization, phosphatization, and rip-ups
(Nicolaides and Wallace, 1997; Railsback et al., 2003; Nalin and Massari, 2009). The
geochemical expression of hardground surfaces is variable. Since lithification at hardground
surfaces presumably occurs in equilibrium with seawater, they may contain heavy δ13C and δ18O
values (Marshall and Ashton, 1980). Conversely, if the surface has a high influx of organic
matter (Mutti and Bernouli, 2003), or if sulfate reduction occurs at the surface (Dickson et al.,
2008), a negative δ13C shift may be present. However, any surface that did not form through
subaerial processes would not be expected to display a negative carbon isotope excursion in a
86
zone displaced below the surface, because one would not expect there to be significant
downward advection of water (James and Choquette, 1983; Railsback et al., 2003).
Positive Linear Covariance Trends (PLCTs)
Positive covariance in 13C and 18O cross-plots are interpreted as mixing lines between two
phases of carbonate, the diagenetic component (micrite) and the primary component (Allen and
Matthews, 1982; Mitchell et al., 1997). This phenomenon is interpreted as being caused by
"infiltrating meteoric water and evaporative losses to the atmosphere" (Suarez et al., 2009) or
by phreatic-marine mixing zone interactions (Allen and Matthews, 1982). Positive linear
covariant trends in Permian-Triassic boundary sections have been used to infer surfaces of
subaerial exposure and subsequent meteoric diagenesis (Heydari et al., 2001). Isotope values
from confirmed subaerial exposure surfaces in the Ordovician Nashville Dome and Creaceous
Tlayua Formation of Mexico show positive covariance as well (Railsback et al., 2003; Suarez et
al., 2009). Allen and Matthews (1982) report an increase in covariance with increasing depth
beneath a surface of subaerial exposure in the Upper Mississippian Newman Limestone of
northern Kentucky. They interpret the positive linear covariance in the Newman Limestone as
resulting from marine-meteoric mixing, and interpret the thin overlying 12C-enriched subaerial
exposure zone as representing meteoric-only alteration with little covariance. Hence, while the
meteoric-only zone displays little positive linear covariance, covariance increases deeper in the
formation due to alteration in a marine-meteoric mixing zone environment. Regardless of the
mechanism, positive linear covariance is interpreted herein as representing diagenetic
alteration.
87
Oceanic Anoxia Events (OAEs)
Schlanger and Jenkyns (1976) proposed a model to explain the widely distributed black organicrich Late Cretaceous sediments observed from a large range of paleoenvironments. These
organic-rich deposits have important implications for the formation of oil, and may account for
about 60% of the world’s oil supply (Irving et al., 1974; Jenkyns, 1980). OAEs have been
associated with positive δ13C excursions in both bulk carbonate and organic carbon due to the
deep ocean burial of 12C-rich organic carbon and subsequent global enrichment of 13C in total
dissolved carbon (TDC) (Scholle and Arthur, 1980; Arthur et al., 1987; Coccioni and Galeotti,
2003; Bowman and Bralower, 2005; Jarvis et al., 2006). The magnitude of the excursions are
+0.5‰ to +0.8‰ in bulk carbonate during the Middle Cenomanian Event (MCE) (Coccioni and
Galeotti, 2003; Jarvis et al., 2006), and roughly +2.5‰ to +4‰ in bulk carbonate and organic
carbon, respectively, during Oceanic Anoxic Event 2 (OAE2) (Pratt and Threlkeld, 1984; Tsikos et
al., 2004; Bowman and Bralower, 2005; Jarvis et al., 2006; Parente et al., 2007).
Schlanger and Jenkyns (1976) note that the Late Cretaceous experienced marked
increases in seafloor spreading rates. Volumetric increase of mid-ocean ridges combined with
the warm, tropical Cretaceous climate to produce widespread transgression. This transgression
greatly increased the surface area of epicontinental seas, and provided much more volume to
the photic environment. Increased organic production in the oceans would have resulted from
this expansion of the photic zone, which would have increased the absolute amount of organic
matter escaping to deeper water. The warm high latitude seas during the Late Cretaceous
would not have had the oxygen-carrying capacity of today’s seas, which would have limited the
recharge of oxygen-rich water to the deep ocean. These effects combined to create widespread
expansion of the oxygen minimum layer to within 300 m of the ocean surface during times of
88
increased organic productivity. Thus, the widespread preservation of black organic-rich
sediments from a broad range of paleoceanographic environments during the Late Cretaceous is
at least partly attributable to major transgressive episodes along with a very warm climate.
Arthur et al. (1987) point out that increased freshwater influx to relatively restricted
waterways such as the Cretaceous Western Interior Seaway may have created low-salinity
surface “cap” layers which prevented oxygen interchange between the atmosphere and the
lower ocean layers. Oxidation of pelagic organic material would have gradually decreased
dissolved oxygen levels in these lower ocean layers, leading to dysoxia or anoxia. However,
Arthur et al. (1987) stress that the common factor in the deposition of organic material is marine
transgression and high productivity.
Recently, OAE2 has been linked to eruptions of ~107 km3 of Caribbean oceanic plateau
basalts, which depleted already low oceanic O2 levels through oxidation of reduced metals
(Sinton and Duncan, 1997; Kerr, 2005; Elrick et al., 2009). These eruptions may have released
large quantities of bio-limiting elements, enhancing primary productivity. Intriguing evidence
for this phenomenon has been recovered in the Guerrero-Morelos platform of southern Mexico
by Elrick et al. (2009). They showed that the isotopic onset of OAE2 in the Morelos Formation is
also associated with concentration anomalies of trace metals 5-20X those of background levels,
thought to have been derived from hydrothermal effluents. In their study, two mysterious
negative δ13C excursions, where the carbon isotope value can drop below 0‰ VPDB, were
measured the Morelos Formation beneath the onset of OAE2. If this signal is indeed regionally
correlable, it could be used to identify pre-OAE2 Upper Cenomanian sediment.
