Journal of Sedimentary Research, 2011, v. 81, 0–0
Research Article
DOI: 2110/jsr.2011.35
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SEDIMENTOLOGICAL CHARACTERIZATION OF THE SEGO SANDSTONE (NW COLORADO, USA): A NEW
SCHEME TO RECOGNIZE ANCIENT FLOOD-TIDAL-DELTA DEPOSITS AND IMPLICATIONS FOR
RESERVOIR POTENTIAL
CARLY C. YORK,1 CLAYTON S. PAINTER,2
AND
BARBARA CARRAPA2
1
2
University of Wyoming, 1000 East University Avenue Laramie, Wyoming 82071, U.S.A.
University of Arizona, Gould-Simpson Building #77, 1040 East 4th Street, Tucson, Arizona 85721, U.S.A.
e-mail: carly.york@gmail.com
ABSTRACT: Marginal-marine deposits are typically excellent hydrocarbon reservoirs around the world. Certain depositional
environments may provide natural, selective, physical processes that produce better reservoir rocks than others depending on
associated energy, hydraulic regime, etc. With this paper we present new detailed sedimentological, petrographic, and grainsize data of ancient flood-tidal-delta deposits that provide means to characterize this environment in the ancient as well as to
determine reservoir characteristics. In this study, we compare grain size, grain and cement composition, and the ratio of pore
space to cement from thin sections between tidal, shoreface, and flood-tidal-delta facies of the Sego Sandstone in northwest
Colorado. The intent of this comparison is to provide an evaluation of different physical processes and the degree of diagenesis
related to different environments to better understand the relative controls on reservoir characteristics. We find that flood-tidaldeltas show a statistically larger average grain size than either shoreface or other tidal facies. Also, flood-tidal-deltas have a
smaller ratio of pore-space to cement. Shoreface facies have a larger ratio of pore space to cement overall than that of floodtidal-delta or other tidal facies. We propose that this may be due to chemistry and higher weathering rates in the backbay
environment in contrast to the shoreface environment, where higher mechanical reworking may lead to selective removal of
chemically unstable minerals. To test this hypothesis, we conducted statistical analyses of these petrophysical and grain size
characteristics. Our results show that flood-tidal-deltas are characterized by clean upper-fine-grained cross-bedded sandstone
that is well sorted and subrounded to rounded with a majority of landward paleocurrent directions and a presence of tidal
bundles, mud drapes, and reactivation surfaces.
INTRODUCTION
The Sego Sandstone in western Colorado (Fig. 1) is a marginal-marine
sandstone that has been interpreted as a tide-influenced to tide-dominated
delta in Utah and southwestern Colorado (Yoshida et al. 1996; Willis and
Gabel 2001; Willis and Gabel 2003). The Sego Sandstone was deposited
during the late Campanian at the end of the Sevier Orogeny and the
beginning of the Laramide Orogeny on the western edge of the
Cretaceous Interior Seaway (Fouch et al. 1983; Van Wagoner 1991,
1995; Dickinson 2004; Horton et al. 2004; Dickinson and Gehrels 2008).
Although regional studies have detailed time-equivalent deposits in the
Book Cliffs, Utah, the tide-influenced and marginal-marine facies
observed north of Rangely, Colorado, are distinctly different from the
dominantly fluvial and tide-influenced delta facies of Book Cliff outcrops
to the southwest (see Fouch et al. 1983; Lawton 1986; Miall 1993; Van
Wagoner 1991, 1995, 1998; Yoshida et al. 1996; Robinson and Slingerland 1998; Willis 1997, 2000; Miall and Arush 2001; Willis and Gabel
2001, 2003; Wood 2004). This study describes the interval in the Sego
Sandstone that contains the flood-tidal-deltas near Rangely, Colorado as
largely being the result of a back-barrier system, with focus on a detailed
description of the flood-tidal-delta deposits in that system. Understanding the shape and size of these tide-dominated marginal-marine
sandstones as well as the processes that contribute to their deposition
and physical characteristics such as grain size and morphology is
Copyright E 2011, SEPM (Society for Sedimentary Geology)
imperative when estimating their potential as petroleum reservoirs. Tidal
and tide-influenced reservoirs have traditionally been good hydrocarbon
reservoirs, including barrier-island systems (Barwis 1990; Barwis and
Hayes 1979; Wood 2004), yet the dimensions of these systems are often
difficult to predict owing to the large range of shapes and sizes in the
barrier-island complex.
Several works have explored modern-day flood-tidal-delta deposits and
barrier-island systems (Morton and Donaldson 1973; Fitzgerald 1976;
Hubbard and Barwis 1976; Barwis and Hayes 1979; Hubbard et al. 1979;
Hayes 1980; Hodge 1982; Boothroyd et al. 1985; Israel et al. 1987;
Reynolds 1999; Morales et al. 2001; Davis et al. 2003) as well as some in
the ancient (Brenner 1978; Galloway and Cheng 1985; Barwis 1990;
Longhitano et al. 2010), but few provide detailed data on the nature of
these deposits, specifically in the ancient. More importantly, no previous
work quantitatively describes the differences between these deposits and
associated marginal-marine facies and explores the possible different
characteristics controlling reservoir quality and the reason for such
differences. For example, a key question that this paper aims to answer is:
are shoreface deposits and tide-influenced deposits significantly different
from one another in regards to preserving good reservoir characteristics?
And if yes, why? In answering these questions this study identifies the
relative controls on reservoir characteristics in this type of back-barrier
environment, which have previously been difficult to predict.
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FIG. 1.—Regional map of study area and general geology with locations of five outcrop study areas. Kh, Hunter Canyon Fm; Ki, Illes Fm.; Km, Mancos Shale; Kmv,
Mesverde Fm.; Ksc, Sego Sandstone; Kw, Williams Fork Fm.; Qa, Alluvium; Qe, Aeolian deposits; Tf, Ft. Union Fm.; Tgl, Green River Fm.; Tu, Uinta Fm.; Tw,
Wasatch Fm.
Additionally, this paper provides new detailed sedimentological data
including centimeter-scale stratigraphic sections detailing the flood-tidaldelta facies, detailed grain size statistical analysis, sandstone petrography,
cement, pore space, and matrix percentages, paleocurrent data, and
cement analysis. We use data and observations from modern systems on
the east coast of the United States as an analog for comparisons. These
comprehensive analyses contribute to both assessment of reservoir quality
and recognition and description of these deposits in the ancient. Also, we
hypothesize about the type of coastal system, including tidal range, in
which these flood-tidal-deltas were deposited. This contributes to the
characterization and understanding of ancient tidal systems as well as the
nature of a portion of the coastline (in present-day Colorado; Fig. 1) of
the Cretaceous Interior Seaway during the late Campanian.
