Sequence Stratigraphy of Comanchean Cretaceous ...

Sequence Stratigraphy of Comanchean Cretaceous Outcrop Strata of Northeast and South-Central Texas: Implications for Enhanced Petroleum Exploration Ernest A. Mancini1 and Robert W. Scott2 1

Center for Sedimentary Basin Studies and Department of Geological Sciences, Box 870338, University of Alabama, Tuscaloosa, AL 35487 2

RR 3 Box 103-3, Cleveland, OK 74020

ABSTRACT An integrated sequence stratigraphic classification of the Comanchean Series, Cretaceous strata in the Gulf Coast is developed for northeast and south-central Texas by using the eastern Gulf Coast scheme of Mancini and Puckett and the western Gulf Coast scheme of Scott et al.. New outcrop biostratigraphic data are integrated with lithostratigraphic data to identify four Aptian-Albian to lower Cenomanian, second-order, transgressive-regressive (T-R) sequences in Texas. The lower T-R K1 sequence consists of the Sycamore, Hammett and Cow Creek formations. The base of this sequence in Texas is defined by the unconformity (Ap SB PR1 of Scott et al.) between the Paleozoic basement and the Sycamore Sandstone. The T-R K2 sequence includes the Hensel, lower Glen Rose, and upper Glen Rose units and the lower beds of the Paluxy Formation. The base of this sequence is marked by the Cow Creek-Hensel unconformity (Ap SB PR2 of Scott et al.). The Hensel and lower Glen Rose comprise a lower, higher-order sequence, and the upper Glen Rose and lower Paluxy constitute an upper, higher-order sequence. These higher-order sequences are separated by a disconformity (Al SB GR2 of Scott et al.) a few meters below the base of the Corbula bed. The T-R K3 sequence consists of the upper beds of the Paluxy and the Walnut, Goodland (Comanche Peak and Edwards), and Georgetown Limestone or lower part of the Washita Group. The base of this sequence is defined by an intraformational unconformity (Al SB FR1 of Scott et al.) in the Paluxy Formation. The upper part of the Paluxy, Walnut and Goodland comprise a lower, higher-order sequence, and the Georgetown Limestone and lower part of the Washita represent an upper, higher-order sequence. These higher-order sequences are separated by a disconformity (Al SB WA1 of Scott et al.) at the base of the Georgetown or Kiamichi Formation. The T-R K4 sequence includes the upper Washita Grayson/Del Rio and Buda formations. The base of this sequence is defined by the Georgetown/Main Street-Grayson/Del Rio unconformity (Ce SB1.1, Al SB WA6 of Scott et al.). The top of this sequence is marked by the regional unconformity (Ce SB3 of Scott et al.) that separates the Comanchean Series (upper Washita beds at its top) from the Gulfian Series (Woodbine or Lake Waco/ Pepper Shale at its base). This unconformity correlates with the mid-Cenomanian un-

Mancini, E. A., and R. W. Scott, 2006, Sequence stratigraphy of Comanchean Cretaceous outcrop strata of northeast and south-central Texas: Implications for enhanced petroleum exploration: Gulf Coast Association of Geological Societies Transactions, v. 56, p. 539-550.

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conformity in the Gulf of Mexico. These T-R sequences can be correlated from northeast to south-central Texas and from the western to eastern Gulf. Because the stratal architecture of these sequences delineates potential hydrocarbon reservoir facies, this classification has implications for developing enhanced petroleum exploration strategies for Comanchean Cretaceous strata in the Gulf Coast.

INTRODUCTION The stratigraphy and biostratigraphy of the Comanchean Series strata as observed from surface exposures in Texas have been studied extensively by numerous workers. The purpose of this paper is to build on these published works by using the evolving concept of chronostratigraphic correlation through an integrated biostratigraphic and sequence stratigraphic approach in order to further the understanding of the Comanchean Cretaceous section in the Gulf Coast (Fig. 1) and to enhance petroleum exploration for potential Comanchean Cretaceous hydrocarbon reservoirs. These objectives are achieved by characterizing the physical surfaces dividing these strata and by characterizing the nature of the lithologic boundaries of these units as observed in outcrops from northeast to south-central Texas. The transgressive-regressive cycle scheme published by Mancini and Puckett (2002) for Lower Cretaceous strata of the eastern Gulf Coast and the sequence stratigraphic classification of Scott et al. (2000, 2003) for the Comanchean Cretaceous section for the western Gulf Coast are used to construct the stratigraphic framework for this study.

