High-resolution stratigraphy and facies ... - GeoScienceWorld

High-resolution stratigraphy and facies architecture of the Upper Cretaceous (Cenomanian–Turonian) Eagle Ford Group, Central Texas Michael D. Fairbanks, Stephen C. Ruppel, and Harry Rowe

ABSTRACT Rock-based studies of the Eagle Ford Group of Central Texas demonstrate that mudrock deposition is more complicated than previously supposed. X-ray diffraction, x-ray fluorescence, total organic carbon (TOC), and log data collected from eight cores and two outcrops demonstrate that bottom-current reworking and planktonic productivity are primary depositional controls, acting independently from eustatic forcing. Central Texas Eagle Ford facies include (1) massive argillaceous mudrock, (2) massive foraminiferal calcareous mudrock, (3) laminated calcareous foraminiferal lime mudstone, (4) laminated foraminiferal wackestone, (5) cross-laminated foraminiferal packstone–grainstone, (6) massive bentonitic claystone, and (7) nodular foraminiferal packstone–grainstone. High degrees of lateral facies variability, characterized by pinching and swelling of units, lateral facies changes, truncations, and locally restricted units, are observed even at small lateral scales (50 ft [15 m]). At 10 mi (16 km) and greater lateral spacings, core and geochemical data significantly underestimate intraformational facies variability. Approximately 73% of units can be successfully correlated across a distance of 500 ft (152 m), 35% are traceable across 1 mi (1.6 km), and only 16% of beds are correlative across 10 mi (16 km). Geochemical proxies (enrichment in molybdenum and other trace elements) indicate that maximum anoxia occurred within the Bouldin Member despite being composed of the most calcareous and high-energy facies. Comparison of total gamma ray (GR) logs to computed GR logs is requisite, because GR alone may provide misleading determination of

Copyright ©2016. The American Association of Petroleum Geologists. All rights reserved. Manuscript received October 8, 2014; provisional acceptance January 7, 2015; revised manuscript received August 10, 2015; final acceptance December 7, 2015. DOI:10.1306/12071514187

AAPG Bulletin, v. 100, no. 3 (March 2016), pp. 379–403

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AUTHORS Michael D. Fairbanks ~ Apache Corporation, 2000 Post Oak Boulevard, Suite 100, Houston, Texas 77056-4400; michael. [email protected] Michael D. Fairbanks is a petroleum geologist for Apache Corporation. He graduated from The University of Texas at Austin in 2012 with an M.S. in geology, and he received his B.S. in geology from Brigham Young University, Utah, in 2010. His interests include sedimentological and depositional controls in unconventional reservoir systems and chemostratigraphy of source rock intervals. Stephen C. Ruppel ~ Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924; [email protected] Stephen C. Ruppel has over 30 years of experience in sedimentological and geochemical characterization of carbonate and mudrock reservoir systems. He holds a Ph.D. from the University of Tennessee as well as B.S. and M.S. degrees from the University of Illinois and the University of Florida, respectively. He has published more than 200 scientific papers and reports and is director of the Bureau’s Mudrocks Systems Research Laboratory. Harry Rowe ~ Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924; [email protected] Harry Rowe is a geologist at the Bureau of Economic Geology, Austin, Texas. His main interest is in developing chemostratigraphic records of paleoceanographic change. ACKNOWLEDGMENTS This paper comprises parts of the thesis of Michael D. Fairbanks at The University of Texas at Austin, under the direction of Stephen C. Ruppel and William L. Fisher. Funding for this project came from the industry members of the Mudrock Systems Research Laboratory Consortium at the Bureau of Economic Geology. Industry members include Anadarko, Apache, BP, Chesapeake, Chevron, Cima, Cimarex, ConocoPhilips, Cypress, Devon, Encana, EOG,

EXCO, Husky, Marathon, Pangaea, Penn Virginia, Penn West, Pioneer, Shell, StatOil, Texas American Resources, The Unconventionals, US EnerCorp, Valence, and YPF. The Jackson School of Geosciences and the Bureau of Economic Geology provided the opportunity, resources, and facilities for the study. Thanks go to Timothy Kearns, Brett Huffman, Robert Nikirk, and Jessica Buckles for their assistance in acquisition and analysis of x-ray fluorescence data while at The University of Texas at Arlington, to Necip Guven of The University of Texas at San Antonio, and to Henry Francis of the Kentucky Geological Survey, Lexington, for integrated analysis of x-ray diffraction data. Thanks also go to Ryan Harbor, who laid the ´ foundation, and Greg Frebourg and Bob Loucks, whose continual insights contributed to the progression of this work.

facies, TOC content, depositional environment, and sequence stratigraphic implications.

INTRODUCTION The Upper Cretaceous Eagle Ford Group has long been recognized as an important source rock for productive reservoirs throughout Texas. Heightened industry focus on the Eagle Ford is a result of recent discoveries of producible unconventional petroleum resources in this emerging play. However, little has been published on the facies and facies heterogeneities within this mixed-carbonate–clastic mudrock system. A rock-based study is fundamental to understanding the controls, types, and scales of inherent heterogeneities, which have implications for enhanced comprehension of the Eagle Ford Group and other mixedcarbonate–clastic mudrock depositional systems worldwide. This study uses a unique data set of densely distributed cores to constrain the controls on inherent facies heterogeneities and variabilities, which helps to answer the following questions regarding mudrock systems: What is the continuity of units expressed by wireline log response? What causes variable production response, and how can production be optimized across large play areas? What controls the distribution of reservoir properties? The primary objectives of this study are to (1) define Eagle Ford lithofacies present in the Austin, Texas, area; (2) determine lithofacies continuity on various scales, along with the processes that control intraformational heterogeneities; (3) explore the effectiveness of elemental abundance data in identifying lithological variations within the Eagle Ford system; (4) use geochemical data to refine lithofacies interpretations; and (5) evaluate the applicability of total gamma ray (GR) logs in defining facies and rock properties. The eight cores and two outcrops of this rare data set span a nearly 11 mi (18 km) transect, with seven of the cores located within a 2 mi (3 km) area, providing a uniquely high-resolution perspective for mudrock stratigraphy and facies continuity (Figure 1). Energy-dispersive x-ray fluorescence (XRF), x-ray diffraction (XRD), Rock-Eval total organic carbon (TOC), and thin-section synthesis further enhance this study.

REGIONAL GEOLOGY The southwesternmost expression of the Grenville orogeny is the 1.15-1 Ga Llano uplift in Central Texas, resulting in vast exposures of deformed crystalline basement, consisting of gneisses, schists, and metavolcanic rocks (Culotta et al., 1992). An extension 380

Eagle Ford Stratigraphy and Facies Architecture

Figure 1. Location map of the study area showing cores and outcrops in the Austin, Texas, area and cores used for regional correlation. The Eagle Ford outcrop belt spans the Texas–Mexico border, passes through the study area, and continues across the Texas–Oklahoma border and into Arkansas. Eagle Ford outcrop belt adapted from Barnes (1992). 204 = BT-204; 221 = BT-221 core; 222 = BT-222-PTPZ core; 301 = BO-301-PTPZ core; 302 = BO-302-PT core; 500 = BI-500-PT core; 514 = BI-514-PTPZ core; ACC = ACC 1 core; Blumberg = J. W. Blumberg 1-B core; Brechtel = W. Brechtel 1 core; Hendershot = C. J. Hendershot 1 core; Orts = H. P. Orts 2 core; Schauer = F. T. Schauer et al. 1 core; SH = state highway; St = street. Fairbanks et al.

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Figure 2. Paleogeographic map of Texas at the Cenomanian–Turonian time boundary. The study area lies on the northeastern flank of the San Marcos arch, which separates the Maverick Basin from the East Texas Basin. The Sligo and Stuart City shelf margins form a physiographic break toward the downdip extent of the Eagle Ford. Modified from Ruppel et al. (2012).

of the uplift, the San Marcos arch, trends southeast– northwest (Dravis, 1980; Young, 1986) and, in the Early Cretaceous, was a minor topographic high (Tyler and Ambrose, 1986; Figure 2) where a large carbonate platform developed (Donovan and Staerker, 2010). During most of the Cretaceous, the San Marcos arch Figure 3. Generalized cross section through the Cretaceous Gulf of Mexico Basin. The Eagle Ford overlies thick progradational packages of the Lower Cretaceous Comanche shelf. Modified from Ruppel et al. (2012).

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was a low-lying subaerial terrain, receiving and supplying little sediment, similar to modern-day Florida (Young, 1986). The study location in Austin lies on the northeastern flank of the San Marcos arch (Figure 2). Thick progradational carbonate packages comprising the Comanche shelf developed during the

Table 1. Description of Data Set, Identifying Cores, and Outcrop Locations Austin Area Cores* Core Number

Name in Study

Eagle Ford Depths (ft [m])

County

BT-222-PTPZ BI-500-PT BI-514-PTPZ BT-204 BO-302-PT BO-301-PTPZ BT-221 ACC 1

222 500 514 204 302 301 221 ACC

51–88 (16–27) 55–94 (17–29) 52–89 (16–27) 80–116 (24–35) 52–90 (16–27) 51–88 (16–27) 52–88 (16–27) 80–125 (24–38)

Travis Travis Travis Travis Travis Travis Travis Travis

Regional Cored Wells Wells C. J. Hendershot 1 W. Brechtel 1 H. P. Orts 2 F. T. Schauer et al. 1 J. W. Blumberg 1-B Burkland 1

Name in Study Hendershot Brechtel Orts Schauer Blumberg Burkland

Eagle Ford Depths (ft [m]) 4734–4774 3280–3315 7684–7757 8093–8159 4175–4225 935–977

(1443–1455) (1000–1010) (2342–2364) (2467–2487) (1273–1288) (285–298)

County

API

Operator

Caldwell Wilson Gonzales Gonzales Wilson Caldwell

4217730218 4249330208 4217730203 4217730394 4218730532 4205534144

Tesoro Petroleum Prairie Producing Co. Transocean Oil, Inc. Geological Res Corp. Prairie Producing Co. Vista Energy Corp.