Geologic Background
The Sego Sandstone, equivalent to the Upper Castlegate Sandstone
and part of the larger Upper Cretaceous Mesaverde Group (Fig. 2), crops
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FIG. 2.—Stratigraphy of region and names of
formations in the Mesaverde Group east and
west of Green River, Utah. Note that in some
locations the Anchor Mine Tongue of the
Mancos Shale splits the Sego Sandstone into
upper and lower Sego. Note also that the Neslen
Fm. is called the Williams Fork in Colorado
(from Willis 2000, with updated timescale from
Obradovich 1993).
out extensively in Utah and in limited but well exposed outcrops in
northwestern Colorado (Van Wagoner 1991, 1995; Horton et al. 2004).
Widespread research exists on the Castlegate type section in the Book
Cliffs, Utah (Fouch et al. 1983; Lawton 1986; Miall 1993; Van Wagoner
1991, 1995, 1998; Yoshida et al. 1996; Robinson and Slingerland 1998;
Willis 1997, 2000; Miall and Arush 2001; Willis and Gabel 2001, 2003;
Wood 2004). Regionally, these sand deposits were derived from the active
thrusting and fluvial systems resulting from the Sevier fold and thrust belt
to the west, creating a clastic wedge that extended , 275 km eastward
into the Cretaceous Interior Seaway (Fouch et al. 1983; Dickinson 2004;
Dickinson and Gehrels 2008). Horton et al. (2004) specifically target the
unroofing of the Santaquin culmination in the Charleston–Nebo salient
as the sediment source for the Castlegate–Price River units, which are
interpreted as being the proximal equivalent to the Sego in Colorado
(Fig. 2) (Willis 2000; Willis and Gabel 2001). Previous work on equivalent
deposits in the Book Cliffs, Utah, describes the Upper Castlegate and
Sego Sandstone as fluvial to estuarine or shallow marine. For example,
Van Wagoner (1991) describes incised-valley fills in the Castlegate and
Sego Sandstone in Utah, and other authors detail features such as tidedominated deltas (Willis and Gabel 2003), and braided river systems
(Yoshida et al. 1996); these studies go as far east as Grand Junction,
Colorado. Two masters’ theses researched the outcrops around Rangely,
Colorado (Stancliffe 1984; Noe 1984). These theses describe the Sego
Sandstone near Rangely, Colorado as part of a barrier-island system, but
neither gives detailed description of the flood-tidal-delta deposits, specific
ranges of grain size and sandstone petrography between different
marginal-marine facies, nor discuss the relative control on sedimentological and petrophysical characteristics.
Stratigraphy of the Sego Sandstone in Northeastern Colorado
Previous workers (Yoshida et al. 1996; Willis and Gabel 2001, 2003)
have described the Sego Sandstone in southwestern Colorado and eastern
Utah as a tide-dominated delta system and place a sequence boundary at
the contact between the base of the Sego Sandstone and the top of Buck
Tongue member of the Mancos Shale (Yoshida et al. 1996) (Fig. 2). Van
Wagoner (1991) interprets the Sego Sandstone in Utah to have nine highfrequency sequence boundaries and characterizes the lowstand deposits as
incised valleys filled with tide-dominated deltas and tidal bars; the
highstand deposits are interpreted as wave-dominated deltas.
Several stratigraphic sections (Fig. 1) were identified in this study for a
detailed sedimentological analysis and for comparison with modern
analogs and previous work. In the following we describe three sections,
the Amphitheater, Taylor Draw, and Canyon Pintado, from proximal
(NW) to distal (E).
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In the Amphitheater section, one of the most northwestern extents (and
farthest landward in this study) of the Sego Sandstone outcrop (Figs. 1,
3), the flood-tidal-deltas overlie a coal facies. These coal facies cap a
prograding shoreface section. The basal boundary of this prograding
shoreface section is marked by a flooding surface, and inclined
heterolithic sand bodies lie below the flooding surface (Fig. 3). The basal
boundary of the inclined heterolithic sand body is regionally recognized
as the sequence boundary with the Buck Tongue member of the Mancos
Shale (Fig. 2). Additionally, the Amphitheater section lies shoreward of a
shoreface succession.
Taylor Draw is the easternmost outcrop (Figs. 1, 4) and thus most
seaward deposit; while the facies and facies relationships are similar if not
identical to the other sections in the Sego Sandstone, flooding surfaces are
not as distinct as the more landward sections (Fig. 4). For example, in
this more distal location, a flooding surface may still be present, but it
may be a shale-on-shale contact. Consequently, we do not see the same
distinct flooding surface in Taylor Draw compared to the more landward
deposits in the Amphitheater (Fig. 3).
Canyon Pintado (Figs. 1, 5) shares similar facies of both the Taylor
Draw and the Amphitheater sections. Here, the flood-tidal-delta caps a
coal, which Noe (1984) interprets as the Anchor Mine Tongue of the
Mancos Shale. This coal shares a basal boundary with an aggradational
lower-shelf to shoreface sand, and underneath this shoreface lies the Buck
Tongue sequence boundary (Fig. 2, ‘‘SB’’ in Fig. 5; Willis and Gabel
2001; Willis 2000).
Modern Flood-Tidal-Deltas
Many authors present data on modern flood-tidal-deltas, and most
agree that a variety of factors contributes to flood-tidal-delta initiation,
growth, facies distribution, and duration, including tidal prism, tidal-inlet
shape, lagoon size, sediment supply, littoral drift, tidal range, and
bedrock shape and distribution (Hayes et al. 1973; Morton and
Donaldson 1973; Hubbard and Barwis 1976; Barwis and Hayes 1979;
Hubbard et al. 1979; Hayes 1980; Hodge 1982; Boothroyd et al. 1985;
Israel et al. 1987; Reynolds 1999; Morales et al. 2001; Davis et al. 2003).
Here we describe morphological and sedimentological characteristics of
modern flood-tidal-deltas and use these observations to compare to the
ancient flood-tidal-delta deposits in the Sego Sandstone.
Hayes et al. (1973) pioneered morphological nomenclature for features
commonly found in flood-tidal-deltas that subsequent work has used to
describe and reference flood-tidal-delta morphology, including the basis
for Figure 6. Barwis and Hayes (1979) give a review of barrier-island
deposits, including flood-tidal-delta idealized stratigraphic successions, in
which they describe a dominant landward current direction (although
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FIG. 4.—Full stratigraphic section from Taylor Draw and corresponding
detailed flood-tidal-delta section. Sample locations are denoted. Note that samples
were not taken from the full section, only in the flood-tidal-delta at , 62.2 meters.
SB 5 sequence boundary, FS 5 flooding surface.
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FIG. 5.—Full stratigraphic section from Canyon Pintado and corresponding
detailed flood-tidal-delta section. Sample locations are denoted. Note that the full
section was taken in a location slightly different than where the flood-tidal-delta
section was measured, so the sections do not match up exactly. In the location
where the full section was measured, we see more tidally dominated facies and the
flood-tidal-delta exists , 400 meters to the north. The reason for measuring the
flood-tidal-delta in a location different from that of the full section is to capture the
tidal bundles. T 5 tidal, SF 5 shoreface, and FTD 5 flood-tidal-delta.
SB 5 sequence boundary, FS 5 flooding surface.