89
Mitchell et al. (2008) have tied OAE 1d, the Mid-Cenomanian Event (MCE), and OAE2 to
Milankovitch cyclicity in the Umbria-Marche Succession of Italy. They propose that OAEs were
relatively extreme events which were superimposed on a much smaller-scale background cycle
of oxic/anoxic conditions driven primarily by seasonality in the Cretaceous climate. They assert
that these OAEs, and likely others, are orbitally driven, and may represent 2.45 Myr cycles of
"extremely weak insolation variation brought about by coeval nodes in precession, obliquity,
and eccentricity.” The 2.45 Myr cycle, caused by interaction between Earth - Mars orbital
cycles, creates especially reduced seasonality which ultimately leads to stagnation of the global
oceanic oxygen cycle. They point out that this cyclicity explains why the MCE occurred
approximately 2.4 Myr before OAE2.
Timing of the Middle Cenomanian Event (MCE) and Oceanic Anoxia Event 2 (OAE2)
In the Scaglia Bianca Formation in the Umbria-marche basin, the MCE occurs in sediment
containing H. reussi, Dicarinella sp., W. praehelvetica, and W. brittonensis. The Middle
Cenomanian Event is placed within the D. algeriana subzone of the R. cushmani Zone (Coccioni
and Galeotti, 2003). It is worthy to note that the D. algeriana Subzone is herein interpreted as
limited to the Upper Cenomanian rather than Middle Cenomanian. The discrepancy is very
small, however, as the MCE may actually begin at the uppermost Middle Cenomanian, very
close (within 0.08 Myr) to the base of the Upper Cenomanian (Lugowski et al., 2009). This
discrepancy may lead to speculation that the FAD of D. algeriana slightly predates the Middle
Cenomanian - Upper Cenomanian boundary or that the MCE really occurs in the Upper
Cenomanian (Fig. 16). OAE2 spans the Cenomanian - Turonian boundary during the Whiteinella
archeaocretacea Partial Range Zone (Lugowski et al., 2009). It is most notably associated with
the extinction of the rotaliporid group of planktonic foraminifera (Arthur et al., 1987).
90
Figure 15 – Idealized C-isotope profile of a meteorically-altered paleoexposure surface. Adapted from Theiling et
al. (2007).
91
Figure 16 - Chart showing the geochronologic distribution of Oceanic Anoxia Events. Created with TimeScale
Creator.
92
Results - Paleoexposure
Dagger Flat Dataset 1
At Dagger Flat DS1, the largest transect of Buda Limestone was analyzed. The δ13C curve is
relatively stable over the entire vertical transect (Table 7a; Fig. 6, 18). The largest magnitude
δ13C shift between any two adjacent samples is only 0.23‰ and occurs between samples DFDS1-24 (4.6 m above the base of the sampled outcrop) at 1.54‰ VPDB and DF-DS1- 26 (5 m) at
1.77‰ VPDB in a 40 cm interval. The absolute range of values varies from a maximum of 1.97‰
VPDB at the very lowest sample, DF-DS1-1 (0 m), to a minimum of 1.54‰ VPDB at DF-DS1-24
(4.6 m), representing a maximum range of 0.43‰. Another notable minimum of 1.58‰ VPDB
occurs at DF-DS1-18 (3.4 m). Neither minimum occurs with enough magnitude to warrant direct
evidence for meteoric effects. This is unsurprising given the likely paleo-depth of these samples.
If there exists a disconformity in the middle of the Buda of BBNP, then isotopic evidence for
subaerial exposure is lacking. Furthermore, any hiatus within the Buda of BBNP is apparently
too short to show up biostratigraphically.
The δ18O signature at Dagger Flat varies roughly between -5‰ and -7‰ VPDB (Table 7a;
Fig. 6, 18). Like the carbon signature, the heaviest values are found at the bottom of the
sampled interval. Interestingly, the only two samples lighter than -7‰ VPDB are the same
samples which gave the lightest δ13C values, DF-DS1- 18 (3.4 m) and DF-DS1-27 (5.2 m). This
covariance will be further discussed below, in the Positive Linear Covariance section. It is worth
noting that oxygen isotope values are more susceptible late diagenesis than carbon isotopes
(Heydari et al., 2001), so this signature likely represents diagenetic values, either from subaerial
exposure or some other process.
93
Dog Canyon
At Dog Canyon, the top 2.6m of Buda was sampled and analyzed (Table 6a; Fig. 3, 17). A slight
negative shift in δ13C values is observed. The largest magnitude δ13C shift is 0.62‰ and occurs
from samples DC-DS1-8 (1.2 m above the base of the sampled outcrop) at 1.88‰ VPDB to DCDS1-6 (1.6 m) at 1.26‰ VDPB in a 40 cm interval. The magnitude of this shift exceeds the
minimum requirement set by this study of 0.5‰ to infer subaerial exposure, but does so over a
slightly larger vertical distance than previous authors have established (Goldstein, 1991;
Railsback et al., 2003).
The average δ13C value of all samples taken from above 1.4 m (DC-DS1-1 to DC-DS1-7) is
1.46‰ VPDB (Table 6b). The average δ13C value from taken from below 1.4 m (DC-DS1-8 to DCDS1-14) is 1.78‰ VPDB. The magnitude of the shift of 0.325‰ falls short of the required
minimum shift. However, the fact that the average δ13C shift is less than the maximum δ13C shift
is to be expected. Areas of the rock that lie above or below the horizon of the maximum δ13C
shift were less affected by presumed meteoric processes. Alone, the observed negative δ13C
shift in the top 2.6 m of the Buda Limestone does not conclusively confirm the existence of
paleoexposure, since the average magnitude is not large. Nonetheless, the presence of any
negative δ13C shift in the top of the Buda Limestone does lend evidence for subaerial exposure.