*Outcrop: West Bouldin Creek, approximate coordinates: N30°1599.88860 and W97°45941.31660; Walnut Creek, approximate coordinates: N30°24928.25940 and W97°42931.79520.

Early Cretaceous (Figure 3). During the Hauterivian Stage of the Early Cretaceous, the Sligo shelf margin developed (Salvador and Muñeton, 1989), forming a raised rim shelf margin profile (Galloway, 2008; Figure 3). Flooding during the Aptian Stage initiated a landward shift in reef development, forming the Stuart City shelf margin (Salvador and Muñeton, 1989; Phelps, 2011). Until the Cenomanian, rudist–coral communities acted as constructers and formed baffles to normal wave action, which formed the nearly continuous carbonate reef of the Stuart City rimming the Gulf of Mexico (Sohl et al., 1991; Scott, 2010; Figure 2). A major transgression of the Comanchean shelf in the middle-to-late Cenomanian initiated the deposition of the Eagle Ford Group (Sohl et al., 1991; Phelps, 2011; Figure 3). To the northeast, sedimentation in the Woodbine delta system resulted in deposition of the coeval Tuscaloosa and Woodbine Formations in a fluvial and deltaic setting (Sohl et al., 1991; Dubiel et al., 2010), as well as incised-valley-fill, nearshore marine, and wave-dominated-delta deposits (Hentz et al., 2014). During the Cretaceous, much of the riverborne detritus included illite (Pratt, 1984),

which contributed a substantial clay mineral component within the Eagle Ford facies. Figure 2 summarizes the paleogeographic setting near the Cenomanian–Turonian boundary. Eagle Ford deposition was immediately followed by Coniacian– Santonian Austin Chalk deposition and subsequently the rest of the Upper Cretaceous Series strata (Hentz and Ruppel, 2010; Hentz et al., 2014; Figure 3).

STUDY AREA In Central Texas, the Eagle Ford outcrop belt passes through Travis County and directly through the city of Austin (Figure 1). An outcrop located along West Bouldin Creek forms the type locality for the Bouldin Member (Adkins and Lozo, 1951; Jiang, 1989) and has been described in detail by previous authors (Feray and Young, 1949; Adkins and Lozo, 1951; Pessagno, 1969; Young, 1977; Liro et al., 1994; Lundquist, 2000). Another outcrop in north Austin is encountered along a cutbank of Walnut Creek, south of Park Bend Road (Figure 1). Fairbanks et al.

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Located between the West Bouldin Creek and Walnut Creek outcrops, 11 cores comprising the entire Eagle Ford interval were also used in this study (Figure 1). One core, ACC 1, was recovered during the development of an Edwards aquifer monitoring well in north Austin. Also, 10 additional geotechnical cores form a tightly spaced, 1-mi (1.6-km)-long transect along Waller Creek in Austin (Figure 1). To obtain a regional perspective and evaluate the regional correlatability of the Eagle Ford in the Austin study, this work includes cored subsurface wells, which were described by Harbor (2011; Figures 1, 2). Two cross-section lines span the San Marcos arch, one extending to the southwest from Austin across the arch, and the other extending to the south along the dip of the arch (Figure 1). The subsurface Eagle Ford extends in a dip section from the shallow surface and outcrop belt to the Late Cretaceous shelf margin, with depths reaching 14,000 ft (4267 m) (Harbor, 2011). Table 1 summarizes the data set used in the current study and the cores studied by Harbor (2011).

METHODS AND DATA Handheld energy-dispersive XRF is quickly becoming an industry standard for acquiring inorganic geochemical data from cores and cuttings of mudrock intervals. Elemental calibrations involved reference materials from five internationally accepted standards, SDO-1, SGR-10, SCo-1, GBW-0717, and SARM-41, along with 86 reference materials from diverse mudrock systems, including the Woodford, Smithwick, Barnett, Eagle Ford, and Ohio Shale (Kearns, 2011; Rowe et al., 2012). All analyses were undertaken using a Bruker AXS Tracer III-V XRF unit, equipped with an Rh x-ray tube. Major element analyses were conducted at 15 kV, with the beam and detector under vacuum. Trace element analyses were conducted at 40 kV, with an Al–Ti–Cu beam filter. Core samples for XRF analysis were taken at a 1 ft (31 cm) interval and were analyzed by placing the flat slab side down on the nose of the instrument. Raw sample XRF spectra were calibrated using proprietary Bruker software (Rowe et al., 2012). Dominant mineral abundances are quickly determined from major element composition (e.g., calcium (Ca), aluminum (Al), and silicon (Si) serve 384


as proxies for calcite, clay minerals, and quartz, respectively) to aid in facies definition. Trace elements (e.g., molybdenum (Mo), uranium (U), vanadium (V)) help to evaluate levels of oxygenation during deposition (Tribovillard et al., 2006). Elemental abundances of U, thorium (Th), and potassium (K) can be used to create the equivalent of a total GR for purposes of comparing core data to borehole log data through the formula ðU8Þ + ðTh4Þ + ðK16Þ = GR where U is the uranium content (ppm), Th is the thorium content (ppm), and K is the weight percent of potassium (Doveton and Merriam, 2004). An equivalent computed GR (CGR) log can be calculated by omitting the U, which ensures that the signal is representative of the clay mineral content and reflects lithological changes. To replicate the appearance of downhole logs, discrete XRF data points are smoothed and plotted. Samples for thin-section, organic geochemistry, and XRD analysis were removed from the intact core and prepared and processed by National Petrographic Services, GeoMark Research Ltd., and Necip Guven, respectively. Sample locations were selected based on representative facies identification and distributed throughout the cores. A thorough discussion of the methods involved in acquiring these data are provided by Harbor (2011) and Fairbanks (2012).

RESULTS Facies Although deceptively homogenous at the core-face scale because of their grain size and dark color, mudrocks are extremely heterogeneous; this has important implications for depositional environment and sediment delivery processes. The following seven distinctive facies are identified within the Eagle Ford group: (1) massive argillaceous mudrock, (2) massive foraminiferal calcareous mudrock, (3) laminated calcareous foraminiferal lime mudstone, (4) laminated foraminiferal wackestone, (5) cross-laminated foraminiferal packstone–grainstone, (6) massive bentonitic claystone, and (7) nodular foraminiferal packstone–grainstone. A summary of Eagle

Fairbanks et al.

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Massive, planar and cross-laminated

84% calcite, 13% clay minerals, 2% quartz

69% smectite, 12% kaolinite, 8% illite

Globigerinid foraminifera, inoceramid, and pelecypod fragments

Light-to-medium gray

Whitish gray to medium rust orange

Rare globigerinid foraminifera

Massive

Massive bentonitic claystone

Nodular foraminiferal packstone– grainstone

Unnamed unit, Bouldin Member

Light-to-medium gray


86% calcite, 5% quartz, 3% clay minerals

Thin subplanar and cross-laminations, scours

Cross-laminated foraminiferal packstone– grainstone

Bouldin Member, unnamed unit

Primarily Bouldin Member

0.5–1 ft (15–30 cm)

Bouldin Member

Medium gray with white-to–light-gray bands



Very thin planar–subplanar laminations, slump folds, fine ripple laminations, scours

Laminated foraminiferal wackestone

1–5 ft (0.3–1.5 m)

Unnamed unit, South Bosque Formation

Medium gray



Very thin planar laminations, scours

Laminated calcareous foraminiferal lime mudstone

1–6 in. (2–15 cm)

0.5–6 in. (1–15 cm)

0.5–6 in. (1–15 cm)

1–5 ft (0.3–1.5 m)

Unnamed unit, South Bosque Formation

Medium-dark gray

53% clay minerals, 32% calcite, 17% quartz

4–6 ft (1.2–1.8 m)

Thickness



Massive

Massive foraminiferal calcareous mudrock

Occurrence (Formation) Pepper Shale

Color Dark gray to medium-dark gray

Dominant Fauna Rare globigerinid foraminifera

Massive

Massive argillaceous mudrock

Mineralogy

Sedimentary Structures

Facies Name

Table 2. Summary Table of Central Texas Eagle Ford Facies and Associated Attributes

Figure 4. Composite stratigraphic chart reviewing nomenclature used in the Central Texas Eagle Ford intervals, compared with those used in West Texas and near Waco, Texas. This study adopts the names of Pepper Shale, unnamed unit of the Lake Waco Formation, Bouldin Member of the Lake Waco Formation, and South Bosque Formation.

Ford facies and their associated attributes is found in Table 2.