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FIG. 3.—Full stratigraphic section from the Amphitheater and corresponding detailed flood-tidal-delta section. Sample locations taken for point counting are denoted. Key
symbols are used for all stratigraphic sections (Figs. 3–5, 7, 8). All samples are used in the grain-size study except for Amp 1.11_FTD and Amp 1.3_T. T 5 tidal, SF 5 shoreface,
and FTD 5 flood-tidal-delta. SB 5 sequence boundary, FS 5 flooding surface. Idealized flood-tidal-delta section is modified from Barwis and Hayes (1979).
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FIG. 6.—Morphology of the flood-tidal-delta
of the Essex Estuary, Massachusetts. 1, flood
ramp; 2, flood channels; 3, high area; 4, ebb
shield; 5, ebb spit; 6, spill-over lobes; 7, ebb
channels. (Modified from Boothroyd and
Hubbard 1975)
with some bimodality owing to the nature of tidal currents) (Fig. 3C).
The idealized stratigraphic sequence in terms of environment includes,
from bottom to top: bay fill, ebb channel, flood channel, flood ramp, ebb
shield or spit, and tidal flat (see Fig. 6 for physical locations of these
features and Fig. 3C for stratigraphic section). Most authors agree that
the flood-tidal-delta facies successions consist mostly of both trough and
planar cross-strata generated from megaripples or sand waves with a
landward preference in bedform direction (Hubbard and Barwis 1976)
resulting from the deposition of sand on the flood ramp (Figs. 3C, 6).
Other modern case studies include specific areas of the east coast
including Virginia (Morton and Donaldson 1973), North Carolina
(Hubbard et al. 1979), South Carolina (Hubbard and Barwis 1976;
Hodge 1982), Georgia (Hubbard et al. 1979), Florida (Davis 1987; Davis
et al. 2003), and some studies in the Texas gulf coast (Israel et al. 1987).
Morton and Donaldson (1973) describe flood-tidal-deltas on the coast of
Virginia in three dimensions to characterize the modern sediment
distribution and evolution of the flood-tidal-deltas. They found that
most of the sediment supply for their specific flood-tidal-delta came from
reworking of the barrier-island sediment. Their three-dimensional boring
samples showed that the flood-tidal-delta formed during a transgression,
on top of drowned lagoonal fill. Boothroyd et al. (1985) and Israel et al.
(1987) report a coarsening-upward sequence in the flood-tidal-deltas in
their study areas, Rhode Island and Texas, respectively. In particular,
these studies report shell beds or shell parts in the base (, 2 m) of the
flood-tidal-delta (Barwis and Hayes 1979).
Below we use these modern features to assist in recognizing basic facies
associations and paleocurrent directions when identifying flood-tidaldelta deposits in the ancient. Figure 3C shows the Barwis and Hayes
(1979) idealized flood-tidal-delta stratigraphic succession for comparison
with the Sego.
METHODS
Well-exposed, nearly horizontal (dipping 1 to 5u to the N–NW) Sego
Sandstone crops out just north and 35 km south of Rangely, Colorado
(Fig. 1). Detailed stratigraphic sections were measured at five locations
(Figs. 1, 3–5, 7, 8) on a trend that extends generally perpendicular to the
NE–SW paleoshoreline (Stancliffe 1984) targeting changes in the
depositional dip along the entire Sego Sandstone outcrop extent from
the most proximal to most distal deposits. We used a Jacob’s staff to
measure five full sections ranging from 19 to 65 m thick at a scale of 1 m
(in the field) 5 1 cm (in the figure) (Figs. 3–5, 7, 8). We used these full
sections only to place the flood-tidal-delta deposits in a stratigraphic
context. The more detailed sections that we identify as ‘‘flood-tidal-delta’’
were described at 1 cm (in the figure) 5 20 cm (in the field) scale
(Figs. 3–5). We measured a total of 50 sections, each, 3 to 6 m thick to
document any lateral changes in the flood-tidal-delta outcrops: 21 in the
Amphitheater, 12 in Taylor Draw, and 17 in Canyon Pintado, though the
flood-tidal-delta sections shown in Figures 3 through 5 are the most
characteristic sections out of the 50 measured. Some of the stratigraphic
sections were of poor quality either because of weathering or because they
were mostly buried in weathered material. Samples for grain size analysis
and sandstone petrography were collected to characterize different facies
and environments. Paleocurrents were measured in the field from the axes
of troughs of cross-beds in the three flood-tidal-delta locations (Fig. 1);
results are plotted in Figure 9, and the raw data are shown in Table 1.
Thirty samples were analyzed for sandstone petrography (Table 2,
sample locations Figs. 3–5, 7, 8) following the Gazzi-Dickinson method
(Gazzi 1966; Dickinson et al. 1983; Dickinson and Suczek 1979; Ingersoll
et al. 1984); we counted 400 framework grains per sample. Pore space,
cement, and matrix were also counted, where pore space was stained blue
and cement and matrix were differentiated by color, shape, and optical
properties (Fig. 10). Matrix was identified as dark amorphous material
that commonly did not conform to grain boundaries, and cement was
generally composed of birefringent calcite or silica.
Sixty samples (Table 3A; locations in Figs. 3–5, 7, 8) were prepared for
grain size analysis through crushing of three to five grams of sandstone
sample in a porcelain mortar and pestle. Samples were examined under
the microscope after crushing to ensure that individual grains were
separate but not broken. Separated samples were then sent to the Denver
USGS, laboratory where carbonates were removed using 10 percent
HCL; organic matter and clays were not removed or deflocculated. The
, 2 mm grain fraction was mixed in an aqueous solution and pumped
through the Malvern Mastersizer 2000 from Malvern Instruments
(Campbell 2003; Eshel et al. 2004) to measure particle diameter in
accordance with the Wentworth scale (Wentworth 1922).
DESCRIPTION OF TIDAL AND SHOREFACE FACIES
Table 4 outlines all facies and facies characteristics examined in this
study. To compare between the different facies (tidal, flood-tidal-delta,
and shoreface) and the respective environmental energies, it is important
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to identify the facies observed in the Sego Sandstone. Typical tidal facies
in the Sego Sandstone consist of laminated sand and mud interbeds, much
like the tidal bedding described in Nio and Yang (1991). Specific
sedimentary structures that are present include flaser and linsen bedding
where the muddy intervals contain syneresis cracks. Mud drapes are also
present, usually interbedded with rippled sand beds (Fig. 11C). Characteristically, the tidal facies are very muddy and the tidal facies are usually
found in inclined heterolithics, below the flood-tidal-delta facies
(Fig. 11D).
Shoreface facies consist of hummocky, tabular, and planar crossstratified sandstone; in a few places we see planar lamination. Shoreface
facies are a characteristically competent, cliff-forming sand that consists
of meters of inter-stacked trough and tabular cross-stratification and
hummocky cross-stratification (Fig. 11A, B). Typically, the base of the
hummocky-stratified sandstones contain gutter casts. Ophiomorpha that
range from 5 to 20 cm in height are common in planar and tabular crossstratified sandstone beds. Overall, the shoreface facies have fewer mud
interbeds than either the tidal facies or flood-tidal-delta facies.
