AN ARCHITECTURAL ANALYSIS AND DEPOSITIONAL INTERPRETATION OF THE
DOCKUM GROUP IN THE WEST TEXAS HIGH PLAINS
by
Grayson Hayworth Lamb
Bachelor of Science, 2017 The University of Arkansas Fayetteville, Arkansas
Submitted to the Graduate Faculty of the College of Science and Engineering Texas Christian University in partial fulfillment of the requirements for the degree of
Master of Science
May 2019
ACKNOWLEDGMENTS
This work would not have been possible without the guidance and support provided by
Dr. John Holbrook. He inspired me to “shake the bag” and see what falls out. I would like to thank my roommates and fellow geologists, Rodney Stieffel and Ben Ryan for their continuous encouragement, and inquisitive spirits. Thank you to Dr. Richard Denne for helping me keep my main goals in sight and pushing me towards them. Thank you to Dr. Walter Manger who has always been one of my biggest fans and my personal geologic hero. Lastly, a special thank you to my ever loving and caring parents, Mike and Desiree Lamb for being my rock in the ups and downs before, during and after this thesis process.
ii TABLE OF CONTENTS
ACKNOWLEDGMENTS………………………………………….……………………………. ii
TABLE OF CONTENTS………………………….…………………………...... iii
LIST OF FIGURES………………………………………………………………………...... v
LIST OF TABLES………………………………………………………………...... vii
CHAPTER 1 INTRODUCTION…………………………………………………………...... 1
1.1 Dockum Group Exposure…………………………………………………………...... 2
1.2 Previous Works………………………………………………………………………..5
1.3 Tectonic Depositional Setting………………………...... ……………………………..6
1.4 Triassic Paleoclimate…….....……………………………………………………...... 8
1.5 Dockum Group Stratigraphy………………………………………………………....12
1.6 Source Parameters………………………………………………………………...... 16
CHAPTER 2: METHODS…………………………………………………………………...... 17
2.1 Field Work……………………………………………………………………...... 17
2.2 Photogrammetry…………………………………………………………………...... 18
2.3 Architectural Analysis……………………………………………………………….20
CHAPTER 3: RESULTS………………………………………………………………………...21
3.1 Measured Sections………………………………………………………………...... 21
3.2 Lithofacies…………………………………………………………………………....24
3.3 Lithofacies Assemblages………………………………………………………...... 33
3.3.1 Upper Flow Regime Channel Assemblage………………………………………...33
3.3.2 Upper Flow Regime Sheets Assemblage....……………………………………...... 34
iii 3.3.3 Perennial Channel Assemblage………………………………………………...... 34
3.3.4 Floodplain Assemblage………………………………………………………...... 36
3.3.5 Paleosol Assemblage…………………………………………………………...... 36
3.3.6 Lacustrine Assemblage………………………………………………………….....37
3.4 Hierarchy of Surfaces………………………………………………………………..39
CHAPTER 4: DISCUSSION……………………………………………………………...... 45
4.1 River channel and flow processes……………………………………………...... 45
4.1.1 Upper Flow Regime Channels………………………………………………...... 45
4.1.2 Upper Flow Regime Sheets …………………………………………………….....52
4.1.3 Perennial Channel Belts and Bars …………………………………………...... 54
4.2 Depositional Context and paleogeography of the Dockum Group………………...... 57
4.3 Megamonsoon Climate Hypothesis……………………………………………….....66
CHAPTER 5: CONCLUSIONS…………………………………………………………………72
REFERENCES………………………………………………………………………………...... 73
VITA
ABSTRACT
iv LIST OF FIGURES
Figure 1: Map of Dockum Group subsurface extent and surface exposure……………………….4
Figure 2: Paleozoic structural highs and lows……………………………………………….…....7
Figure 3: Structural, depositional styles of Dockum Group Sediments and climate patterns...... 11
Figure 4: Triassic stratigraphic column for Dockum Basin..………………………………...... 13
Figure 5: Map of measured section locations……………………………………………………18
Figure 6: Sparse cloud, point cloud, and orthomosaic…………………………………………...19
Figure 7: Measured section along Palo Duro Canyon road cut………………………………….22
Figure 8: Measured sections throughout the study area……………………………………….....23
Figure 9: Images of lithofacies………………………………………………………………...... 30
Figure 10: Image of five paleosol types and separating horizons………………………………..37
Figure 11: Detailed architectural analysis of the three channel types…………………………...41
Figure 12: Lithofacies Assemblages along Highway 207……………….………………………43
Figure 13: Lithofacies along Highway 207……………………………………………………...44
Figure 14: Antidunes in upper flow regime channels…………………………………………....48
Figure 15: Chute and Pool structure in upper flow regime channels…………………………….49
Figure 16: Cyclic steps in upper flow regime channels….………………………………………50
Figure 17: Perennial dual flow system...... 56
Figure 18: Santa Rosa Sandston deposition diagram..………….………………………………..58
Figure 19: Upper flow regime sheets within lacustrine assemblage…...………………………...59
Figure 20: Depositional cartoon of the Tecovas Formation……………………………..………60
Figure 21: Depositional cartoon of the Trujillo Formation……………………………...………62
Figure 22: Cartoon of Cooper Canyon Formation deposition……………………………...……63
v Figure 23: Glacio-fluvial delta depositional process………………………………………….....65
Figure 24: Monsoon circulation diagram…...……………………………………………………67
Figure 25: Megamonsoon circulation pattern……………………………………………………69
vi LIST OF TABLES
Table 1: Table of lithofacies and descriptions...... 29
vii
CHAPTER 1: INTRODUCTION
The Dockum Group of the western Texas High Plains is a relatively understudied basin that has not received an in-depth systematic sedimentological investigation. A comprehensive study of the northern portion of the Dockum Group exposure in the west
Texas Panhandle is undertaken in order to better understand its fluvial architecture and depositional environments through time.
While the Dockum Group is not a productive petroleum target, its deposition into the
Midland Basin drove much of the strata into the oil window (Brown, 2016). Additionally, the basal Dockum has become a target of water resources used in secondary and tertiary petroleum recovery methods. Understanding the depositional styles of the Dockum Group is quintessential to a complete burial history interpretation of the Midland Basin. Furthermore, in terms of producing from the Dockum aquifer, the depositional trends directly relate to porosity, permeability, and water confinement. Accurate interpretations will aid in determining effective reservoir connectivity and drainage methods.
Previous studies addressed the Dockum Group from paleontological and stratigraphic perspectives. Paleontology of the Dockum Group and time equivalent Chinle Formation are studied extensively because they are a rich source of Upper Triassic fossil vertebrates (Green,
1954; Chatterjee 1983, 1984, 1985, 1986a, 1991, 1993; Small, 1985, 1989a, b, 1997, 2002;
Davidow-Henry, 1987, 1989; Long and Murry, 1995; Simpson, 1998; McQuilkin, 1998;
Edler, 1999; Bolt and Chatterjee, 2000; Atanassov 2002; Martz 2002, 2008; Weinbaum,
2002, 2007; Hungerbuhler et al. 2003; Houle and Mueller, 2004; Lehman and Chatterjee,
2005; Lehane, 2006; Mueller and Parker, 2006). However, little has been done recently in terms of the Dockum Group depositional system, and what has been done lacks the details
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that this study provides. Three depositional models currently attempt to explain the depositional styles of the Dockum Group. McGowen et al. (1983) suggest a large Dockum
Lake dominated the depocenter within a complex fluvial, deltaic and lacustrine system.
Lehman and Chatterjee (2005) suggest only small ephemeral floodplain lakes existed while prominent deposition was in braided and meandering fluvial systems. Brown (2016) presented a model that combined both theories, suggesting lacustrine systems were understated by Lehman and Chatterjee (2005) and overstated by McGowen et al. (1983).
Collectively, these works have interpreted the fluvial systems only as lower flow regime and provided few details as to the climatic mechanisms that would drive the fluvial and lacustrine systems.
This study offers a new interpretation for the fluvial lithofacies and a more encompassing paleoclimatic model for the deposition of the Dockum Group fluvial and lacustrine systems. This study does this using detailed outcrop analyses to identify and interpret the individual lithofacies that make up lithofacies assemblages. Depositional patterns unique to lithofacies assemblages in the Dockum Group were used to predict depositional extent of the identified assemblages, and provide paleoclimate interpretations related to the lithofacies assemblages.
1.1 Dockum Group Exposure
In both subsurface and outcrop the Dockum Group approximately extends 246,050 km2 (96,000 mi2) into Texas, New Mexico, Colorado, Kansas, and Oklahoma. Its thickness ranges from tens of meters to more than 610 meters (2,000 feet) (McGowen et al., 1983). It crops out nearly continuously around the Caprock Escarpment of the southern High Plains in
2
Texas, along the Canadian River in Texas and New Mexico, and along the Pecos River valley in New Mexico and Texas (Lehman and Chatterjee, 2005). This study focuses on outcrops along the eastern Caprock Escarpment, specifically in Palo Duro Canyon State Park, north and south-facing exposures along the highway TX 207 road cut, and the road cut in
Tule Canyon (Figure 1).
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Figure 1: Regional extent of the Dockum Group across west Texas and eastern New Mexico. The counties outlined in pink are the study area. Generalized locations where the Santa Rosa, Tecovas, Trujillo and Cooper Canyon formations crop out. Blue: Santa Rosa, Red: Tecovas, Yellow: Trujillo, Purple: Cooper Canyon, Green: Dockum Group undifferentiated. A-A’ is a generalized cross section of the Dockum Group from New Mexico to the study area. (Modified from Lehman and Chatterjee, 2005) 4
1.2 Previous Work
The Triassic Dockum Group was first identified by Cummins (1890) and named after
Dockum Creek in Dickens County, TX. Later, Drake (1891) divided the Dockum Group into three separate units. Two of the units from Drake (1891) were determined to be time equivalent to the Santa Rosa Sandstone and Chinle Formation of New Mexico by Adams
(1929). Gould (1907) elevated the Dockum interval to group status and identified the Trujillo as the upper formation and the Tecovas as the lower formation, he did not define the Cooper
Canyon. Lehman and Chatterjee (1995, 1997, 2005) have used Gould’s nomenclature repeatedly making it the stratigraphic standard. Green (1954) identified the Dockum Group as a collection of non-marine deposits of aeolian, fluvial, and lacustrine origin. The Llano
Uplift region of central Texas was determined to be the likely sediment source area for the
Dockum Group by Kiatta (1960) and Cazeau (1960). Additionally, Cramer (1973) identified the Ouachita Uplift as a potential source area. He further implied a fluvial paleoenvironment for the Dockum Group. A thesis produced by Hood (1977) provided a stratigraphic synopsis of the Dockum Group sediments, underlying Permian Quartermaster Formation, as well as the overlying Tertiary and Quaternary sediments within the Fortress Cliff Quadrangle,
Randall County, TX with much of the work being conducted in Palo Duro Canyon.
Graduate work and publications from the Bureau of Economic Geology (Seni, 1977;
Granata, 1981; McGowen et al. 1979; 1983) presented a depositional framework for the
Dockum Group. Together they proposed a working theory that the dominant depositional environment was a large ephemeral lacustrine system with periods of lacustrine delta deposition. Their work sparked an alternative hypothesis by Frelier (1987) and May (1988), which stated that the Dockum Group was entirely composed of fluvial deposits, and proposed
5
that the delta and lacustrine deposits McGowen et al. (1983) had identified were fluvial floodplains. Lehman and Chatterjee (2005) further developed Frelier (1987) and May’s
(1988) lithofacies identification. The Dockum Group was recently revisited by Brown
(2016), who agreed with both McGowen et al. (1983) and Lehman and Chatterjee (2005).