Massive Argillaceous Mudrock Facies Restricted to the basal Pepper Shale of the Eagle Ford Group (Figure 4), this facies is dark-to–medium-dark gray in core (Figure 5), weathers recessively in outcrop, and displays a distinctively smooth or soapy texture (Table 2). It splits into thin sheets along planar as well as irregular partings, with common ammonite impressions in the partings. The mineral composition, as revealed by XRD and XRF data, is primarily clay minerals (average 53%, range 32%–68%), which in decreasing abundance include illite, kaolinite, illite and smectite mixed, and smectite. Calcite and dolomite constitute an average of 32% (range 3%–60%). Although not apparent 386


from the scope of this study, previous studies have shown that much of the fine silt and clay-sized carbonate in Eagle Ford rocks is composed of coccolith debris. The depositional environment of this facies is interpreted to have been in anoxic marine conditions below storm weather wave base, as suggested by the paucity of benthonic fauna, bioturbation, and wavegenerated structures.

Massive Foraminiferal Calcareous Mudrock Facies Massive foraminiferal calcareous mudrock is the most common facies in the Eagle Ford succession in Central Texas, and it is found in the unnamed member as well as the South Bosque Formation (Figure 4). Medium-to-dark gray in core, this facies is dominantly structureless and weathers recessively (Figure 5). Compared with

(A)

(C)

(B)

(D)

the massive argillaceous mudrock facies, this facies is coarser grained and more calcareous, and it contains a higher quantity of planktonic (forams) and benthonic (inoceramid and bivalve fragments) fauna (Table 2). The XRF and XRD data indicate that this facies is composed of carbonate (average 50%, range 36%–68%) and clay minerals (average 39%, range 21%–67%). The dark-gray color, presence of low-oxygen benthonic fauna, and lack of sedimentary structures

Figure 5. Core photographs, thin-section photomicrographs, and summaries of characteristic features of massive argillaceous mudrock facies (A and B) and massive foraminiferal calcareous mudrock facies (C and D). (A) Slab photograph showing the massive, dark-gray fissile character. (B) Thin-section photomicrograph depicting the fine-grained nature and rare globigerinid foraminifera. (C) Slab photo showing the massive, medium-dark-gray character. (D) Thin-section photomicrograph showing fine-grained nature and abundant globigerinid foraminifera, inoceramid, and bivalve shells.

suggest that the depositional environment was in oxygen-poor marine conditions below storm weather wave base.

Laminated Calcareous Foraminiferal Lime Mudstone Facies This facies is composed of very thinly planar-tosubplanar laminated medium-dark gray (Figure 6), calcareous mudrock-to-lime mudstone that is present in the unnamed unit and the South Bosque Formation (Figure 4). Laminations are primarily composed of Fairbanks et al.

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grain-sized globigerinid foraminifera, with lesser amounts of inoceramid and other bivalve fragments. Calcite and dolomite content from XRD analysis average 62% (range 57%–73%), whereas clay mineral content measures an average of 24% (range 18%–31%) (Table 2). These rocks are interpreted to have been deposited below storm weather wave base where suspension settling supplied planktonic components and bottom-current-winnowing–created planar laminations.

Figure 6. Core photographs, thin-section photomicrographs, and summaries of characteristic features of laminated calcareous foraminiferal lime mudstone facies (A–C) and laminated foraminiferal wackestone (D and E). (A) Slab photograph showing the laminated, medium-gray character. (B) Thin-section photo showing fine lamination and abundant globigerinid foraminifera. (C) Thin-section photomicrograph showing erosional scouring. (D) Slab photograph showing the fine ripple laminations and mediumgray character. (E) Thin-section photomicrograph showing disturbed bedding, abundant globigerinid foraminifera, and inoceramid fragments.

(A)

(D)

(B)

(E)

(C)

388

Laminated Foraminiferal Wackestone Facies Restricted to the Bouldin Member (Figure 4), this facies is characterized by higher calcite (average 76%, range 75%–80%) and abundant globigerinid foraminifera compared with previously described facies (Figure 6; Table 2). Although generally a wackestone, these rocks locally grade into foraminiferal packstones or grainstones. Based on the high degree of current-induced structures (very thin planar and subplanar laminations, fine ripple laminations,


(A)

(C)

(D) (B)

scours) and slump folds, this facies is interpreted to have been deposited below storm weather wave base on a gradually dipping seafloor with bottom-current reworking.

Cross-Laminated Foraminiferal Packstone–Grainstone Facies Highly calcareous (XRD average 86%, range 82%–97% calcite) (Table 2), this facies is encountered solely in the Bouldin Member (Figure 4) and contains

Figure 7. Core photographs, thin-section photomicrographs, and summaries of characteristic features of cross-laminated foraminiferal packstone–grainstone (A and B) and massive bentonitic claystone facies (C and D). (A) Slab photograph showing the cross-laminated and light-gray nature. (B) Thin-section photomicrograph showing the abundant globigerinid foraminifera, inoceramid fragments, bioclasts, pyrite, and grain-rich texture. (C) Slab photograph showing the structureless, poorly lithified nature. (D) Thin-section photomicrograph showing rare globigerinid foraminifera and fine-grained texture.

mostly fine sand-sized globigerinid foraminifera tests, as well as highly abraded inoceramid and other bivalve fragments (Figure 7). Cross-laminated sedimentary structures and pervasive scouring suggest that this facies records the highest energy of deposition within the study area. Based on sedimentary structures and paleoredox proxies from XRF, the depositional environment of this facies is interpreted to have been an oxygen-poor sediment–water interface in a marine basin where Fairbanks et al.

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bottom-current reworking was the prominent sediment dispersal mechanism and sediment supply was accelerated by stimulated organic productivity in the photic zone.

Massive Bentonitic Claystone Facies In cores and outcrop, thin (0.5–6 in. [1–15 cm]) bentonitic claystone beds display a characteristically recessive and poorly lithified nature (Figure 7; Table 2), are structureless, and contain only trace foraminifera. The XRD analysis reveals that clay minerals, dominated by smectite and kaolinite, are their dominant constituent, averaging 91% (range 88%–93%). Figure 8. Core photographs, thin-section photomicrographs, and summary of characteristic features of nodular foraminiferal packstone–grainstone (A–D). (A and B) Slab photographs depicting characteristic expression in core, where (A) displays inclined bedding at the terminus of a nodule, and (B) displays a gradational contact. (C) Outcrop photograph depicting the oblate ellipsoid geometry (12-in. [31-cm] hammer for scale). (D) Thinsection photomicrograph showing abundant globigerinid foraminifera and cemented nature.

(A)

These rocks are present primarily in the Bouldin Member (Figure 4), with few occurrences in the unnamed unit and South Bosque Formation. Several studies have interpreted these rocks as volcanic ash deposits (Jiang 1989; Liro et al., 1994; Dawson 1997) potentially sourced from northern Mexico and the Balcones igneous province (Pierce, 2014).

Nodular Foraminiferal Packstone–Grainstone Facies Resistant in outcrop, this facies comprises horizontal lenses that exhibit oblate ellipsoidal cross sections and differential compaction in adjacent mudstones

(B)

(C)

(D)

390


(Figure 8). The primary allochem within this facies is globigerinid foraminifera (Table 2). This facies differs from the cross-laminated foraminiferal packstone–grainstone facies largely in that it is laterally discontinuous in outcrop and even at the core scale. In core, it is difficult to differentiate between this facies and cross-laminated foraminiferal packstone– grainstone facies. These rocks are most abundant in the Bouldin Member, but they are also present ´ in the unnamed unit (Figure 4). Frebourg et al. (2016) proposed that similar nodular foraminiferal packstone–grainstones in West Texas were deposited by bottom currents as sediment waves and dunes and that they represent reworked accumulations of planktonic debris.

Stratigraphy In Central Texas, the Eagle Ford Group is typically divided into two or three units (Figure 4). A recent study (Fairbanks, 2012) has proposed that the section can be subdivided into four units: a basal Pepper Shale, an unnamed unit that has been informally referred to as the Waller member (Fairbanks, 2012; Denne and Breyer, in press), an overlying Bouldin Member, and an uppermost South Bosque Formation (Figure 4). The Pepper Shale (Figure 4) is a recessive, dark gray, argillaceous claystone, comprised solely of the massive argillaceous mudrock facies (Figure 6; Table 2). This interval is relatively thin; it is 4–6 ft (0.6–1.0 m) thick in the study area (Figure 4). This highly siliceous and argillaceous unit is interpreted by recent authors as distal deltaic and prodelta facies (Hentz et al., 2014), suggesting time equivalence to the Woodbine section (Adkins and Lozo, 1951; Childs et al., 1988; Dawson et al., 1993) facies. Overlying the Pepper Shale is an unnamed member of the Lake Waco Formation (Figure 4). Visually, the basal contact of this unit is gradational and subtle. The contact is marked by the transition to a gritty calcareous mudrock composed of massive foraminiferal calcareous mudrock and laminated calcareous foraminiferal lime mudstone (Table 2). Minor amounts of cross-laminated foraminiferal packstone–grainstone, nodular foraminiferal packstone– grainstone, and massive argillaceous mudrock are also found within this unnamed member (Table 2). This unit is poorly exposed in outcrop, and the full succession is observed only in core,