LITHOSTRATIGRAPHY Field work included study of four transects (Fig. 2) across the outcrop belt in northeast to south-central Texas: Trinity River, Leon River, Colorado River, and Guadalupe-Blanco River (Fig. 3). The vertical distribution of macrofossils was observed and recorded in the field. Field samples for calcareous microfossil and palynomorph analyses were collected and analyzed. Field observations are as follows: Sycamore Sandstone is an alluvial and fluvial, poorly sorted conglomerate and coarse-grained sandstone that unconformably overlies Paleozoic rocks (Ap SB PR1 of Scott et al., 2000) (Fig. 1). The Sycamore-Hammett contact is sharp and irregular representing transgression (Fig. 4A). The Hammett Shale is a marine shale and wackestone. The Hammett-Cow Creek contact is gradational and consists of interbedded shale and wackestone (Fig. 4B). This contact represents the change from transgression to progradation. The Cow Creek Limestone is a beach to shoal carbonate unit consisting chiefly of fossiliferous grainstone with packstone and wackestone and containing quartz grains. The grainstone and packstone facies are potentially high-quality reservoirs. The Cow Creek-Hensel contact is unconformable, sharp and irregular (Scott, 1993; Yurewicz et al., 1993). It was designated as Ap SB PR2 by Scott et al. (2000). The Hensel Sandstone is an alluvial-fluvial and estuarine coarse-grained sandstone, shale, and sandy limestone containing pebbles at the base of the unit (Fig. 5A). The Hensel-Glen Rose contact is gradational and consists of interbedded shale and limestone representing marine transgression (Fig. 5B). The Glen Rose Limestone comprises mainly limestone, dolostone and thin interbeds of marlstone and calcareous shale, and overlies the Hensel Sandstone and underlies the Fredericksburg Group (Stricklin et al., 1971). The Glen Rose has been divided into informal lower and upper members separated by the top of the regionallypersistent Corbula marker bed (Stricklin et al., 1971). The Corbula bed consists of one or more resistant, brown, iron-stained limestone ledges about 1 ft (0.3 to 0.5 m) thick. The lower Glen Rose member (informal term) includes cycles grading from intertidal to tidal lime mudstone and marlstone to rudist and coral boundstone and fossiliferous packstone and grainstone. Biostromes are well developed in this unit. The rudist boundstone and fossiliferous grainstone facies are potentially high-quality reservoirs. In the uppermost part of the lower Glen Rose a few meters below the Corbula bed is a bored hardground contact that represents a regional sequence boundary designated as Al SB GR2 by Scott et al. (2000) (Fig. 4C). The Corbula bed consists of a fossiliferous packstone to grainstone rich in Corbula martinae (Fig. 4D).

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Sequence Stratigraphy of Comanchean Cretaceous Outcrop Strata of Northeast and South-Central Texas

Figure 1. Sequence stratigraphy of Comanchean Cretaceous strata of the western Gulf Coast of Scott et al. (2000) and of Lower Cretaceous stratigraphy of the eastern Gulf Coast of Mancini and Puckett (2002) (modified after Mancini and Puckett, 2002, their Figures 2 and 4).