Brown identified both lacustrine and deltaic deposits and floodplain facies across the
Dockum Group outcrops.
1.3 Tectonic and Depositional Setting
The Dockum Group was deposited during the Triassic by fluvial systems that drained highlands of the supercontinent Pangea (Figure 2). During the Late Paleozoic, the collision of
North America with South America and Africa resulted in a long suture with associated mountain chains, most proximal being the Ancestral Rocky Mountains and the Ouchita-
Marathon orogenic belt. The Matador Arch of the Texas Panhandle and the Amarillo-Wichita
Uplift of the Texas Panhandle and southern Oklahoma were minor uplifts associated with these major orogenic event (Figure 2). The Midland and Palo Duro Basins also developed as part of this orogeny and persisted into Dockum Group deposition (Kluth and Coney 1981;
Walper, 1977; Dickinson, 1981; Viele and Thomas, 1989). These basins are separated by the
Matador Arch which remained a positive feature throughout the Triassic (May, 1988).
Accordingly, the Dockum Group is thicker to the north and south of the Matador Arch but thins over the arch. Pangea began to rift in Late Triassic time with the opening of the Gulf of
Mexico (Van der Voo et al., 1976; Salvador, 1987, 1991). The opening of the rift provided accommodation space to the Dockum Group sediments by reactivating subsidence of the
Midland and Palo Duro basins and causing thermal uplift of the Ouachita Marathon Belt.
6
(Figure 2). Thus, Dockum Group sediments shedding from the Ouachita-Marathon Belt filled the relict Midland and Palo Duro basins, collectively called the Dockum Basin, during the
Late Triassic (McGowen et al., 1983).
Figure 2: Paleozoic structure highs (red) and lows (blue) that influenced Dockum Group sedimentation. Palo Duro Basin is outlined in green. (Modified from McGowen et al., 1983)
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1.4 Triassic Paleoclimate
Most of the present paleoclimatic understanding of western North America during
Late Triassic time stems from studies of the Chinle Formation (Dubiel et al., 1989; Ash and
Creber, 1992; Therrien and Fastovsky, 2000; Tanner, 2003).The Chinle Formation is the western equivalent of the Dockum Group. The Chinle Formation was deposited during the
Upper Triassic as a non-marine clastic and carbonate system that reached across south- central Utah, southwestern Arizona and eastern New Mexico (Dickinson, 1981; Dickenson et al., 1983; Blakely and Gubitosa, 1983, Busby-Spera, 1988; Saleeby and Busby-Spera, 1992;
Lawton, 1994). The Chinle Formation was deposited in a complex system of fluvial, deltaic, and lacustrine environments. Sediments originated from a magmatic arc that extended across the modern Colorado Plateau and also from uplifts associated with the Ancestral Rocky
Mountains (Stewart et al., 1986, Bilodeau, 1986; Dubiel, 1994). At its initial depositional phase, much of the Chinle depositional area was inundated. Large marsh lands and mudflats filled a lacustrine basin with sediment provisionally provided by lateral fluvial systems.
Vertisols and gleysol paleosols of the associated floodplain deposits indicate that the floodplain at times was poorly drained. On the other hand, evidence of water table fluctuations from these strata point to dry spells. Van der Voo et al. (1976) reconstructed the paleolatitude of the Late Triassic Chinle using paleomagnetism and determined that the latitude and continental configuration is most consistent with a tropical monsoonal climate.
The variety of fluvial, deltaic, and lacustrine environments supported a diverse invertebrate and vertebrate fauna, which is also consistent with a tropical monsoonal climate. Monsoons would have provided the necessary moisture to form the streams, marshes, and lakes that are generally interrupted to record internment dry and wet seasons (Dubiel, 1989).
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Similarities in paleontological and sedimentological evidence argue that the Dockum
Group paleoclimate was similar to the semiarid to subhumid climate of the Chinle Formation
(McGowen et al., 1983, Frehlier, 1987; May, 1988; Lehman and Chatterjee, 2005). The
Dockum Group deposits show evidence for both lacustrine and fluvial deposits with associated floodplain facies (Brown, 2016). The total extent of lacustrine facies is uncertain
(McGowen et al., 1983). Much of the mud dominated deposits are identified as poorly drained floodplain facies (Lehman and Chatterjee, 2005) similar to the Chinle floodplain deposits (Dubiel, 1989). Thus, the Dockum is similarly argued to record seasonal climate with several shifts from arid to humid conditions occurring throughout deposition of the
Dockum Group (Seni, 1977; May, 1988; Martz, 2008). A climate shift from arid to semi humid caused the fluvial systems to experience highly seasonal flows. During seasonally wet times the lake volumes increased as did the floodplain facies thicknesses, while dry seasons saw the reduction of lakes and thinning of floodplain mud. The fluctuations between the dry- wet seasons in Dockum strata could be explained by the same tropical monsoons that drove
Chinle deposition. The size and longitudinal orientation of the supercontinent of Pangea is predicted to have established “megamonsoonal” circulation (Parrish, 1988). These megamonsoons drew moisture from the eastern Triassic Tethys Sea (Parrish, 1988). The uplands would have received the maximum levels of precipitation and in turn were densely vegetated. But further to the north, away from the uplands in the south, the rainfall would have decreased and vegetation would have been restricted to the fluvial pathways (Demko et al., 1998).
Early rifting of the Gulf of Mexico resulted in thermal domal uplift of the Ouachitas and thus increased the precipitation levels over the uplands (McGowen et al., 1983; Figure
9
3). At this time Pangea was almost symmetrically aligned at the equator, so maximum seasonality of wet-dry is expected. Climates across Pangea may also have varied on time scales that exceeded seasons, meaning lengthy periods of dry spells would follow lengthy periods of humidity (Parrish, 1988).
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3A Easterlie Prevailing Wind Climate Pattern
Megamonsoon Climate Pattern 3B
Figure 3A: A southeast to northwest representation comparing the structural and depositional styles of Triassic sediments. Dockum basin subsidence is reactivated following domal uplift of early Gulf of Mexico rifting. Shallow depocenter then receives sediment sourced from the southeast highlands. Climate patterns were associated with the prevailing winds and non megamonsoon circulation (Modified from McGowen et al., 1983) Figure 3B: A west to east schematic depicting megamonsoon rains moving eastward. Majority of precipitation falls over the Chinle Formation with minimal precipitation reaching the Dockum Group further inland. (Modified from McGowen et al., 1983).
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Additionally, due to the large size and the alignment of Pangea along the equator, maximum monsoonal circulation may have been strong enough to reverse normal eastern equatorial flow and draw moisture from the west (Parrish, 1988). Highlands along the western coast of Pangea, the Ancestral Rockies to the north and Ouachita orogenic belt to the south and east, may have confined the monsoon low pressure trough allowing for extended periods of rainfall (Parrish, pers. comm.). These counter regional circulations patterns were probably irregular with time scales exceeding typical seasonal fluctuation (Parrish, pers. comm.).
Outcrop work of this study agrees with the previous workers in regard to a prevailing tropical monsoon climate. However, its intensity and development throughout Dockum deposition is understated. As a result, this study only partially agrees with the lithofacies interpretations made by previous workers, as the flow regime of the discussed fluvial complexes is also understated.
1.5 Dockum Group Stratigraphy
The Dockum Group is confined between Permian rocks below and Cretaceous and
Tertiary rocks above. The Permian “Red Beds” of the Quartermaster Formation underlie the
Dockum Group across the Dockum Basin. To the north, the contact is unconformable, but to the south, it is gradational (Drake, 1891; Gould 1906, 1907; Seni, 1977; Frelier, 1987; May
1988). The Quartermaster Formation consists of red to brown siltstone and mudstone with local interbedded gypsum layers (May, 1988).Cretaceous strata and the Tertiary Ogallala unconformably overlie the Dockum Group.
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Figure 4: Triassic stratigraphic column for the Dockum Basin. Yellow represents sandy channel complexes within each formation. Modified from Lehman and Chatterjee (2005)
The Dockum Group has undergone extensive nomenclatorial reviews and alterations, but for this study, nomenclature of the Dockum Group will follow that of Lehman (1994a,
1994b) (Figure 4). The Dockum Group is divided into the Santa Rosa Sandstone, Tecovas,
Trujillo, and Cooper Canyon formations (Figure 4). These units are mapped in outcrop throughout the extent of the Dockum Basin (Figures 1 and 2). This study focuses on the outcrops along the eastern escarpment of the High Plains in the Texas Panhandle, north of the
Matador Arch. The formations discussed in this study are the Tecovas, Trujillo, and, specific to the south Texas Highway 207 location, the Cooper Canyon Formation. These formations
13
were the focus because of their excellent exposure. In general, these formations remain thick in the north but thin southward as they cross the Matador Arch with the exception of the
Santa Rosa Sandstone. Lehman and Chatterjee (2005) identified the Santa Rosa in Palo Duro
Canyon, southwest Randall County, as unconformably overlying the Permian Quartermaster
Formation. However, this study did not identify the Santa Rosa Sandstone in Palo Duro
Canyon.
Darton (1922, 1928) described the base of the Dockum Group as a distinctive thick sandstone in the Pecos River valley of New Mexico, which he named the Santa Rosa
Sandstone. Extending into Texas, the Santa Rosa Sandstone is described as a sand dominated interval, and is interpreted as a basal, fluvial channel sandstone that intertongues with the
Tecovas Formation (Fritz, 1991). It reflects multiple episodes of channel incision, lateral migration and aggradation in the pattern of amalgamated channels. The Santa Rosa and overlying Tecovas are both completely truncated by the Trujillo Formation in Briscoe
County (Lehman and Chatterjee, 2005). Lehman and Chatterjee (2005) described the
Dockum Group as two alluvial depositional sequences defined respectively by the Santa
Rosa-Tecovas and Trujillo-Cooper Canyon intervals. Both sequences represent fining upward sections with a thick basal fluvial sandstone. Thus, they interpreted the Santa Rosa
Sandstone and Tecovas Formation as a singular alluvial depositional sequence in the
Dockum exposures of Texas. Santa Rosa is the sand dominated portion of the sequence and fines upward into the mud dominated Tecovas Formation.
The Tecovas Formation was originally named after Tecovas Creek located in Potter
County, Texas (Gould, 1907). Sandstone at the base of the Tecovas Formation is described as either fluvial (May, 1988) or lacustrine delta deposits (Seni, 1977). This basal sand is what
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Lehman and Chatterjee (2005) describe as the Santa Rosa Sandstone. Within the Tecovas
Formation are prominent greenish to reddish paleosols and mudstones that serve as unique criteria distinguishing it from the Santa Rosa Sandstone. Overall, the Tecovas is composed of reddish siltstone and mudstone beds and fines upward. The Tecovas sequence thins southward toward the Matador Arch and is truncated by the overlying Trujillo Formation in
Briscoe County (Frelier, 1987; Lehman and Chatterjee, 2005). South of the Matador Arch the
Tecovas Formation remains thin and difficult to identify. Following Lehman’s depositional sequence interpretation, the Tecovas is the mud dominated upper portion of the Santa Rosa-
Tecovas sequence. The Tecovas Formation is interpreted to be flood plain deposits with localized lacustrine deposits (May, 1988). Alternatively, the Tecovas Formation is interpreted as strictly lacustrine deposits (McGowen et al., 1983; Brown, 2016).