CHARACTERIZATION OF ANCIENT FLOOD-TIDAL-DELTA DEPOSITS
Overall, the facies and facies associations of what we interpret to be
flood-tidal-delta deposits consist of bright white, well sorted, subrounded
to rounded, sandstone beds, 3 to 6 m thick, characterized by tidal bundles
and double-mud-draped high-angle planar cross-beds that range from 0.2
to 1 m high (Figs. 12A, 13) with occasional rippled intervals in between
cross-bedded intervals. In places, the top of the planar and trough crossbedded foresets are rippled. A few reactivation surfaces and double mud
drapes were also identified throughout the flood-tidal-deltas (Fig. 13). In
places, these reactivation surfaces and mud drapes are found on the top of
a trough or cross-bed, but usually they were observed on top of rippled
beds (Fig. 13E). Most of the cross-bedded foresets are unidirectional with
planar lower bounding surfaces (Fig. 12A). The sections described below
highlight the specific differences between the three flood-tidal-delta
outcrops.
Amphitheater
The Amphitheater section crops out in the westernmost section of the
Sego Sandstone northwest of Rangely, Colorado (Fig. 1) for , 500 m
along depositional dip and expresses a sharp base with a coal or
carbonaceous shale underneath (Figs. 3, 13D). Mud drapes are present
throughout, but the thickest (, 2 cm) and most continuous mud drapes
and mud rip-up clasts are found in the bottom 1 m of the planar crossbedded facies (Fig. 13F). Bioturbation is minimal, although we find
occasional vertical Ophiomorpha, , 10 cm long and , 1 cm wide,
throughout all levels of the flood-tidal-delta. A thin oyster bed is present
at the same stratigraphic level as the flood-tidal-delta (Fig. 3), but
approximately 100 m away from the exact location of our stratigraphic
sections. Heights of bedforms do not increase or decrease consistently up
section, as commonly described in stratigraphic profiles from modern
flood-tidal-delta examples (Barwis and Hayes 1979). Paleocurrents
measured in the 3 to 6 m of the flood-tidal-delta from the trough axes
of tabular cross-beds show a bimodal distribution with a preferential
landward component (Fig. 8). Above this sand body in the Amphitheater
section, the outcrop ends after 3 m of cross-bedded sandstone (Fig. 3).
Taylor Draw
FIG. 7.—Full section from Dead Dog. Samples taken for grain-size analysis and
point counting are denoted. T 5 tidal, SF 5 shoreface, and FTD 5 flood-tidaldelta. SB 5 sequence boundary, FS 5 flooding surface.
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The section measured at Taylor Draw (Fig. 4) crops out for , 150 m
along depositional strike and , 15 km east of the Amphitheater in the
easternmost extent of the Sego Sandstone east of Rangely, Colorado
(Fig. 1). Overall, there are fewer mud drapes and more bioturbation
(, 10%) than what is observed in the Amphitheater (, 20%), and the
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dominant sedimentary structure observed is tabular cross-bedding
(Figs. 3, 4, 12). This hypothesized flood-tidal-delta does not have a
sharp base with the sandstone below. We identify the base of the floodtidal-delta outcrop and distinguished it from the sandstone units below on
the basis of relative bioturbation. The sand below the hypothesized floodtidal-delta facies forms a low-angle bench and is , 30–40% bioturbated
with Ophiomorpha. There is a gradual but distinguishable change to a
cliff-forming sand with , 0–5% bioturbation (Ophiomorpha). The
outcrop ranges from 4 to 6 m thick with both high-angle planar and
tabular cross-beds where the thickness of sets ranges from 0.3 to 1 m
height (Fig. 12B). Both types of cross-bed set thicknesses vary in height
up section, and there is no systematic increase or decrease in set height or
systematic distribution of tabular or planar cross-beds throughout the
sandstone body. Overall, planar cross-beds are slightly more abundant
with respect to the Amphitheater. We see more bidirectional foresets than
what is observed in the Amphitheater flood-tidal-delta section; bedding
planes between foreset beds are both tangential and non-tangential.
Paleocurrents measured from the trough axes of cross-beds are generally
northwest oriented (Fig. 9). Above this sand body in Taylor Draw, the
outcrop quickly grades into slope-forming fluvial deposits of the Williams
Fork or Neslen Formation (Willis 2000; Lawton 1986).
Canyon Pintado
Located approximately 35 km south of Rangely, Colorado, Canyon
Pintado (Fig. 5) crops out as a three-dimensional, tan sandstone, 3 to 6 m
thick, that is exposed for , 100 m along depositional strike (Fig. 1).
Overall, this section is characterized by planar cross-beds and a larger
relative amount of rippled intervals (, 30% of outcrop) with respect to the
Amphitheater (, 10%) and Taylor Draw sections (, 10%) (Figs. 3–5,
13C). Cross-bed sets range from 0.2 to 0.6 m thick, and the ripples are
asymmetrical. As in the Amphitheater and Taylor Draw, the bedding
planes between the foresets are nontangential, but in Canyon Pintado there
is typically a thin (, 0.5 cm), planar or flaser muddy interval between the
sandy beds (Fig. 13C). Although the muddy layer is thin, there are more of
these thin muddy layers distributed throughout this section compared to
the thicker mud observed at the base of the Amphitheater (Fig. 13F).
Bioturbation is minimal, and no obvious Ophiomorpha are observed. This
is the only location where we find tidal bundles which lie , 1.5 m above the
base of the first outcrop (Figs. 5, 12C). Paleocurrents measured from the
axes of troughs show a more pronounced bimodal direction than either the
Amphitheater or Taylor Draw sections, though the majority of paleocurrents are landward oriented and slightly oblique to the NE–SW
paleoshoreline (Fig. 9; Stancliffe 1984). This is probably the result of the
three-dimensional nature of this exposure. Overall, the dominant
sedimentary structure is trough and planar cross-bedding, although more
rippled intervals are observed here than in either the Amphitheater or the
Taylor Draw sections. Above the flood-tidal-delta outcrop, the section is
eroded because of faulting in the area, but other exposures in the area place
fluvial formations above the Sego Sandstone in Canyon Pintado (Willis
2000; Lawton 1986).
Interpretation
Megaripples and sand waves, which, in two dimensions form highangle cross-beds, are generally associated with lower flow regimes
r
FIG. 8.—Full section from White River North 2. Sample locations taken for
point counting are denoted. All samples are used in the grain-size study except
2.10_SF, 2.12_SF, and 2.13_T. T 5 tidal, SF 5 shoreface, and FTD 5 floodtidal-delta. SB 5 sequence boundary, FS 5 flooding surface.
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TABLE 1.— Paleocurrent and Gutter-Cast Data.
FIG. 9.—Paleocurrents taken from the trough axes of cross-beds. A) Amphitheater, N 5 22; B) Taylor Draw, N 5 9; C) Canyon Pintado, N 5 20 D)
Combined flood-tidal-delta paleocurrents, N 5 51.