If the Buda Limestone were exposed to subaerial effects at its top, one would expect the
standard deviation between all samples taken from the top of the Buda (i.e. the 2.6 m section at
Dog Canyon) to be higher than the standard deviation from samples taken from the rest of the
Buda (i.e. the Dagger Flat section). The standard deviation of all δ13C and δ18O values taken
from the top of the Buda are 0.207 and 0.822, respectively (Table 6b). The entire Dagger Flat
94
section has a standard deviation in δ13C and δ18O values of 0.096 and 0.709, respectively (Table
7b). The standard deviation in the measured oxygen isotope values from the top of the Buda at
Dog Canyon is 1.16 times larger than those from Dagger Flat. The standard deviation in the
measured carbon isotope values from the top of the Buda at Dog Canyon is 1.93 times larger
than those from Dagger Flat. This result lends supporting evidence for a subaerial exposure
surface between the Buda and Boquillas Formations, since there is higher variation in measured
carbon isotope values closer to the unconformity.
The oxygen isotope signature from Dog Canyon shows two negative shifts, one at the
base of the sampled section at -6.85‰ VDPB, and one at 1.4 m at -6.83‰ VDPB, which
correlates with the negative δ13C shift. This lends further support in favor of a meteoric
diagenetic origin for the shifts, because meteoric waters are enriched in the lighter isotopes of
both carbon and oxygen. Assuming the horizon at 1.4 m containing the negative shifts was the
primary zone of re-cementation, one would expect both oxygen and carbon to follow the same
negative trend as observed. Above 1.4 m, the δ18O signature oscillates, and then shifts positive
to -4.06‰ VDPB. This positive δ18O shift may indicate evaporation and 16O depletion during
subaerial exposure. Again, the influences of subsequent diagenesis here are unknown.
The micrite infill of a large gastropod floatstone found resting on the Buda - Boquillas
contact at Dog Canyon was also analyzed. The δ13C value is 1.39‰ VPDB and the δ18O value is 4.06‰ VPDB, which are not appreciably different from the sample taken directly beneath, in the
topmost Buda Limestone. That material above the subaerial exposure surface yields similarly
light isotopic vales as below the exposure is expected if there has been transgressive reworking
of material underlying the exposure. This result suggests that chemical components derived
from the Buda Limestone may have been incorporated into the Boquillas Formation.
95
Positive Linear Covariance Trends (PLCTs)
Isotope data from Dagger Flat DS1 shows marked positive covariance when plotted on a δ13C
and δ18O cross-plot (Table 7b; Fig. 21). Covariance is 0.053 and R2 is 0.669. Isotope data taken
from Dog Canyon yields different results. In the top 1.4 m of Buda Limestone, where meteoric
alteration is interpreted to be the greatest, there is no positive linear covariance trend.
Covariance in this interval is -0.011 and R2 is 0.014 (Table 6b; Fig. 20). Beneath 1.4 m, positive
linear covariance is observed. Covariance in the interval below 1.4 m is 0.034 and R2 is 0.154.
These results show that positive covariance in the Buda Limestone increases beneath
the zone of apparent meteoric alteration. Thus, the Buda may have experienced diagenesis in a
marine-meteoric mixing zone overlain by a thin zone of meteoric-only alteration much like the
Newman Limestone of Allen and Matthews (1982). Because data for this analysis was
composited from two separate sections, Dog Canyon and Dagger Flat DS1, local diagenesis or
lateral variation on covariance may have influenced the results. The section at Dagger Flat may
simply have stronger positive covariance due to local diagenetic effects rather than being lower
in the Buda section. However, there is still demonstrable covariance at the Dog Canyon locality
below 1.4 m. Regardless of uncertainties, positive linear covariance from Dog Canyon and
Dagger Flat shows that the data is derived from more than one phase of carbonate with a
significant diagenetic component, possibly associated with a mixing zone environment and/or
subaerial exposure.
Dagger Flat Dataset 2
The Dagger Flat DS2 section is somewhat unusual because two samples from above the Buda
give anomalously negative δ13C and δ18O values (Table 8, Fig. 19). One of these samples, DF-
96
DS2-m10 (0.1 m), was taken from the anomalous black carbonate immediately overlying the
Buda Limestone in this area, and yielded δ13C and δ18O values of -1.22‰ VPDB and -6.74‰
VPDB, respectively. The highest sample, DF-DS2-p120, was taken from "normal" Boquillas flaggy
carbonate, and yielded δ13C and δ18O values of -1.27‰ VPDB and -9.67‰ VPDB, respectively.
Between these two negative samples lies another sample, DF-DS2-p80, again taken from
"normal" Boquillas flags, which yielded more expected δ13C and δ18O values of 1.82‰ VPDB and
-5.50‰ VPDB, respectively. Since the Dagger Flat Dataset 2 locality is within a roughly a
kilometer of several small igneous intrusions, it is possible that some form of hydrothermal
alteration has occurred in this area. It is also possible that contamination from organic
components in the rocks have artificially driven down bulk-rock isotopic compositions. Finally,
the reworking of meteorically altered material which eroded before or during transgression can
lead to negative isotopic shifts in basal transgressive sediments. Each explanation for the low
δ13C values seems plausible. For example, black shale was present in this sampling interval. The
reworking of meteorically-altered hiatal chemical components is also likely, presuming the basal
Boquillas Formation was deposited during transgression. Furthermore, the very negative δ18O
values, especially in sample DF-DS2-p120, may very well indicate some form of alteration.