Overlying the Tecovas Formation is the highly channelized, multi-story, sand- dominated Trujillo Formation, named after Trujillo Creek in Oldham County, Texas (Gould,
1907). The Trujillo Formation is similar to the amalgamated channel belts of the Santa Rosa
Sandstone. The incisional nature of the Trujillo Formation is evident in the Tule Canyon area as it completely truncates the underlying Tecovas Formation (Lehman and Chatterjee, 2005).
The Trujillo Formation extends across the Dockum Basin, ultimately thinning onto the
Matador Arch. Trujillo sands make up the base of the Trujillo-Cooper Canyon depositional sequence and fine upward into the overlying mud-dominated Cooper Canyon Formation
(Lehman and Chatterjee, 2005).
The Cooper Canyon Formation is not described in detail for this study, but is regarded as the mudstone facies of the Trujillo-Cooper Canyon depositional sequence
(Lehman and Chatterjee, 2005). While predominately muddy, the Cooper Canyon Formation
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comprises single-story fluvial sandstones. Much debate exists whether the fine-grained strata of the Cooper Canyon Formation represent floodplain facies (Frehlier, 1987; May, 1988;
Lehman, 1994; Lehman and Chatterjee, 2005) or lacustrine facies (Seni, 1977; McGowen et al 1983, Brown, 2016). In the one locality of the Cooper Canyon studied here, this formation represents a complex interaction of channel belts and bars, floodplain, and lacustrine systems.
1.6 Source Parameters
Whereas the depositional environments of the Dockum Group in the High Plains is currently debated, the source provenance is relatively accepted. McGowen et al (1983) postulate that the Dockum Group records multiple sediment sources, from highlands to the south, east, and north. Fluvial systems in turn drained to the west, south, and northwest, and filled old Paleozoic basins associated with the Ouachita--Marathon orogeny. The rifting of
Pangea reactivated the subsidence of these Paleozoic basins and caused thermal uplift of the
Ouachita-Marathon Belt. The initial phases of Dockum Group sedimentation began with this phase of uplift and subsidence (McGowen et al., 1983). Large fluvial systems flowed north into the Dockum Basin, with minor additional sediment input from the Amarillo-Wichita uplifts to the north (Figure 2).
Lehman and Chatterjee (2005) argued that Santa Rosa-Tecovas sediments were derived from the north, northeast, and east based on petrographic and paleocurrent data. The source area changed, however, to the south and southeast during Trujillo-Cooper Canyon deposition. They suggested this change was a response to tectonism, climatic changes, or base level changes. .
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CHAPTER 2: METHODS
2.1 Field Work
Seven locations exposing extensive outcrops of the Dockum Group were selected for study across a 1,300 square mile area crossing Randall, Armstrong, and Briscoe counties
(Fig. 5). Four locations are within Palo Duro Canyon State Park in Randall County. They are designated the “Road Cut,” “Amphitheater,” “MS 3,” and “Graveyard.” Two locations are along Texas Highway 207 in Armstrong County; these are referred to as “North Highway
207” and “South Highway 207.” The last location is in Tule Canyon of Briscoe County, referred to simply as “Tule Canyon.” Measured sections were conducted at each of these locations. The Palo Duro Road Cut had six measured sections, while the Amphitheater, MS
3, and Graveyard outcrops had one each. North Highway 207 had one, whereas South
Highway 207 had three. One measured section was conducted in Tule Canyon. These measured sections were used to identify and describe lithofacies. The lithofacies descriptions are based on lithology, grain size, and bedforms. These locations were photographed using a drone and a Cannon EOS Rebel T3i handheld camera with either an 18-55 mm or 55-250 mm lens. Photos were merged into orthophoto pans (see below). Lithofacies defined in measured sections were then mapped across photopans.
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Figure 5: Left- zoomed in look at the four measured section locations in Palo Duro State Park. Right- three other measured section locations. Total of 7 measured sections across the study area. (Images from OnX Maps)
2.2 Photogrammetry
The two Texas Highway 207 locations were photographed using a drone to collect roughly 700 photos. The drone photographs were stitched together using Agrisoft Photoscan
3D PRO version 1.4.3. Using the focal length of the images the software determines the distance of the outcrop from the camera. Agrisoft then identifies similar pixels across the photos to align them. With the alignment complete the pixels are added from the photos to create a dense point cloud. Surfaces are generated between three adjacent pixels to establish a textured surface. Lastly, the images are overlain onto the established surfaces and smoothed to create the final product, an orthophoto. All orthophotos were exported into Adobe
Illustrator for further analysis. Due to Texas state law, drone flights are prohibited in state parks, thus the Palo Duro Canyon State Park outcrops were photographed with the handheld
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camera. Tule Canyon outcrops were also photographed with the hand-held camera. Smaller photosets were taken with hand-held camera were pieced together using Adobe Illustrator.
A
B
C
Figure 6: A). A sparse cloud is used after aligning the photos to insure that the photos are properly aligned. B). Interpolation between the individual points of the sparse cloud generates a dense point cloud. C). The final product is an orthomosaic produced by layering the photos over the dense point cloud. Photo is from the south Highway 207 road cut (All images from Agisoft Photoscanner 3D)
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2.3 Architectural Analysis
Architectural analysis was conducted on the three different sand dominated channel types that were identified by lithofacies analysis. Their associated architectural elements were determined by applying the principles of architectural element analysis (Miall, 1985,
1988, 1996). The bounding-surface relationships were drawn onto the stitched drone images following guidelines described in detail by Holbrook (2001) and summarized here:
1. Each surface is assumed to be unique and laterally continuous until truncated by
another surface or determined indiscernible
2. The unique surfaces can truncate one another but cannot cross.
3. Surfaces can develop diachronously however if the surface cuts another surface it
must be younger than the surface it cuts and older than the surfaces it bounds.
Each surface was then assigned a surface order. Holbrook (2001) further describes the methodology of ranking the order of surfaces, summarized below.
1. First order surfaces are considered to bound lamina bed sets.
2. Lower order surfaces are bounded by high order surfaces
3. A surface’s order is one higher order of the highest ordered surface it bounds.
4. Surfaces can only truncate surfaces of equal or higher order.
5. A set of equal ordered surfaces will be bounded by a higher ordered surface.
Not all of the photo pans had adequate resolution, or necessary exposure to define and rank fluvial surfaces. Thus, detailed architectural analysis focused on drone images taken at the
South Highway 207 location because of both quality images and surface exposure. The surface hierarchies identified from those images were then considered representative of similar lithofacies assemblages across the study area.
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CHAPTER 3: RESULTS
3.1 Measured Sections
Seven sections were measured: four in Palo Duro State Park, two along Texas
Highway 207, and one in Tule Canyon (Figures 5). Outcrop height and erosional effects presented complications to acquire traditional vertical measured sections, therefore measured sections were taken in segments along a horizontal trend. A Trimble Geo7x range finder was used to acquire the thicknesses of the intervals. The results were then displayed in the cross section below using Easy Core 1.3.3.
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Figure 7: Measured section 1 divided along the Palo Duro State State Duro Palo the along divided 1 section Measured 7: Figure lithofacies the to correspond Colors sections. 6 into cut road Park sedimentary and contacts, as well as key, the in noted paleosols and 1.3.3) Core (Easy line datum m 9 structures.
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well as contacts, and and contacts, as well
4 in Palo Duro State Park, North end of of Highway Park,end North Palo in State Duro 4
-
Measured sections 2 sections Measured
Left to Right, to Left
igure 8:
F Canyon (7). Tule and 3 sections, into divided 207 207 south (5), cut road(6) of end Highway key, the as in noted paleosols lithofaciesthe to correspond Colors and 1.3.3) Core sedimentary(Easy structures.
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3.2 Lithofacies
The associated lithofacies within each assemblage were categorized by its grain size and sedimentary structure. Grain size ranged from mud to gravel. Pedogenic structures identified were blocky to platey. Sedimentary structures identified were planar bedding, ripples, cross-bed sets, accretion sets, anti-dune, chute and pool, and cyclic steps. Lithofacies are summarized in
Table 1 and Figure 9 A thought N and detailed below.
The basal Dockum Group is a thick and extensive paleosol unit subdivided into five paleosol types over most of the study area. They are distinct, colored horizons separated by calcrete (CH) or silcrete horizons (SH). Type 1 consists of dark red soils with large angular blocky pedologic structure and little to no preserved sedimentary structures (Figure 9 b). It unconformably overlies the Permian Quartermaster Formation. Type 2 consists of bright red soil with small sub angular blocky pedologic structure containing manganese nodules, faint laminations and dark red mottling. Type 3 consists of brown soils with platy soil structure and yellow and dark red mottling with root halos. Type 4 consists of yellow soils, with both blocky and platy soil structures, containing chert nodules. Type 5 consists of gray soils that are massive with minor laminations and large purple clay clasts (Figure 9 a). Type 5 is scoured by an overlying cross-bedded sandstone lithofacies of fine tabular sandstone (FT). The calcrete horizon is light in color, with no preserved sedimentary structure or evidence of organic material (Figure
9 c). The silcrete horizon is also light in color, with little preserved sedimentary structures. It consists of hard, rounded chert nodules (Figure 9 d). The collection of these lithofacies is interpreted as a fluvial floodplain with extensive paleosol development. The paleosol horizons are reserved to the Tecovas Formation and uninterrupted by other lithofacies. It unconformably
24
overlies the Permian Quartermaster. They are readily identifiable in Palo Duro Canyon and pinch out to the south.
Three lithofacies are interpreted as of lacustrine deposits. Lacustrine laminated mudstone
(M1) comprises mud- to clay-sized grains that are typically highly laminated and well sorted with only minor bioturbation (Figure 9 f). It occurs independently or in conjunction with the other two lacustrine lithofacies. Where it is independent, this lithofacies fills scour related depressions, interpreted as muddy lake fill in small floodplain lakes. When associated with the other lacustrine lithofacies it is interpreted as the muddy lake fill part of relatively deeper lakes.
Lacustrine lithofacies (M2) consists of silty, tabular sandstone interbedded with mud. The sandstone consists of massive to rippling sedimentary structures. Locally, this lithofacies has fluid escape structures (Figure 10 e). Where present, it underlies M1 and M3 (Figure 9 f). SR is interpreted as lake bottom mud interspersed with hyperpycnites related to periodic, low energy, lake bottom turbidity currents. Lacustrine lithofacies (M3) consists of silt to fine sandstone and locally medium to coarse sandstone. The beds are thin to thick and inclined, reflecting progradational accretion sets (Figure 9 f). This lithofacies was identified in the Tecovas
Formation in Tule Canyon and Cooper Canyon Formation along Texas Highway 207. They interfinger with M1, and tend to lap onto themselves and downlap onto the underlying SR lithofacies. Specific to the Cooper Canyon Formation (M3) has upper flow regime structures.
This lithofacies is interpreted as sand pulses deposited on a delta face in terminal lakes by lower and upper flow regime hyperpycnal conditions.