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Figure 2. Location map of measured sections in Texas: (1) Trinity River section, (2) Leon River section, (3) Colorado River section, and (4) Guadalupe-Blanco River section. The upper Glen Rose member consists of shallowing-upward cycles grading from subtidal to intertidal and supratidal fossiliferous lime mudstone and wackestone to miliolid and peloidal grainstone and packstone. Rudist biostromes are localized. The grainstone facies are potentially high-quality reservoirs. The Glen Rose-Walnut (Bull Creek) contact consists of interbedded sandstone, limestone and dolostone (Fig. 5C). This contact is sharp and represents the change from a restricted tidal flat regime to a transgressing marine lagoonal regime. It is considered to be a regional sequence boundary (Scott, 1993; Yurewicz et al., 1993) and was designated as Al SB FR1 by Scott et al. (2000). The Paluxy Formation consists of lower and upper members separated by an unconformable surface marked by a soil horizon that is considered to be a sequence boundary (Al SB FR1) separating the Trinity and Fredericksburg groups (Scott et al., 2003). The lower member includes marginal marine-to-marine, fossiliferous, very fine to fine-grained sandstone, siltstone and claystone. The upper member includes unfossiliferous, bioturbated friable quartz sandstone, siltstone and claystone interbedded with fossiliferous grainstone. The Paluxy-Walnut contact is gradational and consists of interbedded sandstone and limestone (Fig. 5D). The Walnut Formation includes the Bull Creek Limestone Member, Bee Cave Marl Member, Cedar Park Limestone Member, Whitestone Limestone Member, Keys Valley Marl Member, and upper marl member. The Bull Creek Limestone, Cedar Park Limestone and Whitestone Limestone members consist of fossiliferous wackestone and fossiliferous, peloidal, ooid grainstone and packstone. Hardgrounds are common in these units. These grainstone and packstone facies are potentially high-quality reservoirs. The Bee Cave Marl, Keys Valley Marl, and upper marl members consist of fossiliferous marlstone, calcareous claystone, lime mudstone, and wackestone. These units comprise a series of higher-order cycles. The Walnut-Goodland (Comanche Peak, Edwards) contact is gradational and consists of interbedded marl and limestone.

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Figure 3. Correlation of measured sections from northeast to south-central Texas using sequence stratigraphic analysis.


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Figure 4. Outcrop photographs: (A) Contact between the Sycamore Sandstone and Hammett Shale, Route 3238, Travis County, Texas; (B) Contact between the Hammett Shale and Cow Creek Limestone, Hamilton Creek, Travis County, Texas; (C) Hardground near the base of the Glen Rose Limestone, Blanco River, Blanco County, Texas; and (D) Contact between the lower and upper members of the Glen Rose Limestone, Blanco River, Blanco County, Texas.

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Figure 5. Outcrop photographs: (A) Hensel Sandstone, Route 1471, Burnet County, Texas; (B) Contact between the Hensel Sandstone and Glen Rose Limestone, Route 1471, Travis County, Texas; (C) Contact between the Glen Rose Limestone and the Bull Creek Limestone Member of the Walnut Formation, Route 360, Travis County, Texas; and (D) Contact between the Paluxy Formation and Walnut Formation, Highways 80/180, Parker County, Texas.


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The Goodland Formation includes fossiliferous marlstone and wackestone. The Comanche Peak Limestone includes fossiliferous marlstone and wackestone, packstone, and grainstone. The Edwards Limestone consists of fossiliferous (rudist) wackestone, packstone and grainstone. The Edwards-Georgetown contact is sharp and irregular (Fig. 6A). This surface is a hardground in places and usually is iron stained and bored (Scott, 1993; Scott et al., 2003). This is a sequence boundary and was designated Al SB WA1 by Scott et al. (2000). The Georgetown Limestone in central Texas grades north to the Trinity River at Fort Worth into the Kiamichi, Duck Creek, Fort Worth, Denton, Weno, Pawpaw, and Main Street formations. These units comprise a series of higher-order, shale-limestone cycles (Scott et al., 2000, 2003). The shale facies are calcareous and the limestone facies are fossiliferous wackestone to grainstone. The grainstone and packstone facies are potential reservoirs. An unconformable surface, which is irregular and iron-stained, occurs at the top of the Main Street (Fig. 6B) and separates the limestone and marlstone beds of the Main Street from the claystone and shale beds of the Grayson/Del Rio Formation (Fig. 6C). It is marked by a pebble bed and approaches a hardground in composition. This surface represents an unconformity (Scott et al., 2003) and was designated Ce SB1.1 (Al SB WA6) by Scott et al. (2000). In northeast Texas, the upper Washita Group includes the Buda Limestone, which rests conformably on the Grayson Formation and is overlain unconformably by the Woodbine Formation and Lake Waco/Pepper Shale. This unconformity (Ce SB3 of Scott et al., 2000) is marked by an irregular surface characterized by the presence of reworked fossils and pebbles (Fig. 6D).