(Reineck and Singh 1975); bimodal paleocurrents, coupled mud drapes,
and tidal bundles imply tidal currents (Hubbard and Barwis 1976; Nio
and Yang 1991; Visser 1980). The presence of mud drapes indicates
periods of low energy and possibly slack tide (Reineck and Singh 1975;
Visser 1980). Mud rip-up clasts imply a relatively high-energy environment where waves, wind, or increased current velocity disturb muddy
layers, which are then reworked and deposited in sandier units.
Ophiomorpha are often associated with marginal-marine environments,
but they are not diagnostic, in that they have been reported from a range
of energies, sedimentation rates, and water depths (Gingras et al. 2007;
Hauck et al. 2009). The presence of 28 countable tidal bundles makes this
a tidally deposited unit and implies a tidal signal (Fig. 11C; Nio and Yang
1991). Mud rip-up clasts and thick mud drapes in the bottom 1.5 meters
in the Amphitheater locations are interpreted to be from the initial
barrier-island breach during a storm event (Fig. 13F). We interpret the
decrease in height up section in a few isolated locations in our study area
as the result of the fact that as sediment accumulates on the flood ramp, it
reduces the water depth, producing smaller-scale bedforms up section, in
agreement with Hubbard and Barwis (1976) (Fig. 3C). Additional
indicators that these sand bodies are flood-tidal-deltas include the
reactivation surfaces and double mud drapes that occur throughout the
height of each flood-tidal-delta section. Visser (1980) details the tidal
cycle, where the erosional or reactivation surface occurs during the
subordinate current—in this case, the ebb current (Fig. 13E). The
presence of these reactivation surfaces and coupled mud drapes implies
an alternating subordinate and dominant current, which can occur only in
subtidal environments.
Canyon Pintado bedforms vary between trough and tabular crossbedded, rippled, flaser, and wavy bedded beds with a larger proportion of
rippled and wavy beds than either of the two other flood-tidal-delta
sections. We interpret this as either the result of a lower-tidal-energy
environment where the flood current is of lower velocity than either of the
two northern outcrops, or the fact that this is the top of a thicker (6 to 9 m
Journal of Sedimentary Research sedp-81-05-06.3d 7/4/11 22:25:05
Sample
Location
Measurement
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Trough axis
Gutter Cast
Gutter Cast
Gutter Cast
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
AmpFTD
TDFTD
TDFTD
TDFTD
TDFTD
TDFTD
TDFTD
TDFTD
TDFTD
TDFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
CPFTD
Amp2
NSD4
Amp1
65
80
56
20
46
24
35
8
100
65
52
57
25
201
245
255
265
260
265
205
190
236
300
310
318
286
240
275
292
185
181
110
245
275
260
120
100
135
90
275
285
340
265
274
245
285
270
275
150
255
105
110
335
125
thick) flood-tidal-delta where rippled, wavy, and flaser bedding is most
prevalent (Hubbard and Barwis 1976). We favor the latter scenario, in
which the top of the flood-tidal-delta was preserved. Evidence for this is
that the grain size is consistent with the two northern sections, implying
that the tidal current had high enough velocity to carry and deposit these
larger sand sizes behind the barrier-island. In many locations, bedding is
disrupted toward the top of the Canyon Pintado section, possibly because
of increased bioturbation, which is common in the upper sections of
flood-tidal-deltas (Hayes and Kana 1976). We also see increased
bioturbation in the more distal flood-tidal-delta deposits in Taylor Draw.
We interpret these sedimentary structures to be dominantly megaripples
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TABLE 2.— Modal Point-Count Data.
Sample
Q
F
L
Cement Pore space
Matrix
Flood-tidal delta
AmpFTD 1.1
AmpFTD 1.5
AmpFTD 2.3
AmpFTD 3.3
AmpFTD 4.3
AmpFTD 5.1
AmpFTD 5.4
TDFTD 1.1
TDFTD 1.3
TDFTD 1.4
TDFTD 1.5
TDFTD 1.7
TDFTD 1.9
TDFTD 1.12
Average
98.8
95.1
93.3
96.6
96.1
98.5
96.6
95.6
96.6
97.9
94.1
94.3
93.5
93.8
95.8
1.0
3.6
4.0
2.5
3.2
1.2
2.7
3.2
1.7
1.4
1.2
2.6
2.3
1.7
2.3
0.2
1.2
2.7
1.0
0.7
0.2
0.7
1.2
1.7
0.7
4.6
3.1
4.2
4.5
1.9
12.0
14.5
15.2
9.6
13.8
14.2
13.8
10.6
10.9
14.3
12.5
10.6
10.0
18.2
12.9
4.2
8.5
8.9
19.0
6.5
9.4
7.8
9.5
5.6
5.3
5.8
9.5
9.6
5.4
8.2
2.5
2.5
1.3
1.5
1.3
0.6
1.3
0.0
0.0
0.0
0.0
0.0
0.9
0.0
0.9
Tidal
CP 1.2
CP 1.3
CP 1.4
CP 1.5
CP1.8
CP 1.10
Amp 1.1
Amp 1.2
Amp 1.4
Amp 1.8
WRN 2.5
WRN 2.6
WRN 2.7
WRN 2.8
WRN 2.9
Average
94.6
95.4
94.3
93.5
95.8
96.6
92.9
87.4
94.1
96.9
94.5
98.3
91.8
94.5
93.4
94.3
4.9
4.6
4.2
5.8
3.7
3.4
2.1
6.2
4.2
2.9
3.8
1.2
5.5
5.3
5.6
4.2
0.5
0.0
1.5
0.7
0.5
0.0
5.0
6.4
1.6
0.2
1.7
0.5
2.7
0.2
0.9
1.5
22.7
12.2
11.6
7.4
8.0
12.6
3.4
13.3
13.4
6.4
5.8
6.0
14.5
13.4
18.4
11.3
1.9
11.5
6.3
10.2
6.3
4.3
11.7
10.5
10.4
12.4
15.8
14.2
13.5
11.9
5.6
9.8
2.9
1.6
2.5
9.2
11.7
1.5
8.1
6.1
16.8
3.6
0.0
0.0
0.0
0.0
0.2
4.3
Shoreface
CP 1.1
CP 1.6
CP 1.7
CP 1.9
WRN 2.1
WRN 2.2
WRN 2.3
WRN 2.4
WRN 2.11
Amp 1.5
Amp 1.6
Amp 1.7
Amp 1.9
Amp 1.10
Amp 1.12
Average
96.0
95.2
94.1
95.9
94.6
93.7
97.3
97.4
96.4
96.3
92.3
98.5
96.3
94.6
99.1
95.8
3.7
2.5
5.7
4.1
3.7
4.3
1.5
1.4
3.6
0.5
4.3
0.2
2.2
2.9
0.0
2.7
0.2
2.3
0.2
0.0
1.7
1.9
1.2
1.2
0.0
3.2
3.5
1.2
1.5
2.4
0.9
1.4
12.8
13.6
18.2
13.8
8.9
5.0
5.2
4.5
5.7
1.4
4.5
0.6
8.7
7.3
1.5
7.5
9.3
6.2
5.9
6.7
16.4
15.8
16.2
16.2
10.8
16.5
17.7
15.6
12.0
14.7
12.1
12.8
0.6
0.7
2.5
0.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.7
3.0
3.1
0.8
and sand waves, based on previous studies (Boothroyd and Hubbard
1975, p. 230; Visser 1980; Nio and Yang 1991).