One lithofacies was associated to floodplain deposition. MM consists mud, with minor preservation of sedimentary structures that include planar to faint lamina and ripples with minor to abundant bioturbation. Locally, red to orange mottling and blocky pedologic structures were
25
observed indicating poor soil development. However, this lithofacies lacks well-defined soil horizons and is therefore, considered distinct from the paleosol lithofacies unit at the base of the
Tecovas Formation. Thickness of this facies ranges from 1-3 m. This mudstone lithofacies varies in color from dark to light red, indicating it was a well-drained floodplain. The minor sedimentary structures, poor soil development and thickness suggest this lithofacies is aggradational, floodplain mud associated with poor soil development.
Two different types of perennial channel fills were identified, each with unique lithofacies. Perennial lithofacies (FT) consists of white, fine-grained sand interbedded with mud.
Sedimentary structures within the sandstone include fine parallel lamina to small-scale ripples, tabular cross-laminated bedding, bounded by scours. This sandstone fines upward into the interbedded mudstone (MF). The MF lithofacies consists of lenticularly bedded faintly parallel laminated mudstone. The mudstone drapes the underlying coarser grained sandstone, and is interpreted as muddy channel fill. FT lacks bar development (Figure 9 g). Thus, the sandstone- dominated lithofacies is interpreted as a low flow regime floodplain channel fill preceding muddy channel fill. Perennial lithofacies (STC) consists of medium to coarse sand, with well- defined ripples, trough cross bedding, and abundant scour surfaces (Figure 9 h). This lithofacies is predominantly sand and is not interbedded with MF but the unit, as a whole, fines upward into overlying mud (MM). STC is interpreted as an amalgamated trunk channel complex with multiple stories under low flow regime conditions (Bridge, 2003).
Often associated with perennial lithofacies STC are well defined bar developments, referred to as perennial lithofacies (SA). This lithofacies consists of medium to coarse sand beds separated by mud layers. Sedimentary structures include large ripples and tabular cross bedding.
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Beds are inclined and fine upward into mud layers, reflecting bar accretion sets. SA represents trunk channels forming accretionary bars under low flow regime conditions.
Lithofacies (SS) records upper flow regime sandstone sheets. Upper flow regime sheets are medium to coarse-grained sands with upper plane bed stratification and minor antidune development. They are singular, thick to medium tabular beds with a scour base, typically less than a meter in bed thickness. These sheets are continuous laterally for tens of meters. This lithofacies fines upward into mud, either as discrete sheets confined in mud or stacked sheets separated by mud layers (Figure 9 i, j). The upper flow regime strata and bed structure suggests this lithofacies represents sheet sands deposited by unconfined sheet flows under upper flow regime conditions (Miall, 1996; Fielding, 2006).
Upper flow regime channel fills (SUF) are similar to upper flow sheets, however, their sedimentary structures include more complex antidunes, chutes and pools, and cyclic steps
(Figure 9 k, l). Antidune cosets are the predominant sedimentary structure, but chutes and pools and cyclic steps occur locally along the south Highway 207 location. Both the chute and pool and cyclic step bedforms are roughly 1-2 m high. This lithofacies is thickly bedded, averaging 3-
5 m and records discrete scour and fill events. SUF lithofacies is laterally extensive, and mappable for 10’s of meters. Given the bedforms and bedding structure, this lithofacies is interpreted as large channels carrying a high sediment load under upper flow regime conditions
(Alexander et al., 1999; Fielding, 2009).
Upper flow regime channels contained two gravel-dominated lithofacies that are associated with the upper flow channels. Gravel lithofacies (G1) consists of large, sub-rounded clasts within a coarse sand matrix, with trough cross bedding. Locally, gravel beds fine upward into either mudstone or medium sandstone. These gravel beds are inclined and resemble bar
27
accretion sets (Figure 10 m). G1 is interpreted as gravel bar deposits following a hydraulic jump during upper flow regime conditions. Gravel lithofacies (G2) also consists of large, sub-rounded clasts confined in a coarse sand matrix. This lithofacies appears as thin antidune or breaking anti- dune lenses (Figure 9 n). G2 is interpreted as gravel lenses deposited under prevailing upper flow regime conditions.
28 Thickness Lithofacies Lithofacies (Code) Characteristics Interpretation Range Assemblage (m)
Moderately bioturbated, poor-well soil Varying degrees of paleosol Mudstone, paleosols (MP) Paleosol development, some nodules, blocky-platy 1-20 m development pedologic structure, root halos
White, CaCO3 cemented, no preserved Caliche Horizon in paleo 0.3 - 0.4 Calcrete Horizon (CH) Paleosol pedologic structure aridosol m
Silty mud, off white to yellow, red orange, Silicrete Horizon, hot, humid 0.2 – 0.3 Silcrete Horizon (SH) Paleosol SiO2 cemented, cherty consistency environment m
Mudstone, Clay to mud grain size, laminar bedding to (M1) Lacustrine Lacustrine mud fill 0.5- 3.0 m laminated local rippling, little to no bioturbation
Massive mudstone with little to no preserved sedimentary structure to faint laminar to Mudstone, massive (MM) Floodplain Flood plain mud 1-3 m ripple laminations locally, reddish orange, blocky pedologic structure, bioturbated
Faintly laminated, lenticular bedding, clay- Muddy channel fill and mud 0.3 - 0.5 Mudstone, lenticular (MF) Perennial channels sized grains, often drapes coarser sediment drapes in channels m
Laminar to small scale ripples, silty sand, Silty sandstone, Hyperpycnites along lake (M2) Lacustrine thinly bedded, interbedded with mud, locally 2 m rippled bottom fluid escape structures
Progradational sets, inclined and thin to Silty, sandstone, Pulses of silty sand across (M3) Lacustrine thickly bedded, interbedded with thick mud 2-8 m inclined prograding delta foresets beds, locally upper flow regime structures
White, fine sand grains, tabular to Fine sandstone, Perennial floodplain channels (FT) Perennial channels lenticular, cross bedding, scouring surfaces, 1-4 m lenticular draining the floodplain fines into mud drapes
Medium-Coarse Amalgamated trunk channel Abundant scour bases, trough cross bedding, sandstone, trough (STC) Perennial channels fills forming multistory 1-6 m ripples, fines into mud cross- stratified channel belts
Medium-Coarse Bar accretion sets, fines into floodplain mud, Trunk channel forming (SA) Perennial channels 0.5 – 1 m sandstone, accretion tabular cross bedding, lengthy ripples accretionary bars
Singular tabular beds, locally packages separated by mud layers and laterally Medium to Coarse Upper flow regime Upper flow regime sheet (SS) extensive (10s of meters), scour base, upper 1-2 m sandstone sheets sheets deposits plane bedding structures, average bedding < 1 m, fines into mud
Scour base, laterally extensive (10s of Medium to Coarse Upper flow regime meters), thick bedsets >1 m, predominantly sandstone, upper (SUF) Upper-flow-regime channels 1-18 m channels anti-dune bedforms, less common chutes and flow structures pools (1-2 m) and cyclic steps (1-2 m)
Gravel clasts, medium-coarse sand matrix, 0.5 - 2.0 Coarse sandstone to Upper flow regime accretionary bar sets, trough cross bedding, Gravel bars associated with (G1) m gravels, accretion channels inclined beds, fines into coarse sandstone or upper flow regime channels
mudstone
Gravel anti-dune and Gravel sized sediment, antidune lenses, Coarse sandstone to Upper flow regime breaking anti-dune deposits (G2) breaking anti-dune lenses, fines into coarse 0.1- 0.2 m gravels, lenses channels in upper flow regime sand channels
Table 1: Table of lithofacies and associated descriptions. Column 1 is the lithofacies name and code. Column 2 is the characteristics of the lithofacies. Column 3 is the interpretation of the lithofacies identified. Column 4 gives the observed thickness range of the lithofacies.
29
e bottom of a bottom e
horizon, separating separating horizon,
c). Caliche (K) (K) c). Caliche paleosol Palo Image horizons. from Duro. silica within d).staining Laterite Image nodules. chert cemented horizon, Palo from Duro.
e). Fine silty, sand laminae, with fluid fluid with laminae, sand e).silty, Fine as Interpreted escape features. at th deposits hyperpycnite in Tecovas the lacustrine system 207. Hwy along Formation. Image outcrop showing f).Canyon Tule yellow facies by hyperpycnitebounded muddy lacustrine lines. Underlying Tule Image delta. Canyon. from
Figure 9: Facies images of lithofacies images lithofacies of Figure Facies 9: classified above. clasts, clay purple with a). soil Gray Palo Image soil. poorly from drained Duro. halos, few with root b). soil, blocky Red Duro. Palo from wellImage soil. drained
F
B
D
C
E A 30
acies acies -
Palo from e
by separated is sheet each Tecovas,
i). Upperflow regime sheets within flood flood within sheets i). regime Upperflow is geometry Internal plain mud. bed consistent with plane upper 207. TX along stratification. Hwy Image in sheets regime flow j).upper of Stacks the 207. TX Hwy Image mud. along
k). Upper flow regime channel complex, complex, k). flow channel regime Upper antidune showing from bedforms. Image Palo Duro. flow upper regime l).meters) Large (18 in cosetsthe channel antidune with Trujillo Imag Formation. Duro.
fills drape cross sets of the perennial sets of cross fillsperennial the drape Formation. Tecovas the in flow facies Duro. Palo Image from well with h).facies channel Trunk of bar sets indicative defined accretion f perennial flow development the of from Image in Formation. Trujillo the Palo Duro. channel muddy channels, g). Floodplain
J
L H
I
G K 31
m). Gravel, accretionary bar sets atbase the sets bar accretionary Gravel, m). Image channel. regime of flow an upper along207. TX Hwy antidune breaking and antidune n). Gravel channel. regime flow upper lensesan in Duro. Palo Image from
N
M 32
3.3 Lithofacies Assemblages
Lithofacies are grouped into six assemblages based on common environments of deposition and field association. Lithofacies assemblages include: upper flow regime channels, upper flow regime sheets, perennial channel and bar complexes, floodplain, paleosol, and lacustrine deposits (Figure 12, 13).
3.3.1 Upper flow Regime Channel Assemblage
Upper flow regime channels consist of the medium to coarse sandstone lithofacies with upper flow regime structures. These channels consist almost entirely of lithofacies SUF. Locally,
G1 and G2 are associated with this assemblage. The G1 lithofacies are confined to the base of the channel bodies. The antidunal gravel lithofacies occurs locally throughout the channel bodies. The upper flow channels scour into the upper flow regime sheets, perennial channel and bars, floodplain, and lacustrine assemblages. The channels are several meters (3-5 m) thick with a scoured base. Long-wavelength, antidune cosets are the dominant sedimentary structure. Less commonly chute and pool, and cyclic steps coexist with the antidune cosets. The upper flow regime lithofacies and coarse-grained gravel lithofacies indicate flow was critical to supercritical carrying a coarse sediment load. These lithofacies are constrained within discrete scoured intervals recording different cut and fill events. The various upper flow regime bedforms represent varying fill styles. Upper flow regime channel bodies occur in both the Tecovas and
Trujillo Formations across the study area but are more abundant in the Trujillo Formation.
Collectively, this assemblage records discrete meter scale cut and fill events in broad channels under upper flow regime flooding conditions (Figure 11b, 12, 13).
