STRATIGRAPHY OF THE ERNST MEMBER OF THE UPPER CRETACEOUS BOQUILLAS FORMATION, BLACK GAP WILDLIFE MANAGEMENT AREA, BREWSTER COUNTY, TEXAS
BY
ROBERT ALAN WHITE
Bachelor of Science, 2015 Western State Colorado University Gunnison, Colorado
Submitted to the Graduate Faculty of The College of Science and Engineering Texas Christian University In partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
December 2019
Stratigraphy of the Ernst Member of the Upper Cretaceous Boquillas Formation, Black Gap Wildlife Management Area, Brewster County, Texas
Thesis submitted by
Robert Alan White
Thesis approved:
Dr. Helge Alsleben
Advisor Dr. Richard Denne
Dr. Arthur Busbey
Bo Henk
Dean of College of Science & Engineering
iii
Copyright © by Robert Alan White 2019
iv
ACKNOWLEDGEMENTS
Firstly, I would like to express my sincere appreciation and gratitude to my thesis advisor
Dr. Helge Alsleben, for his guidance, support, knowledge, patience, and most of all, the opportunity to take on this project and get it completed. I would also like to thank the other members of my committee, Dr. Richard Denne, Dr. Arthur Busbey, and Mr. Bo Henk, for their contributions in the field and in the classroom from start to finish. Additionally, I would like to thank John Alvarez and Black Wawrick for their many hours of contribution in the field and in the lab. I am thankful for the entire TCU geology faculty and staff for making it an enjoyable, comfortable, and rewarding time, it was an amazing experience. I want to thank the other geology students who made this experience much more enjoyable and I appreciate their contributions and guidance throughout the entire process. I want to thank Pioneer Natural Resources for allowing me to use their data from Big Bend National Park to help aid in the findings of the project. I would also like to thank Mr. Bo Henk for the idea for the project and finding of the section. Lastly, I would like to thank my parents Jeffrey White and Mary Ann White, and my sister Anna White for their support through school. I could not have done it without you.
ii
Table of Contents
Acknowledgements ...... ii
Table of Contents ...... iii
List of figures ...... v
List of tables ...... ix
Introduction……………………………...... 1
Previous Studies ………...... 6
Tectonics and Depositional Environment...... 15
Purpose and Objective ...... 18
Methods ...... 19
Outcrop ...... 19
ED-XRF...... 19
Dimpler………...... 21
Spectral Gamma Ray………...... 22
Results ………...... 29
Outcrop ...... 29
Spectral Gamma Ray...... 44
ED-XRF...... 46
Dimpler...... 92
Discussion...... 101
Lithostratigraphy and chemostratigraphy by zone...... 102
Zone A...... 102
Zone B...... 105
Zone C...... 108
iii
Zone D...... 109
Zone E...... 110
Ernst Member...... 112
Rock Strength...... 118
Distal studies comparison…...... 119
South Texas subsurface...... 112
Conclusions...... 126
Bibliography...... 129
Vita
Abstract
iv
List of Figures
1. Late Cretaceous paleogeography of southern Laurentia…………………………………………...3
2. Wheeler diagram and stratigraphic nomenclature of the Eagle Ford Shale and Boquillas
Formation in Texas………………………………………………………………………………...4
3. Texas outline with Trans-Pecos area shaded in black and location of study………………………5
4. Historical nomenclature of the Boquillas Formation in Brewster County in west Texas and in the
Langtry area in Val Verde County in south Texas………………………………………….……14
5. Late Cretaceous paleogeography of the South Texas Shelf showing adjacent structures and
features present during time of Boquillas and Eagle Ford deposition……………………………17
6. Annotated satellite image from Google Earth showing outcrop location and path taken to measure
the section………………….……………………………………………………………...……...23
7. Picture of rock hammer, tape measure, and plastic bags used for section measuring and sample
collecting………………………………………………………………………………………….24
8. Picture of handheld Bruker Tracer IV energy dispersive X-ray fluorescence (ED-XRF)
spectrometer used for study…………………………………..……………………………….….25
9. Generalized diagram showing how X-ray beams from the XRF tool interact with electrons in an
atom………………………………………………………………………………..……………...26
10. Picture of Dimpler tool and layout of tape used to measure dimpler ticks…………………….…27
11. Radiation Solution Incorporated RS-230 BGO Super-SPEC portable gamma-spectrometer used
for study………………………………………………………………………………..…………28
12. Graphical representation of measured section……………………………………………………33
13. Picture of lower portion of measured section 7’-14’ (2.1–4.2 m)………………………...………34
14. Picture of two large ammonites on bedding plane at 12.5’ (3.8 m)………………………….…...35
15. Picture of in situ inoceramid on top of limestone at 18.5’ (5.6 m)………………………….…....36
16. Picture of ash bed at 13.5’ (4.1 m)……………………………………….…………………….…37
v
17. Picture of lower portion of measured section 25’-32’ (7.6-9.8 m)………………………….…....38
18. Picture of lower portion of measured section at 55’-62’ (16.8-18.9 m)……………………….…39
19. Picture of horizontal burrows at 70’ (21.3 m)…………………………………………………….40
20. Picture of thick grainstones with interbedded flaggy wackestones where outcrop continues after
~50’ (15.2 m) of missing section………………...…………………………………….………....41
21. Picture of thin bedded wackestones with thin continuous and non-continuous grainstones
interbedded at 190’-194’ (57.9-59.1 m) in the section…………………………………………...42
22. Picture of thin bedded wackestones with thicker continuous and discontinuous grainstones
interbedded at 218-222’ (66.4-67.7 m) in the section …….……………………………………...43
23. Compensated gamma ray (CGR) and total gamma ray (TGR) from the portable Spectral Gamma
Ray (SGR) tool versus measured section thickness………………………………………………45
24. XRF and SGR compensated gamma ray (CGR) and total gamma (TGR) versus depth for the
measured section……………………………………………………….……………….……..….49
25. Plot of major element abundances in weight percent (wt%) versus measured section
thickness…………………………………………………………………………..…………...….51
26. Plot of trace element abundances in parts per million (ppm) versus measured section
thickness………………………………………………………………………………………..…52
27. Trace metal abundances in Average Shale determined by Wedepohl (1971, 1991).
28. Plot of Enrichment Factors and CGR versus measured section thickness………………………..58
29. Cross plot of Mo enrichment factor versus U enrichment factor………………………………....61
30. Dendrogram produced from TIBCO Spotfire computer software created from Hierarchal Cluster
Analysis (HCA)………...………………………………………………...………………………66
31. Bar graph produced from TIBCO Spotfire software showing depth vs. chemofacies
type……………………………………………………………………………………………..…67
32. Ternary diagram showing organic lithofacies types from Gamero-Diaz et al,
2008…………………………………………………………………………..……….…………..68
vi
33. Ternary diagram showing trends in mineral composition and color identifying
chemofacies……………………………………………………………………………….……....69
34. Cross plot of percent weight silica versus percent weight aluminum…………………………….75
35. Cross plot of percent weight potassium versus percent weight aluminum………………….……76
36. Cross plot of percent weight titanium versus percent weight aluminum……………….………...77
37. Cross plot of parts per million zirconium versus percent weight aluminum……………….…….78
38. Cross plot of parts per million rubidium versus percent weight aluminum……………….……...79
39. Cross plot of parts per million thorium versus percent weight aluminum…………..……………80
40. Cross plot of percent weight calcium versus percent weight aluminum……………….….……...81
41. Cross plot of percent weight calcium versus percent weight magnesium…………………….….82
42. Cross plot of parts per million molybdenum versus percent weight aluminum…………….……83
43. Cross plot of parts per million vanadium versus percent weight aluminum……….……….……84
44. Cross plot of parts per million uranium versus percent weight aluminum………………….……85
45. Cross plot of parts per million nickel versus percent weight aluminum…………………….……86
46. Cross plot of parts per million copper versus percent weight aluminum…………………………87
47. Cross plot of parts per million zinc versus percent weight aluminum…………………….……...88
48. Plot of XRF CGR and XRF Brittleness Index (BI) versus measured section
thickness…………………………………………………………………..………………………90
49. Plot of measured section thickness versus Dimpler UCS (psi) colored by chemofacies
type………………………………………………………………………………………….…….94
50. Cross plot of Dimpler UCS (psi) versus XRF Brittleness Index (BI)………………………….…96
51. Cross plot of Dimpler UCS (psi) versus %Ca……………………………………………………97
52. Cross plot of Dimpler UCS (psi) versus %Si………………………………………………….….98
53. Cross plot of Dimpler UCS (psi) versus %Al………………………………………………….…99
54. Comparison of XRF and SGR between Fry (2015) study and this study (Black Gap)…...….....116
vii
55. Comparison of XRF data between Wehner et al. (2017) study and this study (Black Gap).
………………………………………………………………………………...…………………117
56. Comparison of XRF, SGR, and XRF data between Donovan et al. (2012) Ernst study in Lozier
Canyon in Val Verde County, and this study (Black Gap) in Brewster
County………………………………………………………………………………..………….123
57. Top plot is XRF and SGR plotted versus measured section thickness (in feet) for StatOil
(Equinor) Core-X in Maverick County from (Fry,
2015)………………………………………………………………………………………….…124
58. Comparison of XRF data between the Wokasch (2014) study from two cores in the Maverick
Basin and this study (Black
Gap)……………………………………………………………………………….……………..125
viii
List of Tables
1. Table of average abundances of CGR and TGR API units for grainstones and wackestones
identified in Black Gap measured section from collected samples for the XRF machine and
Spectral Gamma Ray machine……………………………………………………………………50
2. Table of average abundances of major elements in each zone identified in the
section…………………………………………………………………………………………….53
3. Table of average abundances of trace elements in each zone identified in the section
……………………………………………………………………………………….……………54
4. Table of average abundances of major and trace elements for grainstones and wackestones
identified in Black Gap measured section from collected samples………………………………55
5. Table of average enrichment factor values for selected trace elements in selected chemo-
zones………………………………………………..…………………………………………….60
6. Average abundances of trace elements and major elements in each chemofacies identified in the
section…………………………………………………………………………………………….70
7. Table of average abundance of facies in selected chemozones……………………….………….71
8. Table of average Brittleness Index (BI) values for selected brittleness sections within measured
section…………………………………………………………………………………………….91
9. Table of average Dimpler UCS (psi) for selected strength sections within measured section…...95
10. Table of average Dimpler UCS (psi), XRF Brittleness Index (BI), and major elements for
chemofacies……………………………………………………………………………………...100
ix
Introduction
The time-equivalent Boquillas Formation and Eagle Ford Shale (EFS) were deposited on the South Texas Shelf (Fig. 1) in the Late Cretaceous (Cenomanian-Turonian) during a time of widespread marine transgression (Galloway, 2008). The two formations consist of organic-rich shales and marls interbedded with calcareous limestones that vary laterally and vertically in thickness. The organic-rich shales and marls in the lower part of the EFS have been major targets for unconventional production in south Texas and the East Texas Basin since 2008 (EIA, 2015).
Until recently, the EFS was thought to be relatively homogenous throughout, but detailed core and outcrop studies suggest the EFS contains complex lithological and geochemical heterogeneity (Donovan et al., 2012; Fry, 2015; Frebourg et al., 2016; Denne et al., 2016). An understanding of the heterogeneity of the formation at different scales is vital in characterizing the EFS play.
The EFS unconformably overlies the Upper Cretaceous Buda Limestone and is overlain by Upper Cretaceous Austin Chalk (Fig. 2). The Boquillas Formation, which is the EFS equivalent in the Trans-Pecos region of West Texas, overlies the Buda Limestone and is divided into two formal units: the Ernst Member and the San Vicente Member (Maxwell and Dietrich,
1965). The Ernst Member represents deposition in an Eagle Ford-type environment, whereas the
San Vicente suggests deposition in an environment similar to the Austin Chalk. Additionally, the
Ernst Member and EFS were deposited during Oceanic Anoxic Event (OAE) #2. This defines the
Cenomanian-Turonian boundary and represents a time of maximum worldwide sea levels and ocean anoxia (Schlanger and Jenkyns, 1976; Jenkyns, 1980).
With industry interest in the EFS, an understanding of the geology and depositional environment of these rocks is imperative to maximize production results. The Ernst Member of
the Boquillas Formation in southwest Texas is equivalent to the subsurface and exposed EFS to the east. For the study, a section of the Ernst Member of the Boquillas Formation was measured and described in detail in the Black Gap Wildlife Management Area in Brewster County, Texas
(Fig. 3). Lithostratigraphy is determined via outcrop study, chemostratigraphy and mechanical stratigraphy are determined via sample collection and lab analyses. The data is compiled and used to describe and characterize the depositional environment of the Ernst Member in the study area. Additionally, these data are integrated with data from similar, proximal Boquillas outcrop studies (Donovan et al., 2012; Fry, 2015; Frebourg et al., 2016; Pioneer Natural Resources
(PNR) (internal study)) for possible regional correlation and to characterize the depositional environment of the Boquillas in the Trans-Pecos region on the South Texas Shelf.
These rocks can be correlated over moderate distances from 30 to 50 miles (48.3 to 80.5 km) to rocks exposed at Ernst Tinaja and Hot Springs in Big Bend National Park (BBNP).
Correlations can be established with a relatively large sampling interval (every six inches to a foot or every 15 to 30 cm) for XRF, spectral gamma ray (SGR), and mechanical data.
Additionally, correlations can be made from Black Gap Wildlife Management area to the northwest portion of the Maverick Basin in Maverick County. The Eagle Ford in the western portion of the Maverick Basin shows similar characteristics geochemically and lithostratigraphically to the Ernst Member in Black Gap. This Ernst Member of the Boquillas in
Black Gap is therefore interpreted to be deposited in a similar environment as the Eagle Ford in the Maverick Basin.
2
Figure 1. Late Cretaceous paleogeography of southern Laurentia showing Western Interior Seaway connection with the Gulf of Mexico. Light blue is drowned South Texas Shelf (From Fry, 2015).
3
Figure 2. Wheeler diagram and stratigraphic nomenclature of the Eagle Ford in the west Texas outcrop area and south and east Texas subsurface areas (Modified from Denne et al., 2016).
4
Brewster Co.
Figure 3. Texas outline with Trans-Pecos area shaded in black. Red dot shows approximate location of study area in Black Gap Wildlife Management Area (Modified from Williams, 2017).
5
Previous Studies
Since its original definition by Udden (1907), the Boquillas Formation has been the topic of many studies and over the years more refined stratigraphic relationships and depositional models have been reported for these rocks in the Trans-Pecos region of Texas (e.g., Maxwell and
Hazzard (1967); Sanders, 1988; Miller, 1990; Donovan and Staerker (2010); Fry, 2015).
Udden (1907) first defined the Boquillas Formation as the “Boquillas Flags" for the
Boquillas post office on the west flank of the Carmen Range. Udden noted that the formation is
~385 ft (117 m) thick and consists of flaggy, thin beds, composed of calcite and organic material overlying thick-bedded limestones. Additionally, Udden (1907) observed common foraminifera and Inoceramus shell fragments in the formation.
Maxwell and Dietrich (1965) included Udden’s Boquillas Flags and the lower portion of their Terlingua Group as the Boquillas Formation and note that it overlies the Buda Limestone and underlies the Pen Formation in the BBNP area. They separated it into two members; the lower Ernst Member and the upper San Vicente Member. They showed the Ernst Member being
Turonian and partly Cenomanian based on the appearance of ammonites. Additionally, Maxwell and Dietrich (1965) defined the boundary between the Ernst Member and San Vicente on the appearance of the ammonite Allocrioceras hazzardi.
St. John (1966) described and mapped the Boquillas Flags in the Black Gap area. He measured the “flaggy bedded” Ernst Member at 277 ft (84 m) and the chalky limestones and calcareous shales of the San Vicente Member at 274 ft (83 m). In the area, the Boquillas overlies the Buda Limestone and underlies the Pen Clay (St. John, 1966).
Pessagno (1969) divided the Boquillas into two members based on a study of foraminifera in the western Gulf Coast region: the Rock Pens Member and the Austin Chalk. He
6 defined these members in Val Verde and Terrell counties of Texas. He included the top 30 ft (9 m) of the Rock Pens Member as the Langtry Member, consisting of thin bedded shales and marls similar to the overlying Austin Chalk. Additionally, he divided the Late Cretaceous into six different zones based on foraminifera.
Barnes (1977) described and mapped the Boquillas Flags in Val Verde and Terrell counties and noted a “gradual facies change” into the Eagle Ford to the east. Therefore, he placed the nomenclature boundary at the Devil’s River in Val Verde County. Barnes also noted that the
Boquillas overlies the Buda Limestone and underlies the Austin Chalk in Terrell and Val Verde counties.
Sanders (1988) reported on the sedimentology and isotope geology of the Ernst Member in the Hot Springs area of BBNP. Sanders divided the member into five informal sections: the basalt clastic limestone unit, the black shale unit, the lower limestone unit, upper shale unit, and cyclic limestone/shale unit. Sanders concluded that the Ernst represents shallow water deposition on the Coahuila Platform in 150 m of water or less. Additionally, he stated the clastic interval at the base could have been deposited in shallower waters (~60 m or less) followed by a transgression for the remaining Ernst deposition.
Lock and Peschier (2006) measured sections of the EFS (Boquillas) in Val Verde and
Terrell counties and concluded that the Ernst Member was characteristic of upper slope conditions, rather than tidal flat or shallow shelf conditions interpreted from previous work. Lock et al. (2010) divided the EFS in Val Verde County from description of outcrops along Highway
90, into three units: lower, middle and upper. Additionally, Lock and Peschier (2006) further divided the middle unit into a lower, middle, and upper section. Donovan and Staerker (2010) used stratigraphic and biostratigraphic analysis to correlate the EFS outcrops on Highway 90 in
7
Val Verde and Terrell counties to outcrops in the adjacent Maverick Basin. In outcrop, Donovan and Staerker interpreted the Rock Pens Member and the Austin Chalk as each representing a different transgressive-regressive cycle. This is unlike their interpretation for the Eagle Ford in the subsurface, which they interpret as containing a transgressive lower member and a regressive upper member. Donovan et al. (2012) used outcrops along Highway 90 in Val Verde and Terrell counties to show lateral and vertical heterogeneity in the EFS. Donovan et al. (2012) divided the
EFS into four units: the lower unnamed member, the Lower Shale Member, lower unnamed member of the Upper Eagle Ford and Upper Langtry Member. Additionally, Donovan et al.