totaling 10 ft (3 m) of thickness in the ACC 1 core (Table 1). The Bouldin Member of the Lake Waco Formation (Figure 4) averages 10–12 ft (3.5–4.0 m) of thickness in the study area. The Bouldin is more calcite rich than other Central Texas Eagle Ford Units (Figure 4), the calcite being primarily composed of planktonic foraminifera, which is consistent with the findings of recent investigators (Denne et al., 2014). Dominant facies include cross-laminated foraminiferal packstones–grainstones, laminated foraminiferal wackestones, massive bentonitic claystones, and nodular foraminiferal packstones–grainstones (Table 2). A particularly interesting aspect of the bentonitic claystone facies is their strong association with the cross-laminated foraminiferal packstone–grainstones and nodular foraminiferal packstones–grainstones. Except for a few isolated beds in the underlying unnamed unit and the overlying South Bosque Formation, bentonitic claystones are largely confined to the Bouldin Member. Also confined primarily to the Bouldin Member are the laminated foraminiferal wackestones and the cross-laminated foraminiferal packstone– grainstone facies (Table 2). The strong association between abundant planktonic foraminifera and bentonite claystones suggests that these deposits are genetically related. Recent studies have connected surface water nutrient enrichment to volcanic eruptions (Duggen et al., 2007, 2010; Jones and Gislason, 2008; Langmann et al., 2010). It is proposed that nutrients introduced into the ocean water system by volcanic eruptions led to higher productivity within the oxic zone and stimulated planktonic blooms that led to higher rates of carbonate sedimentation. The contact between the Bouldin Member and the underlying argillaceous foraminiferal mudrock of the unnamed unit is picked as the base of the first bed of cross-laminated foraminiferal packstone– grainstone facies. The upper contact is defined by the last occurrence of foraminiferal packstone– grainstone. The South Bosque Formation is composed primarily of medium-dark–to-medium gray silty mudrocks and includes beds of massive foraminiferal calcareous mudrock and laminated calcareous foraminiferal lime mudstone (Table 2). This unit, which forms the top of the Eagle Ford Group in the study area, averages 16 ft (5 m) in thickness. Fairbanks et al.

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Figure 9. North–south lithostratigraphic cross section of the study area. The vertical columns are described cores and outcrops with color shading representing facies continuity. Note the degree of facies and thickness variability within each stratigraphic unit. Location of wells and outcrops is provided in Figure 1. ACC = ACC 1 core; Fm = formation; Mbr = member.

Cyclicity and Facies Continuity This study uses a unique data set that provides an excellent insight into variations in lateral facies continuity. The 10 cores and outcrops in the Austin study area (Figure 1) constitute an approximately 11 mi, north–south transect. The measured sections from these cores and outcrops allow facies continuity to be evaluated at several scales, ranging from 50 ft (15 m) 392


to 10 mi (16 km) (Figure 9). Correlation between cores was conducted based on facies characteristics and associations, as defined previously in the Facies and Stratigraphy sections. High degrees of facies discontinuity are observed within each stratigraphic interval, even in close spacings (50 ft [15 m]). For example, a nodular foraminiferal packstone–grainstone bed in core 514 at 75 ft (23 m), or 14 ft (4 m) above the top of the Buda, is

not present 50 ft (15 m) to the northwest in core 500 (Figure 9). Likewise, a massive bentonitic claystone bed at 79 ft (24 m), or 14 ft (4 m) above the top of the Buda, in core 500 is not present in core 514 (Figure 9). Additionally, two repetitive beds of massive argillaceous mudrock observed in core 204 at 109 ft (33 m), or 7 ft (2 m) above the top of the Buda, are not represented 500 ft (150 m) to the south in core 302 (Figure 9). Facies continuity decreases substantially with distance. For example, 73% of units can be successfully correlated across a distance of 500 ft (152 m), 35% are traceable across 1 mi (1.6 km), and only 16% of beds are correlatable across 10 mi (16 km) (Figure 9). At spacings of 10 mi (16 km) and greater, bed-scale correlations based on inorganic geochemical data (XRD, XRF), organic geochemical data (RockEval TOC), and well-log data (GR) are suspect. It is possible to correlate and define the boundaries of the four Eagle Ford stratigraphic units (Pepper Shale, unnamed unit, Bouldin Member, and South Bosque Formation) with reasonable confidence through the study area (Figure 9). Cyclicity of facies is readily observed within the Eagle Ford, with the highest degrees of alternating units contained within the Bouldin Member of the Lake Waco Formation. Beds ranging in thickness from 1 in. (2.5 cm) to 1 ft (30 cm) alternate between cross-laminated foraminiferal packstone–grainstone and laminated foraminiferal wackestone, with local massive bentonitic claystones (Figure 9; Table 2). Several individual cycles can be laterally traced at scales less than 1 mi (1.6 km), like those between the Walnut Creek outcrop and the ACC core at approximately 95–99 ft (29–30 m), or 26–30 ft (8–9 m) above the top of the Buda (Figure 9).

Chemostratigraphic Analysis Energy-dispersive XRF analysis provided elemental composition data that were used to evaluate the inorganic geochemistry of the Eagle Ford system. Variations in elemental composition have implications for ocean water chemistry as well as depositional setting. Certain trace metals are used as proxies for bottom water anoxia because they are redox sensitive and relatively immobile in the sediment, thus preserving primary signals (Algeo and Rowe, 2012). One such

trace metal, Mo, has been widely used as a proxy for benthonic redox potential because of its generally strong enrichment in organic-rich marine facies deposited under oxygen-depleted, sulfide-rich conditions (Algeo and Lyons, 2006; Tribovillard et al., 2006; Algeo and Tribovillard, 2009). In anoxic marine systems, Mo shows significant variations from normal oceanic conditions. For example, normal open-oceanic conditions contain 80%–100% of the Mo concentration in the Saanich Inlet of British Colombia, Canada; 70%–80% in the Framvaren Fjord of Norway; and just 3%–5% in the Black Sea (Algeo and Rowe, 2012). Other redox-sensitive trace elements include U, V, Mn, and chromium (Cr), which some have linked to changing oxygen conditions during the Cenomanian and Turonian (Tribovillard et al., 2006; Smith and Malicse, 2010; Algeo and Rowe, 2012). In this study, Mo, U, V, Mn, and Cr were all assessed as geochemical paleoredox proxies. Of these trace elements, Mo is selected as a representative proxy for anoxic bottom water conditions. Major excursions and enrichments in these key trace elements occur in the Bouldin Member (Figure 10), suggesting a period of maximum bottom water anoxia during this time. The Mo values, for example, reached 50 ppm within the Bouldin Member, compared with the typical 5–10 ppm throughout the other members (Figure 10). Another small enrichment in the Mo curve is observed within the upper part of the unnamed unit, suggesting another interval of anoxia (Figure 10).

Regional Facies Architecture: Regional Lithostratigraphic Correlation The four-component stratigraphy described in the previous section comprises the major depositional successions of the Eagle Ford Group in Central Texas (Figure 4). In the deeper subsurface where these members have not been defined, the Eagle Ford is typically divided into informal upper and lower units. However, it is possible to correlate the Central Texas stratigraphy into the deeper subsurface by comparing facies defined in the current study to those in the San Marcos arch area defined by Harbor (2011) (see Table 1). This comparison (Figure 11) illustrates that the Pepper Shale, the Lake Waco Formation (unnamed unit and Bouldin Fairbanks et al.

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Figure 10. Composite log and type section for the ACC 1 core showing the borehole gamma ray (GR) log compared with the computed gamma ray (CGR) (potassium-thorium) from x-ray fluorescence (XRF) data, the identified facies from core description, a calcium curve from XRF data suggesting mineralogy, a molybdenum curve from XRF data as a paleoredox proxy, and a percent total organic carbon (% TOC) curve as measured from core. Note that the greatest paleoredox enrichment occurs within the Bouldin Member of the Lake Waco Formation, which is associated with elevated Ca and high energy facies, and not with greatest enrichment of % TOC, which might be expected in the unnamed unit.

Members), and the South Bosque Formation can all be defined regionally in cored wells in the subsurface. Three cores, Hendershot, Orts, and Schauer, are used in conjunction with the ACC core in Austin, Texas, to provide a north–south transect of nearly 70 mi (128 km) (Figure 11). All stratigraphic intervals experience a southward downdip thickening along the axis of the arch. The Pepper Shale thickens from 5 ft (1.5 m) in Austin to approximately 12 ft (3.6 m) in the Schauer core (Figure 11), at which point it undergoes a facies change from massive argillaceous mudstone to laminated calcareous foraminiferal lime mudstone toward the shelf margin (Figures 2, 11). The unnamed unit also displays thickening southward along the arch, from 10 ft (3 m) near Austin to 16 ft (5 m) in the Schauer core (Figure 11). The Bouldin Member not only thickens southward from approximately 13 ft (4 m) (ACC core) to approximately 25 ft (7.5 m) (Schauer core) (Figure 11), but also it displays a general facies change. In the Austin study area, there is a higher ratio of high-energy cross-laminated foraminiferal packstone– 394


grainstone and laminated foraminiferal wackestone relative to lower-energy mudrocks. Southward, that ratio diminishes, such that finer, low-energy mudrock facies become more abundant in the Schauer core (Figure 11). South Bosque lithologies remain fairly unchanged from Austin southward. The Schauer core (Figure 11) reveals that facies of the South Bosque, massive (and laminated) argillaceous foraminiferal mudrock (which are also facies of the unnamed unit), become the dominant facies of the Eagle Ford Group. The general thickening of individual units and the entire Eagle Ford Group toward the south is probably associated with differential accommodation around the San Marcos arch. Distal locations likely experienced greater subsidence than the arch did (Figure 11). The facies change observed in the Bouldin Member unit from high planktonic sediment concentration facies in Austin to the argillaceous and foraminiferal mudrock facies toward the Schauer core (Figure 11) is interpreted to result from a change in planktonic productivity. Oxic zone productivity likely