33
3.3.2 Upper flow Regime Sheet Assemblage
Upper flow regime sheets consist of the medium to coarse sandstone with upper plane beds of lithofacies SS, and are interbedded with the mudstone lithofacies. Locally, the sheets terminate into the lacustrine lithofacies. These sandstone bodies are similar to the upper flow regime channels in grain size; however, their beds are significantly thinner (≤ 1 m). The bedsets are low wavelength, uniform, and mostly upper plane bed stratification. This tabular sandstone is configured either as laterally extensive (10’s of meters) singular or stacked sheets. As singular sheet bodies they scour and interfinger with the floodplain and lacustrine lithofacies. These sheets scoured across the floodplain without channel confinement. Where they are stacked, the sheets are separated by the mud lithofacies. The interbedded mudstone is credited to waning flow that followed upper flow regime deposition of the singular sheets. The stacked sheets are interpreted to have a low degree of channel confinement. The upper flow regime sheet assemblage is identified predominantly in the Tecovas Formation. Given the upper flow regime structures and relatively thin beds, this assemblage is the result of upper flow regime conditions in shallow unconfined channels (Figure 11a, 12, 13).
3.3.3 Perennial Channel Assemblage
The perennial channel assemblage has two distinct depositional patterns, occurring either as floodplain channels or trunk stream channel belts and bars. The floodplain channels consist of perennial lithofacies FT draped by the lenticular-bedded mudstone lithofacies MF. Although the sandstone channel bodies are thin, they are the dominant lithofacies in this perennial flow deposit. The sandstone fines upward into the mudstone lithofacies that fills shallow scour surfaces. The channels reflect aggradational deposition. There was no bar development
34
associated with this type of perennial flow assemblage. These perennial channel belts have a scoured base and locally scour any of the underlying assemblages. The upper flow regime sheet and channel assemblages scour perennial channel belts. Given the bed structures of these channels, deposition occurred during low flow regime conditions. These floodplain channel complexes span the Tecovas and Trujillo Formations in Palo Duro Canyon.
The perennial channel assemblage transitions from floodplain channels to trunk stream channel belts with bars to the south. These trunk stream deposits consist of the perennial channel belt lithofacies STC and perennial bar lithofacies SA. STC is the dominant lithofacies within the trunk stream channel deposits and locally fine upwards into mudstone. Overall these channel belts are sandstone dominated with sandstone channel fill. Channel belts contain only minor contributions from mud. Relative to STC, the bar development described as SA is less common.
Bars of lithofacies SA fine upward into overlying mud layers. Collectively, these trunk channel deposits scour into the upper flow regime channel, sheet, flood plain, and lacustrine assemblages, and are commonly scoured by the upper flow regime assemblages. Deposition of the trunk channel belts and bars indicate low flow regime conditions. The perennial channel complexes occur in the Tecovas and Cooper Canyon Formations, but are more dominant in the Trujillo
Formation (Figure 11c, 12, 13).
Both the floodplain channels and trunk channel belts and bars collectively describe the low flow regime, perennial channel belt and bar assemblage. These channels record deposition under normal seasonal flow, including stable confined channels reflecting a size consistent with bankfull flow conditions.
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3.3.4 Floodplain Assemblage
The floodplain lithofacies assemblage consists entirely of the massive mudstone lithofacies MM, with only local minor soil development. Floodplain deposits have a sharp contact with the other assemblages. They tend to overlay the upper flow regime channel, sheet, or perennial channel assemblages, and interfinger with the upper flow channels and sheets. All three fluvial driven assemblages scour the floodplain assemblage. Commonly, the laterally extensive, upper flow regime sheet assemblage is engulfed by the floodplain lithofacies. The assemblage is aggradational, locally filling scour depressions. The floodplain assemblage is laterally extensive, readily identified throughout the Dockum Group formations and study area
(Figure 12, 13).
3.3.5 Paleosol Assemblage
The paleosol assemblage is an amalgam of the five paleosol types with silcrete and calcrete horizons. The assemblage reflects continual aggradation unaffected by the other assemblages until the floodplain channels of the perennial channel belt assemblage scoured it at the top, or it was locally lapped by the floodplain assemblage. The soil horizons transition between red, yellow, brown and gray, and green mudstones, indicating the floodplain fluctuated from well drained to poorly drained conditions. The silcrete, interpreted as laterite, indicates a mature oxisol horizon developed in a humid tropical climate (Retallack, 1997). The calcrete, interpreted as K horizons, suggest soil development occurred in an arid climate (Retallack,
1997). The repeated occurrence of these two soil horizons implies the climate underwent humid and arid cycles (Figure 10). Likely, there is a pattern of advanced soil development, however, more work is needed to identify paleosol maturity and further detail the pedogenic structures.
36
This paleosol assemblage is restricted to the basal Tecovas Formation. It is thick in Palo Duro
Canyon ranging from 15-20 m, and thins southward to 1 m in Tule Canyon.
FT
Type 5 Silcrete Silcrete Type 4 Calcrete Type 3 Calcrete Type 2 Silcrete
Type 1
Permian Quartermaster
Figure 10: Five types of paleosol horizons, seperated by either silcrete or calcrete horizons. Unconformably overlies the Permian Quartermaster and scoured at the top by Tecovas Floodplain channels (lithofacies FT). Image from Palo Duro Canyon.
3.3.6 Lacustrine Assemblage
The lacustrine assemblage consists of the three lacustrine lithofacies. Regionally, M1 comprises the dominant proportion of the assemblage and represents muddy lake fill, followed by M2, representing lake-bottom hyperpycnite deposits. The sand rich, inclined beds of M3 was found only along Texas Highway 207 and Tule Canyon and represents lacustrine deltaic deposits. This assemblage locally interfingers with the upper flow regime channel and sheet assemblages, and is scoured by the upper flow regime channel, sheets, and perennial channel assemblages. Generally, the floodplain assemblage sharply overlies the lacustrine assemblage
37
unless it is removed by either the upper flow regime channel bodies or the perennial channel assemblage. The lacustrine assemblage is more abundant to the south and its associated lithofacies are thicker there. In Palo Duro Canyon M1 and M2 are the only lithofacies within the assemblage, and the assemblage is relatively thin (0.5-1.0 m). Here, M1 fills small depressions and is possibly the by-product of local floodplain scour events (Willis and Behrensmeyer, 1994).
M1 represents muddy lake deposits filling ephemeral floodplain lakes. Where M2 underlies the muddy lake fill, lakes were deeper with turbidity lake bottom currents depositing the hyperpycnites of lithofacies M2. South of Palo Duro Canyon, both lithofacies M1 and M2 also increase in thickness (1-3 m) and M2 increases in abundance (Figure 12, 13).
In the southern limits of the study area at Tule Canyon, all three lacustrine lithofacies are present. The muddy lake fill and hyperpycnites make up 80% of the assemblage, with the deltaic sandstone beds making up 20%. The muddy lake fill lithofacies is interbedded with the thin, fine deltaic sandstone beds. Each of these deltaic sands were not deposited on the same plane as each other; but appear higher or lower in the section. It is interpreted that lake levels fluctuated in response to changes in climatic conditions. South of Palo Duro, the assemblage is interpreted to be filling deeper, terminal lake basins compared to the shallow floodplain lakes to the north. The lacustrine assemblage is predominantly reserved to the Tecovas Formation, but locally appears in the Cooper Canyon Formation. Here, only M1 and M3 are present. The inclined deltaic sandstone beds are much thicker and contain upper flow regime structures. The muddy lake fill is interbedded with these upper flow regime progradational sets. Similar to the deltaic sands at Tule
Canyon, the presence of these upper flow regime foresets are the response to climate conditions that prevailed during deposition (Figure 12, 13).
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3.4 Hierarchy of Surfaces
Detailed fluvial architectural analyses were conducted for the upper flow regime channel, upper flow regime sheets, and perennial channels assemblage belt part along the southern portion of Highway 207, where exposure was best (Figure 10). A more general analysis of the surface hierarchy was conducted on the northern half of the outcrops, including the road cut in Palo Duro
Canyon. The results from the detailed analyses were used to determine the fluvial style of the three distinct channel types.
Surfaces within the upper flow regime channels, upper flow regime sheets, and perennial flow assemblages were mapped and designated a hierarchy. The upper flow regime sheets possessed zero order lamina, first order, upper plane bedform structures, with sets of upper plane beds bound by second order surfaces. The assemblage as a whole is bound by third order surfaces (Figure 11 a). Upper flow regime channels comprise third order surfaces binding complex packages of antidunes and other upper flow regime structures (Figure 11 b). These are bound within discrete, fourth order scour and fill events. Fifth order surfaces separate storm flood events and the entire assemblage is bound by sixth order scour surfaces that separate it from the other lithofacies assemblages. The perennial flow assemblage consists of zero order, cross and planar lamina, bound into bed sets by first order surfaces recording (Figure 11 c).
Second order surfaces bind simple unit bars, and third order surfaces bind more complex unit bars. Packages of unit bars are bound by fourth order accretion sets in larger compound bars.
Lateral accretion events are bound by fifth order surfaces. Collectively, belts of the perennial flow lithofacies assemblage are bound by sixth order surfaces.
Seventh order valley scour surfaces locally bind sets of channel assemblages but otherwise, assemblages interfinger and are unbound. The seventh order valley scours are
39
interpreted as buffer valleys and record changes in base level (Holbrook et al., 2006). These are the most continuous surfaces where they occur, but more typically, the sixth order surfaces that bind assemblages are the most continuous. Upper flow regime sheets of third order surfaces are typically truncated by upper flow regime channels or perennial channel assemblages or terminate into the floodplain or lacustrine assemblages. Upper flow regime channel sixth order surfaces are truncated by the perennial flow sixth order surfaces. The perennial flow sixth order surfaces are generally truncated by the upper flow regime sheet third order and channel sixth order surfaces.
40 A
B
Figure 11: A). Top: unedited photopan. Bottom: Upper flow regime sheets architectural analysis. 3rd order yellow bind sets of sheets of upper plane beds and minor antidune development. Image from Hwy 207. B). Top: unedited photopan. Bottom: Upper flow regime channel complex architectural analysis. Pink 3rd order cyclic step bedforms, bind a complex assemblage of lower order surfaces. 4th order green bind separate cut and fill events. Blue 5th order surfaces separate flood events, 6th order yellow bind the entire lithofacies assemblage. Image from Hwy 207.
41
C
Channel Belt and Bar
4th order
2nd order 6th order
3rd order
1st order
5th order
Figure 11: C). Top: unedited photopan. Bottom: Perennial Channel belts and bars architectural analysis. 4th order accretion sets bind either 2nd order simple unit bars or complex 3rd order unit bars. 5th order surfaces differentiate between lateral accretion events. The entire assemblage is bound by 6th order. Image from Hwy 207.