(2012) proposed that the boundary between the Rock Pens and Langtry members represents the contact between the Eagle Ford and Austin Chalk in the subsurface in east Texas.
Forkner et al. (2013) correlated outcrop data to core data in the Del Rio area, which shows potential for correlating Boquillas outcrops in west Texas to the Eagle Ford in the subsurface to the east. Forkner and others argue that the Eagle Ford can be internally subdivided into “motifs” that can potentially be correlated over long distances. They defined a “motif” as a recurring depositional element within a succession that represents a lithostratigraphic subset within the evolution of a formation.
Gardner et al. (2013) correlated several outcrops that are 8 miles (13 km) apart in west
Texas at Lozier Canyon and Antonio Creek. They define five distinct lithostratigraphic units (A-
E) and subunits that are laterally continuous between the two outcrops. They found minor thickness changes between the outcrops in the basal A Unit suggesting higher sediment supply in the southeast portion of the study area. Additionally, based on observations of sedimentary structures, they conclude that units A, C, D, and E were deposited within storm wave base and deposition of Unit B was episodically within storm wave base.
8
Wokasch (2014) gathered XRF, TOC, and lithostratigraphic data from two Statoil
(Equinor) cores in the Maverick Basin in La Salle and Live Oak counties. Wokasch divides the section into five chemostratigraphic zones based on major and trace elemental concentrations.
Wokasch correlated the zones between the two cores, which show broad correlations can be made using XRF and lithostratigraphic data.
Denne et al. (2014) conducted a study of the EFS in the subsurface in south Texas and conclude that identification of foraminifera is best with thin section analysis. Denne et al. (2014) show that foraminifera are more dominant in the marl layers and limestones are composed of diagenetic calcite and calcified radiolarians. Based on planktonic foraminifera, benthic foraminifera, and Inoceramus prisms, Denne et al. (2014) divide the EFS into 13 separate groups. Additionally, Denne et al. (2014) conclude the EFS was mostly deposited under anoxic to euxinic conditions.
Pioneer Natural Resources (PNR) (internal study) measured a 283’ (86.3 m) section of the Ernst Member of the Boquillas at Ernst Tinaja in BBNP. The researchers gathered XRF,
XRD, strength, biostratigraphic, and lithostratigraphic data throughout the section. For XRF and
XRD analysis the authors collected 80 samples. However, they did not offer any interpretations as far as dividing the section into units or members.
Fry (2015) used two cores from the Maverick Basin and measured a section of the
Boquillas in the Hot Springs location in the BBNP where Sanders (1988) previously reported on the formation. Fry (2015) used biostratigraphy, chemostratigraphy, and lithostratigraphy to correlate between them. Fry (2015) suggests correlation on an individual bed basis is almost impossible due to the heterogeneity of the EFS. Fry also concludes that OAE #2 might not be preserved in the Hot Springs area.
9
Lyon (2015) measured sections in Lozier Canyon and Antonio Creek in a similar location to outcrops measured by Gardner et al. (2013) to interpret the depositional environment of Facies
A of the Eagle Ford. Lyon (2015) interpreted cross-bedded structures in Facies A as swaley cross-stratification and hummocky cross-stratification and discusses how these structures suggest deposition at or above storm wave base.
Frebourg et al. (2016) conducted a detailed sedimentologic study on outcrops along
Highway 90 in Val Verde County and in BBNP in Brewster County. The authors suggest vertical and lateral facies variability are controlled by sediment productivity under the influence of bottom water currents below storm wave base. Additionally, the authors suggest sequence stratigraphic principles should be used with caution because deposition occurs in a deep-water setting and is not affected by sediment input from a shallow carbonate factory, but rather from pelagic sediment in an open-marine environment that is subject to bottom water currents.
Denne et al. (2016) conducted a comprehensive compilation and vetting project of Eagle
Ford and Woodbine using outcrop and core data throughout Texas. The authors used biostratigraphic, geochemical, and lithologic data and tied it to well data to come up with a better nomenclature system for the formations. Additionally, composite sections of the major areas of
Texas were created to construct age models and better understand regional depositional environments. In the Langtry and Del Rio areas of West Texas, the authors recommend a few changes to the current nomenclature system. They propose that the carbonate-rich lower beds that were originally placed in the Lozier Canyon Member be separated into the new “Terrell
Member”, which corresponds to Lock and Peschier’s (2006) lower Boquillas and Donovan and
Staerker’s (2010) Facies A. Additionally, they suggest the term “Lozier Canyon Member” be restricted to the organic-rich shales, corresponding to subfacies B3-B5 of Donovan and Staerker
10
(2010). Finally, they accept Gardner et al. (2013) Scott Ranch and Langtry Members as described and think that they are a better fit than the terms “Upper and Lower Eagle Ford” for the area. In the BBNP area of West Texas, the authors find the term “Ernst Member” acceptable as originally defined by Maxwell and Dietrich (1965). They note that OAE #2 is partially or completely missing in the BBNP area based on occurrence of microfossils and the lack of benthic fossils. They suggest that continued calcareous nannofossil and carbon isotope work would help better define what record is missing in that area. Lastly, Denne et al., 2016 organized the historical nomenclature of the Boquillas Formation from Brewster County to Val Verde
County (Fig. 4).
Donovan et al. (2016) propose division of the Eagle Ford outcrops in west Texas into five informal stratigraphic units A-E (bottom to top). They suggest that units A and B represent the
Lower Eagle Ford in the subsurface of south Texas and units C, D, and E, represent the Upper
Eagle Ford. In their work, they also propose that the five informal units can be further divided into sixteen subunits. They use the more detailed vertical facies succession to define four genetically related depositional sequences with distinct geochemical and petrophysical characteristics. The authors suggest that these four depositional sequences can be mapped regionally into the subsurface from west Texas to south Texas. The four sequences are termed the lower and upper (allo-) members of the Lower Eagle Ford Formation and lower and upper
(allo-) members of the Upper Eagle Ford Formation. They note that the lower member of the
Lower Eagle Ford is an organic-rich, high-resistivity, uranium-poor mudstone sequence marked at the base by a clay-rich, low resistivity zone. The upper member of the Lower Eagle Ford is characterized as a uranium and bentonite rich, mudstone-dominated sequence. The lower member of the Upper Eagle Ford is characterized to be a uranium-poor interbedded mudstone
11 and limestone succession with an overall low blocky gamma ray (GR) pattern, a distinctive positive carbon isotope excursion, and a low-resistivity clay-rich zone at its base. The upper member of the Upper Eagle Ford is a bentonite-bearing low-TOC interval that is more bioturbated towards the base and interbedded at the top. Additionally, the authors suggest it is characterized by a high GR pattern, low resistivity, and a low-velocity mudstone interpreted to represent the maximum flooding surface of the unit. Finally, the authors note that regional correlations suggest that unconformities are present between each of these four members and below the Austin Chalk. They discuss that any attempt to explain thickness variations and distributions of the four members is dependent on recognition and regional mapping of the unconformities.
Tinnin and Darmaoen (2016) used whole-rock inorganic elemental data from 36 vertical and horizontal wells along the productive Eagle Ford trend in south Texas to characterize Eagle
Ford depositional environments, sedimentary facies, mineralogy, and provenance. Since much of the Eagle Ford is composed of fine-grained mudrock, which are enriched or depleted in certain major and trace elements, the authors suggest the use of chemostratigraphy to understand the geochemical variability within the formation. The authors note that trace elements are used as paleodepositional proxies to target zones of organic-rich shale deposition. In their work, the authors determine that well performance positively correlates with an increase in oxygen-poor paleoenvironment.
Wehner et al. (2017) measured a section of the Boquillas at the Hot Springs location in
BBNP where Sanders (1988) and Fry (2015) previously reported on the formation. Wehner et al.
(2017) collected 172 samples for lithostratigraphic, chemostratigraphic, and isotopic analysis.
Wehner et al. (2017) then correlate rocks at Hot Springs to rocks at Lozier Canyon and to core in
12 the south Texas subsurface in Webb County using geochemical and lithological data. Since correlations can be made and thickness changes do not vary much in the Boquillas and Eagle
Ford from Hot Springs in BBNP to Lozier Canyon to the subsurface in Webb County, Wehner et al. (2017) suggest the formation was deposited in a similar environment. Additionally, the authors suggest the lower 10’ (3.0 m) of the Ernst Member is coeval with the Woodbine Group in East Texas based of presence of hummocky cross stratification and geochemical correlation.
Wehner et al. (2017) also suggest that OAE #2 might not be preserved in the Hot Springs area based on isotope data.
13
Figure 4. Historical nomenclature of the Boquillas Formation in Brewster County in west Texas and in the Langtry area in Val Verde County in south Texas. (Modified from Denne et al., 2016).
14
Tectonics and Depositional Environment
In the late Triassic rifting of the super-continent Pangea initiated and by the early
Cretaceous the continent was split into Laurentia in the northern hemisphere and Gondwana in the southern hemisphere. Subsequently, Laurentia was split into two continents. Rifting of
Laurentia in the late Triassic initiated the opening of the proto-Gulf of Mexico (Pearson et al.,
2010), which affected the setting of the South Texas Shelf. Stern and Dickinson (2010) suggest the proto-Gulf of Mexico was initiated as a back-arc basin related to subduction of the Farallon
Plate, which was occurring along the western portion of Laurentia. The structures proximal to the area of study on the South Texas Shelf are the Coahuila Platform, formed from earlier reef deposition to the west, and the Terrell Arch to the northeast (Fig. 5). The Chihuahua Trough to the west is a back-arc basin related to the subduction further west (Fig. 5), and the Maverick
Basin lies to the east of the study area. In Late Cretaceous time, the seas transgressed further onto Laurentia, drowning the South Texas Shelf, and creating the Western Interior Seaway
(Murray, 1961).
The stratigraphic record on the South Texas Shelf in the Trans-Pecos area includes, from oldest to youngest, the Del Carmen Limestone, Sue Peaks Formation, Santa Elena Limestone,
Del Rio Clay, Buda Limestone, Boquillas Formation, and Austin Chalk (Fig. 2). The Del
Carmen Limestone is the dominant reef-forming formation in the Trans-Pecos area and is equivalent to the Edwards Limestone, which is the major reef facies to the east. Following deposition of the reefs of the Del Carmen, a major transgression occurred and drowned the reef trends on the South Texas Shelf. This allowed for deposition of non-reef forming formations of the Del Rio Clay and Buda Limestone. As the seas transgressed further, deposition of organic- rich carbonate mud of the Boquillas (EFS) occurred (Galloway, 2008). Algeo and Rowe (2012)
15 propose that the reefs deposited earlier created a sill, which resulted in restriction of the basin and prohibited oxygenated ocean water renewal on the South Texas Shelf. Following this major transgression was a regression, that again allowed for healthy carbonate production on the South
Texas Shelf (San Vicente Member and Austin Chalk deposition).
The Ernst Member of the Boquillas Formation in the Trans-Pecos area consists of unburrowed skeletal wackestones and argillaceous to calcareous laminated mudrocks and wackestones (Fry, 2015). Additionally, the Ernst Member lacks bioturbation and benthic fauna with portions of well-preserved sedimentary structures. This suggests the Ernst was deposited in a potentially anoxic environment before oxygen returned into the system during San Vicente and
Austin Chalk deposition (Fry, 2015).
16
MB
Figure 5. Late Cretaceous paleogeography of the South Texas Shelf showing adjacent structures and features present during time of Boquillas (EFS) deposition. Green star is type location for the Ernst Member (section measured by Fry, 2015). Red circle is approximate location of Black Gap section (Modified from Fry, 2015).
17
Purpose and Objective
The purpose of this study is to use lithostratigraphy, chemostratigraphy, and mechanical stratigraphy to characterize the depositional environment and palaeoceanographic conditions during deposition of the Ernst Member of the Boquillas Formation in the Black Gap Wildlife
Management Area in Brewster County, Texas. For the study, a section of the Ernst Member is described in detail. Additionally, samples were collected for chemical and mechanical analysis.
The Ernst Member in the Trans-Pecos region is equivalent to the EFS to the east and is, therefore, well-suited to improve our understanding of the formation and characteristics of these rocks. Complex heterogeneities within the Ernst and EFS have resulted in inconsistent conclusions regarding paleoenvironment of the formations (Donovan et al., 2012; Fry, 2015;
Frebourg et al., 2016). Aspects such as the depth of deposition, depositional environment, and ocean conditions have been argued about in previous studies.
The objective of this study is to interpret the depositional environment and better understand the heterogeneities of the Ernst Member in the study area and to use the data to potentially correlate with data from previous studies. Similar studies have been completed in the
BBNP to the west and in the Terrell Arch and Langtry areas (Maxwell and Hazzard, 1967;
Sanders, 1988; Miller, 1990; Donovan et. al, 2010; Fry, 2015). No similar studies appear in the literature of the Black Gap area of Brewster County. This project will provide a detailed interpretation of the Ernst Member in the study area and will potentially fill in a gap in the understanding of the depositional environments of the South Texas Shelf during EFS time.
18
Methods
Outcrop
For the study, a section of the Ernst Member of the Boquillas Formation was measured in the Black Gap Wildlife Management Area. The section is 5.2 miles (8.4 km) north of the US-
Mexico border off the east side of FMR 2627 (Fig. 6). Thickness was determined using a metric tape measure and/or Jacob’s Staff (Fig. 7). Many portions of the section are covered or broken so correlation across the creek bed is frequently necessary. Additionally, individual bed thickness and continuity, color, grain size, composition, sedimentary structures, fossils, and trace fossils were noted. A detailed stratigraphic section was produced to depict facies distribution and interpret the environment of deposition of the Ernst Member. Samples for geochemical and mechanical analysis were collected at one per six-inch (15 cm) interval throughout the section.
Measurement started in the upper portion of the Buda Limestone (3 feet or 1 meter) and continued up into the Ernst Member for 233 feet (71 m) until the section ended. Samples were put into plastic bags noting section interval (Fig. 7). Samples were then filed down at the TCU
Core Lab with a hand file to create relatively flat surface orthogonal to bedding. Samples were cleaned using water and a brush and dried before geochemical and mechanical analysis. Local structures such as thrust faults, folds, and normal faults (Williams, 2017) were accounted for during section measuring, gamma ray logging, and sampling.
ED-XRF (Energy-dispersive – X-ray fluorescence)
XRF is a nondestructive method of collecting elemental data from rock samples. There are two separate types of XRF spectrometers, energy-dispersive (ED) and wavelength-dispersive
(WD). WD-XRF records element-specific wavelengths of energy and is therefore more precise
19 but requires significant sample preparation and down time between readings, whereas ED-XRF does not. ED-XRF collects an energy spectrum that is characteristic of the elements in question
(Kaiser and Rowe, 2012). The Bruker Tracer IV (Fig. 8) was used to collect raw data and then the data was converted from point counts to weight percent using a mudrock calibration in the
Bruker software that is provided with the device. Weight percent is reasonable to use for major elements, which are usually in higher abundance than trace elements. For trace elements, parts per million (ppm) values are more reasonable to use. To determine ppm values, trace element percent weight is multiplied by 10,000. Using ED-XRF to analyze mudrocks is highly effective because major elements can be associated with specific rock forming minerals. Some elements, such as redox-sensitive trace metals, can also provide additional information about the depositional environment, as for example, bottom water oxygen conditions (Kaiser and Rowe,
2012).
The hand-held x-ray fluorescence device was set on a stationary stand and was used to determine bulk geochemistry of each sample (Fig. 8). Samples from the section were set on a platform on a stationary stand to collect data. Then samples were set on the flat surface to ensure best results. The XRF transmits and receives high-energy X-ray beams that have interacted with the atoms of a sample. Emitted X-rays force an electron to be expelled from the lower-energy inner shell (K shell), resulting in electrons from the higher-energy outer shells replacing the expelled electron and releasing energy in the form of emission X-rays (Fig. 9). The intensity of the released energy is measured by the XRF detector and is specific to certain elements whose quantities are determined (Jenkins, 1988). Each sample collected at a six-inch (15 cm) interval throughout the section was scanned at least once. A total of six samples were collected from the
Buda Limestone and 336 samples were collected and scanned from the Ernst Member.
20
Instrument settings must be adjusted to obtain either trace or major elemental data. Major element analysis consisted of a run time interval of sixty seconds with a 15kV calibration and trace element analysis consisted of a ninety second run time with a 40kV calibration. For major elements a vacuum was used, and for trace elements a vacuum was not necessary. The trace elements (19) were sampled first, and the major elements (10) were sampled second.
Additionally, each day before sampling, a reference sample (SARM-41) was scanned to verify that the machine was functioning properly.
Dimpler
A point load penetrometer (“Dimpler”), introduced by Ramos et al. (2008), was used to determine the strength of the samples in the section. The Dimpler is a good tool to calculate the strength of the rock for this study because it is small, portable, and relatively non-destructive.
The sampling interval for the Dimpler is the same interval used for the XRF (6 inches or 15 centimeters). The points sampled on the rocks were put as close as possible to where the XRF measurement was taken. The strength data collection method was the same as that of Enderlin
(2010). A piece of removable tape was placed on the surface of the sample, then a dye was added on the tip of the Dimpler using a red maker. As the tip of the Dimpler is forced into the surface of the sample, the depth and diameter of penetration are recorded by the red dye on the removable tape (Fig. 10). The tape was then removed and put onto a flat surface and diameter of the dye (dimple) was measured and noted. Three points were collected for each sample and average dimple size was determined. The raw data from the Dimpler was then converted to
Unconfined Compressive Strength (UCS) using an empirical formula to determine strength of the rocks throughout the section (Ramos et al., 2008).