Figure 11. Cross section AA9 based on lithostratigraphy. This subsurface correlation is based on facies observed in core. The C. J. Hendershot 1, H. P. Orts 2, and F. T. Schauer et al. 1 cores were described in detail by Harbor (2011). The section shows a general continuity of stratigraphic intervals along the San Marcos arch and thickening southward. Location of transect is provided in Figure 1. The uppermost unit observed by Harbor (2011) refers to a transitional Eagle Ford–Austin Chalk facies. Modified from Harbor (2011). ACC = ACC 1 core.

decreased to the south, based on the relative lack of planktonic debris in the Schauer core. Harbor (2011) described a transitional Eagle Ford–Austin Chalk interval that is not represented in Austin but overlies the South Bosque Formation in the subsurface (Figure 11). This unit contains disrupted bedded foraminiferal packstone and adjacent cross-laminated foraminiferal packstone–grainstone (Harbor, 2011) (Figure 11). The lack of this unit in the Austin study area suggests the presence of an unconformity at the top of the Eagle Ford, marked by a period of erosion. Alternatively, this unit observed by Harbor (2011) could also represent a facies change from massive foraminiferal calcareous mudrock in the ACC core to disrupted bedded foraminiferal packstone and cross-laminated foraminiferal packstone–grainstone in the Hendershot and Orts cores.

Use of Gamma Ray Logs for Correlation Core–Log Relationships in Study Area Because of the lack of conventional borehole GR logs for the cores in the study, pseudo GR logs were constructed from elemental abundances of Th, U, and K gathered from XRF. Both CGR, which combines the Th and K responses, and total GR, which combines the Th, K, and U responses, were generated (Table 3). Recent studies have shown that spectral GR logs are critical for the calibration and application of GR logs to identifying mineralogy and facies in mudrocks (Rowe and Ruppel, 2013). Because of the abundant K in the minerals, CGR logs are good indicators of clay mineralogy abundance in mudrocks. However, GR logs provide a more complex indication of rock properties, because they combine the mineral response of K in clay minerals with an Fairbanks et al.

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Table 3. Data for the ACC 1 Core, Containing Depth of Sample, Potassium Percentage, Uranium Parts per Million, and Thorium Parts per Million from X-Ray Fluorescence, As Well As Measured Percent Total Organic Carbon Depth (ft) Depth (m) K (%) U (ppm) Th (ppm) TOC (%) Depth (ft) Depth (m) K (%) U (ppm) Th (ppm) TOC (%) 70.4 71.1 71.5 72.0 72.4 72.9 73.4 74.1 74.7 75.5 75.8 76.3 77.0 77.7 78.3 78.8 79.7 80.1 80.5 80.9 81.4 81.9 82.4 82.9 83.5 84.1 84.5 85.6 86.1 86.6 87.5 88.6 89.5 90.4 91.5 92.4 92.8 93.4 94.5 95.5 96.3 96.6

21.46 21.67 21.79 21.95 22.07 22.22 22.37 22.59 22.77 23.01 23.10 23.26 23.47 23.68 23.87 24.02 24.29 24.41 24.54 24.66 24.81 24.96 25.12 25.27 25.45 25.63 25.76 26.09 26.24 26.40 26.67 27.01 27.28 27.55 27.89 28.16 28.29 28.47 28.80 29.11 29.35 29.44

0.545 0.469 0.417 1.548 1.372 1.389 0.433 0.583 0.542 0.382 0.318 0.380 0.783 0.673 0.852 0.520 1.091 1.538 1.866 1.834 2.063 2.133 1.612 1.593 1.711 1.522 1.420 1.627 1.695 1.589 1.650 1.336 1.266 1.405 1.141 1.675 1.614 1.803 1.762 0.433 1.163 1.268

-12 -12 -13 -3 0 1 -14 -12 -11 -16 -11 -9 -8 -12 -4 -14 -5 0 4 0 2 3 1 1 7 0 0 -2 -1 -7 4 2 -2 0 2 9 -2 2 1 -13 0 2

3 2 2 3 4 5 3 3 3 2 2 4 3 3 4 2 4 3 5 3 5 5 5 6 6 5 5 4 4 6 4 3 3 6 5 5 4 4 5 1 4 4

n/a n/a n/a n/a n/a n/a n/a n/a 0.22 n/a n/a 0.15 n/a n/a 0.60 n/a n/a n/a 2.13 n/a n/a n/a 1.34 n/a n/a n/a 2.81 n/a n/a 3.16 n/a 3.75 n/a 3.62 n/a 2.05 n/a n/a 4.34 n/a n/a 0.31

97.5 98.4 98.6 99.4 99.8 100.3 101.2 101.8 102.6 103.5 103.8 104.7 105.5 105.8 106.2 106.5 106.9 107.4 107.9 108.8 109.4 110.1 110.9 111.5 112.4 113.0 113.7 114.3 115.5 116.5 117.3 118.7 119.5 120.5 121.3 122.5 123.6 124.3 124.8 125.2 126.3 127.7

29.72 29.99 30.05 30.30 30.42 30.57 30.85 31.03 31.27 31.55 31.64 31.91 32.16 32.25 32.37 32.46 32.58 32.74 32.89 33.16 33.35 33.56 33.80 33.99 34.26 34.44 34.66 34.84 35.20 35.51 35.75 36.18 36.42 36.73 36.97 37.34 37.67 37.89 38.04 38.16 38.50 38.92

Abbreviations: K = potassium; n/a = no data available; Th = thorium; TOC = total organic carbon; U = uranium.

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0.665 0.148 1.407 0.331 1.278 1.460 0.195 1.118 1.210 1.245 0.147 1.267 1.229 0.718 1.715 1.318 0.280 1.336 0.225 0.564 1.466 1.599 0.969 1.400 1.549 0.234 1.847 1.398 0.981 1.440 1.512 1.449 1.449 1.901 1.813 1.887 1.308 1.212 1.740 0.355 0.098 0.069

3 -18 7 -11 9 -3 -14 -1 1 1 -12 -4 0 7 -1 1 -11 -1 -14 -6 11 11 3 2 9 -14 9 6 -10 16 7 9 15 15 7 7 9 14 12 -13 -20 -14

4 1 1 1 4 4 3 3 3 3 1 4 2 2 2 4 1 3 3 4 3 7 2 6 5 2 5 4 5 6 6 6 13 13 15 15 11 11 15 3 2 0

1.48 2.48 0.24 3.29 0.28 n/a 0.71 4.63 4.56 4.51 0.58 4.07 5.40 n/a n/a 6.48 1.11 3.89 n/a 0.32 6.47 n/a 8.91 7.16 5.37 1.20 7.99 0.88 6.29 5.20 5.92 1.00 1.31 1.20 1.20 1.47 3.96 2.33 n/a 0.20 0.13 0.22

indication of bottom water redox conditions from the U response. Examination of the Central Texas Eagle Ford Group illustrates differences in spectral GR response that can be related to the four-component stratigraphy defined in the ACC 1 core (Figure 10). The basal Pepper Shale is typically defined by a high CGR and GR response, especially compared with the underlying Buda Limestone (Figure 10). The high CGR and GR values exhibited by the Pepper Shale are a function of the massive (and laminated) calcareous foraminiferal lime mudstone facies, which contain abundant illite, kaolinite, and smectite (Figure 10). The overlying unnamed unit is characterized by lower CGR and GR responses caused by higher carbonate and lower clay mineral abundances, compared with the Pepper Shale (Figure 10). The similarity between the values of the CGR and the GR indicates that little U is present in the massiveto-laminated calcareous foraminiferal lime mudstone of the unnamed unit. The Bouldin Member is unique among the Eagle Ford units in that the CGR and GR logs from its rocks display a significant separation (Figure 10). The low CGR values indicate low clay mineral content. This is consistent with the higher carbonate content in the high-energy, highly calcareous, laminated foraminiferal wackestone–packstone and cross-laminated foraminiferal packstone–grainstone facies that constitute this unit. The high frequency oscillations in both CGR and GR curves reflect the cyclic alternations in these two facies. The separation of the CGR and GR in the Bouldin identifies an abundance of U in these rocks (Figure 10), which is reflected in the high total GR. The implications of the high U concentrations are considerable when trying to use total GR logs to interpret or correlate facies. Note that, based solely on a total GR log, most workers would interpret the very high GR response in this interval as an indication that the rocks are higher in clay mineral content than any other part of the Eagle Ford. Instead, the Bouldin contains the highest carbonate and lowest clay mineral concentrations of the entire Eagle Ford (Figure 10). Like the unnamed unit, the South Bosque Formation displays similar low values for both the CGR and GR, suggesting low U content (Figure 10). Like the unnamed unit, the dominant facies are massive

foraminiferal calcareous mudrock and laminated calcareous foraminiferal lime mudstone. The CGR values for both of these units are higher than those of the Bouldin, indicating higher clay mineral abundance (Figure 10).

