FIELD TRIP GUIDE
PENNSYLVANIAN FOSSILS, FACIES AND ANCIENT
DEPOSITIONAL ENVIRONMENTS OF NORTH-CENTRAL
TEXAS
FIELD TRIP GUIDE FOR THE 2010 TEXAS ACADEMY OF SCIENCE MEETING
MARCH 4-6, 2010
TARLETON STATE UNIVERSITY
EDITOR
JOREE BURNETT
BO ALLEN
BEAUX JENNINGS
BEAU BEREND
CLIFTON HOLLAND
SPENCER BUSCH
KAYLI LAGOW
CODY CHRISTIAN
JAMIE LANE
CALVIN CLARY
CHANCE MARTIN
SHAWN FOYT
MICHEAL PRATT
TAD GASS
ROBERT SCHERMERHORN
PATRICK GLEASON
LYNN SMITH
CHRISTINE GOFFINET
MATTHEW WATERMAN
DEDRICK HAIL
BRITTANY WHITE
MARK WIDMER
OVERVIEW
This field trip guide will concentrate on the Pennsylvanian system of rocks deposited in
the North-Central Texas area. These facies were deposited in the Pennsylvanian Period and are
named for the coal-bearing rocks found in the state of Pennsylvania. The Pennsylvanian System
is divided into five series (oldest to youngest): the Morrowan, Atokan, Desmoinesian,
Missourian, and Virgilian. For the purpose of this field trip, the study will be confined to the
Desmoinesian and Missourian Series (Middle to Late Pennsylvanian Age), and concentrate on
the formations that represent the Strawn Group.
The outcrops seen today were worked by Tarleton’s sedimentology class for class field
trips, and the class’s interpretations are represented in the guidebook. The interpretations are
based on the delta models outlined by Bradshaw, Mazzulo, Brown, Cleaves and Erxleben.
INTRODUCTION
North Texas was a very active area during the Middle and Upper Pennsylvanian Period.
Several tectonic, or mountain building events, were occurring and influencing North Texas. The
Amarillo-Wichita-Arkbuckle Uplifts were to the north, along with the southern extension of the
Ouachita Uplift to the east, and the Llano-Pecos-Marathon Uplift to the south. These tectonic
events allowed for a shallow seaway to extend over the Kern, Val Verde and Midland basins.
Large river systems transported loads of sediment from the erosion of the uplands. Throughout
the Pennsylvanian, and by the Upper Pennsylvanian, these rivers systems had created a large
coastal plain that extended from the Dallas-Fort Worth area westward to north-central Texas.
As these large river systems entered the shallow seaway, they deposited their sediment loads
of sand, silts and clays. These sediments were deposited along the shelf area as fan-shaped
delta deposits. The localities that will be seen will each be pertaining to a different section of a
fluvial- dominated delta and shallow shelf environment.
To envision some of the things we will be seeing, a little bit about deltas needs to be
explained. In a delta environment, as the sediment-rich waters from the channel enter the
seaway, the velocity decreases, thereby releasing the sands, silts and clays out of suspension.
As more and more sediment comes into the sea, the delta builds out (progrades) onto the shelf.
On the shelf, at a substantial distance from the incoming channel, prodelta silts and clays are
deposited. Prodelta mudstones are generally thick and laminated with very little fossil content,
except for a few plant fragments that have washed down the channel. As the delta progrades
further out to sea, coarser sediments (fine to medium-grained sands) are deposited on top of
the previous prodelta deposits of thick mudstones. The sands that are deposited on the delta
front as the delta progrades are generally fine to medium-grained and thinly laminated.
Laminations and thin-bedded ripple marks are common in delta front environments as well as
deformation structures and growth faults associated with loading sands on top of weakly
consolidated sediments. These faults continue to grow as more and more sediment is
deposited. At the point where the river pours into the sea, energy decrease is drastic and
channel-mouth bars are deposited. Channel-mouth bars are typically moderately well-sorted,
fine to medium-grained, cross-bedded sandstones. Further up in the channel is where medium
to coarse-grained sandstones will be found. Cross-bedding will be common in the channel fill
areas.
As a delta progades it is common to see a vertical sequence that consists of prodelta
mudstones, overlain by fine-grained sands of delta fronts, overlain by fine to medium-grained
channel mouth bars and upward into the coarser-grained sandstones of the channels.