Overall, it appears that diagenetic overprinting or contamination from organic matter has
affected the isotopic results in at least two of the four samples at this locality, complicating
interpretation. If the signal measured here is not significantly altered, then there is a possibility
that this negative δ13C and δ18O peaks may correspond to one or both negative isotopic trends
measured in the Upper Cenomanian of the Morelos Formation of southern Mexico by Elrick et
al. (2009). However, too little data has been gathered to make formal conclusions as of the time
of this writing.
97
Interestingly, the topmost Buda Limestone sampled in this locality, DF-DS2-m30, yielded
δ13C and δ18O values of 1.72‰ VPDB and -6.54‰ VPDB, respectively. The δ13C value is 0.27‰
higher than the highest sample from the Buda taken at Dog Canyon, and the δ18O value is
2.48‰ lower. A δ13C value of 1.72‰ VPDB is more in line with samples taken beneath the
negative shift in the topmost Buda observed at Dog Canyon. Still, these values fall within the
normal range of values taken from the Buda Limestone. Since the rest of Buda Limestone is
beneath the surface at this location, no lower samples could be taken. It is unknown if the
values grow heavier with increasing depth at this location, as would be expected if the contact
underwent subaerial exposure.
Results - Oceanic Anoxia Events
Dog Canyon and Dagger Flat Data Set 1 (Buda)
Given the Lower Cenomanian age of the Buda Limestone, the unit was probably deposited
before the onset of the MCE or OAE2 (Fig. 16). The Boquillas Formation, however, was likely
deposited during one or both events. This study affirms the conclusion drawn by Frush and
Eicher (1975) that the basal Boquillas was deposited in an oxygen-poor environment, due to the
presence of black shale layers and absence of benthic foraminifera. Given the reported
presence of dicarinellids in sediments bearing evidence of the MCE (Coccioni and Galeotti,
2003), it is possible that the base of the Boquillas was deposited at or near time of the MCE.
OAE2 peaks in the W. archaeocretacea Zone, which is defined by the LAD of R. cushmani (and all
rotaliporids). The presence of rotaliporids in the sampled interval of Boquillas sediment
suggests that OAE2 may manifest higher in the section than analyzed by this study.
98
Since the Dog Canyon and Dagger Flat sections are probably highly influenced by
diagenesis, it is unlikely that a reliable primary isotope signal is preserved. However, carbon
isotope values slightly less than 2.0‰ VPDB, comparable to values from the Buda Limestone in
this study, have been reported from other formations of similar Cenomanian age (Price et al.,
1998; Coccioni and Galeotti, 2002; Bowman and Bralower, 2005; Jarvis et al., 2006). At the very
base of the Dagger Flat DS1 section, the δ13C value is 1.97‰ VPDB, the most positive value
recorded. At three higher locations in the section (DF-DS1-12, DF-DS1-18, DF-DS1-24), three
minima are observed, each of which fall below is 1.6‰ VPDB. When comparing the maximum
and minimum values measured in this section, a ~-0.4‰ shift δ13C is observed. Excluding the
effects of diagenesis discussed above, this negative isotopic trend could correlate to several
negative δ13C excursions noted in the late Lower Cenomanian of England by Jarvis et al. (2006).
However, given the strong evidence of diagenetic alteration in the Buda and the distance
separating these sections, isotope correlation based on this dataset is tenuous at best.
99
Dog Canyon Sample
DC-DS1-16
DC-DS1-1
DC-DS1-2
DC-DS1-3
DC-DS1-4
DC-DS1-5
DC-DS1-6
DC-DS1-7
DC-DS1-8
DC-DS1-9
DC-DS1-10
DC-DS1-11
DC-DS1-12
DC-DS1-13
DC-DS1-14
Height (m)
2.8
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
δ13C VPDB
1.39
1.45
1.42
1.49
1.43
1.68
1.26
1.47
1.88
1.76
1.80
1.80
1.56
1.95
1.71
σ
0.01
0.04
0.03
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.02
0.01
0.02
0.01
δ18O VPDB
-4.06
-4.06
-5.07
-4.86
-5.49
-4.93
-4.89
-6.83
-5.16
-4.61
-5.26
-4.53
-5.24
-4.53
-6.85
σ
0.02
0.03
0.01
0.04
0.03
0.02
0.03
0.02
0.04
0.04
0.04
0.03
0.03
0.01
0.02
Table 6a - Sample number, height abiove base of outcrop, and isotope data for the Dog Canyon DS1 data set. DCDS1-16 is gastropod floatstone found resting on Buda - Boquillas contact.
Avg. δ13C ‰ VPDB top 1.4 m
1.46
Dog Canyon Std. Dev 13C
0.208
Avg. δ13C ‰ VPDB below 1.4 m
1.78
Dog Canyon Std. Dev. 18O
0.822
Difference (δ13C ‰ VPDB shift)
0.325
Avg. δ18O ‰ VPDB
-5.16
Covariance top 1.4 m
-0.011
Covariance below 1.4 m
0.034
Table 6b - Average carbon isotope values, standard deviations, and covariance from above and below the zone of
inferred meteoric alteration, and average oxygen isotpe value.