The mean paleocurrent direction varies slightly between the three
outcrops, but overall they show a landward direction (Fig. 9). According
to Barwis and Hayes (1979), depending on inlet shape and specific
morphologic construction of the back-barrier system (i.e., the distribution
of flood and ebb channels, and the size and shape of the lagoon),
paleocurrents may show an oblique but still high-angle direction to the
shoreline. The Amphitheater (Figs. 1, 3) section shows slightly more of an
ebb-current direction. We speculate that the outcrop exposure preserves
more parts of the ebb shield, spit, or channel section of the flood-tidaldelta as detailed in the idealized flood-tidal-delta sequence in Figure 3C.
Our facies interpretations agree with modern examples (Hayes and Kana
FIG. 10.—QFL plot (note scale). A) shoreface and tidal, B) tidal and floodtidal-delta.
1976; Barwis and Hayes 1979; Hayes 1980; Israel et al. 1987; Davis et al.
2003).
SANDSTONE PETROGRAPHY
Sego Sandstone samples from flood-tidal-delta, tidal, and shoreface
sections are all quartz arenites and fall above the 90% quartz category
(Fig. 10) (Williams et al. 1982). We counted 14 thin sections in the floodtidal-delta facies, 12 in the shoreface facies, and 15 in the tidal facies and
averaged the pore space, matrix, and cement between the three facies. The
thin sections in Figure 14 are the most representative of the facies. In
Figure 14A, the tidal thin section is from the Amphitheater, the shoreface
thin section in Figure 14B is from the Amphitheater, and the flood-tidaldelta thin section in Figure 14C is from Taylor Draw.
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TABLE 3A.— Grain Size Statistical Data.
Sample Name
Shoreface
AMP-1.12_sf
AMP-1.5_sf
AMP-1.6_sf
AMP-1.7_sf
AMP-1.9_sf
CP-1.1_sf
CP-1.6_sf
CP-1.7_sf
CP-1.9_sf
DD/1.3_sf
DD/1.7_sf
DD/1.8_sf
DD/1.9_sf
WRN-2.1_sf
WRN-2.11_sf
WRN-2.2_sf
WRN-2.3_sf
WRN-2.4_sf
Tidal
DD/1.1_t
DD/1.2_t
DD/1.4_t
DD/1.5_t
DD/1.6_t
CP-1.2_t
CP-1.3_t
CP-1.4_t
CP-1.5_t
CP-1.8_t
CP-1.10_t
WRN-2.5_t
WRN-2.6_t
WRN-2.7_t
WRN-2.8_t
WRN-2.9_t
AMP-1.1_t
AMP-1.2_t
AMP-1.4_t
AMP-1.8_t
Modes
%
115
230
137
96.5
137
96.5
193.5
163
193.5
163
96.5
193.5
137
230
81
230
230
230
13.8
12.8
13.8
13.6
15.4
13.4
9.7
10.7
9.3
13.6
7.5
11.7
10.8
13.5
11.9
11.9
13.1
11.4
137
163
193.5
163
137
96.5
163
163
193.5
163
193.5
230
230
193.5
193.5
209.1
230
137
81
137
9.5
10.4
11.3
7.0
9.2
11.3
10.9
11.5
10.7
9.3
9.7
14.1
12.2
10.3
10.0
10.7
12.2
12.0
8.4
15.8
P-value
Tidal vs. SF
Tidal/SF vs. FTD
0.47
0.04
Modes
%
Flood-tidal delta
AMPFTD/1.1
AMPFTD/1.4
AMPFTD/2.3
AMPFTD/3.3
AMPFTD/4.3
AMPFTD/5.1
AMPFTD/5.4
CPFTD/1.1
CPFTD/1.5
CPFTD/1.8
CPFTD/2.1
CPFTD/3.1
CPFTD/3.4
CPFTD/4.1
TDFTD/1.1
TDFTD/1.3
TDFTD/1.4
TDFTD/1.5
TDFTD/1.7
TDFTD/1.9
TDFTD/1.12
230
58
163
230
230
230
230
163
163
163
163
163
163
163
230
230
193.5
230
230
193.5
193.5
9.6
1.0
9.4
9.9
9.3
8.8
8.8
11.8
177.0
149.0
11.7
11.2
9.6
11.2
16.0
13.3
13.2
11.4
250.0
177.0
10.9
The flood-tidal-delta facies are subrounded to rounded, moderately
sorted, and fine grained, and have a larger amount of cement or intragrain material than either the tidal or shoreface facies. Tidal facies are
subrounded to rounded, moderately sorted and lower fine to fine grained.
Shoreface facies are subrounded, well sorted, and lower fine grained, and
have the most pore space of either tidal or flood-tidal-deltas (Fig. 14;
Table 2). Note the distinct difference in between the shoreface and the
flood-tidal-deltas’ relative percent pore space and percent cement as they
TABLE 3B.— Grain Size t-test Statistical Results.
Data used
Sample Name
TABLE 4.— Sego Lithofacies Characteristics.
Lithofacies assemblage
Shoreface
Tidally Influenced
Flood-tidal delta
Relative Energy
Sedimentary Structures
Hummocky-stratified sandstone
Lithofacies and Facies
Upward-coarsening, very fine sand
Grain Size
Low to moderate
Trough to planar cross-stratified
sandstone
Wavy and lenticular sandstone
with muddy interbeds
Upward-coarsening, lower-fine to
fine sand
No trend, fine sand
Moderate
Trough and planar cross-stratified
sand
Occasionally upward-fining,
upper fine sand
Hummocky cross-stratification,
gutter casts
Trough and planar crossstratification, rippled tops
Flaser, wavy, and rippled
bedding, syneresis cracks,
double mud drapes
Trough and planar crossstratification, ripples,
reactivation surfaces, mud
drapes
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Low to High
Low to High
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seem inversely proportional to each other. Although this difference is
apparent, we recognize that this pore space may be slightly altered
because of postdepositional dissolution of labile grains from groundwater migration or leaching of surface water.
Overall, we observe approximately 20–30% less quartz fraction in QFL
diagrams with respect to the Castlegate Sandstone in Utah (Fig. 10)
(Miall and Arush 2001; Horton et al. 2004). This is consistent with a
longer sediment transport for the sediment deposited in the study area
than for the Castlegate Sandstone deposits in Utah.
GRAIN SIZE STATISTICAL DATA
Sixty samples from five locations (Fig. 1; Table 3A) and depositional
environments in the Sego Sandstone were collected and processed for
grain size analysis. Seven samples were taken across each flood-tidal-delta
section, for a total of 21 flood-tidal-delta samples, and 39 samples were
taken from full stratigraphic sections to compare tidal, shoreface, and
flood-tidal-delta facies (See Figs. 3–5, 7, 8 for sample locations). Tidal
and shoreface facies were specifically sampled in the field for grain size
analysis.