Therefore, it is typical to say that a delta sequence will coarsen upward in grain size.
Delta-forming rivers carry a tremendous amount of sediment. As the delta progrades
seaward, it is common for the outlets into the sea to become clogged with sands, silts and
clays. During a flooding event, when the channel is clogged, it is common for a channel to
breach its levee and create a crevasse splay. When the breach occurs, the depositional energy
decreases drastically, thus depositing a sheet of silt to fine-grained sands, or may even create a
new channel. So it is common to see a sequence of mud, sand, mud in an area where there has
been a crevasse splay. When a new channel is created and the old channel abandoned, then
subsidence can occur. As the large sediment loads are coming down the channel and piling up,
there has not been a substantial amount of dewatering and compaction. When a channel is
abandoned, and the input of sediment has ceased, then compaction and dewatering occurs
which leads to the area sinking. As the area sinks, and sediment inputs decrease, it creates a
stable clastic platform or a shallow shelf environment where calcite-secreting organisms may
live, called carbonate banks. When the sedimentation rate on these platforms is low, then the
surface may be completely bioturbated, where the organisms have had time to burrow and dig
without being covered up. The presence of marine sediments overlying delta facies suggest
that there has been a transgression, when actually the delta itself has sunk and caused the
influx of water.
Locality 1: 1.8 miles west of 281 on Hwy 4
32° 31’ 41’’ N
98° 9’ 30’’ W
Presenters:
Clifton Holland-lower mudstone unit
Beaux Jennings-fossils present
Bo Allen-Brannon Bridge Member
Grindstone Creek Formation
At this locality the Grindstone Creek Formation is present. The description at this
location will focus mainly on two different lithologies. At the base of the locality is a strongbrown silty mudstone to muddy siltstone that is plant fossil rich. The probable depositional
environment for these mudstones is a delta lobe with a marsh or a swamp, somewhere off to
the side of a channel.
Sedimentary structures of this unit consist of ironstone concretions. The composition of
these concretions is the mineral hematite. These nodules preserve the imprints of plant fossils
that flowed down the delta channel. These fossils include plants such as, sphenopsids
(horsetails), ferns, and lycopods. As subsidence, flooding, or transgressions occurred, the
plants would be washed downstream. Transport was minimal, as evidence by plants still being
somewhat intact, like fronds still attached to fern stems, but no whole plants are found. The
plants were washed down and deposited in a marsh or a swamp area. In these marshes or
swamps, where the mudstones were formed, were somewhat oxidizing conditions with some
water circulation. As these plants washed in and were buried in the muds, iron precipitated out
and formed the ironstone concretions that encase the plants, thus leaving imprints. The
oxidizing conditions that were present did not allow for the formation of coal deposits.
Almost due north at a vertical distance of 19 feet (5.8 m) from the fossil outcrop, a
distinct bench can be seen. The bed that forms this escarpment is the Brannon Bridge Member
of the Grindstone Creek Formation, and is a marker bed used for correlation between localities
due to the distinct scarp lines. The Brannon Bridge Member can be interpreted, by the
sedimentology class, in two ways: as a clastic and as a carbonate. As the vertical and lateral
facies change, the amount of sand and the activity of organisms changes as well. As a clastic
rock, the bed can be classified as a medium bedded, very-fine, fossiliferous quartz arenite (less
than 15% clay) to fossiliferous quartz wacke (more than 15% clay). As a carbonate rock it can
be classified as a medium bedded sandy biomicsparrudite and a sandy crinoid-bearing lime
mudstone. The member has a light olive weathered surface and a yellowish-brown fresh
surface and is 0.65 ft (20 cm) thick. As the bed extends laterally, over miles, the composition
changes, and the presence of large amounts of sand and calcite make the lithology
questionable.
Sedimentary structures seen in the Brannon Bridge includes burrows. Fossils can also
be seen. Crinoid columnals are abundant in the member, and show only a small amount of
transport since lengthy columnals can be found. The Brannon Bridge can be interpreted as
being deposited on a shallow shelf, since large amounts of sand and calcite are found.
At this locality, the sequence goes from marsh deposits on bottom to shelf deposits on
top. This is possibly due to subsidence of the delta from channels migrating, causing the water
level to rise, and giving the appearance of a transgression.