21
Spectral gamma ray
A Radiation Solution Incorporated RS-230 BGO Super-SPEC portable gamma- spectrometer was used to collect percent potassium, uranium parts per million, and thorium parts per million at one scan per six-inch (15 cm) interval throughout the section (Fig. 11). The scanner was placed orthogonal to bedding flush against a flat surface of the outcrop for sixty seconds. The scan was performed in the same area in which samples were collected. Scans started in the upper portion of the underlying Buda for logging purposes. Formulas were used to convert raw data to API units and determine both compensated gamma ray (CGR) and total gamma ray (TGR) using the following equations (Luthi, 2001; Doveton and Merriam, 2004;
Fairbanks et al., 2016):
CGR (API) = (4*Th ppm) + (16*K (%))
TGR (API) = (4*Th ppm) + (8*U ppm) + (16*K (%))
A total of six scans were taken in the underlying Buda Limestone and 336 scans were taken in the Ernst Member.
22
Mex.
N
ormal fault from from fault ormal U.S.
miles 2
and path taken to measure the section, just off the east side side east the off just section, the measure to taken path and
the of top and base of location show tacks Green border. Mexico -
inplace measuring when taken path represents line Blue section. the of rtion
500 feet 500
Annotated satellite image from Google Earth showing outcrop location outcrop showing Earth Google from image satellite Annotated
. .
6
Figure US from north km) (8.4 miles 5.21 isapproximately Section 2627. ofFMR po missing of top and base show tacks red and Member Ernst n inferred and fault Normal section. repeat causes that fault ofnormal location approximate shows line black section.Dotted (2017). Williams
23
thickness and bags to collect collect to bags and thickness
. (Left): Picture of rock hammer used to collect collect to used rockhammer of Picture (Left): .
Figure 7 Figure at device measuring thickness a as used and samples Tape (Right): measure. tape touse toocovered locations measure to used measure assistant. field the is Alvarez John samples.
24
Figure 8. Handheld Bruker Tracer IV energy dispersive X-ray fluorescence (ED-XRF) spectrometer (Bruker, 2017).
25
Figure 9. Generalized diagram showing how X-ray beams (incident radiation) from the XRF tool interact with electrons in an atom. K shell electron is ejected and replaced by higher energy orbital electrons and an emission X-ray beam (K X-ray) is released (From Bruker, 2017).
26
Figure 10. (Left): Picture showing how a Dimpler is used on samples. (Right): Picture shows layout of dimples on tape and zoomed in view through the lenses to determine dimple size. Right two pictures from Taylor (2017).
27
Figure 11. Radiation Solution Incorporated RS-230 BGO Super-SPEC portable gamma- spectrometer used to collect percent potassium, uranium parts per million, and thorium parts per million at one scan per six-inch (15 cm) interval throughout the section.
28
Results
In this section lithostratigraphic data, biostratigraphic data, chemical data, and strength data are described, cross-plotted and graphed to aid in interpretation of the measured section. The sections are broken up into outcrop (lithostratigraphy), XRF (chemostratigraphy), and Dimpler
(mechanical stratigraphy).
Outcrop
A section of the Buda Limestone and the Ernst Member of the Boquillas Formation was measured and described in the Black Gap Wildlife Management Area (Fig. 12). Additionally, samples were collected from both formations for chemical and mechanical analysis. Three feet (1 m) were measured and six samples were taken from the Buda. In the Ernst Member, 233 feet
(71.0 m) were measured and 336 samples were taken.
The top three feet of the Buda are composed of white, fossil-rich, burrowed skeletal wackestone that is mottled and massive. The contact between the Buda and overlying Boquillas
Formation is abrupt and wavy. The overlying Boquillas has a much thinner bedded, flaggy appearance and is dark gray to orange and pink (Fig. 12).
From the top of the Buda to about 24’ (7.30 m), the Ernst is mostly composed of dark gray to light gray, thin bedded (1-5 inches) (2-12 cm) low-angle cross stratified and planar laminated grainstones and wackestones. Minor amounts of argillaceous material are incorporated in some of these wackestones, which make them more fissile and more recessive than Ca-rich wackestones (Fig. 12-13). The beds are contorted, wavy, and commonly discontinuous with few beds being continuous and laterally extensive. Discontinuous grainstones have a “pinch and swell” or lenticular geometry to them. At 12.5’ (3.8 m) into the section an ammonite-rich bed is seen (Fig. 14) with numerous nearby ammonites encased in ash. Denne (personal
29 communication, 2018) suggests that these ammonites belong to the genus Acanthoceras and species Acanthoceras amphibolum defined by Morrow (1935) in the Graneros Shale in Kansas.
Additionally, in situ inoceramid fossils occur at 13.5’ (4.1 m) and 18.5’ (5.6 m) in the section on top of bedding planes (Fig. 15). Nine ash beds ranging from 0.5 to 6.0 inches (1.3-15 cm) thick are also present in this interval (Fig. 16). On average an ash bed occurs every 2.7’ (0.8 m) in this part of the section.
From ~24’ to ~55’ (7.30-16.8 m) into the Ernst the color changes abruptly to orange and pink and the grainstones become less frequent and thin flaggy wackestones beds become more frequent (Fig. 17). Some continuous grainstones (15-25 cm thick) that can be followed laterally, and discontinuous thin (10-15 cm) grainstone beds occur between thinly bedded Ca-rich wackestones and argillaceous wackestones. Discontinuous “pinch and swell” grainstones occur in the lower portion of this section and continuous blocky grainstones occur more frequently in the upper portion of this section. Additionally, 26 thin ash beds (0.5-1 cm) that are laterally extensive for more than 3 meters occur throughout. On average an ash bed occurs every 1.2’
(0.37 m) in this part of the section.
From 55’-71’ (16.8-21.6 m) grainstone beds become thicker (0.2-0.4 m) than the lower sections. Thin (2-10 cm) flaggy pink to orange wackestones are interbedded with the thicker grainstones (Fig. 18). Grainstones are both continuous and discontinuous laterally in this portion of the section and some contain low-angle cross stratification. Some grainstones are completely recrystallized and no sedimentary structures are seen. Ten ash beds occur in this part of the section and on average an ash bed occurs every 1.6’ (0.5 m).
At 71’ (21.6 m) into the Ernst Member, the first trace fossils are seen. Horizontal burrows on a bedding plane of a calcareous wackestone are seen at 71’ (21.6 m) into the section (Fig. 19).
30
No trace fossils are seen in the entire measured section until this point. An inoceramid fossil hash is seen at 71.5’ (21.8 m) in the section on the top of the bedding plane.
From 71’-122’ (21.6-37.2 m) thin (2-10 cm), recrystallized, continuous and discontinuous low-angle cross stratified nodular shaped grainstones are interbedded with thin (1-6 cm) wackestones. Grainstone and wackestone beds occur frequently in this section and stay thin bedded. The thin style of bedding in this section is similar to the 24’-55’ (7.30-16.8 m) section, but there are much more interbedded thin grainstones in this section (Fig. 12). Unfortunately, the section is covered with debris or outcrops are poor within the interval from 85’-100’ (25.9-30.5 m), the missing thickness was measured with a tape measure using a similar dip as outcrop before the missing section. Therefore, measurements of missing section could be off slightly if any faulting occurred that cannot be detected via outcrop study. Similarly, exposures are covered or poor from 122’-170’ (37.2-51.8 m). A thin ~10’ (3.1 m) interval with increased amounts of argillaceous material (recessed beds) is seen from 110-120’ (33.5-36.6 m) and in a few other, even thinner intervals in Zone C. From 170’-184’ (51.8-56.1 m) thick bedded (0.3-0.9 m) laminated calcareous wackestones occur interbedded with thin (1-4 cm) flaggy wackestones (Fig.
20). At 181’ (55.2 m) the top of a bedding plane is seen with inoceramid fossils. The interval in the section from 71-184’ (21.6-56.1 m) contains a smaller number of ash beds, totaling only 15 beds. On average an ash bed occurs every 8.9’ (2.7 m) in this part of the section. However, two missing sections totaling 63’ (19.2 m) occur in the interval from 71-184’ (21.6-56.1 m), which likely contain additional ash beds that would change the average abundance of ash beds in this section.
From 184’-213’ (56.1-64.9 m), zones of 0.5-1.0 inch (0.2-0.6 m) laminated grainstones with interbedded thin 0.5-1.5-inch (1-4 cm) wackestones and zones of flaggy thin 1.0-2.0 inch
31
(2-5 cm) grainstones and wackestones occur (Fig. 21). Overall, this interval is much thinner bedded than the previous interval and contains more argillaceous material (Fig. 11).
Additionally, seven thin 1.0-1.5-inch (2-4 cm) ash beds occur in the upper portion of this section.
On average an ash bed occurs every 4.1’ (1.2 m) in this portion of the section.
For the rest of the measured section from 213’-233’ (64.9-71.0 m), the lithologies and bedding styles are the same as the previous section, but this section has a higher ratio of grainstones to wackestones than the previous interval and beds are generally coarser grained than beds in the previous interval (Fig. 22). Additionally, grainstones beds are thicker than the previous section ranging from 1.0-2.5’ (0.3-0.8 m). Inoceramid fossils are seen in three grainstone beds in the lower 10’ (3.1 m) of the interval. No ash beds are observed in this zone.
After studying the lithostratigraphic data, the section is split into five “zones”. Each zone has similar ratios of grainstones to wackestones, color, fossils, trace fossils or lack of trace fossils, and sedimentary structures. The five lithostratigraphic “zones” are named Zones A-E.
Zone A is from 0-24’ (0-7.32 m), Zone B is from 24-71’ (7.32-21.6 m), Zone C is from 71-184’
(21.6-56.1 m) and includes both missing portions in the entire measured section, Zone D is from
184-213’ (56.1-64.9 m), and Zone E is from 213-233’ (64.9-71.0 m). Based on samples that were physically collected from the section every six inches (15 cm), a ratio of grainstone to wackestone in each of the five selected zones is calculated. In Zone A 38.30% of samples collected are grainstones, 12.91% are grainstones in Zone B, 53.91% are grainstones in Zone C,
7.89% are grainstones in Zone D, and 35.0% are grainstones in Zone E. These same five “zones” are also used in the XRF section to compare geochemical data (see below).
32
i i
i
Inoceramid fossil Inoceramid
Trace fossil Trace
Ammonite fossil Ammonite
ą ţ
i
i
Parallel Laminations Parallel
Lenticular Bedding Bedding Lenticular swell) and (Pinch
WispyLaminations
Finer Grainstone Finer
Coarser Grainstone Coarser
i
(Argillaceous)
(Massive)
(Laminated)
ţ
i
Wackestone
Grainstone
Ash Bed Ash
Grainstone (Calcareous) Wackestone
i
i
ą
Buda Fm.Buda
Ernst Ernst Mbr.
Measured section Measured
. .
in Black Gap Wildlife Wildlife Gap Black in Management Area. Big Area. Management missing or represent “X’s” section. covered 33 12 Figure
Figure 13. Picture of lower portion of measured section 7’-14’ (2.1–4.2 m). The lower portion of the measured section (Zone A) is from base to ~24’ (0-7.30 m) and consists of dark gray to light gray thinly bedded wavy grainstones with interbedded thin wackestones similar to this interval.
34
Figure 14. Picture of two large ammonites (Acanthoceras amphibolum) on bedding plane at 12.5’ (3.8 m).
35
Figure 15. Picture of in situ inoceramid (green arrow) on top of calcareous wackestone at 18.5’ (5.6 m).
36
Figure 16. Picture of ash bed (green arrow) at 13.5’ (4.1 m). Ash beds occur throughout entire section and can be less than a centimeter to only a few centimeters thick.
37
Figure 17. Picture of lower portion of measured section 25’-32’ (7.6-9.8 m). Lower portion of section from 24’ to ~55’ (7.30-16.8 m) (Zone B) consists of yellow to orange thin bedded wackestones and with few grainstones interbedded.
38
Figure 18. Picture of lower portion of measured section at 62’ (18.9 m). Lower portion of section from ~55’ to ~71’ (16.8-21.6 m) (Zone B) consists of yellow to orange thicker bedded grainstones with thin wackestone interbeds.
39
Figure 19. Picture of horizontal burrows, possibly Chondrites (green arrows) at 71’ (21.6 m). These are the first noticeable trace fossils/burrows in entire section.
40
Figure 20. Picture of thick laminated grainstones (indurated) with interbedded flaggy wackestones (recessive). Picture is where outcrop continues after ~50’ (15.2 m) of missing section. This type of lithology occurs ~170-184’ (51.8-56.1 m) (Zone C) in section.
41
Figure 21. Picture of thin bedded wackestones with thin continuous and discontinuous grainstones interbedded. Picture is from 204’-209’ (62.2-63.7 m) in the section. This type of lithology is seen in the upper portion of the section from ~184’- 213’ (56.1-64.9 m) (Zone D).
42
Figure 22. Picture of thin bedded wackestones with thicker continuous and discontinuous grainstones interbedded. Picture is from 218-222’ (66.4-67.7 m) in the section. This type of lithology is seen in the upper most portion of the section from ~213’- 233’ (56.4-64.9 m) (Zone E).
43
Spectral Gamma Ray (SGR)
A total of 342 scans were taken with the Radiation Solution Incorporated RS-230 BGO
Super-SPEC portable gamma-spectrometer, six in the Buda Formation, and 336 in the Ernst
Member of the Boquillas Formation. The uranium, thorium, and potassium values collected by the device are used to calculate both compensated gamma ray (CGR) and total gamma ray
(TGR).
The values from these calculations are plotted versus measured section thickness (depth) and used as analogues for downhole logs (Fig. 23). Additionally, other outcrops that have been scanned with a spectral gamma ray device are correlated from section to section. To eliminate effects of uranium and gain a better understanding of clay elements, the CGR log can be used, but uranium can still be used to provide lithological information (Fairbanks et al., 2016). Since both CGR and TGR provide useful, but different information, it is important to compare them and not base determinations of facies from one alone (Fairbanks et al., 2016). Average SGR API values for TGR and CGR are calculated and are mentioned in the “XRF Calculated Gamma
Ray” section immediately following this section.
44
Figure 23. Plot of compensated gamma ray (CGR) and total gamma ray (TGR) from the portable Spectral Gamma Ray (SGR) tool versus measured section thickness.
45
ED-XRF
XRF Calculated Gamma Ray
XRF data can also be used to calculate gamma ray values. CGR and TGR are calculated and plotted versus measured section thickness to compare to CGR and TGR values from the spectral gamma ray (SGR) (Fig. 24). For each lithology present in the measured section
(grainstone and wackestone), the average gamma ray API value for both CGR and TRG for the
Spectral Gamma Ray and XRF machine are calculated (Table 1). All gamma ray API average values calculated are lower in grainstones and higher in wackestones (Table 1). Data from the
SGR shows grainstones have average CGR and TGR values of 19.8 API and 62.8 API, respectively, compared to wackestones that have average values of 20.0 API and 69.1 API, respectively (Table 1). Data from the XRF shows grainstones have average CGR and TGR values of 11.6 API and 56.0 API, respectively, compared to wackestones that have average values of 20.6 API and 125.0 API, respectively (Table 1).
XRF Elemental Abundances
Major elements are plotted versus measured section thickness with XRF CGR, to identify trends in elemental composition, and to potentially identify “zones” with similar elemental characteristics (Fig. 25 and 26). The five major “zones” identified lithostratigraphically (A-E), also show distinct chemical characteristics. Average abundances of major and trace elements for
“Chemo-zones” A-E are in Tables 2 and 3. Additionally, the percent weight of major elements and ppm of trace elements are calculated for each lithology identified in the measured section
(grainstones and wackestones) (Table 4). Grainstones have the highest amount of Ca and Mg
46 compared to wackestones, but the lowest amount of all other major and trace elements compared to wackestones (Table 4).
Zone A in this study is from 0-24’ (0.0-7.3 m) and has overall higher average XRF CGR values and greater abundance of Ca and Mg than the overlying Zone B. Abundance of these elements generally decreases in concentration upwards towards Zone B. Al, K, and Ti behave in an opposite manner as XRF CGR API, Ca, and Mg in Zone A. However, two thin intervals 3’
(~1.0 m) thick at 3-5’ (0.9-1.5 m) and 6-8’ (1.8-2.4 m) into the section exist in Zone A, where Ca and Mg decrease and Si, Al, K, and Ti increase. Mo and V in Zone A are in higher concentrations compared to other portions of the section and generally increase upwards towards
Zone B. U and Zr in Zone A are in higher concentrations compared to other portions of the section and generally decrease upwards towards Zone B. Ni, Zn, Th, and Rb are less abundant in
Zone A than overlying Zone B and generally increase in abundance as you move through Zone A towards Zone B.
The rocks in Zone B from ~24’ to ~71’ (7.30-21.6 m) contain the lowest average abundance of Ca and Mg in the measured section and higher overall averages of Mo, V, U, Ni,
Zn, Zr, Th, and Rb compared to the overlying Zone C. This could explain the very low ratio of grainstones to wackestones and overall much finer-grained appearance of beds observed in Zone
B compared to the underlying and overlying zones. Additionally, Ca and Mg behave in an inverse fashion compared to Si in Zone B. Ca and Mg decrease from the base of Zone B to ~50’
(15.2 m), and then start to increase towards Zone C. Si behaves in the opposite fashion. Si increases in abundance from the base of Zone B to ~50’ (15.2 m), then decreases towards the top of Zone C. Al, K, Ti, Mo, V, Ni, Zn, Zr, Th, and Rb are all more abundant in the lower 25’ (7.6 m) of Zone B and generally decrease upwards towards Zone C. U is the only elements that is
47 more abundant in the lower 25’ (7.6 m) of Zone B and generally increases upwards towards
Zone C.
Zone C is from 71-184’ (21.6-56.1 m) and includes both missing portions in the entire measured section. This section shows increased abundance of Ca and Mg and decreased abundance of Si, Al, K, and Ti compared to the underlying Zone B. Additionally, Zone C contains lower amounts of all trace elements compared to Zone B. Mo is almost absent in Zone
C except in an ~10’ (3.0 m) meter interval from 110-120’ (33.5-36.6 m). This interval also shows increased abundance of Si, Al, K, Ti, and all trace metals compared to the rest of Zone C.