100
Dagger Flat DS1 Sample
DF-DS1-31
DF-DS1-30
DF-DS1-29
DF-DS1-28
DF-DS1-27
DF-DS1-26
DF-DS1-24
DF-DS1-23
DF-DS1-22
DF-DS1-21
DF-DS1-19
DF-DS1-18
DF-DS1-16
DF-DS1-15
DF-DS1-14
DF-DS1-13
DF-DS1-12
DF-DS1-10
DF-DS1-8
DF-DS1-6
DF-DS1-4
DF-DS1-3
DF-DS1-2
DF-DS1-1
Height (m)
6.0
5.8
5.6
5.4
5.2
5.0
4.6
4.4
4.2
4.0
3.6
3.4
3.0
2.8
2.6
2.4
2.2
1.8
1.4
1.0
0.6
0.4
0.2
0.0
δ13C VPDB
1.74
1.82
1.73
1.70
1.77
1.77
1.54
1.67
1.71
1.68
1.70
1.58
1.65
1.71
1.67
1.79
1.58
1.73
1.82
1.73
1.71
1.76
1.91
1.97
σ
0.01
0.01
0.01
0.02
0.01
0.02
0.02
0.01
0.02
0.01
0.01
0.01
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.02
0.01
0.03
0.01
δ18O VPDB
-5.65
-5.43
-6.20
-6.70
-6.13
-6.29
-7.28
-6.14
-5.61
-5.62
-5.84
-6.94
-6.52
-6.36
-6.09
-6.31
-7.11
-6.06
-5.38
-5.88
-5.14
-5.04
-4.95
-4.34
Table 7a - Sample number, height abiove base of outcrop, and isotope data for the Dagger Flat DS1 data set.
Dagger Flat DS1 Avg. δ13C ‰ VPDB
1.73
Dagger Flat DS1 Avg. 18O
-5.96
Dagger Flat DS1 Std. Dev. 13C
0.108
Dagger Flat DS1 Std. Dev. 18O
0.799
Covariance
0.053
Table 7b – Average isotpe values, standard deviation, and covariance for the Dagger Flat DS1 data set.
101
σ
0.02
0.01
0.03
0.01
0.03
0.02
0.01
0.02
0.02
0.02
0.02
0.03
0.03
0.02
0.02
0.02
0.02
0.02
0.00
0.03
0.01
0.02
0.03
0.02
Dagger Flat DS2 Sample
DF-DS2-p100
DF-DS2-p60
DF-DS2-m10
DF-DS2-m20
Height (m)
1.2
0.8
0.1
0.0
δ13C VPDB
-1.27
1.82
-1.22
1.72
σ
0.01
0.01
0.01
0.01
δ18O VPDB
-9.67
-5.50
-6.74
-6.54
Table 8 - Sample number, height abiove base of outcrop, and isotope data for the Dagger Flat DS2 data set.
102
σ
0.04
0.02
0.01
0.02
Figure 17 – Stable isotope profile for Dog Canyon. Top most point is from gastropod floatsone found resting on the
Buda - Boquillas contact.
103
Figure 18 – Stable isotope profile for Dagger Flat data set 1.
104
Figure 19 – Stable isotope profile for Dagger Flat data set 2. Bottom most point is Buda Limestone, while higher
points are from basal Boquillas Formation.
105
13
18
Figure 20 - δ C and δ O cross-plot showing positive linear covariance trend (PLCT) in the top 1.4 m of Buda
13
Limestone at the Dog Canyon DS1 locality. Below 1.4 m, there is no PLCT. Also note the lighter average δ C values
above 1.4 m.
106
13
18
Figure 21 - δ C and δ O cross-plot showing positive linear covariance trend (PLCT) in the Buda Limestone at the
Dagger Flat DS1 locality.
107
CHAPTER 6 - DISCUSSION AND CONCLUSION
Interpretation of the Commanchean - Gulfian of Big Bend National Park
Foraminiferal biostratigraphy and paleoenvironment
The Buda Limestone was deposited during the late Lower to early Middle Cenomanian Th.
globotruncanoides Zone. The sea that deposited the Buda Limestone was virtually devoid of
clastic influx, representing a temporary hiatus in terrigenous deposition between the Del Rio
Clay and Boquillas Formation. This sea was deep enough for the foraminiferal assemblage to be
dominated by shallow to intermediate depth planktonics, with only about 20% of the
assemblage represented by benthics. It was also deep enough to remain beneath normal wave
base, as evidenced by the lack of ripples and high-energy deposits. On the other hand, this sea
was still shallow enough to truncate the habitat of deep-dwelling planktonic foraminifera such
as rotaliporids. This open shelf environment was likely between about 50 to 100 m in depth.
The deposition of the Boquillas Formation in Big Bend National Park began with mixed
carbonate clastic sedimentation during a major transgression. Water depth was greater than
during the deposition of the Buda Limestone, and could accommodate the deep-dwelling
rotaliporid species. Bottom waters during Boquillas time were likely oxygen-deficient, which
severely reduced benthic habitats (Frush and Eicher, 1975). This study places the basal ±1 m of
the Boquillas in the Late Cenomanian D. algeriana Subzone of the Rotalipora cushmani Zone.
The duration of the Buda - Boquillas unconformity is therefore roughly equivalent to the
duration of the missing Th. reicheli and Th. greenhornensis Biozones, which represent a sizable
portion of the Middle Cenomanian. The foraminiferal age determination of the basal Boquillas
Formation presented herein is in disagreement with the late Early Cenomanian - Middle
Cenomanian age given by ammonites (Cobban et al., 2008; Cooper, 2008). Whether this
discrepancy results from misidentification, reworking, or errors in the biozonations of either
foraminifera or ammonites is uncertain. This study does suggest that because the basal
Boquillas is likely a transgressive deposit, the incorporation of previously deposited or hiatal
ammonite fauna in a transgressive lag is possible. The fact that ammonites restricted to the
basal Boquillas are often silicified and abraded may support this interpretation (Young, 1958).
Sequence stratigraphy and paleoexposure of the Buda Limestone
The Buda - Boquillas unconformity is interpreted as resulting from subaerial processes by: 1)
The maximum negative δ13C isotope shift in the 1.4 m beneath the unconformity; 2) The average
negative δ13C isotope shift in the 1.4 m beneath the unconformity; 3) The increase in variability
between measured δ13C values in the 1.4 m below the unconformity; 4) The positive linear
covariance trend observed in δ 13C and δ 18O cross-plots beneath the top 1.4 m of Buda; and 5)
The presence of a dm-scale microkarst unconformity surface (Lock and Peschier, 2006).