Regional Log-Based Correlations Comparison of GR log trends in the ACC cored well with other subsurface wells extending to the south across the San Marcos arch suggests that general correlations of facies may be possible (Figure 12). However, the GR logs available for these wells do not closely follow core-based lithological relationships (Figure 12). Based on these observations, it must be concluded that facies determination founded solely on total GR logs (K–Th–U) is liable to be misleading. As illustrated in Figure 10, a moderate total GR response in the ACC core corresponding to the unnamed unit that is followed by a high GR zone corresponding to the Bouldin Member could be interpreted as a carbonate-rich interval succeeded by a clay mineral– rich interval. However, core, CGR, and total GR response reveal the opposite facies relationships (Figure 10). Thus, GR logs alone are insufficient for facies determination in the Eagle Ford, and CGR logs must be used for accurate facies recognition. Because sequence stratigraphic interpretations hinge on accurate determination of facies and accommodation, correlations and facies designations based solely on total GR logs are likely to produce erroneous results. For example, although the high total GR of the Bouldin Member appears generally correlative, what, if anything, can be reliably interpreted about sediment supply, water depth, accommodation, or even mineralogy from the total GR log? Without knowledge of the concentration of and variations in U in the unit (made possible by comparison of CGR and total GR logs), none of these key factors that are necessary for stratigraphic correlation (using sequence concepts or not) can be reliably defined. Total Organic Carbon Analysis The Upper Cretaceous Eagle Ford is a proven source rock in Texas (Robison, 1997). Samples collected in the current study for source rock analysis vary in TOC stratigraphically (Figure 10; Table 3) but contain an average of 2.4% (range 0.1%–8.4%) TOC. At the base of the Eagle Ford, a dramatic increase in Fairbanks et al.

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Figure 12. Cross section BB9 based on gamma ray (GR) and pseudo computed GR (CGR) log (ACC core). The GR log signatures are somewhat recognizable to the southwest across the San Marcos arch. ACC = ACC 1 core; Fm = formation; Mbr = member.

TOC is observed in comparison with the underlying Buda Limestone (Figure 10). The Pepper Shale displays a general upward increase in TOC, with an average of 2.9% (range 1.7%–6.3%) TOC (Figure 10). The overlying unnamed unit displays a large range in organic enrichment but contains the highest overall average of 4.0% (range 0.1%–8.4%) TOC (Figure 10). In contrast, the Bouldin Formation is characterized by the lowest TOC, with an average of only 2.3% (range 0.4%–4.1%) TOC (Figure 10), a finding that is corroborated by Corbett and Watkins (2013), who observed sharp declines in TOC within the Bouldin. The overlying South Bosque Formation displays an average increase to 2.4% (range 0.2%–5.4%) TOC and then gradually decreases upward into the Austin Chalk (Figure 10).

DISCUSSION According to conventional wisdom in mudrock successions, an association exists between a restricted 398


basin environment and ideal conditions for organic matter preservation (Demaison and Moore, 1980). Recognition criteria for restricted basin conditions include the presence of black, organic rich shales containing high TOC; a lack of benthonic fauna; and enrichment in redox-sensitive trace elements. However, the correspondence of these characteristics is not observed in the Central Texas Eagle Ford succession, substantiating the recent findings that paleoceanographic changes controlling anoxia were more complex than previously suggested (Corbett and Watkins, 2013). Maximum TOC enrichment is observed in the unnamed unit (average TOC 4%, maximum 8.4%) (Figure 10). Facies within the unnamed unit dominantly consist of massive foraminiferal calcareous mudrock and laminated calcareous foraminiferal lime mudstone (Figure 10). The association between relatively low-energy facies and high TOC would suggest that this interval represents the most distal setting and the greatest degree of basin restriction. However, geochemical proxies for anoxia (enrichment in Mo, U, Mn, V, Cr) indicate that greatest bottom

water anoxia occurred during deposition of the Bouldin Member (Figure 10). Surprisingly, the Bouldin Member is composed of the highest energy grain-rich facies (Figures 7, 10). These grain-rich facies are not traditionally expected to form within restricted basin conditions. The Bouldin Member is also the interval characterized by the highest total GR and highest U concentrations but the lowest TOC (Figure 10). Hence, two unlikely associations are observed: (1) high GR signatures with corresponding calcareous highenergy facies, and (2) lowest TOC values corresponding to maximum basin restriction (Figure 10). Planktonic globigerinid foraminifera are the dominant grain type within the Bouldin Member. This sediment type was sourced from the photic zone and reflects surface water environmental conditions, not bottom water conditions. The high concentration of planktonic debris indicates accentuated productivity during this interval, providing high rates of carbonate sediment supply. As mentioned earlier, a strong association exists between the massive bentonitic claystone layers and calcareous foraminiferal units. In fact, the massive bentonitic claystones (interpreted to result from volcanic ash settling into marine waters) are generally restricted to the Bouldin Member. This close relationship suggests that heightened productivity in the oxic zone of the water column is stimulated by nutrient enrichment from volcanic input, and it drives sediment supply independent of eustatic forcing. Notwithstanding paleoredox conditions suggesting maximum bottom water anoxia and ideal conditions for organic preservation (Figure 10), TOC enrichment is lowest in the Bouldin Member, averaging 2.3% compared with a 4.0% average in the unnamed unit (Figure 10). The discrepancy between high preservation potential and low TOC during deposition of the Bouldin Member is interpreted to result from carbonate dilution. Frequent influx, higher associated sedimentation rates, and relatively rapid deposition of foraminiferal tests resulted in grain-rich carbonate facies within which very little TOC was preserved. The high proportion of carbonate beds to mudrock beds within the Bouldin Member resulted in low TOC values overall for this interval. Additionally, higher calcite content from the planktonic material caused the Bouldin Member to become more resistant to postdepositional compaction. Greater compaction in other Eagle Ford units allowed for differentially concentrated TOC

when compared with the Bouldin Member. Recent investigators (Ratcliffe et al., 2012) suggested that TOC can be estimated from paleoredox proxies, particularly enrichment in Mo. However, in the current study, the disparity between paleoredox indicators and low TOC in the Bouldin Member suggests that TOC estimation from inorganic chemistry data may provide misleading results. High-energy facies within the Bouldin Member are recognized in core and outcrop by crosslaminations, dune foreset laminations, low angle ripple laminations, planar-to-subplanar laminations, mud rip-up clasts, pebble and bioclastic lags, and scour and truncation surfaces. Facies variability is also most pronounced in the Bouldin Member, even at small scales (50 ft [15 m]). Bedding thickness variations, pinchouts, erosional scouring and truncations, and localized nodular facies are all recognized as facies variability in the Bouldin Member. These current-induced sedimentary structures and facies variabilities do not reflect shallow water processes but are interpreted to result from bottom-current reworking. Bottom currents, which are capable of transporting grains up to fine sand and even gravels ´ (Faugeres and Stow, 1993; Stow et al., 2002; ´ Frebourg et al., 2016), can be independent of eustatic control. Furthermore, many of the minor components, consisting of benthonic fauna (mainly inoceramids), live in dysoxic environments and can be transported into more anoxic settings, as supported by the fragmented nature of the grains. Although extremely calcareous and represented by the highest energy facies, the Bouldin Member is interpreted to have been deposited in a marine setting with substantial bottom water anoxia. This interpretation contrasts with classical sequence stratigraphic perspectives that interpret such high-energy, carbonate-rich deposits as forming proximal to an active carbonate platform. Expected characteristics of a deposit from such a shallow water setting would be increased carbonate sediment supply, high-energy facies from storm waves or turbidity currents, and decreased TOC resulting from sediment dilution and poor preservation in an oxygenated environment. The Bouldin Member displays these characteristics. However, the presence of current-induced sedimentary structures from deep water bottom-current reworking (Frébourg et al., 2016), high degrees of facies variability, and paleoredox proxies demonstrate that this Fairbanks et al.

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traditional model does not apply to the Central Texas Bouldin Member. Defining mineralogies and the facies they represent is not simple, because over-reliance on total GR logs (K–Th–U) can provide very misleading concepts of mineralogy and facies distribution. As observed in Figure 10, the unnamed unit and South Bosque Formation exhibit moderate total GR values, whereas the intervening Bouldin Member displays much higher values. Based solely on these GR patterns, the following interpretations could justifiably but erroneously be drawn: (1) the unnamed unit and South Bosque successions contain the highest levels of carbonate in the Eagle Ford and should contain low levels of TOC, and (2) the Bouldin Member contains lower levels of carbonate and higher amounts of clay minerals, suggesting that these rocks accumulated in a more distal, low-energy setting, where high levels of TOC would be expected. Core data show the relationships to be just the opposite. Even when mineralogy is well constrained (e.g., by spectral GR logs), the conditions under which these facies were deposited cannot be readily defined from logs. Conventional thinking regarding many carbonate-rich successions is that carbonate is sourced from shallow water settings and that high levels of carbonate indicate relatively shallow water sedimentation regardless of the setting. Abundant carbonate in the Eagle Ford, however, is the result of two processes not interpreted to be connected with water depth: nutrient driven productivity and bottomcurrent reworking. Accordingly, attempts to place Eagle Ford rocks into a simple sequence stratigraphic framework based on an assumed connection between mineralogy (i.e., carbonate content) and accommodation or water depth are of questionable merit. Alone, GR profiles are insufficient in the Eagle Ford for determination of (1) facies, (2) TOC content, (3) depositional environment, and (4) sequence stratigraphic correlations. Comparison of the GR curves with CGR curves provides Eagle Ford investigators with more accurate means for rock character determination. Primary controls on Eagle Ford character are (1) sediment supply from planktonic foraminifera and (2) depositional reworking by bottom-current activity. Globigerinid foraminifera comprise the dominant carbonate sediment type and originate in the photic zone of the water column. Changes in carbonate sedimentation rates are driven by surface 400


water nutrient enrichment, stimulated in part by volcanic activity. Bottom-current activity results from the interplay of thermohaline and wind-driven currents because of both Coriolis forces and seafloor topography (Stow et al., 2002; Frébourg et al., 2016). These two primary controls are processes that may be independent from eustatic fluctuation, rendering classical sequence stratigraphic applications unreliable in the Eagle Ford system of Central Texas.