Zone D is from 184’-213’ (56.1-64.9 m) and contains an overall lower amount of Ca,
Mo, and Zr compared to the underlying zone. Every other major and trace element is greater in average abundance in Zone D compared to Zone C. The low ratio (less than 10%) of grainstones in Zone D is supported by the increase in trace metals and lower abundance of Ca. Mo is in low abundance and only shows up in a less than 3’ (~1 m) interval in the zone. However, Mg is highest in Zone D, potentially indicating higher dolomite concentrations.
Zone E is from 213’-233’ (64.9-71.0 m) and contains a higher average amount of Ca,
Mo, and Ni compared to the underlying Zone D. All other major and trace elements are in less average abundance in Zone E compared to Zone D, and generally decrease as you move from the base to the top of Zone E towards the top of the measured section (Ca and Mg decrease upwards). The higher percentage of grainstones in Zone E (35.0%) could explain the increase in the average of Ca. Additionally, higher concentrations of Mo and V are observed throughout
Zone E compared to the underlying zone.
48
Figure 24. Plot of XRF and SGR compensated gamma ray (CGR) and total gamma (TGR) versus depth for the measured section.
49
Table 1. Average abundances of CGR and TGR API units for grainstones and wackestones identified in Black Gap measured section from collected samples for the XRF machine and Spectral Gamma Ray machine. Red shading shows lowest value of each row, green shows greatest value of each row. Standard deviation (SD) is shown in parentheses after average value.
Gamma Ray Type Grainstones Wackestones
XRF CGR 11.57 (1.74) 20.56 (7.48)
XRF TGR 56.01 (50.3) 125.0 (52.92)
SGR CGR 19.80 (12.8) 19.97 (14.5) SGR TGR 62.82 (27.3) 69.07 (33.8)
50
C E
B
A D
E are shaded in blue and black to show measured measured toshow black and inblue shaded are E -
that was not accessible. Interpreted “zones” A “zones” Interpreted accessible. not was that
Plot of major element abundances in weight percent versus measured section thickness. Blank spaces represent represent spaces Blank thickness. section measured versus percent in weight abundances element major of Plot
.
Figure 25 Figure outcrop or section missing coverage. section
51
C
B
A E D
Plot of trace element abundances in parts per million versus measured section thickness. thickness. section measured versus million per inparts abundances element trace of Plot . . 52
Figure 26 Figure Table 2. Average abundances of major elements in each zone identified in the measured section. Red shading shows lowest value of each column, green shows greatest value of each column. Standard deviation (SD) is shown in parentheses after average value. Zone Interval Ca Si Mg Al K Ti
E 213-233’ 29.40 (7.6) 9.880 (5.5) 1.392 (0.42) 1.886 (1.4) 0.468 (0.35) 0.103 (0.08)
D 184-213’ 27.04 (7.0) 12.16 (5.0) 1.434 (0.42) 2.408 (1.2) 0.636 (0.38) 0.145 (0.09)
C 71-184’ 31.11 (7.9) 7.994 (6.1) 1.394 (0.56) 1.431 (1.3) 0.290 (0.26) 0.085 (0.09)
B 24-71’ 22.79 (6.1) 13.05 (5.1) 0.780 (0.37) 2.078 (1.0) 0.282 (0.17) 0.109 (0.05)
A 0-24’ 23.62 (9.8) 12.56 (8.5) 0.961 (0.51) 1.850 (1.8) 0.103 (0.10) 0.098 (0.09)
53
Table 3. Average abundances trace elements (ppm) in each zone identified in the measured section. Red shading shows lowest value of each column, green shows greatest value of each column. Standard deviation (SD) is shown in parentheses after average value.
Zone Interval Mo V U Ni Zn Zr Th Rb
E 213-233’ 3.918 126.9 11.86 52.05 57.16 19.03 3.333 18.38
(10.1) (62.2) (6.16) (30.2) (62.3) (17.7) (1.08) (16.6)
D 184-213’ 0.141 137.7 12.37 41.90 72.04 21.56 3.458 20.79
(1.07) (91.0) (5.19) (25.8) (53.8) (12.4) (0.76) (11.3)
C 71-184’ 0.302 113.8 9.176 31.38 39.20 19.54 2.797 11.13
(1.60) (63.5) (6.11) (22.9) (42.6) (19.3) (0.69) (10.8)
B 24-71’ 11.51 237.9 12.45 51.23 79.95 42.55 3.265 17.21
(12.8) (117) (5.88) (20.8) (52.2) (19.1) (0.77) (9.95)
A 0-24’ 6.073 236.1 14.55 36.14 52.15 37.74 2.796 8.638
(9.91) (148) (9.68) (18.4) (38.7) (25.3) (0.45) (5.78)
54
Table 4. Average abundances of major and trace elements for grainstones and wackestones identified in Black Gap measured section from collected samples. Red shading shows lowest value of each row, green shows greatest value of each row. Standard deviation (SD) is shown in parentheses after average value.
Major Elements Grainstones Wackestones
%Ca 33.98 (4.40) 23.76 (7.65)
%Si 4.968 (2.61) 13.57 (5.69)
%Al 0.733 (0.55) 2.388 (1.25)
%Mg 1.327 (0.52) 1.099 (0.54) %K 0.120 (0.06) 0.443 (0.32)
%Na 0.123 (0.07) 0.209 (0.10)
%Ti 0.042 (0.02) 0.133 (0.08)
%Fe 0.636 (0.28) 1.026 (0.57) Trace Elements Grainstones Wackestones
Cr (ppm) 24.32 (21.6) 49.91 (30.70)
Mn (ppm) 0.021 (0.01) 0.022 (0.06)
Ni (ppm) 29.59 (17.3) 47.19 (25.5)
Zn (ppm) 23.79 (29.4) 74.24 (52.2)
As (ppm) 6.788 (22.9) 7.621 (20.4)
Pb (ppm) 8.941 (5.24) 11.81 (5.38)
Th (ppm) 2.418 (0.30) 3.400 (0.77)
Rb (ppm) 5.459 (3.17) 19.07 (11.7) U (ppm) 7.357 (6.20) 13.34 (6.20)
Sr (ppm) 727.9 (337) 910.7 (298)
Mo (ppm) 7.884 (5.87) 14.93 (12.6)
V (ppm) 95.95 (30.4) 203.6 (122) Zr (ppm) 13.02 (8.03) 35.46 (22.1)
Cu (ppm) 5.337 (5.30) 21.19 (16.3)
Ba (ppm) 493.1 (547) 760.1 (3120)
Nb (ppm) 3.1320 (0.94) 4.7361 (1.04)
55
XRF Enrichment Factors
The trace elements used to evaluate productivity (Zn, Ni, Cu) and redox conditions (Mo,
V, U) typically have a poor correlation to %Al, and therefore, they are thought to be non-detrital in origin and are used to interpret palaeoceanographic conditions (Tribovillard et al., 2006). The significance of these trace metals is determined by calculating their level of enrichment relative to the average shale (Wedepohl, 1971; 1991), known as an “Enrichment Factor” (Equation 1)
(Fig. 27).
Equation 1
Aluminum is used in the equation to normalize the concentrations because it is not affected by diagenesis or biological activity but is delivered to the system with other detrital material
(Tribovillard et al., 2008). Enrichment factors for redox sensitive elements and for productivity- sensitive elements are plotted versus measured section thickness (Fig. 28). Any value over 1 means the element is enriched compared to the average shale (Tribovillard et al., 2008).
Redox sensitive trace elements (U, V, Mo) are mobile in oxic systems and are forced to precipitate in oxygen depleted systems (Tribovillard et al., 2006). Manganese is also a redox sensitive trace element, but it behaves in the opposite way as V, U, and Mo. Mn is mobile in anoxic systems and is forced to precipitate in oxygenated systems (Tribovillard et al., 2006).
Average enrichment of Mo is highest in chemo-zones A and B averaging at 93.45 and 84.53, respectively. Additionally, average enrichment of V and U is highest in chemo-zone A and lowest in chemo-zone D like Mo. Average enrichment of Mn in lowest in chemo-zones A, B, and D averaging at 6.446 (A), 4.412 (B), and 2.552 (D), respectively (Table 5).
56
Paleoproductivity is best estimated using elements that are delivered to the sediments bound in organic matter. Ni, Cu, and Zn are important bio-essential trace elements that can be used for this purpose (Tribovillard et al., 2006). Average enrichment of Zn and Cu is also highest in Chemo- zones A and B. Zn has an average Enrichment Factor of 6.332 in Chemo-zone A and 6.594 in B.
Cu has an average Enrichment Factor of 2.924 in Chemo-zone A and 4.195 in B (Table 5).
However, Ni enrichment is highest in zones C and E averaging at 12.50 and 17.37, respectively.
Enrichment Factors of Zn and Cu are highest in zones A and B and decrease in zones C and D.
Zone E shows and increase in these elements compared to zones C and D (Table 5).
Patterns of uranium–molybdenum covariation in marine sediments can potentially provide insights into depositional conditions and processes in paleoceanographic systems, specifically bottom water redox conditions (Tribovillard et al., 2012). Molybdenum enrichment factor is plotted versus uranium enrichment factor to better understand paleoceanographic conditions present during deposition of sediment in the section (Fig. 29). Most points plot in the anoxic to euxinic areas with only a few points plotting in the Particulate Shuttle area. One thing to note is that 125 samples have no Mo and are not plotted. Additionally, the points are colored by chemofacies type, which are described in the next section. Most all of the samples that contain Mo are siliceous carbonate mudstones (Facies 1) and mixed mudstones (Facies 2), with only a few being carbonate-dominated mudstones (Facies 3) and mixed carbonate mudstones
(Facies 4).
57
Figure 27. Trace metal abundances in average shale determined by Wedepohl, 1971,1991 (green box) (From Tribovillard et al., 2006).
58
E
D
C B
A
e”. Any value value Any e”.
are shaded in blue and black. in shaded blue are
E E
-
Plot of enrichment factors and CGR versus measured section thickness. Enrichment factors are comparisons to the “Average Shal theto“Average comparisons are factors Enrichment thickness. section measured and versus CGR factors enrichment of Plot . .
59
Figure 28 Figure A zones Selected enriched. considered is over 1
Table 5. Table of average Enrichment Factor values for selected trace elements in selected chemo-zones (A-E). Red shading shows lowest value of each column, green shows greatest value of each column. Standard deviation (SD) values are shown in parentheses after the average value.
Zone Interval MoEF VEF UEF MnEF NiEF ZnEF CuEF
E 213-233’ 41.89 (203) 24.22 (52.2) 68.19 (138) 15.62 (56.1) 17.24 (36.8) 6.301 (10.3) 1.263 (1.20)
D 184-213’ 0.744 (5.66) 9.256 (9.28) 38.69 (33.9) 2.571 (3.79) 6.281 (8.45) 5.548 (3.37) 2.011 (1.86)
C 71-184’ 1.758 (8.34) 21.96 (26.7) 48.98 (75.3) 7.409 (10.1) 12.36 (17.4) 5.521 (5.85) 1.783 (3.03)
B 24-71’ 84.53 (125) 17.94 (16.1) 43.04 (51.6) 4.431 (16.1) 7.908 (8.52) 6.594 (4.40) 4.195 (3.25)
A 0-24’ 93.45 (364) 31.34 (44.7) 69.09 (87.5) 6.446 (14.3) 12.20 (21.6) 6.332 (6.42) 2.924 (3.95)
60
MoEF vs. UEF
euxinic
EF
Mo anoxic
Facies 4
Facies 3 Facies 2 suboxic Facies 1
UEF
Figure 29. Cross plot of molybdenum enrichment factor versus uranium enrichment factor. Both scales are logarithmic. Blue lines represent Mo/U values of modern seawater and multiples thereof. One thing to note is these points represent samples that contain Mo, samples that do not contain Mo are excluded from this graph. Additionally, the points are colored by chemofacies type which is discussed in the following section.
UEF
61
XRF Chemofacies
Chemofacies can be determined through Hierarchal Cluster Analysis (HCA), which uses all available elements identified by the XRF and groups them together based on certain relationships with each other. The HCA was carried out using Tibco Spotfire software and within the software, Ward’s method (Ward, 1963), half square Euclidean distance, and no normalization was used (Turner et al., 2015). In Ward’s method, each value is given a grouping number and then each group is compared to the group that immediately follows until the total number of groups is reduced by n-1 (Ward, 1963). This is done until n=1 (Ward, 1963). The major and trace elements that were imported into the cluster analysis were Cr, Mn, Ni, Zn, As, Pb, Th, Rb, U, Sr,
Zr, Nb, Na, Mg, Al, Si, P, K, Ca, Ti, V, and Fe. Commonly, a dendrogram is produced to visualize how the groupings cluster together after the analysis is done (Fig. 30). An arbitrary red line is used and slid to the left or right to choose the number of clusters. For this project four facies are identified that split up the section adequately (Fig. 30). Bentonites were initially included in HCA but were removed when determining chemofacies because Ward’s method would classify each bentonite as its own unique chemofacies. Chemofacies type is plotted versus depth to show how facies are distributed throughout the measured section (Fig. 31).
In conjunction with the HCA, the abundance of three primary oxides can be estimated from stoichiometric conversions of Ca, Al, and Si (Brumsack, 1989). These oxides are plotted on a ternary diagram to show variations between each chemofacies (Fig. 32) Marine sediments are most often comprised of biogenic carbonates (CaO), aluminosilicates (Al2O3) and silica (SiO2), and biogenic silica (SiO2). The plotted points can be compared to the calculated values of the
“Average Shale” determined by Wedepohl (1971, 1991). Most of the data plot below the
62
Average Shale. Facies 3 plots near the CaO portion of the diagram and with limited input of
Al2O3, all other facies trend towards the SiO2 endmember of the ternary diagram (Fig. 32).
Chemofacies 1 and Chemofacies 4 contain similar abundances of most major and trace elements (Table 6). The major elements with the highest variability between the two chemofacies are Mg, Ca, K, and Ti. The trace elements in this same category are Zr and V. Chemofacies 1 has less Mg, Ca, K, and Ti than Chemofacies 4 with 0.73% (Mg), 20.5% (Ca), 0.23% (K), and
0.12% (Ti), respectively. For comparison, Chemofacies 4 contains 1.40% (Mg), 25.6% (Ca),
0.64% (K), and 0.16% (Ti). Chemofacies 1 has almost twice as much Zr and V as Chemofacies 4 with 43.6 ppm to 22.4 ppm for Zn and 252.3 ppm to 134.4 ppm for V, respectively. In order to assign a lithofacies name to each chemofacies identified in the measured section, the Gamero-
Diaz et al. (2012) classification scheme for organic mudstones is used (Fig. 32 & 33).
Chemofacies 1 is classified as a carbonate/siliceous mudstone with some mixed carbonate mudstone and Chemofacies 4 is classified as a mixed carbonate mudstone with some mixed mudstone.
Chemofacies 2 has the lowest abundance of Ca at 18.2 %. This chemofacies, however, contains the highest abundance of Al, Si, K, Ti, and Fe averaging at 3.60%, 17.2%, 0.85%,
0.21% and 1.48%, respectively. Additionally, this chemofacies contains the highest abundance for 10 out of 12 trace elements. Only Mn and Sr are more abundant in other chemofacies. This chemofacies is classified as a mixed mudstone with some carbonate/siliceous mudstone.
Chemofacies 3 has the highest abundance of Ca and Mg averaging at 34.7% and 1.4%, respectively. Additionally, this chemofacies also has the lowest abundance of Al, Si, P, K, Ti, Na and Fe averaging at 0.78% (Al), 5.13% (Si), 0.06% (P), 0.15% (K), 0.04% (Ti), 0.11% (Na) and
0.61% (Fe), respectively. This chemofacies also contains the lowest abundance for 10 out of 12
63 trace elements. Only Mn and As are less abundant in other chemofacies. Chemofacies 3 is mostly a carbonate dominated lithotype with some silica-rich carbonate mudstone.
For each chemo-zone the abundance of each chemofacies type is calculated (Table 7).
Zone A contains mostly siliceous carbonate mudstone (Facies 1) and carbonate dominated mudstone (Facies 3) averaging 63.8% and 34.0%, respectively. There is a minor amount of
Facies 4 in Zone A averaging 2.13%. Facies 2 does not appear in Zone A. Zone B contains mostly siliceous carbonate mudstone (Facies 1) with similar amounts of mixed mudstone (Facies
2) and carbonate dominated mudstone (Facies 3) averaging 67.7%, 14.0%, and 17.2%, respectively. Like Zone A, Zone B also contains a small amount of mixed carbonate mudstone
(Facies 4) averaging 1.11%. Zone C is largely composed of carbonate dominated mudstones
(Facies 3) averaging 66.6%. The second most abundant facies are mixed carbonate mudstones
(Facies 4) 18.8%. Siliceous carbonate mudstones (Facies 1) and mixed mudstones (Facies 2) are in similar abundance in Zone C averaging 8.60% and 6.45%, respectively. Zone D is composed mostly of mixed carbonate mudstones (Facies 4) averaging 58.1%. Carbonate dominated mudstones (Facies 3) and mixed mudstones (Facies 2) have similar abundances in Zone D averaging 24.2% and 16.1%, respectively. A minor amount of siliceous carbonate mudstones
(Facies 1) are in Zone D averaging 1.61%. Lastly, Zone E is composed mostly of carbonate dominated mudstones (Facies 3) averaging 52.8%. Mixed carbonate mudstones (Facies 4) are second in abundance in Zone E averaging 36.1%. A minor amount of mixed mudstones occur
Zone E averaging 11.1%. No siliceous carbonate mudstones occur in Zone E.
Finally, each sample collected for geochemical analysis is identified as either a grainstone or a wackestone, and the appropriate chemofacies type for each is identified. Samples that are identified as grainstones consist of 93.88% of carbonate dominated mudstones (Facies 3)
64 and 16.12% of siliceous/carbonate mudstones (Facies 1). Samples that are identified as wackestones consist of 42.06% siliceous/carbonate mudstones (Facies 1), 14.16% mixed mudstones (Facies 2), 15.02% carbonate dominated mudstones (Facies 3), and 28.76% mixed carbonate mudstones (Facies 4).
65
Figure 30. Dendrogram produced from TIBCO Spotfire computer software created from Hierarchal Cluster Analysis (HCA). Dashed red line indicates the arbitrary cutoff line for number of divisions (chemofacies).