This study is in agreement with conclusions drawn by Lock et al. (2007), who interpreted
the Buda Limestone as occupying its own stratigraphic sequence. In Big Bend National Park, the
lower contact of the Buda Limestone with the Del Rio Clay is likely a subaerial exposure. This
109
interpretation is based on the presence rip-ups of Del Rio Clay incorporated into the basal Buda
(Lock et al., 2007) and an upsection decrease in the P:B ratio in the Del Rio Clay to ~0-5%
planktonics at its top, indicating significant regression (Mauldin and Cornell, 1986). Since the
foraminifera have been previously studied in the Del Rio Clay (Mauldin and Cornell, 1986) and
shown to be benthic-dominated, it was not sampled by this study. However, some inferences
can be drawn from the foraminiferal assemblage observed in the lowermost samples of Buda
Limestone. Based on the high percentage of planktonic foraminifera near the base of the Buda
Limestone, the contact of the Buda Limestone with the Del Rio Clay is presumably a P:B break
from roughly 0% planktonics in the uppermost Del Rio Clay to roughly 55-75% planktonics in the
basal Buda Limestone. This data seems to suggest that the lowermost portion of the Buda
Limestone was deposited during transgression following subaerial exposure of the Del Rio Clay.
The rest of the Buda Limestone represents a highstand to regressive package that culminates
with the subaerial unconformity at its top. It is unclear whether Buda sediment records all of
the time during which sea level rose and subsequently fell, or if the Buda is a mostly highstand
deposit with little depositional record existing of the flanking transgression and subsequent
regression. Since the whole Buda Limestone represents an entirely pelagic-dominated
assemblage below storm wave base, it is unlikely that sea level changes during deposition of
existing Buda sediment were of enough magnitude to result in significant environmental or
oceanographic changes.
Oceanic Anoxia Events (OAEs)
The Buda Limestone was deposited in an isotopically quiet period between the major Oceanic
Anoxia Events OAE1d and OAE2, and before the Middle Cenomanian Event. Furthermore,
distinguishing subtle isotopic signals from diagenetic effects is very difficult in the absence of
110
large-scale isotopic trends. However, the observed isotopic values from the Buda Limestone do
compare with other reported values from the late Lower Cenomanian (Price et al., 1998;
Coccioni and Galeotti, 2002; Bowman and Bralower, 2005; Jarvis et al., 2006). The Boquillas
Formation, on the other hand, was likely deposited during the Middle Cenomanian Event (MCE)
and/or Oceanic Anoxia Event 2 (OAE2). If such is the case, then the MCE is likely near the base
of the Boquillas Formation, while OAE2 likely manifests higher in the section than analyzed in
this study, in the W. archaeocretacea Zone (Fig. 14, 16).
Conclusions
1. The overlapping ranges of Favusella washitensis (Carsey), Globigerinelloides bentonensis
(Morrow), G. caseyi (Bolli), Loeblich and Tappan), Parathalmanninella appenninica
(Renz), Praeglobotruncana delrioensis (Plummer), P. stephani (Gandolfi), and Rotalipora
montsalvensis (Mornod) allow for placement of the Buda within the upper portion of
the Early to Middle Cenomanian Th. globotruncanoides Zone.
2. Given the age of the Buda Limestone, the unit was probably deposited in a relatively
isotopically stable time before the onset of the MCE or OAE2.
3. The Buda Limestone contains a shallow water pelagic foraminiferal assemblage roughly
consisting of 46-90% heterohelicids, 3-37% globigerinellids, and 4-22% hedbergellids
(incl. favusellids). Deeper water forms such as praeglobotruncanids and rotaliporids are
present in very low quantities of 0-3 specimens per thin section throughout the section.
111
Benthic foraminifera make up roughly 15-45% of the total foraminiferal assemblage.
This assemblage implies water depths between about 50-100 m (Baker, 1976), which
agrees with Scott and Kidson (1977) and Lock et al. (2007), who assigned an open shelf
paleodepth for the Buda Limestone in Big Bend National Park.
4. Intermediate-depth, open marine globigerinellids show an initial increase followed by
overall decrease in abundance upsection. The accompanied increase in opportunistic
shallow-depth heterohelicids upsection indicates the development of conditions more
favorable to opportunistic species than to other species (Nederbragt, 1991). These
abundance data are interpreted as showing initial sea level transgression followed by
regression, since open marine assemblages would be expected to decrease in
abundance shoreward (Sliter, 1972; Leckie, 1987). Since the Buda is entirely pelagicdominated, it is likely that the unit was deposited entirely within the 50-100 m depth
interval. The peak abundance of globigerinellids is interpreted as a proxy for the
maximum flooding surface (MFS).
5. The largest magnitude δ13C shift values in the top 2.6 m of Buda observed at Dog
Canyon is 0.62‰ and occurs from samples DC-DS1-8 (1.2 m above the base of the
sampled outcrop) at 1.88‰ VPDB to DC-DS1-6 (1.6 m) at 1.26‰ VDPB in a 40 cm
interval. The magnitude of this shift exceeds the minimum requirement set by this
study of 0.5‰ to infer subaerial exposure, but does so over a larger vertical distance
than previous authors have established (Goldstein, 1991; Railsback et al., 2003).
112
6. The average δ13C value of all samples taken from the top 1.4 m of Buda at Dog Canyon
(DC-DS1-1 to DC-DS1-7) is 1.46‰ VPDB. The average δ13C value from taken from below
1.4 m (DC-DS1-8 to DC-DS1-14) is 1.78‰ VPDB. The magnitude of the shift of 0.325‰
falls short of the required minimum shift. However, the presence of any negative δ13C
shift in the top of the Buda Limestone does lend evidence for subaerial exposure.