SUMMARY AND CONCLUSIONS The Upper Cretaceous (Cenomanian–Turonian) Eagle Ford Group of Central Texas contains the following seven distinct facies: (1) massive argillaceous mudrock, (2) massive foraminiferal calcareous mudrock, (3) laminated calcareous foraminiferal lime mudstone, (4) laminated foraminiferal wackestone, (5) cross-laminated foraminiferal packstone–grainstone, (6) massive bentonitic claystone, and (7) nodular foraminiferal packstone–grainstone. These seven facies comprise four stratigraphic packages defined herein as the Pepper Shale, the unnamed unit of the Lake Waco Formation, the Bouldin Member of the Lake Waco Formation, and the South Bosque Formation. The basal Pepper Shale is an argillaceous, finegrained (clay-sized) unit. The unnamed unit, newly designated in this study, is an argillaceous and calcareous (foraminiferal), massive mudrock. The Bouldin Member is a high-energy, carbonate-rich (foraminiferal) interval with high degrees of facies variability. The uppermost unit is the South Bosque Formation, which, like the unnamed unit, is an argillaceous and calcareous (foraminiferal) massive and laminated mudrock. The Bouldin Member is defined by high API total GR values and low API CGR (GR K–Th) values. The disparity between total GR and CGR values is commensurate with U content. Enrichment in U, as well as in other paleoceanographic proxies (i.e., Mo, V, Cr), suggests that the Bouldin Member represents maximum basin restriction, despite being composed of the most carbonate-rich, highest energy facies. The comparison of total GR logs to CGR logs is requisite, because GR alone may provide misleading determination of facies, TOC content, depositional environment, and sequence stratigraphic implications.

High energy deposition within the Bouldin Member resulted from bottom-current activity. Carbonate content was controlled by heightened productivity in the oxic zone, which led to a higher sediment supply of planktonic foraminiferal skeletal debris. Both bottom-current reworking and planktonic sediment supply were controlling factors that were decoupled from eustatic sea-level fluctuations, thus rendering classical sequence stratigraphy unreliable in the Eagle Ford system. Even at a small lateral spacing (50 ft [15 m]), much variability of Eagle Ford facies is observed in cores and outcrops, and this is attributed to bottomcurrent reworking and planktonic productivity. Bottom-current reworking is responsible for erosional scouring, truncation, and localized distribution of facies. Planktonic productivity, possibly resulting from the nutrient enrichment of volcanic ash settling into marine waters, influenced sediment supply. Facies continuity decreases substantially with distance. For example, 73% of units can be successfully correlated across a distance of 500 ft (152 m), 35% are traceable across 1 mi (1.6 km), and only 16% of beds are correlatable across 10 mi (16 km). At spacings of 10 mi (16 km) and greater, bed scale correlations based on inorganic geochemical data (XRD, XRF), organic geochemical data (Rock-Eval TOC), and well-log data (GR) are suspect. However, it is possible at these distances to define and correlate the approximate boundaries of the four Eagle Ford stratigraphic units, at least in the study area. Contrary to conventional schemas, mudrock deposition is complex, involving the interplay of many controlling processes. Facies variability, often overlooked by stratigraphers and explorationists, is a significant aspect of the Eagle Ford system and has implications for source rock quality, seal capacity, reservoir characterization, and hydraulic fracture potential. Caution must be employed when evaluating mudrock systems, because nodular facies, ash beds, and other heterogeneities can dramatically influence correlatability.

REFERENCES CITED Adkins, W. S., and F. E. Lozo, Jr., 1951, Stratigraphy of the Woodbine and Eagle Ford, Waco area, Texas: Southern Methodist University Fondren Science Series 4, p. 105–161.

Algeo, T. J., and T. W. Lyons, 2006, Mo–total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions: Paleoceanography, v. 21, doi: 10.1029/2004PA001112. Algeo, T. J., and H. Rowe, 2012, Paleoceanographic applications of trace-metal concentration data: Chemical Geology, v. 324–325, p. 6–18, doi:10.1016 /j.chemgeo.2011.09.002. Algeo, T. J., and N. Tribovillard, 2009, Environmental analysis of paleoceanographic systems based on molybdenum-uranium covariation: Chemical Geology, v. 268, p. 211–225, doi: 10.1016/j.chemgeo.2009.09.001. Barnes, V. E., 1992, Geologic map of Texas: Austin, Texas, Bureau of Economic Geology, scale 1:500,000, 4 sheets, SM0003. Childs, O. E., G. Steele, and A. Salvador, 1988, Gulf Coast region, correlation of stratigraphic units of North America (COSUNA) project: AAPG, oversize chart, 20 map sheets. Corbett, J. M., and D. K. Watkins, 2013, Calcareous nannofossil paleoecology of the mid-Cretaceous Western Interior Seaway and evidence of oligotrphic surface waters during OAE2: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 392, p. 510–523, doi:10.1016/j .palaeo.2013.10.007. Culotta, R., T. Latham, M. Sydow, J. Oliver, L. Brown, and S. Kaufman, 1992, Deep structure of the Texas Gulf Passive Margin and its Ouachita-Precambrian basement: Results of the COCORP San Marcos arch survey: AAPG Bulletin, v. 76, no. 2, p. 270–283. Dawson, W. C., 1997, Limestone microfacies and sequence stratigraphy: Eagle Ford Group (Cenomanian-Turonian) North-Central Texas outcrops: Gulf Coast Association of Geological Societies Transactions, v. 47, p. 99–105. Dawson, W. C., B. J. Katz, L. M. Liro, and V. D. Robison, 1993, Stratigraphic and geochemical variability: Eagle Ford Group, east-central Texas, in Rates of geologic processes, tectonics, sedimentation, eustasy and climate— Implications for hydrocarbon exploration: 14th Annual Gulf Coast Section of the Society of Economic Paleontologists and Mineralogists Foundation Research Conference, Houston, Texas, December 5–8, 1993, p. 19–28, doi:10.5724/gcs.93.14.0019. Demaison, G. J., and G. T. Moore, 1980, Anoxic environments and oil source bed genesis: AAPG Bulletin, v. 64, no. 8, p. 1179–1209. Denne, R. A., and J. A. Breyer, in press, Integrated stratigraphy and biostratigraphy of the Cenomanian-Turonian interval (Eagle Ford and Woodbine groups), Texas. Part 2: Regional depositional episodes of the Cenomanian-Turonian Eagle Ford and Woodbine groups of Texas, in J. A. Breyer, ed., The Eagle Ford Shale—A renaissance in U.S. oil production: AAPG Memoir 110. Denne, R. A., R. E. Hinote, J. A. Breyer, T. H. Kosanke, J. A. Lees, N. Englelhardt-Moore, J. M. Spaw, and N. Tur, 2014, The Cenomanian-Turonian Eagle Ford Group of South Texas: Insights on timing and paleoceanographic conditions from geochemisty and micropaleontologic analyses: Palaeogeography,

Fairbanks et al.