Locality 2: 4.2 miles west of Hwy 281 on Hwy 180, just east of the Brazos River Bridge
32° 47’ 56’’ N
98° 11’ 50’’W
Presenters:
Brittany White-Village Bend Limestone
Christine Goffinet-East Mountain Shale
Patrick Gleason-Lake Pinto Sandstone
Mineral Wells Formation
Towards the Upper Pennsylvanian, the highland areas to the east and northeast became
worn down, and the erosion of the Ouachita Fold Belt progressed, and their supply of sediment
to the deltas decreased. As the sediment supply diminished and the deltas dewatered and
compacted, causing subsidence, stable platforms formed where many marine organisms could
survive. As a result, more marine invertebrates can be seen in the Upper Strawn Group.
At this locality, within the Mineral Wells Formation, there are three members present:
the Village Bend Limestone, the East Mountain Shale and the Lake Pinto Sandstone. The Village
Bend Limestone sits at the bottom of this sequence and is a 3.5 ft (1.1 m) thick wackestone that
is light grey weathered, with a reddish-yellow fresh surface. This member shows complete
bioturbation, which indicates a very slow sedimentation rate. As the delta was building out, the
organisms were able to keep up with the sedimentation rate and completely burrow the
sediments. Some fossils are visible in this layer, such as brachiopods. The shells of some of
these brachiopods are broken, suggesting a relatively high current activity. The probable
depositional environment for the Village Bend Limestone is open marine, along a shoreface,
shelf limestone. The Village Bend Limestone is also significant because it marks the boundary
between the Middle and Upper Pennsylvanian, and is the top of the Desmoinesian Series.
The next member up in the sequence is the East Mountain Shale. This is a light gray
colored shale unit with a coarse siltstone layer within. The total thickness of the East Mountain
Shale is approximately 33 ft (10.1 m). Within the unit there are many fossils present. Crinoid
columnals, brachiopods, gastropods, bivalves, bryozoans, cephalopods, and echinoid plates and
spines, can all be found within this facies. Fossil abundance is greater within the lower portion
of the unit, suggesting that as the layer built up, sedimentation rate increased, therefore, not
allowing organisms to survive in the sediment rich water. The depositional environment for the
shale unit in the lower part is a stable clastic platform with a prograding prodelta environment
further up in the unit.
The next unit up is the sequence is the Lake Pinto Sandstone. This unit is approximately
34 feet (10.4) thick and is composed of a quartz arenite. The Lake Pinto Sandstone represents a
channel that trends west-northwest. Sedimentary structures within this unit are numerous.
Trough cross-beds are easily seen in the lower portion of the unit. Large amounts of sediment
layering over easily deformable sediment caused load structures such as pseudonodules and
recumbent forsets. Some fossils are also visible. These consist of Casts of logs that were
transported downstream and incorporated within the channel sands.
The absence of a complete delta sequence: shelf > prodelta > delta front > channel
mouth bar > channel, where the delta front and channel mouth bar deposits are missing, can be
explained by a small accommodation space. What this means, is the channel had no room to
build up between the sea floor and the ocean surface, so instead of building on top of these
deposits, it ripped through them, and cut all the way down to the prodelta deposits. The
sequence that is left does represent a regressive sequence.
Locality 3: spillway of Lake Palo Pinto, 1 mile west of Hwy 4 on Lakeview Dr, 10.2 miles south of
Hwy 180
32° 38’ 38’’ N
98° 16’ 13’’ W
Presenters:
Lynn Smith-units 1&2
Mark Widmer-units 3&4
Dedrick Hail-unit 5 and Brazos River Formation
This location has two formations present, the Mingus Formation and the Brazos River
Formation. Looking around at the topography, the Brazos River Formation forms all the scarps
and caps of the mesas and the Mingus Formation forms the slopes for the valleys.
Mingus Formation
For ease of description, the Mingus Formation will be divided into five distinct units.
Unit#1 is the lower-most unit and is a 7+ ft (2.1+ m) muddy siltstone that is interbedded
with thinly bedded quartz wacke. The depositional environment for this unit is interpreted as a
delta front where fine-grained sand, shale sedimentation rates are high and alternations are
common due changes in transport energy.
Unit #2 is a 1.1 ft (0.34 m) thick very fine-grained quartz arenite layer. This layer is
completely bioturbated, which suggests lower sedimentation rates that allowed organisms to
thrive. The depositional environment for unit #2 is a clastic platform shoreface evidenced by
its fine-grained texture, and also its coarser texture than unit #1.