General statistics for this grain size analysis are in Table 5. In all of
these statistics, we used the percentage of the grain size range, because this
is the output of the raw data (Appendix A, see Acknowledgements
section). In Table 5, we compiled the average of each grain size range for
the traditional Udden–Wentworth grain size groupings for the three
facies: shoreface, tidal, and flood-tidal-delta (Wentworth 1922). Table 5
also compiles the standard deviation for each grain size range. The overall
highest average percentage of the grains in the flood-tidal-delta facies are
fine-grained sand at 39.54% out of 100%, the highest average for tidal
facies are fine-grained sand with 38.53% out of 100%, and for shoreface
facies is fine-grained sand with 37.17% out of 100%. Flood-tidal-delta
facies have a higher overall percentage of medium grains at 19.50%,
whereas shoreface and tidal facies have 13.47% and 14.48%, respectively.
Shoreface and tidal facies have higher average percentages of very fine
grains, at 23.72% and 19.88%, respectively, whereas flood-tidal-delta
facies have an average of 14.40% of very fine grains. Silt and mud are
mostly equal among the grain size averages. While these general statistics
are helpful, they are not always conclusive. Therefore, we also conducted
a more rigorous statistical analysis, detailed below.
In addition to a general statistical treatment of the data, this study
performs a statistical t-test using the computation and graphics program
called R (R Development Core Team 2009) to determine if there is a
significant difference between grain size populations in these three
depositional environments. The mode of the range of the grain sizes that
comprise the highest percentage of grains for each representative sample
(Fig. 15A, B, Table 3A) is used here for comparison for assessing
differences between the largest population of grains in a sample.
Figure 15C shows a sample graphical representation where we used the
mode of the range of grain sizes that corresponded with the highest
percentage (or peak) of grain size from each sample. This allows us to
have one associated number for each sample. The samples were
categorized as tidal, shoreface, and flood-tidal-delta (based on our facies
analysis, Table 4) and this dataset formed the basis for the t-test.
A t-test is a statistical test used to determine if two independent
populations tested on the same trait or attribute are statistically different
and are not just different by chance. This test assumes a null hypothesis
r
FIG. 11.—Pictures of the shoreface and tidal facies. A) Shoreface in the
Amphitheater, approximately around meters 4–7 in figure. B) Shoreface in White
River North, approximately around meters 0–14 in figure. C) Tidal in White River
North approximately around meters 15–17 in figure.
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FIG. 12.—Outcrop pictures from three flood-tidal-delta locations. Jacob’s staff is 1.6 m long with 10 cm divisions. Solid lines are traces of planar and tabular crossbedding. Dashed lines are erosional surfaces between beds. A) Amphitheater picture shows unidirectional bedding where both trough and planar cross-beds are present.
Bed height ranges from 0.6 to 0.3 m. B) Taylor Draw picture shows unidirectional trough and planar cross-bedding. Bed height ranges from 0.8 to 0.4 m. C) Canyon
Pintado picture shows arrow pointing to tidal bundles, which are approximately 0.3 m thick. Dog is at bottom of the picture for scale.
and takes into account how much the individual sample (e.g., all floodtidal-delta data) varies compared to the variation across both samples
(Fig. 15D). The test then produces a ‘‘p-value’’ which specifies how likely
we could produce the same results by mere chance. If the p-value is
significantly less than 5%, then we can reject the null hypothesis and
conclude that the two populations are statistically different.
Table 3B presents the statistical data where the t-test between the tidal
and shoreface has a p-value of 0.47 (47%), not less than 5%. Because these
populations are not statistically different, we combined them into one
new larger data set (Tidal and SF in Fig. 15) and tested this new
combined dataset against the flood-tidal-delta data. The p-value from this
test is 0.04 (4%), less than 5%, so we conclude that flood-tidal-delta
samples are statistically different from the other two populations.
The box plot in Figure 15B shows the combined shoreface and tidal
grain size data mean against the flood-tidal-delta data mean without the
outlier (Fig. 15A). It is apparent that the flood-tidal-delta facies are
Journal of Sedimentary Research sedp-81-05-06.3d 7/4/11 22:25:23
statistically coarser than either the shoreface or tidal facies. We assume
that by the time these grains were deposited by long-shore currents, they
had already been substantially sorted and weathered—the local beach and
shoreface environment providing smaller amounts of additional physical
alteration. We thus interpret the grain size difference to be due to the
effect of tidal amplification through a small-width tidal inlet, increasing
the local shear stress relative to the shoreface environment and creating
higher-velocity tidal currents resulting in winnowing. This process has
been described by Longhitano et al. (2010) as responsible for the
preferential suspension and deposition of larger particles in the floodtidal-delta because of the locally amplified tidal current. We acknowledge
that a variety of grain sizes exist in the shoreface, but only the largest
grains from that fraction are preferentially carried and deposited in the
flood-tidal-delta by the higher-velocity tidal current. The smaller grain
fraction either is carried farther down shore or is deposited in the lower
shoreface.
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FIG. 13.—Pictures of the flood-tidal-delta facies. A) Double mud drapes in the Amphitheater. B) Mud drapes on top of ripples in Canyon Pintado. C) Double mud
drapes and mud drapes on ripples in Canyon Pintado. D) Coal at the base of the Amphitheater and tabular cross-beds. E) Reactivation surfaces in Canyon Pintado. F)
Thicker mud drapes on the tops of foresets at the base of the Amphitheater.
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FIG. 14.—Histogram of percent pore space, cement, and matrix. Note that these do not add up to 100%; grains make up remaining percentage. Thin-section pictures in
plane light at 103 magnification. Q, Quartz; Cmt, Cement. A) Amphitheater tidal, B) Amphitheater shoreface, C) Taylor Draw flood-tidal-delta.
DISCUSSION
The facies descriptions provided in this paper (Table 4; Figs. 3–5, 12,
13) provide new details on ancient flood-tidal-delta deposits in the Sego
Sandstone and present a model to recognize ancient flood-tidal-delta
deposits applicable to similar facies in other tectonic settings. The greatest
evidence for these sand deposits to be interpreted as flood-tidal-deltas is
their stratigraphic position in the Sego Sandstone, paleocurrent
orientation, and presence of tidal bundles and coupled mud drapes.
Locally, this portion of the Sego Sandstone lay on the western edge of
the Cretaceous Interior Seaway on a coastline where waves and tides had
complex interactions. Though we interpret this to be a microtidal coast (0
to 2 m tidal range; Davies 1964) there was still considerable tidal influence
seen in the environments behind the barrier-island system. Davis and
Hayes (1984) note that although coasts are generally narrowly classified
as tide or wave dominated, there are often exceptions to the rule, and we
interpret this part of the coast to be wave dominated with tidal-current
influence in the back-bay environment.