66
E
D
Facies 4 Facies 3 Facies 2
Facies 1 C Thickness (ft) Thickness
B
A
Chemofacies type
Figure 31. Bar graph produced from TIBCO Spotfire software showing depth vs. chemofacies type. Black shaded area represents missing or covered section (Facies 2). Chemo-zones A-E are shown on the right side.
67
5 x Al2O3
Facies 4 Facies 3 Facies 2 Facies 1
2 x CaO SiO2
Figure 32. Ternary diagram showing trends in mineral composition and color identifying chemofacies. Data plotted with the “Average Shale” (A.S.) marked by red triangle (Wedepohl, 1971, 1991).
68
Clay
Three primary mudstone classes: - Siliceous (50% < Si < 80%) - Argillaceous (50% < Clay < 80%) - Carbonate (50% < Carb < 80%)
20 80 16 organic mudstone lithofacies
50 50
80 20
20 50 80 QFM Carbonate
Figure 33. Ternary diagram with mudstone classes listed based on relative abundance of clay, carbonate, and quartz/feldspar (From Gamero-Diaz et al., 2012)
69
Table 6. Average abundances of trace elements and major elements in each chemofacies identified in the section. Red shading shows lowest value of each row, green shows greatest value of each row. Standard deviation (SD) values are shown in parentheses after the average value.
Major Elements Facies 1 Facies 2 Facies 3 Facies 4
%Ca 20.55 (6.33) 18.16 (3.58) 34.71 (3.55) 25.64 (5.52)
%Si 14.74 (5.91) 17.22 (3.16) 5.126 (1.92) 13.39 (4.60)
%Al 2.204 (1.27) 3.603 (0.73) 0.775 (0.54) 2.705 (0.90)
%Mg 0.734 (0.40) 0.940 (0.38) 1.433 (0.48) 1.401 (0.45)
%K 0.233 (0.14) 0.846 (0.33) 0.148 (0.08) 0.639 (0.25)
%Na 0.247 (0.09) 0.274 (0.09) 0.110 (0.06) 0.178 (0.07)
%Ti 0.117 (0.07) 0.207 (0.06) 0.042 (0.02) 0.160 (0.09)
%Fe 1.056 (0.67) 1.483 (0.37) 0.606 (0.19) 0.975 (0.46)
Trace Elements Facies 1 Facies 2 Facies 3 Facies 4
Cr (ppm) 52.18 (32.2) 80.89 (39.6) 24.35 (13.5) 40.84 (18.9)
Mn (ppm) 0.017 (0.003) 0.016 (0.002) 0.028 (0.077) 0.020 (0.014)
Ni (ppm) 46.23 (15.7) 70.74 (24.2) 28.67 (15.1) 46.43 (33.6)
Zn (ppm) 71.83 (38.3) 149.6 (60.1) 21.93 (23.9) 65.80 (35.3)
As (ppm) 7.340 (19.9) 9.207 (4.03) 8.005 (28.9) 5.360 (1.38)
Pb (ppm) 13.37 (5.07) 13.58 (3.76) 9.074 (6.31) 9.569 (2.58)
Th (ppm) 3.165 (0.49) 4.718 (0.60) 2.498 (0.39) 3.386 (0.52)
Rb (ppm) 14.33 (7.28) 39.01 (10.8) 6.394 (3.74) 20.40 (7.71)
U (ppm) 14.40 (7.28) 15.65 (4.85) 7.955 (5.78) 12.26 (4.73)
Sr (ppm) 855.4 (361) 790.1 (166) 781.1 (327) 1032 (227)
Mo (ppm) 14.22 (11.7) 18.05 (13.4) 12.54 (15.5) 3.015 (1.69)
V (ppm) 252.3 (118) 305.9 (118) 92.71 (21.1) 134.4 (63.5)
Zr (ppm) 43.62 (20.3) 58.69 (16.4) 12.28 (7.83) 22.38 (9.30)
Cu (ppm) 20.71 (13.4) 40.18 (19.3) 4.791 (4.73) 15.62 (10.6)
Ba (ppm) 586.6 (1016) 362.9 (345.0) 456.0 (267.1) 1404 (5668)
Nb (ppm) 4.548 (0.91) 5.915 (0.87) 3.334 (1.02) 4.755 (0.87)
70
Table 7. Average abundances of facies in selected chemo-zones (A-E). Red shading shows lowest value of each row, green shows greatest value of each row.
Zone Interval Facies 1 Facies 2 Facies 3 Facies 4
E 213-233’ 0.00% 11.1% 52.8% 36.1%
D 184-213’ 1.61% 16.1% 24.2% 58.1%
C 71-184’ 8.60% 6.45% 66.7% 18.7%
B 24-71’ 67.7% 14.0% 17.2% 1.11%
A 0-24’ 63.8% 0.00% 34.0% 2.13%
71
XRF Cross Plots
For this section, several major and trace elements are plotted versus Al to see if they are of detrital origin since Al is typically of detrital origin and is generally thought to be immovable during diagenesis (Tribovillard et al., 2008). Major elements (Ca, Si, K, and Ti) are plotted versus Al, trace elements (Zr, Rb, Th, Zn, U, V, Ni, Cu and Mo) are also plotted versus Al, and
Ca is plotted versus Mg (Figs. 34-47).
The cross-plot of %Si versus %Al (Fig. 34) for the chemical data shows a moderate positive linear trend between these two elements with an R² value of 0.495, which indicates that the Si is potentially contained within the clay (e.g. illite) component. Points that plot well above the trendline suggests an enrichment of silica by either detrital (quartz) or biogenic (radiolarian) origin. Si enrichment can be seen in approximately twenty points, which plot above the illite- silica mixing line. Most points that plot above the mixing line are siliceous carbonate mudstones
(Facies 1) with a few points being mixed mudstones (Facies 2) and mixed carbonate mudstones
(Facies 4). Most other samples plot below the illite-silica mixing line and are diluted by Ca.
A cross-plot of %K versus %Al for the chemical data shows a positive trend (Fig. 35), which possibly indicates that K is associated with the main clay phase (Tribovillard et al., 2006).
Points plotting below this trend could indicate a change in clay chemistry from illite dominated to more rich in clays that do not contain K, such as smectite or kaolinite, which has been found in rocks at Ernst Tinaja using XRD analysis (PNR (internal study)). The cross-plot shows a moderate positive linear trend with a R² value of 0.639 (Fig. 35). Most mixed mudstones (Facies
2) and mixed carbonate mudstones (Facies 4) plot above the trendline. Most of the points that plot along or below the trendline are siliceous carbonate mudstones (Facies 1) and carbonate dominated mudstones (Facies 3) (Fig. 35).
72
A cross-plot of %Ti versus %Al for the chemical data shows a moderate positive linear trend with an R² value of 0.545 (Fig. 36). Ti is reflective of detrital inputs due to its ultra-low seawater abundance and lack of participation in biologic cycling (Tribovillard et al., 2006). It is because of this that Ti, instead of Al, is sometimes used, as a proxy for clay content (Tribovillard et al., 2006). Most points plot along the trendline with five or six plotting well above it that are siliceous carbonate mudstones (Facies 1) and mixed carbonate mudstones (Facies 4) (Fig. 36).
Cross-plots of Zr, Rb, and Th (Figs. 37-39) show moderate positive trends when plotted versus Al. This suggests these trace elements are part of the detrital fraction and are mostly coming into the system with the Al. Siliceous carbonate mudstones (Facies 1) and mixed mudstones (Facies 2) have the most Zr (Fig. 37), mixed mudstones (Facies 2) have the most Rb and Th (Fig. 38 & 39).
A cross-plot of %Ca versus %Al for the chemical data shows a moderate negative linear trend with an R² value of 0.465 (Fig. 40). Al is used as a proxy for the clay content, whereas Ca is used to represent the carbonate component. The moderate negative slope of the Ca relative to the Al suggests the clay content in the system is reduced during carbonate deposition. The cross- plot clearly shows carbonate dominated lithotypes (Facies 3) have more Ca content than the other facies (Fig. 40).
A cross-plot of %Ca versus %Mg for chemical data shows a moderate positive linear trend (Fig. 41). This comparison is used to show the relationship between Mg and the calcium carbonate phase. A higher percentage of Mg could indicate the possibility of another carbonate mineral such as dolomite (CaMg(CO3)2), which has been found in rocks at Ernst Tinaja using
XRD (PNR (internal study)). Carbonate dominated mudstones (Facies 3) mixed carbonate mudstones (Facies 4) contain much more Mg and Ca than the other two facies (Fig. 41).
73
Finally, the cross plots of trace elements Mo, Ni, V, Cu, U, and Zn against Al (Figures
42-47) show that these trace elements are poorly correlated with Al, indicating that their concentration is not related to the abundance of detrital mineral fractions (Tribovillard et al.,
2006). The elements Mo, V, U, Cu, Ni, and Zn are all commonly used as proxies for redox conditions and biological productivity and their lack of correlation with Al in the section suggests that they are appropriate for palaeoceanographic reconstruction (Tribovillard et al.,
2006). Mo is mostly contained in siliceous carbonate mudstones (Facies 1) and mixed mudstones
(Facies 2) with only a few points in the other two facies (Fig. 42). Additionally, siliceous carbonate mudstones (Facies 1) and mixed mudstones (Facies 2) contain more V than the other two facies (Fig. 43). Carbonate dominated mudstones (Facies 3) have the least amount of U with the other three facies containing similar amounts of U (Fig. 44). Mixed mudstones (Facies 2) contain the most Ni with the other three facies containing similar amounts of Ni (Fig. 45). Mixed mudstones (Facies 2) contain the most Cu, siliceous carbonate mudstones (Facies 1) and mixed carbonate mudstones (Facies 4) contain a similar amount of Cu, and carbonate dominated mudstones (Facies 3) contain the least amount of Cu (Fig. 46). Finally, mixed mudstones (Facies
2) contain the most Zn, siliceous carbonate mudstones (Facies 1) and mixed carbonate mudstones (Facies 4) contain a similar amount of Zn, and carbonate dominated mudstones
(Facies 3) contain the least amount of Zn (Fig. 47).
74
%Si vs. %Al
Facies 4 Facies 3 Facies 2
Facies 1
Si %
%Al
Figure 34. Cross plot of weight percent silica versus weight percent aluminum. Solid white line represents overall trend between all samples. Dashed white line represents illite-silica mixing line. Points plotted above line are in the “silica excess” zone.
75
%K vs. %Al
Facies 4 Facies 3 Facies 2
Facies 1
K %
%Al
Figure 35. Cross plot of weight percent potassium versus weight percent aluminum. Solid white line represents overall trend between samples.
76
%Ti vs. %Al
Facies 4 Facies 3 Facies 2
Facies 1
Ti %
%Al
Figure 36. Cross plot of weight percent titanium versus weight percent aluminum. Solid white line represents overall trend between samples.
77
%Zr vs. %Al
Facies 4 Facies 3 Facies 2
Facies 1
Zr %
%Al
Figure 37. Cross plot of parts per million zirconium versus weight percent aluminum. Solid white line represents overall trend between samples.
78
%Rb vs. %Al
Facies 4 Facies 3 Facies 2
Facies 1
Rb %
%Al
Figure 38. Cross plot of parts per million rubidium versus weight percent aluminum. Solid white line represents overall trend between samples.
79
%Th vs. %Al
Th %
Facies 4 Facies 3 Facies 2 Facies 1
%Al
Figure 39. Cross plot of parts per million thorium versus weight percent aluminum. Solid white line represents overall trend between samples.
80
%Ca vs. %Al Facies 4
Facies 3 Facies 2 Facies 1
Ca %
%Al
Figure 40. Cross plot of weight percent calcium versus weight percent aluminum. Solid white line represents overall trend between samples.
81
%Ca vs. %Mg
Ca %
Facies 4 Facies 3 Facies 2 Facies 1
%Mg
Figure 41. Cross plot of weight percent calcium versus weight percent magnesium. Solid white line represents overall trend between samples.
82
ppm Mo vs. %Al %Mg Facies 4 Facies 3 Facies 2
Facies 1
ppm Mo ppm
r2=0.008
%Al
Figure 42. Cross plot of parts per million molybdenum versus weight percent aluminum. Solid white line represents overall trend for between samples.
83
ppm V vs. %Al
Facies 4 Facies 3 Facies 2
Facies 1
ppmV
%Al
Figure 43. Cross plot of parts per million vanadium versus weight percent aluminum. Solid white line represents overall trend for between samples.
84
ppm U vs. %Al Facies 4 Facies 3 Facies 2
Facies 1
ppmU
%Al
Figure 44. Cross plot of parts per million uranium versus weight percent aluminum. Solid white line represents overall trend for between samples.
85
ppm Ni vs. %Al Facies 4 Facies 3 Facies 2
Facies 1
ppmNi
%Al
Figure 45. Cross plot of parts per million nickel versus weight percent aluminum. Solid white line represents overall trend for between samples.
86
ppm Cu vs. %Al Facies 4 Facies 3 Facies 2
Facies 1
r2=0.170 ppmCu
%Al
Figure 46. Cross plot of parts per million copper versus weight percent aluminum. Solid white line represents overall trend for between samples.
87
ppm Zn vs. %Al Facies 4 Facies 3 Facies 2
Facies 1
ppmZn
%Al
Figure 47. Cross plot of parts per million zinc versus weight percent aluminum. Solid white line represents overall trend for between samples.
88
XRF Brittleness Index (BI)
XRF Brittleness Index is used to characterize portions of the Ernst Member that are brittle and favorable for hydraulic fracturing. Various calculations to derive brittleness from geochemical data exist and Wang and Gale (2009) proposed calculations based on the following equation.
However, available data is elemental data and Denne (pers. comm., 2019) suggests that most silica is in the clay minerals. Therefore, major elements used were Mg and Ca for the numerator and the same elements were used for the denominator with the addition of Si, Ti, K, and Al for clays. The average BI value for the entire section is 0.679 with the highest being
0.948 and the lowest being 0.008. Additionally, the average BI is computed for the five zones
(A-E) in the section (Figure 48, Table 8). Zone A is from 0-24’ (0-7.32 m) and has an average BI value of 0.626. Zone B is from 24-71’ (7.32-21.6 m) and has an average BI value of 0.609. Zone
C is from 71-184’ (21.6-56.1 m) and has an average BI value of 0.774. Zone D is from 184-213’
(56.1-64.9 m) and has an average BI value of 0.654. Zone E is from 213-233’ (64.9-71.0 m) and has an average BI value of 0.723. Finally, when comparing average BI values based on lithology
(grainstone versus wackestone), grainstones have a higher BI value (0.859) than wackestones
(0.603).
89
CGR XRF BI
E
D
C
Section thickness (ft) Section thickness
B
A
Figure 48. Plot of XRF CGR and XRF Brittleness Index (BI) versus measured section thickness. Selected zones (A-E) are shaded in blue and black.
90
Zone Interval Brittleness Index (BI) Table 8. Table of average Brittleness Index (BI) values for selected zones within measured section. Red shading shows lowestE value213 of -each233 ’column, green0.723 shows (0.161) greatest value of each column.
D 184-213’ 0.654 (0.152)
C 71-184’ 0.774 (0.174) B 24-71’ 0.609 (0.148)
A 0-24’ 0.626 (0.248)
91
Dimpler
Dimpler ticks (DT’s) were measured to the nearest tenth of a tick and then converted to
UCS using the Enderlin (2017, pers. comm.) empirical equations. If DT’s are less than 5.5, the following equation is used:
-1.86399 2 3 4 UCS=(154978.9*DT )+(-12992.97+(60507.13*DT)+(-27346.5*DT )+(4293.111*DT )+(- 226*DT ))
If DT are 5.5 or greater, the following equation is used:
-1.86399 UCS = 154978.9 * DT
Dimpler UCS values in psi (MPa) are calculated for the entire section from DT’s. The average
UCS for the entire section is 6,566 psi (45.27 MPa) with the lowest value being 2,120 psi (14.62
MPa) and the largest being 20,091 psi (138.5 MPa). Additionally, a mechanical stratigraphy is established and the average UCS is computed for three selected “strength sections (SS)” (1-3)
(Table 9, Figure 49). SS #1 is from 0-55’ (0.-16.8 m) and has an average UCS of 5,242 psi
(36.14 MPa). SS #2 is from 55-208’ (16.8-63.4 m) and has an average UCS of 6,911 psi (47.65
MPa). SS #3 is from 208-233’ (63.4-71.0 m) and has an average UCS of 7,294 psi (50.29 MPa).
Additionally, the average UCS value in psi (MPa) is calculated for each lithology type identified in the measured section (grainstone and wackestone). Grainstones have an average UCS value of
8,779 psi (60.53 MPa) and wackestones are on average weaker with an average UCS value of
5,538 psi (38.18 MPa).
92
Analyzing the cross plots colored by chemofacies, certain trends can be identified when comparing the UCS data to XRF BI and common major elements like Ca, Si, and Al (Figs. 50-
53). UCS versus XRF BI shows a weak positive trend with an R2 value of 0.22. UCS versus %Ca shows a weak positive trend with an R2 value of 0.20. UCS versus %Si shows a weak negative trend with an R2 value of 0.18. UCS versus %Al shows weak negative trend with an R2 value of
0.30. It appears Dimpler UCS data has the best correlation with %Al with an R2 value around
0.30. Carbonate dominated mudstones (Facies 3) have the highest average UCS and XRF BI values at 8514 psi (58.70 MPa) and 0.859, respectively (Table 10). Mixed carbonate mudstones
(Facies 4) have the second highest average UCS value and XRF BI value at 5769 psi (39.78
MPa) and 0.625, respectively (Table 10). Siliceous carbonate mudstones (Facies 1) have the third highest average UCS value and XRF BI value at 5435 psi (37.47 MPa) and 0.556, respectively
(Table 10). Mixed mudstones (Facies 2) have the lowest average UCS and XRF BI values at
3874 psi (26.71 MPa) and 0.477, respectively (Table 10).