Unit #3 is a gray mudstone. Unit thickness varies from 16-30 ft (4.9-9.1 m) due to a
channel (unit #4) that is incising the unit. Clay ironstones are common throughout the unit, as
well as thin detrital coal layers. The depositional environment for unit #3 is likely an
interdistributary bay. A reducing environment was present, as evidence by the thin coal layers
and the gray color of the unit. The coals also signal an interdistributary bay. As plant material
washed in, it was buried and with low enough water circulation, it formed thin detrital coal
lenses.
Unit #4 is a channel that is incised into unit #3. The sand that comprises this channel
varies from a thickness of 2 ft (0.61 m) at the edges of the channel to 18 ft (5.5 m) in the
deepest part. The sands are a fine-grained quartz arenite. Soft sediment deformation
structures are present lower in the channel, due to sediment loading over soft sediments.
Fossils are also present. Log casts of horsetails and lycopods can be seen. Fossil remnants are
broken due to transport within the channel. This channel was likely formed as a channel
avulsed into the interdistributary bay environment of unit #3.
Unit #5 lies directly above the channel. This unit is approximately 92 ft (28.0 m) thick
and is a dark grey color, silty mudstone. The dark grey color indicates reducing conditions,
most likely in a prodelta environment. The thickness of the mudstone implies that there was a
greater accommodation space, than stop #2, to allow for channels to build over it. As a result,
the mudstone is much thicker here than at stop #2.
Brazos River Formation
The Brazos River Formation is the thick sandstone unit that is capping all of the
surrounding hills. This layer is approximately 75 ft thick and is a very fine quartz arenite. This
large sand unit could be a distributary mouth bar or a channel facies. Lamination is common,
which implies that is it is more likely a distributary mouth bar, than a channel.
The sequence of the first 4 units of the Mingus Formation represents a prograding delta.
This is evidenced by the sequence of mudstones and siltstones overlain by fine to medium
sands. The Brazos River Formation shows a progradation of the delta as well.
Locality 4: W.K. Gordon Center, Thurber, TX, approximately 13 west of Hwy 4 along I-20
In 1886, brothers William and Harvey Johnson began mining for coal. After Harvey died,
Robert D. Hunter bought the business, changed the name to the Texas and Pacific Coal
Company and began selling coal to the Texas and Pacific Railroad Company. The coal that was
being mined was formed in a coal swamp, under reducing conditions, and is lateral to unit # 3
of the Mingus Formation.
After resolving union worker strikes by hiring many immigrants from places such as Italy,
Poland, Britain and Ireland, the company branched out into another venture. Large shale
deposits from surrounding hills was determined to be of a good quality for making bricks. The
Green and Hunter Brick Company was founded. Paving bricks from Thurber pave streets in Fort
Worth, Galveston, Houston and many others. Many buildings, such as the Dallas Opera House,
also boast Thurber brick. As asphalt replaced paving bricks, Thurber’s brick making business
suffered.
The switch from coal to oil as an energy source also crippled Thurber. Owners of the
town struck oil on some company leases in 1917, but the revenues were not enough to save
Thurber. By the depression, the coal company and the brick plant had shut down and many of
Thurber’s residents had moved on. By the mid 30’s Thurber was a ghost town.
Acknowledgements
Dr. Phil Murry
HEB
Coca-Cola
Wal-Mart
W.K. Gordon Center
Aaron and Larissa Hutchins
Gresham Ranch
Dr. Roger Wittie, Department of Agribusiness and Agronomy
Bibliography
Bradshaw, S. E., & Mazzulo, J. (1996). Depositional Facies and Environments of the Lower Mineral Wells
Formation, Pennsylvanian Strawn Group, North Central Texas. Transactions of the Gulf Coast Association
of Geological Societies , 47-54.
Brown, J., Cleaves, I. A., & Erxleben, A. (1973). Pennsylvanian Depositional Systems in North Central
Texas: A Guide for Interpreting Terrigenous Clastic Facies in a Cratonic Basin. Austin: Bureau of Economic
Geology.
Erxleben, A., & Cleaves, A. (1982). Middle and Upper Pennsylvanian System of North-Central and West
Texas.
Jones, K. (n.d.). W.K. Gordon Center and the History of Thurber. Retrieved March 4, 2010, from Tarleton
State University: http://www.tarleton.edu/gordoncenter/thurberhistory.html