BIOSTRATIGRAPHY Samples collected were analyzed for calcareous microfossils and palynomorphs. The Grayson-Del Rio contains a typical lower Cenomanian foraminiferal assemblage. The Main Street Limestone contains a typical upper Albian foraminiferal assemblage for this area: a low-diversity fauna dominated by Favusella washitensis, Hedbergella delrioensis and Gavelinella plummerae. The palynomorph data provided the following results for age assignments of the units studied: Pepper Shale (middle Cenomanian), Grayson-Del Rio (lower Cenomanian), Georgetown/Main Street (upper Albian), Edwards (upper Albian), Comanche Peak (middle/upper Albian), Walnut (middle Albian), Glen Rose (lower Albian), and Hammett (Aptian). Planktonic foraminifers are important in identifying Cretaceous stages (Premoli Silva and Sliter, 2002). However, because Gulf Coast Lower Cretaceous shelf facies were generally too shallow for planktonic foraminifers, they are rare and not diverse. Also the first occurrences (FO) are truncated by incompatible facies in the Glen Rose and Paluxy formations. The ranges of key taxa in Gulf Coast formations are compared with ranges in Premoli Silva and Sliter (2002): Favusella washitensis, basal Glen Rose Limestone-Buda Limestone (upper Aptian to lower Cenomanian); Ticinella primula, basal Fredericksburg Group-upper Georgetown Limestone (lower-upper Albian); Rotalipora appenninica, uppermost Georgetown Limestone (upper Albian-middle Cenomanian); and Rotalipora evoluta = Rotalipora brotzeni, Grayson Formation (lower-middle Cenomanian). The complete ranges of many benthic foraminifers have not yet been delineated in the Gulf Coast nor are their global ranges fully known. However, a few taxa have diagnostic ranges and can be identified in both washed samples and thin section. Some useful species were not found in samples of this study, but are useful in offshore carbonate facies (Scott, 2002): Chofattella decipiens, Sligo Formation (Hauterivian-lower Aptian); Orbitolina texana, basal to lower part of upper Glen Rose Limestone (upper Aptian-lower Albian); Paracoskinolina sunnilandensis, Rodessa Limestone (upper Aptian-lower Albian); Dictyoconus walnutensis, Fredericksburg Group (middle-lower upper Albian); Coskinolinoides texanus, Fredericksburg Group-Washita Group (middle-upper Albian); Barkerina barkerensis, Fredericksburg Group-Washita Group (middle-upper Albian); Cribratina texanus, middle-upper Washita Group (upper Albian); Paracoskinolina coogani, middle-upper Washita Group (upper Albian); and Streptalveolina mexicana, middle-upper Washita Group (upper Albian).

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Figure 6. Outcrop photographs: (A) Contact between the Edwards Limestone and Georgetown Limestone, Highway 36, Bell County, Texas; (B) Hardground near the top of the Main Street Limestone Member of the Georgetown Limestone, White Rock Creek, Hill County, Texas; (C) Contact between the Main Street Limestone and the Grayson Formation, White Rock Creek, Hill County, Texas; and (D) Contact between the Buda Limestone and the Woodbine Formation, Bolo Point, Denton County, Texas.


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Marine dinoflagellates are quite abundant and diverse in the Lower Cretaceous outcrop units. Several species have short ranges and became extinct in the early Cenomanian, such as Ovoidinium verrucosum and Ovoidinium scabrosum. Several first appeared in the middle to late Albian: Apteodinium grande, Epilidosphaeridium spinosa, Litosphaeridium siphoniphorum, Ovoidinium verrucosum, and Xiphophoridium alatum. Nannofossils are also uncommon in the updip Lower Cretaceous shelf deposits, and many of the base ranges are truncated by inhospitable facies. Nannofossil zones are generally defined by the first and last appearances of taxa. The FO of Axopodorhabdus albianus defines the base of the upper middle Albian zone IC36, and its last occurrence (LO) is at the top of the Cenomanian zone IC50. In the Texas outcrop, the species ranges through the Washita Group. The FO of Eiffellithus turriseiffelii is the base of the middle upper Albian zone IC39 in the middle of the Washita Group. The FO of Gartnerago nanum defines the base of upper Albian zone IC41, and in Texas this species spans the contact between the Main Street Limestone and Grayson/Del Rio Formation. Ammonites and bivalves provide the basic correlation of Lower Cretaceous strata cropping out in Texas with the European stages (Young, 1986; Scott et al., 2003). These were identified in the field. Important new finds of Mortoniceras sp. cf. inflatum were found in the Main Street Limestone in the Leon River valley.