401

Palaeoclimatology, Palaeoecology, v. 413, p. 2–28, doi:10.1016/j.palaeo.2014.05.029. Donovan, A. D., and S. T. Staerker, 2010, Sequence stratigraphy of the Eagle Ford (Boquillas) Formation in the subsurface of South Texas and outcrops of West Texas: Gulf Coast Association of Geological Societies Transactions, v. 60, p. 861–899. Doveton, J. H., and D. F. Merriam, 2004, Borehole petrophysical chemostratigraphy of Pennsylvanian black shales in the Kansas subsurface: Chemical Geology, v. 206, p. 249–258. Dravis, J. J., 1980, Sedimentology and diagenesis of the Upper Cretaceous Austin Chalk Formation, South Texas and northern Mexico, Ph.D. thesis, Rice University, Houston, Texas, 513 p. Dubiel, R. F., P. D. Warwick, L. Biewick, L. Burke, J. L. Coleman, K. O. Dennen, and C. Doolan et al., 2010, Geology and assessment of undiscovered oil and gas resources in Mesozoic (Jurassic and Cretaceous) rocks of the onshore and state waters of the Gulf of Mexico region, U.S.A.: Gulf Coast Association of Geological Societies Transactions, v. 60, p. 207–216. Duggen, S., P. Croot, U. Schacht, and L. Hoffmann, 2007, Subduction zone volcanic ash can fertilize the surface ocean and stimulate phytoplankton growth: Evidence from biogeochemical experiments and satellite data: Geophysical Research Letters, v. 34, p. L01612–L01617, doi:10.1029/2006GL027522. Duggen, S., N. Olgun, P. Croot, L. Hoffmann, H. Dietze, P. Delmelle, and C. Teschner, 2010, The role of airborne volcanic ash for the surface ocean biogeochemical ironcycle: A review: Biogeosciences, v. 7, p. 827–844, doi: 10.5194/bg-7-827-2010. Fairbanks, M. D., 2012, High resolution stratigraphy and facies architecture of the Upper Cretaceous (CenomanianTuronian) Eagle Ford Group, Central Texas, M.S. thesis, The University of Texas at Austin, Austin, Texas, 120 p. Faugères, J.-C., and D. A. Stow, 1993, Bottom-currentcontrolled sedimentation: A synthesis of the contourite problem: Sedimentary Geology, v. 82, p. 287–297, doi: 10.1016/0037-0738(93)90127-Q. Feray, D. E., and K. Young, 1949, Guidebook, Cretaceous of Austin, Texas area, 1949: 17th annual field trip by Shreveport Geological Society: Shreveport, Louisiana, Shreveport Geological Society, p. 51–56. Frébourg, G., S. C. Ruppel, R. G. Loucks, and J. Lambert, 2016, Depositional controls on sediment body architecture in the Eagle Ford/Boquillas system: Insights from outcrops in west Texas, United States, AAPG Bulletin, doi:10.1306/12091515101. Galloway, W. E., 2008, Depositional evolution of the Gulf of Mexico sedimentary basin, in A. D. Miall, ed., The sedimentary basins of the United States and Canada: New York, Elsevier, p. 505–549, doi:10.1016/S1874-5997 (08)00015-4. Harbor, R. L., 2011, Facies characterization and stratigraphic architecture of organic-rich mudrocks, Upper Cretaceous Eagle Ford Formation, South Texas, M.S. thesis, The University of Texas at Austin, Austin, Texas, 184 p.

402


Hentz, T. F., W. A. Ambrose, and D. C. Smith, 2014, Eaglebine play of the southwestern East Texas basin: Stratigraphic and depositional framework of the Upper Cretaceous (Cenomanian-Turonian) Woodbine and Eagle Ford Groups: AAPG Bulletin, v. 98, no. 12, p. 2551–2580, doi:10.1306/07071413232. Hentz, T. F., and S. C. Ruppel, 2010, Regional lithostratigraphy of the Eagle Ford Shale: Maverick Basin to East Texas Basin: Gulf Coast Association of Geological Societies Transactions, v. 60, p. 325–337. Jiang, M. J., 1989, Biostratigraphy and geochronology of the Eagle Ford Shale, Austin Chalk, and Lower Taylor Marl in Texas based on calcareous nannofossils, Ph.D. dissertation, Texas A&M University, College Station, Texas, 524 p. Jones, M. T., and S. R. Gislason, 2008, Rapid releases of metal salts and nutrients following the deposition of volcanic ash into aqueous environments: Geochimica et Cosmochimica Acta, v. 72, p. 3661–3680, doi:10.1016 /j.gca.2008.05.030. Kearns, T. J., 2011, Chemostratigraphy of the Eagle Ford Formation, M.S. thesis, The University of Texas at Arlington, Arlington, Texas, 254 p. Langmann, B., K. Zaksek, M. Hort, and S. Duggen, 2010, Volcanic ash as fertilizer for the surface ocean: Atmospheric Chemistry and Physics, v. 10, p. 3891–3899, doi: 10.5194/acp-10-3891-2010. Liro, L. M., W. C. Dawson, B. J. Katz, and V. D. Robison, 1994, Sequence stratigraphic elements and geochemical variability within a “condensed section”: Eagle Ford Group, East-Central Texas: Gulf Coast Association of Geological Societies Transactions, v. 44, p. 393–402. Lundquist, J. J., 2000, Foraminiferal biostratigraphic and paleoceanographic analysis of the Eagle Ford, Austin, and lower Taylor Groups (middle Cenomanian through lower Campanian) of Central Texas, Ph.D. thesis, The University of Texas at Austin, Austin, Texas, 544 p. Pessagno, E. A., 1969, Upper Cretaceous stratigraphy of the western gulf coast area of Mexico, Texas, and Arkansas: Geological Society of America, v. 111, p. 59–64. Phelps, R. M., 2011, Middle-Hauterivian to LowerCampanian sequence stratigraphy and stable isotope geochemistry of the Comanche platform, South Texas, Ph.D. dissertation, The University of Texas at Austin, Austin, Texas, 240 p. Pierce, J. D., 2014, U-Pb geochronology of the Late Cretaceous Eagle Ford shale, Texas; defining chronostratigraphic boundaries and volcanic ash source, M.S. thesis, The University of Texas at Austin, Austin, Texas, 144 p. Pratt, L. M., 1984, Influence of paleoenvironmental factors on preservation of organic matter in Middle Cretaceous Greenhorn Formation, Pueblo, Colorado: AAPG Bulletin, v. 68, no. 9, p. 1146–1159. Ratcliffe, K. T., A. M. Wright, and K. Schmidt, 2012, Application of inorganic whole-rock geochemistry to shale resource plays: An example from the Eagle Ford Shale Formation, Texas: Sedimentary Record, v. 10, no. 2, p. 4–9, doi:10.2110/sedred.2012.2.4. Robison, C. R., 1997, Hydrocarbon source rock variability within the Austin Chalk and Eagle Ford Shale (Upper

Cretaceous), East Texas, U.S.A.: International Journal of Coal Geology, v. 34, p. 287–305, doi:10.1016/S01665162(97)00027-X. Rowe, H., N. Hughes, and K. Robinson, 2012, The quantification and application of handheld energy-dispersive x-ray fluorescence (ED-XRF) in mudrock chemostratigraphy and geochemistry: Chemical Geology, v. 324–325, p. 122–131, doi:10.1016/j.chemgeo.2011.12.023. Rowe, H., and Ruppel, S. C., 2013, High-resolution chemostratigraphy of the Bakken Formation, Williston Basin (abs.): AAPG International Conference and Exhibition, Cartagena, Colombia, September 8–11, 2013, accessed June 9, 2014, http://www.searchanddiscovery.com/abstracts/html/2013 /90166ice/abstracts/rowe.htm. Ruppel, S. C., R. G. Loucks, and G. Frébourg, 2012, Guide to field exposures of the Eagle Ford-equivalent Boquillas Formation and related Upper Cretaceous units in southwest Texas: The University of Texas at Austin, Bureau of Economic Geology, Mudrock Systems Research Laboratory Field-Trip Guidebook, 151 p. Salvador, A., and J. M. Q. Muñeton, 1989, Stratigraphic correlation chart of Gulf of Mexico Basin, in A. Salvador, ed., The Gulf of Mexico Basin: The geology of North America, v. J: Boulder, Colorado, Geological Society of America, plate 5. Scott, R. W., 2010, Cretaceous stratigraphy, depositional systems, and reservoir facies of the northern Gulf of Mexico: Gulf Coast Association of Geological Societies Transactions, v. 60, p. 597–609. Smith, C. N., and A. Malicse, 2010, Rapid handheld x-ray fluorescence (HHXRF) analysis of gas shales: AAPG Search and Discovery article 90108, accessed March

16, 2011, http://www.searchanddiscovery.com/pdfz /abstracts/pdf/2010/intl/abstracts/ndx_smith.pdf.html. ´ Sohl, N. F., E. Mart´ınez, P. Salmercon-Urreña, and F. Soto-Jaramillo, 1991, Upper Cretaceous, in A. Salvador, ed., The Gulf of Mexico Basin: The geology of North America, v. J: Boulder, Colorado, Geological Society of America, p. 205–244. Stow, D. A., J.-C. Faugeres, J. A. Howe, C. J. Pudsey, and A. R. Viana, 2002, Bottom currents, contourites and deep-sea sediment drifts: Current state-of-the-art, in D. A. Stow, C. J. Pudsey, J. A. Howe, J.-C. Faugères, and A. R. Viana, eds., Deep-water contourite systems: Modern drifts and ancient series, seismic and sedimentary characteristics: Geological Society, London, Memoirs 2002, v. 22, p. 7–20. Tribovillard, N., T. J. Algeo, T. Lyons, and A. Riboulleau, 2006, Trace metals as paleoredox and paleoproductivity proxies: An update: Chemical Geology, v. 232, p. 12–32, doi:10.1016/j.chemgeo.2006.02.012. Tyler, N., and W. A. Ambrose, 1986, Depositional systems and oil and gas plays in the Cretaceous Olmos Formation, South Texas: Report of Investigations 152: Austin, Texas, Bureau of Economic Geology, University of Texas at Austin, 42 p. Young, K., 1977, Guidebook to the geology of Travis County: Austin, Texas, The University of Texas at Austin, accessed October 6, 2011, http://www.lib.utexas.edu/geo /ggtc/toc.html. Young, K., 1986, Cretaceous marine inundations of the San Marcos Platform, Texas: Cretaceous Research, v. 7, p. 117–140, doi:10.1016/0195-6671(86) 90013-3.

Fairbanks et al.

403