42
ied. Figure depicts lithofacies assemblages assemblages lithofacies depicts Figure ied.
meters
85
60 60 meters
Figure 12: Outcrop along south Highway 207 where hierarchy of surfaces, lithofacies, and lithofacies assemblages identif were assemblages lithofacies and lithofacies, surfaces, of hierarchy where 207 Highway south along Outcrop 12: Figure surfaces. architecture with 43
SA
SU F
SA
SU F
ST C
ST C
M M
SA
meters
60 60 meters
85
M M
SA
SA
M M
M M
M M
M M
M M
SA
SU F
SU F
SA
SU F
M2
SS M1
SU F
Figure 13: Same Highway 207 outcrop. Figure depicts lithofacies with architecture surfaces. architecture with lithofacies depicts Figure outcrop. 207 Highway Same 13: Figure
ST C
44
CHAPTER 4: DISCUSSION
4.1 River channel and flow processes
Fluvial flow directly deposited the upper flow regime channel, upper flow regime sheets, and perennial channel assemblages identified in this study. Fluvial architectural analysis was conducted on these assemblages in order to distinguish the component features of these three types of channels, and interpret how they formed. These three channel types provide a depositional context for the remaining Dockum Group assemblages.
4.1.1 Upper Flow Regime Channels
Upper flow regime channels are thick, laterally extensive channel deposits that record intense singular, unconfined flow events. Bedforms within the channels were deposited under upper flow regime conditions, and include antidune cosets, chute and pools, and cyclic steps.
Each of these bedforms is represent large, meter scale scour and fill events, however, the flow styles of fill contrast within and across these channels.
Antidunes are associated with in-phase surface waves and typically develop as sets of antidune ‘trains’ migrating down-stream while individual antidune bedforms may migrate upstream. Typically within the upper flow regime channels, the upstream antidunes are scoured out by the incoming flow while new antidunes form at the tail of the train (Figure 13). The individual antidunes have shallow foresets dipping upstream (Kennedy, 1961; Guy et al., 1966;
Yokokawa et al., 2010; Cartigny et al., 2014). Antidune sets are the most common and widely distributed bedform in the upper flow regime channels. They appear as uniform sets from channel base to top or in local scours. When filling local scours they are bound by second order surfaces within chutes and pools and cyclic steps (Figure 14). These antidune bedforms indicate
45
paleoflow was nearing critical levels, reaching Froude values (Fr) between 0.84 and 1.0
(Southard and Boguchwal, 1990). This flow intensity was held constant throughout the meter scale deposition of these channel bodies as evidenced by the maintenance of antidune deposition throughout the thickness of these deposits.
Chute and pool structures randomly interrupt the continuous antidune bedforms within the upper flow regime channels. They are the least common bedform identified in the channels.
These bedforms reflect deposition under increased flow velocity compared to adjacent antidunes.
Along the bed, flow rapidly accelerated forming an erosive chute, followed by a pool where the bed area experienced tranquil, but accelerating flow (Simons et al., 1965). The pools are filled by antidunes related to breaking surface waves or large standing waves. This bedform migrates upstream in flow velocities similar to, or higher than, those that formed the surrounding antidunes (Middleton, 1965; Guy et al., 1966; Cartigny et al., 2014). Chutes and pools are considered unstable bedforms and do not have regular intervals of cut and fill. The upper flow regime channels reflect this irregularity of deposition as the chutes and pool structures are formed inconsistently throughout the channel bodies. As the paleoflow neared critical levels, antidunes were deposited until scoured out by a chute, when the flow reached supercritical levels. A surge flow began to migrate upstream in response to the recently developed topography on the channel bottom. This surge flow would have then stabilized as a result of a hydraulic jump. The flow velocity reduction created the tranquil pool structure, where antidune deposition resumed and filled the pool (Figure 15). These chaotic cut and fill events, suggest flow intensity within the upper flow channel assemblage was not constant, but rather periodically increased during channel deposition. This resulted in a complex configuration of antidunes bounded by the third order chute and pool bedforms.
46
Cyclic steps resemble chute and pool bedforms, but record a higher more stabilized flow strength. They migrate slowly upstream in steps, delineated by hydraulic jumps. Each step is a zone of decreasing supercritical flow bounded at the downstream end by a hydraulic jump
(Parker, 1996). Within the upper flow regime channels, cyclic steps erode downstream and deposit upstream against the upstream scour, similar to chute and pools. Cyclic steps however reflect regular intervals of cut and fill (Figure 16). The cyclic step bedforms of the upper flow regime channel assemblage tend to occur in sequence with each other. Each step is separated by third order surfaces that bind complex antidune assemblages. The sequence as a whole develops within a discrete scour. This suggests flow intensity during channel deposition was increased to supercritical, near supercritical levels, and subsequently remained long enough to deposit sequences of cyclic steps. These sequences are bound by fourth order surfaces that represent discrete scours and cyclic-step fill events. Sequences of cyclic steps are fairly common, and are randomly scoured into antidune sets throughout the upper flow regime channels.
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Figure 14: Top: unedited photopan. Middle: Multiple scour and fill events of antidunes occurring within a singular, upper flow regime channel along Hwy 207. Bottom: Schematic of idealized unidirectional supercritical flow depositing antidunes. Flow is from right to left. (Cartigny et al., 2014)
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Figure 15: Top: unedited photopan. Middle: Singular scour and fill event depositing a Chute and Pool structure. Deposition interrupts antidune deposition along Hwy 207. Bottom: Schematic of idealized unidirectional supercritical flow depositing chute and pool structures. Flow is from right to left. (Cartigny et al., 2014)
49
Figure 16: Top: unedited photopan. Middle: Sequences of cyclic steps filling discrete scour surfaces along Hwy 207. Below: Schematic of idealized, unidirectional, supercritical flow, depositing cyclic steps. Flow is from right to left. (Cartigny et al., 2014)
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These supercritical flows exceed flow equilibrium, thus they are considered unstable and highly sensitive to grain mobility, flow velocity, and Froude values (Cartigny, 2014). These relations could explain the irregularity of distribution in chute and pool and cyclic steps in the upper flow regime channels. The abundant antidune sets suggest that critical flow was the most prevailing flow condition during channel deposition. Periodically, supercritical conditions were reached resulting in chute and pools and sequences of cyclic steps.
Collectively, these upper flow regime bedforms imply that deposition occurred during high velocity, near critical to supercritical conditions, carrying a high sediment load. The sets of antidunes are approximately 1-2 meters thick and are continuous throughout the channel’s width, representing single scour and fill events of 1-2 m deep and 10’s of meters wide (Figure 14). The cyclic step bedforms reflect local evacuation of sand approximately one meter deep and 2-3 meters wide, subsequently filling this singular scour. Prior scour and fills were cut by later scour and fills indicating at least locally that these channels had multiple upper flow scour and fill events (Figure 16).
The lateral extensive nature of the channels indicates they were initially unconfined before scouring to initiate a limited degree of confinement. These flows were extremely intense and short-lived events as evidenced by the lack of channel belts and bar development and the pervasive preservation of upper flow regime bedforms. The intense, short duration flows are represented by 5th order surfaces that bind lower order scour events and upper flow structures formed during separate flood events. Collectively the entire upper flow channel assemblage is bound by 6th order surfaces.
Work conducted by Alexander and Fielding (1997) in the dry bed of the Burdekin River in Queensland, Australia provide an analog for the upper flow regime channel assemblage.
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Following tropical cyclone-induced floods, gravelly antidune bedforms are deposited as cosets with wavelengths ranging from 8-19 m and wave amplitudes of 0.25-1.0 m. The Burdekin River resides in a large, semi-arid, semi- humid, tropical catchment, where overbank flooding is less frequent compared to temperate climate rivers. When cyclonic storms reach inland the floodwaters rise very quickly, reaching water depths of 9-18 m. The floods have high discharge flows (maximum recorded discharge over 25,600 m3s-1, February, 1927) remobilizing several meters of sediment (Fielding et al., 1999) and developing antidunes in a matter of days.
Floodwaters then fall very quickly (several meters per day), with the flow being diverted by secondary channels. In doing so, the upper flow regime bedforms were preserved with little reworking by falling stage currents under still supercritical flow conditions. Post-flooding, the
Burdekin River reverts to a smaller perennial state of flow. However, this flow does not scour either deep enough, or laterally, sufficiently to remove prior upper flow structures. These perennial flow deposits are typically removed by the next cyclonic storm flood. With a setting where floods are very large and of too short duration, channel-floodplain systems cannot attain equilibrium conditions during a single event, aiding to the preservation of upper flow regime flow and bedforms.
The upper flow regime channels of the Dockum Group are inferred to be the result of violent storms that generated extreme and short-term flows, analogous to the cyclonic storm related deposition of the Burdekin River.
4.1.2 Upper Flow Regime Sheets
Upper flow regime sheets are laterally extensive, thin, tabular sandstone bodies deposited in shallow rapid sheet flows. Sedimentary structures are predominantly upper plane beds,
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indicative of upper flow regime conditions. Sets of upper plane beds are bound by 2nd order surfaces and ultimately the entire sheet is bound by 3rd order surfaces. The sheet flows were shallow, high velocity, near critical (0.8 - 0.95 Fr) flows, and carried a coarse sediment load.
They lasted only a short duration (days), evident by the preservation of upper plane beds and lack of well-developed, discrete channels (Alexander and Felding, 1997). The thin and laterally extensive nature of these sheets implies that they were for the most part deposited during unconfined flow. The upper flow regime sheets are interpreted as flash flood events occurring in an environment that typically remained arid, and did not regularly receive water influx resulting in channelization.
These sheet bodies form in conjunction with the floodplain, lacustrine, and upper flow regime channel assemblages. Where they are interbedded with the floodplain and lacustrine assemblages, they commonly occur as a singular sheet, suggesting flash flood events perpetuated across an arid floodplain or dry lake bottom. Locally, they terminate into the lacustrine assemblage, suggesting lakes were not completely dry, when storm related flow came in contact with the lake water margins. Where they overlie the upper flow regime channel complexes, the sheets occur as stacked sequences with muddy layers in between the sheets. The stacked configuration suggests the storm flows had some degree of incision that allowed sheets to be deposited repetitively in the same location. The mud layers that separate the sheets are likely the result of the waning flow as the floodwaters receded.
These sheets record upper flow regime conditions, but without sufficient flow to generate abundant and thick antidune, chute and pool, or cyclic step bedforms. Consequently, these flows thus lack the discharge or velocity characterized by the flow rates of the upper flow regime channel assemblage. Flow conditions may have been right for development of antidunes that
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were not preserved. Potentially, antidunes may have formed, migrated, and steepened before becoming unstable, but were then destroyed and replaced by upper plane bed deposition (e.g.,
Strong, 2012). In either case, sheets are internally simple and thin, reflecting minimal scour and fill. Rather than large cyclonic storms, these sheets are interpreted as occasional local frontal storms that induced flash flooding over the semi-arid Triassic landscape.
Upper flow regime sheets are analogous to flash flood-dominated deposits of west Texas.
Much of west Texas currently has semi-arid conditions with sparse vegetation similar to the conditions projected in the Triassic. Due to the lack of precipitation in the area, a defined perennial drainage network is not established. Channels are typically shallow and laterally extensive. Therefore, flash flooding tends to follow local storm front rains. Without the hindrance of channel confined, drainage pathways and vegetation, these shallow flows tend to be wide, and entrain a high sediment load. Floodwaters rise and fall rapidly in a matter of days.
Flow will follow preferred fareways, and locally, incise these wide sheets, and repeat along the same path.
Upper flow regime sheet bodies are predominantly recognized in the Tecovas compared to the Trujillo. The mud-dominated Tecovas more readily preserves these sheet deposits compared to the Trujillo, and has an abundance of these sheets as both single sheets and amalgamated complexes.