SEQUENCE STRATIGRAPHY Three major T-R cycles (sequences) of 5 to 19 million years in duration comprise the Lower Cretaceous subsurface section in the onshore eastern Gulf Coast (Mancini and Puckett, 2002) (Fig. 1). The upper Valanginian to upper Aptian T-R sequence (T-R K1) consists of the Hosston, Sligo, Pine Island, and Donovan/James formations. The Hosston and Sligo formations constitute a lower, higher-order sequence in the T-R K1 sequence, and the Pine Island and Donovan/James comprise an upper, higher-order sequence. The upper Aptian to middle Albian T-R sequence (T-R K2) consists of the Bexar, Rodessa, Ferry Lake, Mooringsport, and Paluxy formations. The Bexar and Rodessa constitute a lower, higher-order sequence in the T-R K2 sequence, and the Ferry Lake, Mooringsport and Paluxy comprise an upper higher-order sequence. The middle to upper Albian T-R sequence (T-R K3) consists of the Andrew and Dantzler formations. In the offshore area of the eastern Gulf of Mexico, a fourth T-R sequence (T-R K4) was recognized in upper Albian to lower Cenomanian strata of the Washita Group. In using the sequences identified in strata of the eastern Gulf Coast, comparative sequences in the western Gulf Coast can be identified. The lower T-R K1 sequence, which includes two higher-order sequences in the eastern Gulf Coast, consists of the Sycamore, Hammett, and Cow Creek formations. The T-R K2 sequence consists of the Hensel, lower Glen Rose, upper Glen Rose, and lower part of the Paluxy units. The Hensel and lower Glen Rose formations represent a lower, higher-order sequence in the T-R K2 sequence, and the upper Glen Rose and lower part of the Paluxy constitute an upper, higher-order sequence. The lower Glen Rose includes significant rudist mound and coral biostrome development. The upper T-R K3 sequence consists of the upper part of the Paluxy, Walnut, Goodland (Comanche Peak, Edwards), and Georgetown or lower part of the Washita Group. The upper part of the Paluxy, the Walnut, and the Goodland (Comanche Peak, Edwards) formations constitute a lower, higher-order sequence within the T-R K3 sequence, and the Georgetown Limestone or lower part of the Washita Group represents an upper, higher-order sequence. This upper, higher-order sequence includes numerous parasequences. The T-R K4 sequence consists of the upper part of the Washita Group (Del Rio/Grayson and Buda formations). Field evidence for this interpretation is as follows: 1) T-R K1 sequence, the base of the sequence is defined by the unconformity (Ap SB PR1 of Scott et al., 2000) at the base of the Sycamore Sandstone. 2) T-R K2 sequence, the base of the sequence is defined by the unconformity (Ap SB PR2 of Scott et al., 2000) at the base of the Hensel Sandstone. 3) T-R K2 upper higher-order sequence, the base of the sequence is defined by the unconformity (Al SB GR2 of Scott et al., 2000) below the Corbula bed and associated lower strata. 4) T-R K3 sequence, the base of the sequence is defined by the intraformational unconformity (A1 SB FR1 of Scott et al., 2000) in the Paluxy Formation.

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5) T-R K3 upper higher-order sequence, the base of the sequence is defined by the unconformity (Al SB WA1 of Scott et al., 2000) at the base of the Georgetown Limestone and Kiamichi Formation. 6) T-R K3 sequence, the top of the sequence is defined by the intraformational unconformity (Ce SB1.1 or Al SB WA6 of Scott et al., 2000) near/in the top of the Georgetown Limestone and Main Street Limestone. 7) T-R K4 sequence, top of the sequence is defined by the unconformity at the base of the Woodbine Formation or Lake Waco/Pepper Shale. This unconformity (Ce SB3 of Scott et al., 2000) is known as the midCenomanian Unconformity (MCU) in seismic sections of the Gulf of Mexico. In using the sequences identified in the strata of the western Gulf Coast, Comanchean Cretaceous measured sections in northeast Texas can be correlated with higher resolution to the sections measured in south-central Texas (Fig. 3). Also, the Comanchean Cretaceous section as observed in outcrop in the western Gulf can be correlated with higher resolution to the Lower Cretaceous section of the subsurface in the eastern Gulf (Fig. 1).