Note that this observation from Davis and Hayes (1984) is made from
modern flood-tidal-deltas, where it may be easier to identify single floodtidal-deltas and know their exact location and orientation with respect to
the barrier-island, tidal-inlet, and other sand bodies in the back-barrier
Journal of Sedimentary Research sedp-81-05-06.3d 7/4/11 22:26:46
system. Many of these modern studies have documented the birth and
growth of the flood-tidal-deltas and as a result have constrained the exact
age. In the Sego Sandstone, however, where the bedforms do not always
display a systematic decrease of bed height up section, we may be
observing the result of the amalgamation of multiple flood-tidal-deltas
over a longer time span. Or, this may be the result of a transgressive vs.
regressive coastline, where their preservation potential and facies stacking
pattern may be the exact reverse of modern examples. The modern
examples we use in this study for comparison are largely formed during
marine transgressions, where preservation potential is low. Ultimately,
the sedimentary records for these modern transgressive systems may look
quite different. Specifically, Hubbard and Barwis (1976) created
hypothetical transgressive and regressive back-barrier inlet sequences
based on their South Carolina inlet study. They proposed that in a
regression, the stratigraphic succession should start with the channel
bottom and progress up to the tidal flat and marsh, where the flood-tidaldelta is preserved directly below the marsh. In a transgression, the
stratigraphic succession would be the exact opposite, where the lagoon or
marsh is at the bottom of the sequence and the stratigraphic succession
ends with offshore marine at the top of the sequence (figs. 6 and 7 in
Hubbard and Barwis 1976). In both of these cases, these facies are
laterally related and may not occur in a simple vertical succession. Also,
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TABLE 5.— Standard deviations and averages for grain sizes.
A
Standard deviation for each facies type
Facies Name
Facies Name
Facies Name
Shoreface
Standard Deviation
Tidal
Standard Deviation
FTD
Standard Deviation
VC
C
M
F
VF
S
M
0.08
0.37
3.76
3.10
4.42
0.94
0.27
VC
C
M
F
VF
S
M
0.00
0.00
3.31
2.77
3.23
0.86
0.29
VC
C
M
F
VF
S
M
0.01
0.34
3.60
2.70
2.78
0.75
0.29
B
Average for each grain size
Grain Size
FTD
Shoreface
Tidal
VC 2000
C 1000
M 500
F 250
VF 125
S 63
M 3.9
0.00
0.10
4.87
9.91
3.60
1.26
0.39
0.02
0.13
3.33
8.68
5.86
1.20
0.38
0.00
0.08
3.90
9.54
4.35
1.25
0.43
any of these facies may be eroded and we may find only the very bottom
or ebb portion of the flood-tidal-delta deposit, or more likely, no evidence
of flood-tidal-deltas at all. As the sea transgresses onshore, these deposits
will likely be submerged under water and summarily eroded. A number of
other factors could be responsible for this observation, including shifting
of the tidal-inlet location because of modification from wave or storm
action (Fig. 6). As well, shifting of the tidal-inlet location or orientation
may have shifted the orientation of paleocurrents, and this is why we see
sometimes shore-normal to shore-parallel paleocurrents (Fig. 9).
Furthermore, we find that flood-tidal-deltas have a statistically larger
average grain size because they are preferentially sourcing most of their
material from the coarser grain size fraction from the shoreface or
barrier-island sand, indicating that flood-tidal-delta deposits have higher
overall energy than shoreface and tidal environments. While the energy is
high enough in both the flood-tidal-delta and the shoreface environment
to carry these larger grains, the tidal currents that deposit the flood-tidaldelta are such that they may carry a variety of grain sizes into the back
bay and during the flood tide only the coarser grains are deposited while a
majority of the very fine to fine grains cannot settle so are again carried
seaward with the ebb tide. During slack tide, the mud is able to settle, and
in some instances these ebb and flood tide events can form tidal bundles,
which we see in Canyon Pintado (Fig. 12; Visser 1980). This conclusion
has important implications for reservoir characteristics and could be
helpful in recognizing flood-tidal-deltas in core or well logs, as well as the
potential for a larger proportion of pore space due to decreased grain
packing. The higher degree of cement in our flood-tidal-delta deposits
might be explainable by postdepositional dissolution of cement and intergrain material by groundwater movement in the shoreface and tidal
facies. We propose that the amount of cement and inter-grain material in
the tidal and shoreface facies was similar to that in the flood-tidal-delta
facies, but diagenesis removed those materials resulting in an secondary
porosity that was not original to deposition.
CONCLUSION
The flood-tidal-deltas in the Sego Sandstone discussed in this paper
present examples for future ancient and modern studies of flood-tidaldeltas. Flood-tidal-deltas are some of the most preservable sand bodies in
the back-barrier system and are potentially important hydrocarbon
reservoirs. Newly researched grain size data and detailed sedimentological
analysis concludes that flood-tidal-deltas have a statistically larger grain
size than either shoreface or tidal facies. This conclusion has larger
implications for the significance of tidal energy in a traditional ‘‘wavedominated’’ or microtidal coast where tidal energy is thought to be at a
minimum.
Overall, we interpret the flood tidal delta facies in the Sego Sandstone
to be the result of a larger barrier-island system, creating back-barrier
environments, where these flood-tidal-deltas were preferentially preserved. This preferential preservation was likely due to the progradation
of the shoreline as the Cretaceous interior seaway regressed.
This barrier-island system is a good analog for recognizing future
ancient marginal-marine deposits that had a complex interaction between
wave and tide influence. We see many examples such as the Florida,
South Carolina, and Georgia coasts in the modern to compare to the
Sego Sandstone (Hubbard et al. 1979; Hodge 1982; Davis 1987; Davis et
al. 2003), and now we may use the Sego Sandstone as comparison for
future studies of the ancient in both regressive and transgressive
scenarios.
r
FIG. 15.—Grain-size statistical data. FTD, flood-tidal-delta; SF, shoreface. A) Box plots of range of grain size of modes including outlier in flood-tidal-delta (FTD,
dot at bottom). Thick black line represents mean of modes. Upper and lower limits of grain sizes represented by dashed lines or box boundaries (for FTD). B) Box plot
combining tidal and shoreface data and comparing to FTD without outlier. C) Graph of grain size distribution for three samples; each line is a sample. D) Histograms
showing variance, or range of different grain-size modes, in samples.
Journal of Sedimentary Research sedp-81-05-06.3d 7/4/11 22:27:08
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ACKNOWLEDGMENTS
This research was funded by a generous grant from Shell facilitated by J.
Michael Boyles. We are also grateful for the helpful discussions in the field
with J. Michael Boyles. Additionally, we give sincere thanks to the reviewers,
AE Greg Ludvigson, Anthony Walton, and an anonymous reviewer, who put
a great effort into the comments for this paper. These three reviews
dramatically improved the organization and content of the paper and we are
truly thankful for their edits. Appendix A is available from JSR’s Data
Archive, URL: http://sepm.org/pages.aspx?pageid5229.
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Received 29 April 2010; accepted 16 February 2011.
19
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