4.1.3 Perennial Channel Belts and Bars
The perennial channel assemblage is belts and bars of “typical” fluvial channel belts formed under low flow regime conditions. Two distinct types of channels/belts include floodplain channels and trunk channel belts.
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The floodplain channels are fine-sandstone dominated but consist of a greater mud concentration than the perennial trunk channels found in the south. The mud drapes are interpreted as channel fill, and occur repeatedly in the channel belts, suggesting channels were small, and discrete with significant mud fill components encased in floodplain deposits. Thus, they are single small channels crossing the floodplain with intermittent flow during fill events recording differing energy during flood flow events. The channels drained the floodplain into a trunk channel or lacustrine settings, during humid climate periods, when water influx was more continuous.
The perennial trunk channels dominate in the south and are sandstone with well-defined channel belts that include bars recording a storm-dominated parental system. Channel fill is sand rich with minor mud content, and overall the channel belts fine upward into mud, that overlies the belt. The bars are interpreted as simple unit bars bound by 2nd order surfaces to 3rd order- bound complex unit bars. The unit bars make up laterally accreting bars bound by 4th order surfaces, and are preserved on channel thalwegs. The best exposure of these bars is along Texas
Highway 207. Here, the bars reflect a dual flow system separated by 5th order surfaces. Initially, big washes incised wide, shallow channels and formed unit bars along the bottom. Following this initial depositional phase, secondary bars laterally accreted over the top of the underlying bar complex. These flows did not deposit upper flow regime structures, suggesting flow was continuous under low flow regime conditions. These perennial flow related channel belts and bars are likely storm driven. Partly, they are analogous to most natural temperate climate meandering rivers that receive water influx continuously year round, forming channel belts, lateral bars and effective drainage networks (Bridge, 2003). During storms, the channels widen and scour. They do not fill with upper flow regime after these events, but instead, partly-fill with
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lower-flow-regime, unit bars along the channel base. Lateral-accretion elements from unit bars fill the channel remnant, and replace part of the basal fill during interim perennial flow conditions. Potentially, these perennial channels could be the distal flow of upper flow regime channels. Intense flows that deposit the upper flow channels upstream begin to normalize and deposit the belts and bars under low flow regime conditions downstream. Alternatively, these channels represent an independent and periodic flow condition unrelated to the other channel types.
Figure 17: Top: unedited photopan. Bottom: Perennial dual-flow channel belts and bars. Yellow- lithofacies assemblage boundaries, Green- simple/complex lateral accretionary bars, downlapping onto initial, wide, channel scour and thalweg fill (Blue). Initial channel is filled with complex unit bars (Pink, Orange, Black and Red). Drone photopan from Highway 207.
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4.2 Depositional Context and paleogeography of the Dockum Group
The Dockum Group as a whole is a complex fluvial system with lakes forming and filling at different times and locations in the depositional cycle. The Santa Rosa, Tecovas, Trujillo, and
Cooper Canyon Formations represent four distinct time intervals, and reflect their respective climate at deposition. Climate cycles were the driving factor of the fluvial and lacustrine landscape.
Regionally, the basal unit of the Dockum Group is the Santa Rosa Sandstone. It is identified as a fluvial channel sandstone unit, with multiple stages of scouring, migration and aggradation. Lehman, (2005) described the Santa Rosa strata similarly as a lithofacies assemblage from perennial channel belts with bars. The basal Dockum Group appears to be channel belts deposited by channels migrating across a floodplain with a prevailing tropical climate providing adequate precipitation to allow for continual deposition of channels and bars.
The predominant paleocurrent directions are from the north, northeast and east (Lehman and
Chatterjee, 2005). However, the south-southeast flow preserved by the overlying formations, implies that flow direction changed. The Santa Rosa grades into the mud-dominated Tecovas
Formation (Lehman and Chatterjee, 2005). The Santa Rosa Sandstone was not identified in Palo
Duro Canyon; instead the basal thick paleosol unit grades into the Tecovas Formation. During deposition of the Santa Rosa, the well-defined paleosols were deposited on an elevated floodplain adjacent to the Santa Rosa trunk channel and associated floodplain (Figure 17).
Demko et al, (1998) identified a similar association with the basal Shinarump Member and lateral soils in the equivalent basal Chinle Formation in the Colorado Plateau. The paleosols are recording aggradational floodplain deposition, ranging from well-drained to poorly drained soils.
The soil horizons are separated by either a laterite horizon, signifying humid tropical climatic
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conditions or K horizons, signifying arid climatic conditions. Presence of these alternating soil types suggest the Triassic climate was cycling between humidity and aridity. These climatic cycles are interpreted as reflecting variation in the intensity of the monsoonal circulation patterns.
Figure 18: Schematic cross section of Santa Rosa deposition. Modified from Demko et al. (1998).
The Tecovas Formation records abundant, fine-grained, floodplain and lacustrine deposits. During deposition of the Tecocas in Palo Duro Canyon, the floodplain lithofacies assemblage coexisted with perennial floodplain channels. Lacustrine assemblages filled floodplain lakes. The floodplain and lake assemblages are interbedded with upper flow regime channels and sheets. However, south of Palo Duro Canyon, the Tecovas Formation consists predominantly of the lacustrine lithofacies assemblage filling larger terminal lake basins. These basins are evident along Texas Highway 207 with muddy, lake-fill and hyperpycnites. In Tule
Canyon, a lake delta that deposit of interbedded, muddy lake-fill and sandy foresets overlies hyperpyncal deposits. This delta system prograded to the north. During Tecovas deposition, a
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relatively large, shallow terminal lake extended from the Texas Highway 207 outcrops to Tule
Canyon. A floodplain surrounded this lake that extended north to Palo Duro Canyon.
Tecovas lake levels fluctuated with the seasons as evidenced by the upper flow regime sheet sands that interfinger with lake deposits along Texas Highway 207. Occasional storm fronts induced flash floods that carried these sheets across the desiccated lake bottom as indicated by isolated upper flow-regime sheets enveloped within lacustrine assemblages (Figure
18). Seasonality is also reflected in the fluctuating elevation of delta foresets in Tule Canyon.
The inclined sandstone beds appear at different heights in the section as a result of fluctuating lake levels. Higher lake levels led to foreset deposition higher in the section; while a drop in lake levels moved the foresets distally, and allowed the section to onlap higher foresets. During wet seasons the floodplain lakes and larger lakes were full, and perennial floodplain channels drained the floodplain into lacustrine systems or a main trunk channel system. However, when the climate shifted to arid conditions, the floodplain channels dried, and no longer fed into the lacustrine systems or main trunk channel. The lack of precipitation recharge caused both the lakes and trunk stream water levels to drop, producing an arid floodplain, desiccated lake shorelines, and dry channel bottoms (Figure 19).
Figure 19: Upper flow regime sheet lithofacies assemblage enveloped by lacustrine assemblage when lake levels were low due to arid climate conditions. Image from Tule Canyon.
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Figure 20: Depositional cartoon depicting depositional environment for the Tecovas Formation. Flow from the south, and south east. A Tecovas lake dominated the southern portion of the study area with a muddy lake delta at the Tule Canyon location, while a prominent flood plain existed in Palo Duro, indicated by the abundant paleosol and floodplain assemblages. Images not to scale (Image from OnX Maps)
During deposition of the Trujillo, paleocurrents indicate flow was from the south, and southeast. Sediment was sourced from the Ouachita Orogenic Belt (Lehman and Chatterjee,
2005). The perennial channel belt and bar, and upper flow regime channel assemblages were the dominant lithofacies assemblages preserved at this time. No major lake bodies were identified in the Trujillo, indicating preserved deposits were strictly related to these channel assemblages. The system is interpreted as arid to semiarid given the dominant lithofacies assemblages that were
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identified. During the wet seasons, perennial channel belts and bars dominated the region. When the megamonsoon moved inland, it occasionally brought in cyclonic storms that resulted in the intense upper flow regime conditions that deposited the upper flow regime channel assemblage.
These large erosive events scoured out the wet-season perennial flow deposits locally. The upper flow regime channel assemblage is more prevalent in the Trujillo Formation than either the
Tecovas or Cooper Canyon Formations, suggesting the perennial moisture related to the monsoonal wet seasons infrequently reached the Dockum Group during deposition of the
Trujillo. The only moisture that reached the area was related to cyclonic storms (Figure 20).
Alternatively, the high amalgamation of channels in the Trujillo Formation favored preservation of the upper flow regime channels that had the tendency to scour and remove all prior deposits within their scour-and-fill space.
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Figure 21: Depositional cartoon depicting depositional environment for the Trujillo Formation. Flow predominately from the southeast. Sediment being sourced from the Ouachita Orogeny southeast of the depocenter. Sediment was transported by large upper flow regime channels activated by cyclonic storms during the monsoon season. Images not to scale (Image from OnX Maps)
Although Cooper Canyon deposits were not extensively analyzed in this study, perennial channel belt and bar, floodplain and lacustrine assemblages were identified locally along Texas
Highway 207. The channel belt and bar complex reflect a dual flow system of laterally accreting unit bars. These channel complexes migrated across a floodplain that expanded regionally during the wet seasons under low flow regime conditions. The lacustrine assemblage consists of thick, inclined, interbedded mudstone and sandstone beds that downlap onto the underlying strata.
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Occasionally, those sandstone beds terminate into the mudstone. These bedsets are interpreted as deltaic deposits within a terminal lake. The sandstone foresets have upper flow regime structures of antidunes, chutes and pools, and cyclic steps. This suggests that the upper flow regime channels migrated across the delta and entered the lake under upper flow regime conditions.
These flows maintained a sufficient velocity to deposit upper flow regime structures along the delta foreset and extend as basal hyperpycnal upper-flow regime flow across the lake bottom before dissipating (Figure 21). The flow scoured the muddy delta foreset before back-filling the scour surfaces with coarser grained upper flow regime bedforms.
Figure 22: Schematic cartoon and cross section of the depositional characteristics identified in the Cooper Canyon Formation along northern Highway 207 road cut. Left: Depositional environment during humid conditions. Right: Depositional environment during arid conditions.
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The intensity of upper flow regime flows that entered Cooper Canyon lakes is analogous to glacio-fluvial jet washes. The Middle Pleistocene glacio-fluvial Porta Delta in northern
Germany depicts a glacier in a lacustrine setting. These jet washes produced a delta under high energy, supercritical density flows of melt water flowing beneath the glacier (Figure 22.1).
Continual flow provides sediment that progrades further into the lake in the form of a lacustrine delta (Figure 22.2). Under surge flow conditions, these flows scour the sediment directly in front of the jet wash before back-filling the surface with coarser material, which has upper flow regime bedforms along the delta foresets (Figure 22.3) (Lang et al., 2017). Although not glacial,
Cooper Canyon lakes have surge event deposits similar to glacial jet washes that were likely produced by cyclonic storms that carried a high enough flow energy at supercritical density to deposit upper flow regime bedforms across the delta face. As the flow energy stabilized, deltaic mud deposition returned until the next cyclonic storm induced a new surge event.
Overall, the Dockum Group assemblages were driven by climatic conditions ultimately dictated by megamonsoonal circulation patterns. Stratigraphic control is presently insufficient to determine whether the various channel types and lake systems record an alternation of systems or a coexistent system of the six assemblages. It is therefore undetermined whether these climate conditions represent clear cyclicity through time or if different, climate-driven depositional systems prevailed at specific times.