7. The standard deviation of all δ13C and δ18O values taken from the top of the Buda are
0.207 and 0.822, respectively. The entire Dagger Flat section has a standard deviation in
δ13C and δ18O values of 0.096 and 0.709, respectively. The standard deviation in the
measured oxygen isotope values from the top of the Buda at Dog Canyon is 1.16 times
larger than those from Dagger Flat. The standard deviation in the measured carbon
isotope values from the top of the Buda at Dog Canyon is 1.93 times larger than those
from Dagger Flat. This result lends supporting evidence for a subaerial exposure surface
between the Buda and Boquillas Formations, since there is higher variation in measured
carbon isotope values at the top of the Buda Limestone.
8. The oxygen isotope signature from Dog Canyon shows two negative shifts, one at the
base of the sampled section at -6.85‰ VDPB, and one at 1.4 m at -6.83‰ VDPB, which
correlates with the negative δ13C shift. This lends further support in favor of a meteoric
diagenetic origin for the shifts, because meteoric waters are enriched in the lighter
isotopes of both carbon and oxygen. Assuming the horizon at 1.4 m containing the
negative shifts was the primary zone of re-cementation, one would expect both oxygen
and carbon to follow the same negative trend as observed. Above 1.4 m, the δ18O
signature oscillates, and then shifts positive to -4.06‰ VDPB. Positive δ18O shifts (often
113
overlying negative shifts) have been reported from the surfaces of subaerial exposure
that experience intense evaporation, which preferentially removes 16O in a thin zone
near the surface (Allen and Matthews, 1982; James and Choquette, 1983).
9. The top 1.4 m of Buda Limestone, where meteoric alteration is interpreted to be the
greatest, contains no positive linear covariance trend. Covariance in this interval is 0.011 and R2 is 0.014. Beneath 1.4 m, positive linear covariance is observed. Covariance
in the interval below 1.4 m is 0.034 and R2 is 0.154. A δ13C and δ18O cross-plot of Dagger
Flat DS1 data from lower in the Buda shows a distinct positive linear covariance trend,
with covariance of 0.053 and R2 value of 0.669. These results show that positive
covariance in the Buda Limestone increases beneath the zone of apparent meteoric
alteration. Thus, the Buda may have experienced diagenesis in a marine-meteoric
mixing zone overlain by a thin zone of meteoric-only alteration much like the Newman
Limestone of Allen and Matthews (1982). Because data for this analysis was composited
from two separate sections, local diagenesis or lateral variation on covariance may have
influenced the results. The section at Dagger Flat may have stronger positive covariance
due to local diagenetic effects rather than being lower in the Buda section. However,
positive linear covariance from Dog Canyon and Dagger Flat does show that the data is
derived from more than one phase of carbonate with a significant diagenetic
component, possibly associated with a mixing zone environment and/or subaerial
exposure.
10. In Big Bend National Park, the lower contact of the Buda Limestone with the Del Rio Clay
is likely a subaerial exposure (Lock et al., 2007). Based on the high percentage of
114
planktonic foraminifera near the base of the Buda Limestone, the contact of the Buda
Limestone with the Del Rio Clay is presumably a marked P:B break from near 0%
planktonics in the uppermost Del Rio Clay (Mauldin and Cornell, 1986) to approximately
80% planktonics in the basal Buda Limestone . The lowermost portion of the Buda
Limestone was likely deposited during transgression following subaerial exposure of the
Del Rio Clay. The rest of the Buda Limestone probably represents a highstand to
regressive package that culminates with a subaerial microkarstic unconformity at its top.
Based on this interpretation, the Buda Limestone it bounded by subaerial exposure
surfaces and represents its own sequence.
11. This study places the basal Boquillas Formation in the Upper Cenomanian D. algeriana
Subzone of the R. cushmani Zone based on the overlapping ranges of Dicarinella sp.,
Dicarinella algeriana (Caron), Rotalipora sp., Rotalipora cushmani (Morrow),
Thalmanninella sp. cf. Th. greenhornensis (Morrow ), and Whiteinella baltica Douglas
and Rankin. The duration of the unconformity is therefore roughly equivalent to the
duration of the missing Th. reicheli and Th. greenhornensis Biozones, which represent a
sizable portion of the Middle Cenomanian.
12. The foraminiferal age determination of the basal Boquillas Formation presented herein
is in slight disagreement with the late Early Cenomanian - Middle Cenomanian age given
by ammonites (Cobban et al., 2008; Cooper, 2008). Whether this discrepancy results
from misidentification, reworking of fauna, or errors in the biozonations of either
foraminifera or ammonites is uncertain.
115
13. The δ13C and δ18O values of the micrite infill of a large gastropod floatstone found
resting on the Buda - Boquillas contact at Dog Canyon is 1.39‰ VPDB and -4.06‰ VPDB,
respectively. These values are not significantly different from the sample taken from
topmost Buda Limestone. This result suggests that chemical components derived from
the Buda Limestone may have been incorporated into the Boquillas Formation (Theiling
et al., 2007). The incorporation of previously deposited or hiatal macrofossil fauna in a
transgressive lag is possible. The fact that ammonites restricted to the basal Boquillas
are often silicified and abraded supports this interpretation (Young, 1958).
14. The original zonation of the Boquillas Formation given by Frush and Eicher (1975) is
herein considered obsolete. It is now known that the first appearance of Heterohelix
globulosa (Cushman) occurs in the Upper Cenomanian D. algeriana Subzone of the R.
cushmani Zone (Nederbragt, 1991). The first appearance of Whiteinella aprica (Loeblich
and Tappan) also occurs in the D. algeriana Subzone (Premoli Silva and Sliter, 2002).