93
3
2
Thickness
1
UCS 94 Figure 49. Plot of measured section thickness versus Dimpler UCS (psi) colored by chemofacies type for Black Gap measured section in this study. Table 9. Table of average Dimpler UCS (psi) and Brittleness Index (BI) for selected “strength sections” within measured section. Red shading shows lowest value of each column, green shows greatest value of each column. Standard deviation (SD) values are shown in parentheses after the average value.
Section Interval Dimpler UCS (psi) Dimpler UCS (MPa)
3 208-233’ 7294 (3480) 50.29 (23.99)
2 55-208’ 6911 (2778) 47.65 (19.15)
1 0-55’ 5242 (2058) 36.14 (14.19)
95
UCS (psi) UCS
XRF BI
Figure 50. Cross plot of Dimpler UCS (psi) versus XRF BI value.
96
Dimpler UCS vs. %Ca
Facies 4 Facies 3 Facies 2
Facies 1 Dimpler UCS (psi) UCS Dimpler
%Ca
Figure 51. Cross plot of Dimpler UCS (psi) versus %Ca colored by chemofacies type.
97
Dimpler UCS vs. %Si
Facies 4 Facies 3 Facies 2
Facies 1 Dimpler UCS (psi) UCS Dimpler
%Si
Figure 52. Cross plot of Dimpler UCS (psi) versus %Si colored by chemofacies type.
98
Dimpler UCS vs. %Al
Facies 4 Facies 3 Facies 2
Facies 1
Dimpler UCS (psi) UCS Dimpler
%Al
Figure 53. Cross plot of Dimpler UCS (psi) versus %Al colored by chemofacies type.
99
Table 10. Table of average Dimpler UCS (psi), XRF Brittleness Index (BI), and major elements for chemofacies. Red shading shows lowest value of each row, green shows greatest value of each row. Standard deviation (SD) values are shown in parentheses after the average value.
Strength & Facies 1 Facies 2 Facies 3 Facies 4
Brittleness
Dimpler UCS (psi) 5436 (1940) 3874 (879.0) 8514 (2983) 5769 (1850)
XRF BI 0.556 (0.164) 0.477 (0.082) 0.859 (0.051) 0.625 (0.123)
%Ca 20.55 (6.33) 18.16 (3.58) 34.71 (3.55) 25.64 (5.52)
%Si 14.74 (5.91) 17.22 (3.16) 5.126 (1.92) 13.39 (4.60)
%Al 2.204 (1.27) 3.603 (0.73) 0.775 (0.54) 2.705 (0.90)
%Mg 0.734 (0.40) 0.940 (0.38) 1.433 (0.48) 1.401 (0.45)
%K 0.233 (0.14) 0.846 (0.33) 0.148 (0.08) 0.639 (0.25)
%Ti 0.117 (0.07) 0.207 (0.06) 0.042 (0.02) 0.160 (0.09)
%Fe 1.056 (0.67) 1.483 (0.37) 0.606 (0.19) 0.975 (0.46)
100
Discussion
For the discussion section, data obtained for this study from the Ernst Member in the
Black Gap Wildlife Management Area are compared and interpreted separately according to lithological data, geochemical data, and mechanical data. Results and interpretations from this study are then compared to proximal and distal studies that have obtained similar data. Four other investigations exist that have gathered geochemical, lithological, and mechanical data from
Ernst Member outcrops that are proximal to this study in the Trans-Pecos area. Fry (2015) gathered XRF, XRD, spectral gamma ray (SGR), biostratigraphic, and lithological data from the
Hot Springs Boquillas outcrops in Big Bend National Park (BBNP) ~23 miles (37.0 km) southwest of this study and divided the Ernst Member into upper and lower sections. However,
Fry (2015) only obtained 35 samples for XRF and XRF analysis in 275’ (83.8 m) of Ernst
Member measured. PNR (internal study) gathered XRF, XRD, mechanical, spectral gamma ray, biostratigraphic, and lithological data from the Ernst Tinaja Boquillas outcrops in BBNP ~17 miles (23.4 km) southwest of this study. PNR (internal study) did not separate the Ernst Member into sections and a sampling interval of every meter resulted in a total of 80 samples in 283’
(86.3 m) of measured Ernst Member. Frebourg et al. (2016) conducted a detailed sedimentological study on the Ernst Member at Ernst Tinaja. Finally, Wehner et al. (2017) collected 172 samples for lithostratigraphic, chemostratigraphic, and isotopic analysis at Hot
Springs where Fry (2015) reported on the Ernst Member. Wehner et al. (2017) divide the Ernst
Member at Hot Springs into the Woodbine, Lower Eagle Ford, and Upper Eagle Ford and correlate the sections eastward to Lozier Canyon and Webb County in the south Texas subsurface. After comparisons are made to proximal studies in BBNP, they are made to previous studies conducted on the Ernst Member farther away.
101
Lithostratigraphy and chemostratigraphy by zone
Outcrop and geochemical data obtained in this study suggests that the Ernst Member in the Black Gap area contains only three lithofacies (wackestones, grainstones, bentonites) and four chemofacies (Facies 1-4). Wackestones vary in amount of argillaceous and calcite material throughout the section. These lithofacies and chemofacies can then be grouped into five zones
(1-5). Additionally, UCS data from the Dimpler shows the Ernst Member can be grouped into three distinct strength sections (1-3).
Beds in the section in Black Gap can be laterally extensive for 20-30+ meters, or for less than a meter. Ash beds and most wackestones are commonly laterally extensive for several meters, whereas grainstones can be less than a meter in length, laterally pinch-out, then re- appear. Additionally, faint sedimentary structures can be seen in grainstones, which protrude out of the more recessive wackestones. Sedimentary structures cannot be seen in wackestones due to weathering. Fossil and trace fossil content is rare in the Ernst in Black Gap and the entire section is heavily weathered to dark gray to pink and orange color.
Zone A
Zone A is interpreted to represent deposition in an environment below storm-wave base in bottom waters that were mostly anoxic, but experienced periods of re-oxygenation as supported by inoceramid fossils and ammonites in outcrop. Overall, the lack of trace fossils and small numbers of benthic organisms along with the presence and enrichment of Mo, V, and U, suggests anoxic/euxinic bottom waters. Over one-third (38.3%) of the samples collected in this zone are grainstones, and wackestones appear more calcite rich and coarser-grained than the wackestones in the overlying Zone B. The high ratio of grainstones to wackestones along with the high calcite content in the wackestones, suggests surface to intermediate waters were rich in
102 organisms containing CaCO3. This is supported by a high average enrichment of Ni, an important paleoproductivity indicator, which suggests productivity in surface and intermediate waters was high compared to the overlying zone. Additionally, this is confirmed by higher average amount of Ca and lower amounts of Al, K, and Ti in Zone A compared to Zone B. Clay material is minimal in this zone compared to the above zone (Zone B) and generally increases further up towards Zone B suggesting calcite input became diluted with clay towards the upper portion of zone A. There are two ~5- 10’ (~1.5-2.0 m) thick intervals in Zone A, where Ca and
Mg decrease and all trace elements identified by XRF increase. These two intervals are predominantly composed of wackestones, which are siliceous/carbonate mudstones
(Chemofacies 1) based on HCA. Cross stratification in grainstones is unidirectional in this zone
(and all zones), and no hummocky cross stratification is seen, which suggests sedimentary structures were caused by reworking of pelagic material by bottom-water currents.
Two large ammonites (Acanthoceras amphibolum; R. Denne, pers. comm., 2018) encased in ash at 12.5 (3.8 m) in the measured section are 95.81 Ma (Middle Cenomanian) based on work from Sageman et al. (2014). This allows for correlation of the lower portion of the Ernst
Member in the Black Gap area to places in the Western Interior Seaway where this species of ammonite has been found. Cobban and Scott (1973) identified fragments of Acanthoceras amphibolum in the Pueblo, Colorado area at the Rock Canyon measured section and note that the species is widely distributed in the western interior region from East Texas and New Mexico to
Montana.
Zone A in the Black Gap area can be correlated to the lower section measured at Ernst
Tinaja in BBNP (PNR (internal study)). Zone A-E terminology is used for both studies since zones from Black Gap area can be correlated to eastern BBNP. Rocks in Ernst Tinaja consist of
103 stratified and wavy limestones interbedded with laminated marls from 0-25’ (0-7.62 m). PNR
(internal study) note hummocky cross stratification (HCS) in six limestone beds in this zone
(Zone A). HCS is not seen in Zone A or anywhere in the entire Black Gap section. Additionally, both sections in this zone appear very contorted and wavy with loading structures and are mostly grey to dark gray compared to the underlying white Buda and overlying pink/orange beds. In both locations, Zone A shows an overall higher SGR API response than the underlying Buda and an overall lower API response than the overlying Zone B. Furthermore, in both studies SGR API generally increases from the base of the Ernst Member upwards. At Ernst Tinaja, Zone A contains the lowest average K in the section like Zone A in the Black Gap area, and contains higher average amounts of Ca and Mg and lower average amounts of Si, Al, Ti, Rb, and Th than the overlying chemo-zone B. Zone A also contains high average amounts of Mo in both areas, which potentially indicates the rocks in Zone A at Ernst Tinaja were deposited below storm- wave base in euxinic bottom waters. Zone A in both studies shows similar characteristics lithologically and chemically. In the Fry (2015) study of the Ernst Member in the Hot Springs area of BBNP, the laminated skeletal packstone-grainstone facies lies directly on top of the underlying Buda Limestone and crops out within the lowest Ernst Member from ~11-24’ (3.4-
7.3 m) in the measured section (11’ equals top of Buda). Fry (2015) notes the facies contains cyclic laminations varying between winnowed grain laminations and mud laminations that are carbonate-dominated with some argillaceous material. Additionally, within the facies Fry (2015) notes soft-sediment deformation, on a lamination-scale, which suggests rapid deposition of sediments that exceeded the rate of lithification, compaction, and dewatering. Also, Fry (2015) notes rocks in this interval are thin-bedded and do not follow the same indurated-recessive lithofacies motif observed in the rest of the Ernst Member. Observations in the Black Gap area
104
(this study), at Ernst Tinaja (PNR (internal study)), and Hot Springs (Fry, 2015) show similarities between the three sections which suggests the rocks have been deposited in similar environments, but the presence of HCS at Ernst Tinaja suggests deposition in a shallower environment, under the influence of waves at times. The thin-bedded nature of the rocks in Zone
A, seen at Hot Springs by Fry (2015) matches well with rocks seen in Black Gap and Ernst
Tinaja, but the data obtained from thin section analysis from Fry (2015) cannot be confirmed at
Ernst Tinaja or in Black Gap.
Zone B
Zone B, like Zone A, is interpreted to represent deposition in an environment below storm-wave base in mostly anoxic bottom-waters that experienced brief periods of reoxygenation. This is based on the presence and enrichment of Mo and V in Zone B, which is higher than that of Zone A. Argillaceous material in wackestones in Zone B is noticeably more abundant than in wackestones in Zone A, but decreases up-section in Zone B towards Zone C.
The lower ~25’ (7.6 m) of Zone B from 24-50’ (7.3-15.2 m) are dominated by wackestones and beds that appear to contain overall finer material than rocks immediately above and below the lower portion of Zone B. The greater abundance of argillaceous wackestone in the lower portion of Zone B explains the increased abundance of most trace elements and decreasing amounts of
Ca and Mg. This suggests deposition of the lower portion of Zone B was dominated by detrital clay material and lime-mud, which diluted pelagic sand to silt-sized carbonate deposition.
Above 50’ (15.2 m) grainstone beds become more frequent and thicken towards the upper portion of Zone B. Coinciding with an increase in grainstone is a decrease in Al, K, Ti, and all trace elements, besides U, and an increase in Ca and Mg. Additionally, an increase in the average enrichment of Ni is seen in this interval compared to the underlying interval in Zone B
105 suggesting surface and intermediate waters were more favorable for organic production than the underlying interval. The geochemical data paired with the increased grainstones to wackestones ratio suggests the upper portion of Zone B was deposited in mostly anoxic/euxinic bottom- waters, but surface and intermediate waters were commonly oxygenated and contained life composed of carbonate material that eventually settled. Additionally, detrital clay and lime-mud input was lower in the upper portion of Zone B, which suggests pelagic sand and silt-sized carbonate material was in greater supply and/or detrital clay material was less abundant.
Zone B can be correlated to rocks at Ernst Tinaja (PNR (internal study)), where rocks between 25’ (7.62 m) and 92’ (28.0 m) record the highest SGR API values and contain the highest average amounts of Mo, Si, Al, Ti, Rb, Th and lowest average values of Ca and Mg.
Zone B in both studies is dominated by more detrital clay and ash material than the underlying and overlying zones. This suggests that detrital clay, lime mud, and ash were being mixed into the water column potentially diluting pelagic carbonate deposition. The top of these zones, in both locations, occurs where Mo is drastically reduced, and the first trace fossils are found. This spot occurs at 92’ (28.0 m) in the Ernst Tinaja section and at 71’ (21.6 m) in the Black Gap area and is interpreted as the boundary between the lower and upper Ernst Member defined by Fry
(2015) in BBNP at Hot Springs and the western Maverick Basin, and Donovan et al. (2012) in
Terrel County at Lozier Canyon further away from BBNP (see below).
Fry (2015) divides the Ernst Member in BBNP at Hot Springs into an upper and lower sections based on the occurrence of Heterhelix globulosa, which have Turonian-style globular tests that are wider than the Heterohelix moremani tests found in the lower Ernst. Additionally,
Fry (2015) pairs the foraminifera data with a lower spectral gamma ray API response that potentially indicates the Cenomanian-Turonian boundary (lower-upper Ernst Member and lower-
106 upper Eagle Ford boundary), at approximately 104’ (31.7 m) into the section. The boundary between the upper and lower Ernst identified in the Fry (2015) study is in approximately the same location as the boundary between zones B and C defined in this study. SGR stays high in the lower Ernst and decreases upwards towards the upper Ernst Member (Fig. 54). Additionally, as in Ernst Tinaja and Black Gap, Mo enrichment at Hot Springs is seen in the lower Ernst and eventually drastically diminishes where the lower-upper Ernst is defined (Fig. 54). Therefore,
Zones A and B in this study correlate with the lower Ernst of the Fry (2015) study at Hot Springs in BBNP.
Wehner et al. (2017) divide the Ernst Member in BBNP at Hot Springs into three sections: Woodbine, Lower Eagle Ford, and Upper Eagle Ford. In the lower 10’ (3.0 m) Wehner et al. note hummocky stratified grainstones and the authors interpret this interval as being coeval with the Woodbine Group in the East Texas Basin. No hummocky cross stratification is seen in the Ernst Member in the Black Gap area. Geochemical comparisons between the Hot Springs section and Black Gap section can be made despite different sampling intervals (Fig. 55). Zones
A and B interpreted in this study correlate to the Woodbine and Lower Eagle Ford of the Wehner et al. (2017) study (Fig. 55). In both studies these zones show similar abundances and plots of abundance versus depth show similar concentrations of several major and trace elements suggesting the rocks in the zones were deposited in a similar environment. Wehner et al. (2017) placed the boundary between the lower and upper Eagle Ford at ~99’ (30.2 m) at Hot Springs based on a distinct gamma ray drop driven by a decrease in U content which corresponds with an increase in carbonate content (Fig. 55). The same character of the gamma ray, U content, and carbonate content is seen in the study in the Black Gap area (Fig. 55). Although Fry (2015) and
Wehner et al. (2017) interpret the lower-upper Eagle Ford boundary to be in the same spot, they
107 disagree on the thickness of the Upper Eagle Ford and location of the Allocrioceras hazzardi zone.
Zone C
At the boundary between Zone B and Zone C at 71’ (21.6 m), horizontal burrows on a bedding plane of a calcareous wackestone are seen (Fig. 18). No trace fossils are seen in the entire measured section until this point. Additionally, an inoceramid fossil hash is seen at 71.5’
(21.8 m) on the top of the bedding plane. This suggests the boundary between Zone B and C represents a period of time where the depositional system was reoxygenated.
Zone C is interpreted to represent deposition in an environment below storm-wave base in mostly suboxic/anoxic bottom-waters that experienced periods of reoxygenation. The presence and enrichment of V and U suggests anoxic bottom-water conditions during the majority of deposition of Zone C. The high ratio of grainstones to wackestones combined with high Ca concentrations in Zone C (53.91%) suggests that CaCO3 bearing organisms such as foraminifera were abundant in surface and intermediate waters and carbonate material dominated pelagic sediment delivered to the seafloor. The higher average enrichment of Ni supports this and suggests surface and intermediate waters experienced higher rates of productivity than the underlying and overlying zones. A thin ~10’ (3.1 m) interval with increased amounts of argillaceous material in recessed beds is seen from 110-120’ (33.5-36.6 m). In this interval, Ca decreases and all other major and trace elements increase. This suggests Zone C was deposited during a time where bottom-waters were mostly suboxic/anoxic and not euxinic. A few thin zones dominated by wackestones suggest only rarely did detrital clay and lime-mud material dilute the carbonate material being deposited in Zone C.
108
Zone C correlates to the zone from ~92-229’ (28.0-69.8 m) at Ernst Tinaja (PNR (internal study)), where thicker bedded limestones are interbedded with thin marls. Overall, in both studies a higher ratio of grainstones to wackestones (limestones to marls) is seen. Burrows are found at the base of Zone C in both areas. SGR API generally stays low throughout the section and gets higher towards the top of the zone. Both locations contain high average amounts of Ca and Mg, the lowest amounts of each trace element sampled, and low SGR API values. Mo is lowest in this zone in both studies, which suggests bottom waters were not as oxygen depleted as the underlying and overlying zones.