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2
3 A B
C D E
Figure 23: 1). Depositional model for the gravel- and sand-rich, subaqueous ice contact fans deposited by glacial jet flows. 2). Depositional model for the glacio-fluvial Porta Delta, showing the foreset bed deposited by surging supercritical density flows. 3). Images A-E show the sedimentary facies and architecture deposited in the Porta delta. A). Delta foresets with gravel and sand scour fills related to cyclic steps. B). Foresets with sandy cyclic step deposits. C). Gravel-filled scours related to cyclic steps along the delta foresets. D). Small scale scours related to cyclic steps. E). Sandy antidunes within the delta foreset. (Lang et al., 2017)
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4.3 Megamonsoon Climate Hypothesis
Monsoons are the result of temperature imbalances between the ocean and land. Due to the high heat capacity of water, oceans will warm slowly, but retain heat longer. Comparatively, land masses have a lower heat capacity, and tend to absorb and transmit heat more quickly.
During warm summer months, as the sun transmits heat to both the oceans and land, the land warms and the air mass above expands, developing a low pressure zone. The ocean heats more slowly, and thus, is cooler at this time and produces a high pressure air mass. The pressure differential between the land and ocean causes a sea breeze to move landward (North Carolina
Climate Office, 2019). The air mass over the ocean is heavily saturated with moisture due to intensive evaporation. Therefore, the sea breezes brings moist air inland. The moist air is warmed over the land and rises. As it rises, it is cooled. This, in turn, decreases the air’s ability to retain water vapor resulting in monsoonal precipitation (Figure 23) (Krishnamutri, 1998).
Monsoons can be intensified by highlands, as is the case with the Asian Monsoons against the uplifted Tibetan Plateau. Moist air brought in by the sea breezes is thrust to higher altitudes, cooled rapidly over short distances, and loses nearly all their retained moisture over areas that would otherwise be hot and dry. Modern day monsoonal precipitation within the subtropical latitude zone, such as the Asian monsoon, occurs during the months of July to September and typically moves westward (Central Weather Bureau of Taiwan, 2019).
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Figure 24: A schematic diagram representing the air mass circulation patterns and resulting precipitation events associated with summer and winter monsoons. (Modified from the North Carolina Climate Office)
Chinle Formation deposition in New Mexico and Arizona is accredited to monsoon rains
(Dubiel et al., 1991), and is age equivalent to the Dockum Group of Texas (Brown, 2016). The
Chinle preserves abundant, perennial channel complexes capable of draining the extreme amount of rainwater. The upperflow regime complexes reported here for the Dockum Group have yet to be reported in the Chinle Formation.
The Dockum depocenter reflects a dominance of the impact of rare and isolated monsoonal-driven cyclonic storms. The presence of upper flow regime complexes indicates episodes of intense rain events lasting several days between long dry spells, compared to typical monsoon rains that come with lasting intensity over several months. During the summer
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monsoon season, the enormous land mass of Pangea was heated, especially in the equatorial zone where the Dockum Formation was deposited (3°- 16°; Hinsbergen et al., 2015). This caused a substantial pressure imbalance between it and the massive Triassic ocean to the west. This large imbalance in turn developed megamonsoons, capable of moving inland against the easterly prevailing winds (Parrish, 1993). Because of the surrounding highlands of the western Pangea coast, the Ancestral Rockies to the north, and Ouachita Orogeny to the east, this low pressure monsoon trough was confined inland (Figure 24). Exceptionally wet air masses pulled onto land by the megamonsoons released moisture evaporated from the ocean over the Chinle and periodically the Dockum. Perpetual rainfall resulted in confined channel complexes that could adequately drain the basin. Because the monsoon trough was contained, it was able to continually recharge by drawing in squall storms from the ocean.
With the Dockum being so far inland from the western sea, it did not regularly receive the monsoon rains as the Chinle. Most of Dockum remained arid throughout deposition.
However, large cyclonic storms developing in the oceans were occasionally pulled into the
Dockum Basin by the low pressure inland monsoon. The moisture associated with these storm events periodically reached the Dockum depocenter and released massive amounts of rain, over the typically hot, dry and sparsely vegetated surface. Lacking a drainage system to handle the influx of rain, the run-off formed large unconfined upper flow regime channels. These channels are developed in both the Tecovas and Trujillo Formations.
As a model, similar monsoonal troughs draw in intense cyclonic storms in north
Queensland, Australia (Alexander et al. 1999). The Burdekin River records these seasonal depositions of upper flow regime antidunes consisting of coarse gravels brought about by tropical cyclone-induced floods. The storm floods last only days and record meter scale erosion
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(Alexander and Fielding, 1997; Fielding et al., 2006). As in the case of the Dockum, the
Burdekin drainage area is normally a semiarid to arid system with minimal vegetation, and minor channel drainage, and does not experience frequent monsoonal rains. The system similarly does not maintain perennial drainages sufficient to contain or pass the large floods produced by the occasional cyclones drawn in by the local monsoon. The modern channel of the Burdekin River at bank-full width ranges from 200 – 750 m wide and roughly 25 m in depth. Floods produce peak discharges of more than 35,900 m3s-1 with a mean annual maximum discharge of 9784.4 m3s-1. These flood waters flow very fast with calculated velocities of 4.4 – 5.4 ms-1. These significant flows develop standing waves with wavelengths of 13 – 19 m in water depths of 2.0 –
3.5 m (Alexander and Fielding, 1997). Similarly, the Dockum upper flow channels record lengthy antidune bedsets of 10’s of meters and 1 – 2 m in thickness. Deposition of analogous antidunes suggests paleodischarge was comparable to the flood discharge rates and flow velocities of the modern Burdekin River.
Figure 25: The North American continent during the Late Triassic on the western margin of Pangea. The warm Pangea landmass creates a substantial pressure imbalance capable of developing a sea breeze that moves counter to the prevailing wind direction. The low pressure monsoonal trough is then confined by the Triassic highlands noted by the black dashed line. An interpreted equatorial position is indicated by the yellow line. Dockum Group extent is outlined by the red box. (Map © Ron Blakey)
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During the colder winter months the monsoonal cycle is reversed. The land radiates heat faster, thereby cooling much more quickly than the oceans. This develops a higher pressure air mass over the land relative to the warm ocean during the fall and winter, with an expanded, low pressure air mass over the ocean. The pressure imbalance causes air masses to flow seaward. As this warm, humid air reaches the ocean it rises and cools, releasing the retained moisture over the ocean (Figure 19). In the case of the Asian Winter monsoons, the Himalayas halt the flow of cool air. Thus, the leeward side of the Himalayas remains hot and dry, often in drought conditions.
The winter monsoon is restricted largely to the areas south of the Himalayan front.
Arid conditions prevailed during Dockum deposition, thus vegetation was sparse and precipitation rare. These landscape conditions were ideal for rain events to create flash floods.
When the occasional local storm front formed over the arid Dockum depocenter, rains produced flood events on a smaller scale than floods induced by cyclonic storms, which subsequently deposited the upper flow regime sheets.
The intensity and frequency of the Triassic monsoon climate may have been variable
(Parrish, pers. comm.), possibly causing extended periods of precipitation or drought. Lengthy periods of precipitation could explain the presence of perennial channels and bars. Continuous flow may have developed the perennial flow assemblages and effectively washed out prior deposits. Additionally, they would provide an adequate drainage network that could handle intense periods of precipitation brought in by cyclonic storms or local storm fronts. Conversely, if lengthy periods of drought existed there would not be a floodplain drainage network that could handle the extreme precipitation events and perennial channels could not form. These conditions are conducive to upper flow regime channel and sheet deposition.
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The complex layering in the paleosol assemblage can be explained by alternating periods of extended aridity and humidity tied to variable intensity of the monsoon cycle. Aridisols indicate extended periods of aridity, suggesting monsoon rains did not reach the Dockum depocenter. The presence of laterites imply that monsoon related rains frequently reached the depocenter. The same conclusion can be extended to the lacustrine systems. During lengthy periods of drought, the lake levels dropped and the occasional storm front deposited upper flow regime sheets across the dried lake bottom. In the case of the delta at Tule Canyon, the fluctuations between periods of aridity and humidity are evident in the rising and falling of the delta foresets. Additionally the Cooper Canyon delta along Texas Highway 207 suggests that the megamonsoon trough periodically drew in cyclonic storms that generated large floods and deposited the upper flow regime foresets on delta faces before returning to mudstone deposition under more arid conditions. The presence of upper flow regime delta faces is consistent with deposition from the coexistent upper flow regime channels.
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CHAPTER 5: CONCLUSIONS
1. The Dockum Group consists of five lithofacies assemblages: upper flow regime channels,
upper flow regime sheets, perennial flow channels, floodplain, and lacustrine.
2. Both the Tecovas and Trujillo Formations include upper flow regime channels previously
unidentified by other workers in the Dockum Formation and generally uncommon in the rock
record. These channels record megastorms of the megamonsoon.
3. A megamonsoonal climate persisted throughout the Dockum Group deposition that varied in
frequency and intensity and was intensified locally by orographic lifting from the
surrounding mountain ranges.
4. The megamonsoonal trough typically brought in storms, but minimal persistent rain. Large
cyclonic megastorms were responsible for the deposition of upper flow regime channels.
Upper flow regime sheets are credited to monsoon-related, localized storm fronts. Perennial
channel complexes record alternating storm-driven scour of wide channels followed by
perennial channel and bar development, and were sustained by a combination of normal
monsoon rains, cyclonic storms, and local storm fronts. All of these channels transported
mud to the floodplain, low-lying floodplain lakes, and the extensive and shallow Tecovas
lakes.
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VITA
Personal Grayson Hayworth Lamb Background Prosper, Texas Son of Mike and Desiree Lamb
Education Diploma, Prosper Highschool, Prosper, Texas, 2013 Bachelor of Science, Geology, University of Arkansas, 2017
Experience Research Assistant, University of Arkansas 2014- 2016 Geology Intern, EOG Resources, Inc. 2017 Geology Teaching Assistant, Texas Christian University, 2017-2019 Geology Intern, Occidental Petroleum Corporation, 2018
Professional American Association of Petroleum Geologists, Memberships Fort Worth Geological Society Sigma Gamma Epsilon
ABSTRACT
AN ARCHITECTURAL ANALYSIS AND DEPOSITIONAL INTERPRETATION OF THE DOCKUM GROUP IN THE WEST TEXAS HIGH PLAINS
by Grayson Hayworth Lamb, M.S., 2019 Department of Geological Sciences Texas Christian University
Thesis Advisor: Dr. John M. Holbrook, Professor of Geology
The Triassic Dockum Group of the west Texas high plains represents the Permian-
Triassic boundary. This group has been extensively studied paleontologically but lacks a detailed sedimentological overview.
Six lithofacies assemblages were identified across three formations: upperflow regime channels, upperflow regime sheets, perennial channel belts and bars, floodplain, paleosol and lacustrine deposits. The assemblages consist of unique lithofacies bedforms. These bedforms were previously unidentified in the Dockum Group. Fluvial architectural analysis was conducted on three channel assemblages in order to understand the fill patterns.
The depositional patterns of each assemblage were driven by a megamonsoon climate that perpetuated throughout the Triassic. This study aims to clarify the previous lithofacies interpretations and discuss the climatic implications that ultimately controlled the Dockum
Group depositional environments.