Therefore it appears that the lower 0-60 feet of the Boquillas is constrained to the D.
algeriana Subzone given both the data of Frush and Eicher (1975) and of this study.
15. A composite count of all thin sections made from the lowermost 1.2 m of Boquillas
yields 22% dicarinellids, 44% rotaliporids, 16% whiteinellids, 13% praeglobotruncanids,
and 22% all others (hedbergellids, etc.). Just one broken benthic foraminifer was
recognized out of all thin sections. The Boquillas Formation is dominated by deepdwelling rotaliporids and intermediate-dwelling dicarinellids and praeglobotruncanids.
116
The Boquillas Formation was almost certainly deposited in deeper water than the Buda
Limestone. It likely represents deep, open marine, oxygen-poor transgressive
depositional conditions following subaerial exposure of the Buda Limestone.
16. Given the reported presence of dicarinellids in sediments bearing evidence of the MCE
(Coccioni and Galeotti, 2003), it is possible that the base of the Boquillas was deposited
at or near the time of the MCE. Furthermore, OAE2 probably occurs higher in the
section than analyzed by this study, in the overlying W. archaeocretacea Zone.
17. Two samples from the Boquillas Formation at the Dagger Flat DS2 section give
anomalously negative δ13C and δ18O values, including the anomalous black limestone
layer. Since the Dagger Flat Dataset 2 locality is within a kilometer of several small
igneous intrusions, it is possible that some form of hydrothermal alteration has occurred
in this area. It is also possible that contamination from organic components in the rocks
have artificially driven down bulk-rock isotopic compositions. If the signal measured
here is not significantly altered, then there is a possibility that this negative δ13C and
δ18O peaks may correspond to one or both negative isotopic trends measured in the
Upper Cenomanian of the Morelos Formation of southern Mexico by Elrick et al. (2009).
The reworking of meteorically altered material which eroded before or during
transgression can also lead to negative isotopic shifts in basal transgressive sediments
(Theiling et al., 2007).
117
PLATES - BUDA LIMESTONE
Favusella
Pl. 1 - F. washitensis (Carsey)
Pl. 2 - F. washitensis (Carsey)
vii
Pl. 3 - F. washitensis (Carsey)
Globigerinelloides
Pl. 4 - G. bentonensis (Morrow)
Pl. 5 - G. bentonensis (Morrow)
viii
Pl. 6 - G. caseyi (Bolli, Loeblich and Tappan)
Pl. 7 - G. ultramicrus (Subbotina)
Pl. 8 - G. caseyi (Bolli, Loeblich and Tappan)
ix
Hedbergella
Pl. 9 - Clavihedbergella simplex (Morrow)
Pl. 10 - Hedbergella planispira (Tappan)
Pl. 11 - Hedbergella sp. cf. H. delrioensis (Carsey)
x
Heterohelix
Pl. 12 - H. moremani (Cushman)
Pl. 13 - H. moremani (Cushman)
xi
Parathalmanninella
Pl. 14 - P. appenninica (Renz)
Pl. 15 - P. appenninica (Renz)
Pl. 16 - P. micheli (Sacal and Debourle)
xii
Praeglobotruncana
Pl. 17 - P. delrioensis (Plummer)
Pl. 18 - Praeglobotruncana sp. cf. P. stephani (Gandolfi)
Pl. 19 - Praeglobotruncana sp.
xiii
Rotalipora
Pl. 20 - R. montsalvensis (Mornod)
Pl. 21 - R. montsalvensis (Mornod)
Pl. 22 - ?R. montsalvensis (Mornod)
xiv
Thalmanninella
Pl. 23 - ?Thalmanninella globotruncanoides (Sigal)
Miscellaneous
Pl. 24 - Large benthic foraminifer, planktonic foraminifera, calispheres, echinoderm spine (lower right)
xv
Pl. 25 - Benthic foraminifer
Pl. 26 - Uncertain classification (rare, single specimen recovered)
xvi
Pl. 27 - Benthic foraminifer
Pl. 28 - Benthic foraminifer
Pl. 29 - Benthic foraminifer
xvii
PLATES - BOQUILLAS FORMATION
Dicarinella
Pl. 30 - Dicarinella sp.
Pl. 31 - D. algeriana (Caron)
xviii
Pl. 32 - Dicarinella sp.
Pl. 33 - Dicarinella sp. cf. D. algeriana (Caron)
xix
Pl. 34 - Dicarinella sp.
Hedbergella
Pl. 35 - Hedbergella sp.
xx
Heterohelix
Pl. 36 - H. globulosa (Cushman)
Pl. 37 - H. globulosa (Cushman)
xxi
Praeglobotruncana
Pl. 38 - P. stephani (Gandolfi)
Pl. 39 - P. gibba Klaus
xxii
Rotalipora
Pl. 40 - ?R. cushmani (Morrow) (top), ?Anaticinella sp. (bottom)
Pl. 41 - Rotalipords (single keel)
xxiii
Pl. 42 - Rotalipord (single keel)
Pl. 43 - ?R. cushmani (Morrow)
xxiv
Shackoina
Pl. 44 - Shackoina cenomana (Shacko)
Thalmanninella
Pl. 45 - Broken Thalmanninella sp.
xxv
Whiteinella
Pl. 46 - W. baltica Douglas and Rankin
Pl. 47 - ?W. aprica (Loeblich and Tappan)
xxvi
Pl. 48 - Whiteinella sp.
Miscellaneous
Pl. 49 - Photomicrograph of DF-DS2-m20 (anomalous black limestone layer) showing calcispheres and ?rotaliporid
foraminfer.
xxvii
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