Zone D
Zone D is interpreted to represent deposition in an environment below storm-wave base in mostly anoxic bottom-waters that were intermittently reoxygenated. The presence and enrichment of V and U in Zone B supports this interpretation. Only a ~3.5’ (1.1 m) interval contains Mo which suggests bottom-waters may have been euxinic for a brief period during deposition of Zone D. Like in Zone B, the high amount of argillaceous material and low ratio of grainstones to wackestones (7.91%) in Zone D suggests detrital clay material diluted pelagic grainstone deposition and/or productivity of larger CaCO3 bearing organisms decreased during the deposition of Zone D. This is supported by a low average enrichment in Ni compared to the underlying and overlying zones suggesting productivity in the surface and intermediate waters were low. Al, K, and Ti are in greatest average abundance in Zone D compared to all other zones, which suggests wackestones are composed of a higher percentage of detrital clay material than wackestones in all the other zones. Another major difference between this zone and Zone B is the average ash bed frequency. An ash bed only occurs on average every 4.1’ (1.2 m) in Zone
D, compared to every 1.3’ (0.3 m) in Zone B. This suggests volcanic activity may have been
109 higher delivering more ash material to the system during deposition of Zone B. Deposition in
Zone D was most likely dominated by pelagic detrital clay with minor amounts of ash and pelagic silt-sized carbonate material contributing to the system.
Zone D correlates to the zone from ~229-265’ (69.8-80.8 m) at Ernst Tinaja (PNR
(internal study)), where the section consists of thinner bedded marls with lesser amounts of thin beds of limestone in between, which is similar to this study (Zone D in Black Gap). In the Black
Gap area, Zone D also appears to consist of higher amounts of argillaceous wackestones deposited with minor amounts of grainstone beds. Zone D has a more “flaggy” appearance in both locations than the underling and overlying zones. Zones at both locations record lower values of Ca than the underlying and overlying zones and high average values of Mg, Al, Si, K,
U, Th, and Rb. Additionally, a low amount of molybdenum is seen in this zone in both sections.
This suggests Zone D was deposited in a more oxygen-rich environment like the underlying zone but was influenced by more detrital clay deposition.
Zone E
Zone E is interpreted to represent deposition in an environment below storm-wave base in mostly suboxic/anoxic bottom waters that were periodically euxinic. The presence and enrichment of Mo and V suggests bottom waters were mostly anoxic to euxinic, which led to the preservation of organic material in several intervals within the zone. Overall, Zone E contains less fine-grained material than the underlying zone, and generally grain size increases as you move from the base of Zone E to the top of the measured section. Over one-third (35.0%) of the samples collected in this zone are grainstones. The higher ratio of grainstones to wackestones in
Zone E compared to the lower ratio of grainstones to wackestones in Zone D, suggests that
CaCO3 bearing organisms such as foraminifera were periodically more abundant in surface and
110 intermediate waters, which resulted in coarser pelagic carbonate material delivered to the seafloor. These interpretations are supported by the high average enrichment of Ni in the zone suggesting paleoproductivity in the surface waters was high compared to the underlying zone.
Therefore, fine-grained material in the water column from either lime-mud or detrital clay material was possibly diluted by coarser carbonate material from increased amounts of larger organisms. Additionally, the higher average abundance of Ca and lower average abundance of
Al, K, and Ti compared to the underlying Zone D supports greater abundance of grainstones.
This suggests detrital clay and lime-mud material was in greater abundance during early stages of deposition of Zone E supported by increased amounts of Al, K, and Ti. As you move towards the top of the measured section, pelagic grainstones become more abundant and thicker.
Zone E correlates to the zone from 265-298’ (80.8-90.8 m) at Ernst Tinaja (PNR (internal study)) where it consists of thicker bedded (15-60 cm) limestones and chalks interbedded with thin marls. In both areas these zones are characterized by high average amounts of Ca and Mg.
These zones also contain higher amounts of Mo than the two underlying zones in both study areas. These data suggest deposition during a time of lower oxygen levels at the bottom waters.
Towards the top of the zone Mo disappears and the system seems to be reoxygenated at the onset of the deposition of the San Vicente Member of the Boquillas Formation in a well-oxygenated carbonate setting. In both studies, grainstone/limestone beds increase up through the zone toward the top of the measured section. Additionally, in both studies this zone has an overall lower SGR
API response than the underlying zones and the API response generally decreases as you move upwards.
In the upper Ernst Member at Hot Springs, Fry (2015) observes similar lithologies and sedimentary structures in outcrop and thin section as the lower Ernst Member. The main
111 difference between the upper and lower Ernst is in the upper Ernst Member, where Fry (2015) identifies multiple grainstones with keeled planktonic foraminifera. Additionally, Fry (2015) shows the upper Ernst as having an overall lower SGR API response than the lower Ernst that generally decreases upwards to the top of the section. Furthermore, the upper Ernst has an overall higher abundance of Ca and Mg and lower abundance of trace metals. Fry (2015) suggests this is because deep waters were re-oxygenated more often than the lower Ernst, but periods of anoxic to euxinic conditions existed intermittently at the sediment-water interface.
The upper Ernst observed by Fry (2015) seems to correlate to zones C, D, and E recognized in the Black Gap area (Fig. 54).
The Upper Eagle Ford defined by Wehner et al. (2017) at Hot Springs correlates to zones
C, D, and E in this study (Black Gap) (Fig. 55). The interval in both studies is characterized by higher average abundances of Ca and Mg and lower average abundances of Si, Mo, V, U, and
Th. The Upper Eagle Ford and Austin Chalk boundary is picked by Wehner et al. (2017) at 185’
(56.5 m) based on the occurrence of the Allocrioceras hazzardi zone defined by Cooper and
Cooper (2014). The upper Ernst and San Vicente boundary found by Fry (2015) at Hot Springs based on the occurrence of the Allocrioceras hazzardi zone is interpreted to be at 276’ (84.1 m) into the section (265 feet or 80.7 m above the Buda). Both Fry (2015) and Wehner et al. (2017) worked on the same rocks at Hot Springs and interpreted the top of the Ernst Member, or the
Allocrioceras hazzardi zone, to be at different spots or thicknesses into the section. Therefore, correlations between the three studies remain uncertain.
Ernst Member
SEM images from Ernst Tinaja (Frebourg et al., 2016) show clay minerals compose only up to 15% of the mud fraction in Frebourg’s “argillaceous wackestones”. The authors argue that
112 the cyclicity seen in the Ernst Member is due to alternating periods of lower primary productivity with lower sediment accumulation rates (which deposit argillaceous wackestones), and shorter periods of high primary productivity and higher accumulation rates (which deposit pelagic grainstones). They argue primary productivity was controlled by the absence or input of iron released by ash beds and suggest that iron is a necessary element for the reproduction of the primary producers (Fig. 54). Finally, the authors suggest the Ernst Member in the Langtry,
Comstock, and BBNP areas of west Texas was deposited in a deep water setting below storm- wave base, which was commonly under the influence of bottom-water currents that were constantly depositing and reworking pelagic sediment.
Interpretations by Frebourg et al. (2016) are supported by observations made in the Black
Gap area (this study). Outcrops in the Black Gap area are made up of just three lithologies similar to those described by Frebourg et al. (2016). The more indurated (non-recessive) beds that contain either laterally continuous or discontinuous faint low-angle cross stratification and/or planar laminations are similar to the pelagic grainstones described by Frebourg et al.
(2016). Hummocky cross stratification is not seen in the Black Gap area, is not noted by
Frebourg at the Ernst Tinaja section in BBNP, and is not noted at sections studied by Frebourg along Highway 90 in Val Verde County. The more recessive beds that are deposited above and below the grainstones in this study (Black Gap) are interpreted to be comparable to the
“argillaceous wackestones” described by Frebourg et al. (2016), although some wackestones in this study (Black Gap) are more indurated and not as recessive as others. Based on thin section analysis of the “argillaceous wackestones”, Frebourg et al. (2016) conclude that the mud fraction consists of coccoliths, coccolith platelets, and rare coccospheres with 15% of the mud fraction composed of clay minerals. Lastly, Frebourg et al. note that grainstones generally immediately
113 follow ash beds and suggest this is because Fe-enrichment in the water column from ash material allows for increased productivity. Frebourg et al. suggest this increased the amount of planktonic foraminifera in the water column which eventually got reworked, then deposited and became grainstones. In this study (Black Gap), there are several locations in the section where grainstones immediately follow ash beds, but it does not occur after every ash bed that was observed.
The outcrop and geochemical data obtained in the Black Gap area (this study) shows similarities to the data obtained by Fry (2015), PNR (internal study), and Wehner et al. (2017).
The five zones (A-E) identified in the Black Gap area can be correlated to the two sections. No major geochemical differences are seen between the zones as you move from the Black Gap area to Ernst Tinaja and Hot Springs area in BBNP, suggesting no major environmental changes between the sections. Lithostratigraphic, biostratigraphic, and geochemical data obtained from this study (Black Gap) and all four proximal studies conducted in BBNP (PNR (internal study);
Fry (2015); Frebourg et al. (2016); Wehner et al. (2017)) show similarities. All four studies identify three main facies and lithologies: wackestones, grainstones, and bentonites. In all four studies, wackestones make up the more recessive/fine-grained beds and appear to be composed of more argillaceous and/or lime-mud material. Additionally, in all four studies, grainstones are described as the more indurated, non-recessive beds that protrude out of the outcrop. Finally, in all studies, grainstones contain unidirectional cross beds and current structures. However, in the
PNR (internal study) study, researchers note six limestone/grain stone beds within the first 13’
(4.0 m) of the Ernst that contain HCS. No additional beds with HCS are noted in the Ernst
Member at Ernst Tinaja. Additionally, HCS is noted in the first 10’ (1.0 m) measured by Wehner et al. (2017) at the Hot Springs study site which they define as the Woodbine. Thus, these rocks
114 are interpreted to be deposited in similar environments based on outcrop data. The only exception is the lowermost Ernst member at Ernst Tinaja and at Hot Springs, which may have been deposited within storm-wave base for a period of time.
115
E D
C
B A
Figure 54. Comparison of XRF and SGR between Fry (2015) study and this study (Black Gap). Top plot is XRF and SGR plotted versus measured section thickness (in feet) for Hot Springs measured section in Big Bend Nation Park from (Fry, 2015). Bottom plot is XRF and SGR plotted versus measured section thickness (in feet) for Black Gap measured section in Black Gap Wildlife Management Area from this study. Gray and blue shaded areas are interpreted zones A-E that were correlated between studies to compare geochemical data.
116
hello world
E D C
B
A
Figure 55. Comparison of XRF data between Wehner et al. (2017) study and this study (Black Gap). Top plot is XRF plotted versus measured section thickness (in feet) for Hot Springs measured section in Big Bend Nation Park from Wehner et al. (2017). Bottom plot is XRF and plotted versus measured section thickness (in feet) for Black Gap measured section in Black Gap Wildlife Management Area from this study. Gray and blue shaded areas are interpreted zones A-E that were correlated between studies to compare geochemical data.
117
Rock Strength
The three “strength sections” (1-3) selected from visual analysis of Dimpler UCS data versus measured section thickness do not match the five zones (A-E) selected from analyzing outcrop and XRF data. SS #1 matches Zone A and the lower portion of Zone B, where beds are finer grained and a smaller ratio of grainstones to wackestones is seen. Additionally, the average
Ca per sample is lowest in the interval that makes up SS #1 compared to the overlying two strength sections, which is most likely why SS #1 has the lowest average UCS compared to the overlying strength sections. Moving up to SS #2, which covers the top 15’ of zone B, all of Zone
C, and most of Zone D, the average UCS value increases from the lower SS, which is expected, because SS #2 is composed of a higher ratio of grainstones to wackestones than the underlying
SS #1. Additionally, average Ca generally increases upwards in the measured section and from
SS #1 to SS #2. Moving up to to SS #3, grainstones (outcrop) and carbonate dominated mudstones (Facies 3) (ED-XRF) have a higher average UCS than grainstones and carbonate dominated mudstones (Facies 3) in the underlying strength sections. This may be due to the increased amount of Mg in grainstones in the upper portion of the measured section. Although this zone contains several intervals composed of wackestones that have low UCS values, the high UCS values of the grainstones in SS #3 causes the highest average UCS of the three strength sections. Additionally, SS #3 almost matches Zone E (Table 11), which contains the highest average Ca in the entire measured section and may contribute to the high UCS values.
Zones and intervals in the measured section that contain higher amounts of grainstones or are dominated by carbonate dominated mudstones (Facies 3) (ED-XRF) have typically higher average UCS values. Additionally, from the base to the top of the measured section, UCS
118 generally increases which correlates well to an overall average increase in Ca and Mg content from bottom to top.
PNR (internal study) collected mechanical data from the Ernst Member from Ernst Tinaja using a Schmidt Hammer. Raw data were not obtained from the Henk study, but visual correlations were made using a plot of the data versus measured section thickness. The three “strength sections” that are identified in this study can be correlated to the Henk study using mechanical data paired with SGR data. The UCS response in the Henk study shows similarities to the UCS response in this study (Black Gap) and acts similarly in the three selected strength sections in both studies. In both studies, UCS generally increases from the base of the Ernst to the top of the
Ernst towards the San Vicente. Additionally, grainstones in the upper portions of both sections contain a noticeably higher average UCS than grainstones in the lower portion of the measured sections. No noticeable major differences are seen when comparing mechanical data between the two sections. This suggests the minor chemical and lithological differences between the two sections have little effect on the overall UCS of the sections.
Distal Studies comparison
Lozier Canyon, Terrell County
Donovan et al. (2012) used spectral gamma ray (SGR), XRD, lithostratigraphic, and petrophysical data from outcrops and the BP/SLB #1 well in Lozier Canyon, Terrell County and defined four allostratigraphic members called the upper and lower members of the Lower Eagle
Ford and the upper and lower members of the Upper Eagle Ford (Boquillas Formation). The authors correlate the four allo-members to a borehole in Webb County in south Texas based on
119 geochemical and petrophysical logs. Additionally, the authors split up the Boquillas Formation at these two locations into five units and 16 sub-units. Unit A described by Donovan et al. (2012) matches well will chemozone A described in this study as far as lithology and SGR readings.
Unit A at Lozier Canyon is described as consisting of mostly stratified grainstones with an overall lower gamma ray signature than the overlying and underlying units that increases upwards (Fig. 56). High TOC is seen in this interval, which suggests the rocks were deposited in an anoxic/euxinic environment. This aligns with the presence of Mo in chemozone A in this study (Black Gap). The authors see hummocky cross stratification in Unit A, which is not seen in chemozone A in this study, indicating the section at Lozier Canyon was potentially deposited in a shallower water setting. Chemozone B of this study matches well as far as lithology and SGR response of Unit B described in the Donovan et al. (2012) study. SGR readings in both locations start at the highest API value, which decreases as you move upward through the zone. In each study, both zones (B) end where either TOC or Mo decrease significantly. This suggests oxygenation of the system towards the end of deposition of Unit B and Chemozone B. For the rest of the Ernst Member in this study, chemo-zones C, D, and E correlate to the Upper Member of the Boquillas defined by Donovan et al. (2012) as far as containing an overall lower gamma ray response and containing less TOC and Mo with small intervals that contain higher amounts of TOC and Mo. Additionally, the upper member in the Donovan study and chemo-zones C-E in this study show increased amounts of %K, and decreased amounts of ppm Th, and ppm U when compared to the Lower Member by Donovan and Chemo-zones A and B in this study. In the
Upper Member, Donovan et al. (2012) identify more hummocky cross stratification, wave ripples, and burrows, which are not seen in chemo-zones C-E in this study (Black Gap). This
120 suggests, like the lower member, deposition in the upper member may have occurred in a shallower water setting compared to the Black Gap section.
South Texas subsurface
In addition to the section of the Ernst Member in BBNP, Fry (2015) gathered data from two cores in the Maverick Basin, one in Maverick County and one in Za Valla County. The Za
Valla County core is not a complete section through the entire lower Ernst Member defined by
Fry, but the Maverick County core is complete. Zones A and B in this study (Black Gap) correlate well with the lower Ernst Member in Core-X from Maverick County from ~4032-3850’
(Fig. 57). Both zones show high SGR readings, contain high amounts of Si, Al, V, Zr and have lower amounts of Ca and K (Fig. 57). Additionally, both zones contain Mo, but abundances are reversed. The lower portion of the Eagle Ford in the Fry (2015) study (Zone A) contains more
Mo than Zone A in this study, whereas Zone B contains more Mo in this study than Zone B in the Fry (2015) study. The qualitative character of all the previously mentioned elements with the
SGR are able to be correlated visually between the two sections. The upper Ernst Member in
Fry’s (2015) Core-X seems to correlate well with chemo-zones C, D, and E defined in this study
(Fig. 57). In the Upper Ernst in the Fry study, and in chemo-zones C, D, and E, higher amounts of Ca and K coincide with lower amounts of Si, Al, Zr, V, and SGR values with Mo almost absent (Fig. 57). Only small intervals contain Mo in the upper portion of both sections whereas several intervals in the lower portions of both sections contain Mo. Similarities between the datasets suggest that correlation between sections of the Ernst in west Texas, and sections of the
Eagle Ford in the south Texas subsurface are possible, if the data sets are of similar high sample intervals. This suggests chemo-zones A and B of this study (Black Gap) are equivalent to the
121 producing interval in the south Texas subsurface where Mo and high amounts TOC occur. This may also support the idea of deposition of the Ernst Member in the Black Gap area of west Texas occurring in an environment similar to that of the Eagle Ford in the subsurface of the western
Maverick Basin.
Additionally, Wokasch (2014) gathered XRF, TOC, and lithostratigraphic data from two
Statoil (Equinor) cores in the Maverick Basin in La Salle and Live Oak counties. Wokasch divided the section into five chemostratigraphic zones based on major and trace elemental concentrations. Comparing elemental data between this study (Black Gap) and the Wokasch
(2014) study is difficult because of the large distances between the data sets (~250 miles or ~400 km). However, it appears broad correlations can be made between the two sites. The lower three zones in the Wokasch study seem to correlate to chemo-zones A and B in this study (Fig. 58).
The interval contains lower amounts of Ca and K, with higher amounts of Si, Al, Mo, and higher
SGR API values. The upper two chemostratigraphic zones defined by Wokasch (2014) seem to corelate to chemo-zones C, D, and E in this study (Fig. 58). Higher amounts of Ca and K are seen with lower amounts of Si, Al, Mo, and lower SGR API values. Similar to the Fry (2015) study and this study (Black Gap), the upper Eagle Ford in the Wokasch (2014) study contains a zone in the upper portion where these previous mentioned elements are similar to the lower member. However, this zone seems to be thicker than at Black Gap or Hot Springs. Additionally,
Mo never drastically decreases in the entire section of the two cores in the Wokasch study, where it does in this study (Black Gap), and the Fry (2015) study. This potentially indicates the
Wokasch cores were deposited in a deeper water more anoxic/euxinic setting than in Maverick and Brewster counties.