POTENTIAL RESERVIORS From the study of outcrops of Comanchean Cretaceous strata of the western Gulf Coast and from the study of these strata in the subsurface of the eastern Gulf Coast, stratigraphic intervals observed to have potential to be quality reservoirs in shelfal areas include: grainstone and rudist reef beds of the Sligo Formation, grainstone, rudstone and rudist beds of the Cow Creek Limestone (James Limestone), rudist reef and mound beds of the lower Glen Rose member (Rodessa Formation), grainstone and rudist beds of the upper Glen Rose member (Mooringsport Formation), grainstone and rudist beds of the Walnut and Goodland/Comanche Peak and Edwards formations (Andrew Formation and Fredericksburg Group) and Washita Group; and fluvial to shallow-water sandstone beds of the Hosston, Donovan, Paluxy and Dantzler formations and Fredericksburg Group. Stratigraphic intervals that have potential to be quality reservoirs in the slope areas include deep-water sandstone beds of the Hosston and Paluxy formations and rudist reef front and debris deposits of the Sligo Formation, Cow Creek Limestone/James Limestone, lower Glen Rose member/Rodessa Formation, upper Glen Rose member/ Mooringsport Formation, Comanche Peak Limestone and Edwards Limestone/Andrew Formation (Fredericksburg Group), and Washita Group.

CONCLUSIONS The study of outcrops of the Lower Cretaceous strata of the western Gulf Coastal Plain has assisted in better defining the T-R sequences observed in strata across the Gulf Coastal Plain. Stratigraphic intervals observed to have potential to be quality reservoirs in shelfal areas include: grainstone, rudstone, and rudist beds and fluvial to shallow-water sandstone beds. Stratigraphic intervals that have potential to be quality reservoirs in slope areas include deep-water sandstone beds and rudist reef front and debris deposits.

ACKNOWLEDGMENTS This research was funded, in part, by the U.S. Minerals Management Service and the U.S. Department of Energy.

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Premoli Silva, I., and W. V. Sliter, 2002, Practical manual of Cretaceous planktonic foraminifera: Dipartimento di Scienze della Terra, University of Perugia, Perugia, Italy, 462 p. Scott, R. W., 1993, Cretaceous carbonate platform, U.S. Gulf Coast, in J. A. T. Simo, R. W. Scott, and J.-P. Masse, eds., Cretaceous carbonate platforms: American Association of Petroleum Geologists Memoir 56, p. 97-109. Scott, R. W., W. Schlager, B. Fouke, and S. A. Nederbragt, 2000, Are mid-Cretaceous eustatic events recorded in Middle East carbonate platforms?, in A. S. Alsharhan, and R. W. Scott, eds., Middle East models of Jurassic/ Cretaceous carbonate cystems: Society of Economic Paleontologists and Mineralogists Special Publication 69, p. 73-84. Scott, R. W., 2002, Upper Albian benthic foraminifers new in west Texas: Journal of Foraminiferal Research, v. 32, p. 43-50. Scott, R. W., D. G. Benson, R. W. Morin, B. L. Shaffer, and F. E. Oboh-Ikuenobe, 2003, Integrated Albian - lower Cenomanian chronostratigraphy standard, Trinity River section, Texas, in R. W. Scott, ed., Perkins memorial volume: U.S. Gulf Coast Cretaceous stratigraphy and paleoecology: Gulf Coast Section of the Society of Economic Paleontologists and Mineralogists Foundation Special Publications in Geology 1, p. 277–334. Stricklin, F. L., Jr., C. I. Smith, and F. E. Lozo, 1971, Stratigraphy of Lower Cretaceous Trinity deposits of central Texas: Texas Bureau of Economic Geology Report of Investigations No. 71, 63 p. Young, K., 1986, Cretaceous marine inundations of the San Marcos Platform, Texas: Cretaceous Research, v. 7, p. 117-140. Yuwericz, D. A., T. B. Marler, K. A. Meyerholtz, and F. X. Siroky, 1993, Early Cretaceous carbonate platform, north rim of the Gulf of Mexico, Mississippi and Louisiana, in J. A. T. Simo, R. W. Scott, and J.-P. Masse, eds., Cretaceous carbonate platforms: American Association of Petroleum Geologists Memoir 56, p. 81-96.

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