122
Figure 56. Comparison of XRF, SGR, and XRF data between Donovan et al. (2012) Ernst study in Lozier Canyon in Val Verde County, and this study (Black Gap) in Brewster County. Top plot is SGR, downhole log data, and XRD data plotted versus measured section thickness (in feet) for BP SP #1 borehole in Lozier Canyon (Donovan et al., 2012). Bottom plot is XRF and SGR plotted versus measured section thickness (in feet) for Black Gap measured section in Black Gap Wildlife Management Area from this study. Gray and blue shaded areas are interpreted zones A-E that were correlated between studies to compare geochemical data. 123
E D
C
B
A
Figure 57. Top plot is XRF and SGR plotted versus measured section thickness (in feet) for StatOil (Equinor) Core-X in Maverick County from (Fry, 2015). Bottom plot is XRF and SGR plotted versus measured section thickness (in feet) for Black Gap measured section in Black Gap Wildlife Management Area from this study. Gray and blue shaded areas are interpreted zones A-E that were correlated between studies to compare geochemical data.Comparison of XRF and SGR data between the Fry (2015) study and this study (Black Gap)
124
Figure 58. Comparison of XRF data between the Wokasch (2014) study from two cores in the Maverick Basin and this study (Black Gap). Top plot is XRF data plotted versus measured section thickness (in feet) for StatOil (Equinor) Core #1 in La Salle County from (Wokasch, 2014). Bottom plot is XRF and SGR plotted versus measured section thickness (in feet) for Black Gap measured section in Black Gap Wildlife Management Area from this study. Colored shaded areas on each figure are interpreted zones in each study. Each study has five interpreted zones. 125
Conclusion
The goal of this study was to characterize the depositional environment of the Ernst
Member in the Black Gap area of Brewster County and potentially correlate the lithological, geochemical, and mechanical data gathered with the proximal and distal Ernst Member and
Eagle Ford studies. It was determined that the Ernst Member in the Black Gap is composed of four separate chemofacies:
Facies 1 – siliceous carbonate mudstones
Facies 2 – mixed mudstones
Facies 3 – carbonate dominated mudstones
Facies 4 – mixed carbonate mudstones
The section can further be divided into five zones based on lithostratigraphic data paired with major and trace elemental trends:
a) Zone A contains mostly siliceous carbonate mudstones (63.8%) and carbonate dominated
mudstones (34.0%) (Facies 3) averaging at 63.8% and 34.0% respectively. There is a minor
amount of Facies 4 in Zone A averaging at 2.13%. Facies 2 does not appear in Zone A.
b) Zone B contains mostly siliceous carbonate mudstones (Facies 1) with similar amounts of
mixed mudstones (Facies 2) and carbonate dominated mudstones (Facies 3) averaging
67.7%, 14.0%, and 17.2%, respectively. Like Zone A, Zone B also contains a small amount
(1.11%) of mixed carbonate mudstones (Facies 4). Zones A and B are interpreted to
correlate to the lower Eagle Ford in the south Texas subsurface where organic-rich shales
are the main hydrocarbon exploration targets.
c) Zone C is largely composed of (66.7%) carbonate dominated mudstones (Facies 3). The
second most abundant facies is mixed carbonate mudstones (Facies 4) at 18.8%. Siliceous
126
carbonate mudstones (Facies 1) and mixed mudstones (Facies 2) are in similar abundance
in Zone C at 8.60% and 6.45%, respectively.
d) Zone D is composed mostly of mixed carbonate mudstones (Facies 4) averaging at 58.1%.
Carbonate dominated mudstones (Facies 3) and mixed mudstones (Facies 2) have similar
abundances in Zone D at 24.2% and 16.1%, respectively. A minor amount (1.61%) of
siliceous carbonate mudstones (Facies 1) are in Zone D.
e) Zone E is composed mostly of carbonate dominated mudstones (Facies 3) averaging 52.8%.
Mixed carbonate mudstones (Facies 4) are second in abundance in Zone E averaging
36.1%. A minor amount of mixed mudstones occur Zone E at 11.1%. No siliceous
carbonate mudstones occur in Zone E.
It is concluded that detailed correlations can be made between the Ernst Member and other Boquillas/Eagle Ford sections over moderate distances, such as 30 to 50 miles (48.3 to 80.5 km), and units in Ernst Tinaja and Hot Springs in BBNP can be correlated northeastward to the
Black Gap Wildlife Management area. A large sample size (every six inches to a foot) interval for obtaining XRF, spectral gamma ray (SGR), and mechanical data appears to be adequate to correlate between sections with moderate to large distances between each other. Additionally, broad geochemical correlations can be made from Black Gap Wildlife Management area to the
Langtry area in Val Verde County and to the northwest portion of the Maverick Basin in
Maverick County. The Eagle Ford in the western portion of the Maverick Basin shows similar characteristics geochemically and lithostratigraphically to the Ernst Member in Black Gap. The
Ernst Member of the Boquillas in Black Gap is therefore interpreted to have been deposited in a similar environment as the Eagle Ford in the Maverick Basin. The Ernst Member of the
Boquillas consists of alternating layers of low angle cross stratified to laminated grainstones,
127 laminated argillaceous wackestones, and bentonites. The rocks were deposited below storm- wave base in a commonly anoxic/euxinic environment that contained rarely oxygenated bottom waters. Intermediate and surface-waters were commonly oxygenated and sustained life using
CaCO3, which eventually settled and was deposited along with detrital clay material and lime- mud. Additionally, the system was influenced by bottom water currents that constantly reworked and deposited pelagic carbonate and clay material. Finally, mechanical stratigraphy and strength sections determined from UCS values in the section do not readily correlate to the five zones determined from lithostratigraphy and chemostratigraphy.
128
Bibliography
Algeo, T. J., Rowe, H., 2012, Palaeoceanographic applications of trace-metal concentration data:
Chemical Geology, v. 324-325, p. 6-18.
Barnes, V. E., 1977, Del Rio sheet of Texas Geologic Atlas, Robert Thomas Hill memorial
edition: Texas Bureau of Economic Geology, Austin, scale 1: 250,000.
Bruker, 2017. Handheld XRF: How it works. https://www.bruker.com/products/x-ray-
diffraction-and-elemental-analysis/handheld-xrf/how-xrf-works.html. Accessed
December 30, 2017.
Brumsack, H. J., 1989, Geochemistry of recent TOC-rich sediments from the Gulf of California
and the Black Sea. Geologische Rundschau, v. 78, p. 851-882.
Cobban, W. A., and Scott, G. R., 1973, Stratigraphy and ammonite fauna of the Graneros Shale
and Greenhorn Limestone near Pueblo, Colorado: U.S. Geological Survey, Professional
Paper 645, 108 pp.
Cooper, R. W., Cooper, D. A., 2014, Field guide to Late Cretaceous geology of the Big Bend
region: Houston Geological Society Field Guidebook.
Denne, R. A., Hinote, R. E., Breyer, J. A., Kosanke, T. H., Lees, J. A., EngelhardtMoore, N.,
Spaw, J. M., Tur, N., 2014, The Cenomanian-Turonian Eagle Ford Group of south Texas:
Insights on timing and paleoceanographic conditions from geochemistry and
micropaleontologic analyses: Palaeogeography, Palaeoclimatology, Palaeoecology, v.
413, p. 2-28.
Denne, R. A., Breyer, J. A., Kosanke, T. H., Spaw, J. M., Callender, A. D., Hinote, R. E.,
Kariminia, M., Tur, N., Kita, Z., Lees, J. A., Rowe, H., 2016, Biostratigraphic and
geochemical constraints on the stratigraphy and depositional environments of the Eagle
129
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, p. 1-86.
Donovan, A. D., Staerker, T. S., 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.
Donovan, A. D., Staerker, T. S., Pramudito, A., Li, W., Corbett, M. J., Lowery, C. M., Romero,
A. M., Gardner, R. D., 2012, The Eagle Ford outcrops of west Texas: A laboratory for
understanding heterogeneities within unconventional mudstone reservoirs: Gulf Coast
Association of Geological Societies Journal, v. 1, p. 162-185.
Donovan, A. D., Staerker T. S., Gardner, R., Pope, M. C., Pramudito, A., Wehner, M., 2016,
Findings from Eagle Ford outcrops of west Texas and implications to the subsurface of
south Texas, in J. A. Breyer, ed., The Eagle Ford Shale: A renaissance in U.S. oil
production: AAPG Memoir 110, p. 310-336.
Doveton, J.H., and Merriam, D.F., 2004, Borehole petrophysical chemostratigraphy of
Pennsylvanian black shales in the Kansas subsurface: Chemical Geology, v. 206, p. 249-
258.
Enderlin, M.B., 2010, A method for evaluating the effects of stresses and rock strength on fluid
flow along the surfaces of mechanical discontinuities in low permeability rocks:
[Master’s thesis], Texas Christian University, Fort Worth, TX.
Fairbanks, M.D., Ruppel, S.C., and Rowe, H., 2016, High-resolution stratigraphy and facies
architecture of the upper Cretaceous (Cenomanian-Turonian) Eagle Ford Group, central
Texas: American Association of Petroleum Geologists Bulletin, v. 100, p. 379-403.
130
Forkner, R., Minisini, D., Eldrett, J., Ozkan, A., Bergman, S., MacAulay, C., Lusk, M., 2013,
Carbonate depositional motifs and cycle stacking patterns in the Eagle Ford Formation,
Texas: American Association for Petroleum Geologists Search and Discovery Article, no.
50849.
Frebourg, G., Ruppel, S., C., Loucks, R., G., Lambert, J., 2016, Depositional controls on
sediment body architecture in the Eagle Ford/Boquillas system: Insights from outcrops in
west Texas, United States: American Association of Petroleum Geologists Bulletin, v.
100, n. 4, pp. 657-682.
Freeman, V. L., 1961, Contact of the Boquillas flags and Austin Chalk in Val Verde and Terrell
Counties, Texas: AAPG Bulletin, v. 45, p. 105–107.
Fry, K. O., 2015, Lithofacies, Biostratigraphy, Chemostratigraphy, and Stratal Architecture of
the Boquillas Formation and Eagle Ford Group: A Comparison of Outcrop and Core Data
from Big Bend National Park to Maverick Basin, Southwest Texas, USA. [Master’s
Thesis] University of Texas at Austin.
Galloway, W. E., 2008, Depositional evolution of the Gulf of Mexico sedimentary basin, in
Miall, A.D., (ed.), The sedimentary basins of the United States and Canada: New York,
Elsevier, p. 610.
Gamero-Diaz, H., Miller, C., and Lewis, R., 2012, sCore: A Classification Scheme for Organic
Mudstones Based on Bulk Mineralogy. AAPG Search and Discovery article 40951,
AAPG Southwest Section Meeting, 19-22 May 2012, Ft. Worth, Texas, USA.
Gardner, R., Pope, M., Wehner, M., Donovan, A., 2013, Comparative stratigraphy of the Eagle
Ford Group strata in Lozier Canyon and Antonio Creek, Terrell county, Texas: GCAGS,
v. 2, p. 42-52
131
Jenkins, R., 1988, X-ray fluorescence spectrometry: Chemical Analysis: Chicago, Illinois,
Wiley-Interscience Publication, v. 99, 175 p.
Jenkyns, H. C., 1980, Cretaceous anoxic events: from continents to oceans: Journal of the
Geological Society, v. 137, p. 171-188.
Kaiser, B. and Rowe H., 2012, Fundamentals of mudrock chemostratigraphy: Handheld XRF
analysis, calibration, and interpretation, GCAGS short course, Austin, TX, Oct. 20, 77 p.
Lock, B. E., Peschier, L., 2006, Boquillas (Eagle Ford) upper slope sediments, West Texas:
Outcrop analogs for potential shale reservoirs: Gulf Coast Association of Geological
Societies Transactions, v. 56, p. 491-508.
Lock, B. E., Peschier, L., Whitcomb, N., 2010, The Eagle Ford (Boquillas Formation) of Val
Verde County, Texas – A window on the South Texas play: Gulf Coast Association of
Geological Societies Transactions, v. 60, p. 419-434.
Luthi, S.M., 2001. Geological Well Logs: their Use in Reservoir Modeling. Springer-Verlag,
Berlin, 373 pp.
Lyon, S., T., 2015, Determining the depositional environment of the lower Eagle Ford in Lozier
Canyon, Antonio Creek, and Osman Canyon, Texas: An outcrop study of bedding
features at outcrop scale. [Master’s Thesis] Texas A&M University.
Maxwell, R. A., Dietrich, J. W., 1965, Geologic summary of the Big Bend region, in Geology of
the Big Bend area, Texas, with road log and papers on natural history of the area: West
Texas Geological Society Special Publication, October 1965, no. 65-51, p. 11-33.
Maxwell, R.A., and Hazzard, R. T., 1967, Stratigraphy, in Maxwell, R.A., (Eds.) 1967, Geology
of Big Bend National Park, Brewster County, Texas: University of Texas-Austin, Bureau
of Economic Geology Publication, no. 6711, p. 23-156.
132
Miller, R. W., 1990, The stratigraphy and depositional environment of the Boquillas Formation
of southwest Texas: [Master’s thesis] The University of Texas at Arlington, Arlington,
Texas.
Morrow, A. L., 1935, Cephalopods from the Upper Cretaceous of Kansas: Journal of
Paleontology, v. 9, no. 6, pp. 463–473.
Murray, G. E., 1961, Geology of the Atlantic and Gulf Coastal province of North America:
Harper, New York, New York, p. 692.
Pioneer Natural Resources (PNR) (internal study), 2014, Ernst Tinaja, Big Bend National Park
Pearson, O. N., Rowan, E. L., Dubiel, R. F., Miller, J. J., 2010, Mesozoic-Cenozoic structural
evolution of East Texas - constraints and insights from interpretation of 126 regional 2D
seismic lines and structural restoration: Gulf Coast Association of Geological Societies
Transactions, v. 60, p. 571-582.
Pessagno, E. A., 1969, Upper Cretaceous stratigraphy of the western Gulf Coast area of Mexico,
Texas, and Arkansas: Geological Society of America Memoir, v. 111, p. 139.
Ramos, G.G., Chin, L.Y, and Enderlin, M.B., 2008, Up-scaling of Rock Mechanical and
Petrophysical Properties from Grain Scale to Log Scale by Point-Load and Wedge
Indentation Tests. ARMA 08-152.
Sageman, B.B., Singer, B.S., Meyers, S.R., Walaszczyk, I., Siewert, S.E., Condon, D.J.,
Jicah, B.R., Obradovich, J.D., and Sawyer, D.A., 2014, Integrating 40Ar/39Ar, U-Pb,
and astronomical clocks in the Cretaceous Niobrara Formation, Western Interior Basin,
USA: Geological Society of America Bulletin, v. 126; no. 7/8; p. 956–973
133
Sanders, R., B., 1988, Sedimentology and isotope geochemistry of the Upper Cretaceous Ernst
Member of the Boquillas Formation, Big Bend National Park, Texas. [Master’s Thesis]
Texas Tech University.
Schlanger, S. O., Jenkyns, H. C., 1976, Cretaceous oceanic anoxic events: causes and
consequences: Geologie en Mijnbouw, v. 55, p. 179-184.
St. John, B. 1966, Geology of Black Gap Area, Brewster County, Texas. Geological quadrangle
map GQ-0030 Bureau of Economic Geology, vol. 18, University of Texas at Austin
(1966) geologic map scale: 1: 62,500.
Stern, R. J., Dickinson, W. R., 2010, The Gulf of Mexico is a Jurassic backarc basin: Geosphere,
v. 6, no. 6, p. 739-754.
Taylor, C.Z., 2017, Chemostratigraphy, mechanical stratigraphy, and sample size influence on
micro-mechanical tests for two Barnett shale cores, Fort Worth Basin, Texas: [Master’s
thesis], Texas Christian University, Fort Worth, TX.
Tinnin B. M., and Darmaoen, T. R., 2016, Chemostratigraphic variability of the Eagle Ford
Shale, South Texas: Insights into paleoredox and sedimentary facies changes, in J. A.
Breyer, ed., The Eagle Ford Shale: A renaissance in U.S. oil production: AAPG Memoir
110, p. 259-283.
Tribovillard, N., Algeo, T. J., Lyons, T., Riboulleau, A., 2006, Trace metals as paleoredox and
paleoproductivity proxies, An update. Chemical Geology, v. 232, p. 12-32.
Tribovillard, N., Bout-Roumazeilles, V., Algeo, T., Lyons, T. W., Sionneau, T., Montero-
Serrano, J. C., Riboulleau, A., Baudin, F., 2008, Paleodepositional conditions in the Orca
Basin as inferred from organic matter and trace metal contents: Marine Geology, v. 254,
no. 1-2, p. 62–72.
134
Tribovillard, N., T. J. Algeo, F. Baudin, and A. Riboulleau, 2012, Analysis of marine
environmental conditions based on molybdenum–uranium covariation – Applications to
Mesozoic paleoceanography: Chemical Geology, v. 324–325, p. 46–58.
Turner, B.W., Tréanton, J.A., and Slatt, R.M., 2015, The Use of Chemostratigraphy to Refine
Ambiguous Sequence Stratigraphic Correlations in Marine Shales: An Example from the
Woodford Shale, Oklahoma. Unconventional Resources Technology Conference, ID:
2154166.
Wang, F.P., and Gale, J. F. W., 2009, Screening criteria for shale-gas systems: Gulf Coast
Association of Geological Societies Transactions, v. 59, pp. 779-793
Ward, J., 1963, Hierarchical Grouping to Optimize an Objective Function: Journal of the
American Statistical Association, v. 58, n. 301, p. 236-244.
Wedepohl, K.H., 1971, Environmental influences on the chemical composition of shales and
clays, in L.H. Ahrens, F. Press, S.K. Runcorn, and H.C. Urey, eds., Physics and
Chemistry of the Earth, v. 8, p. 305 333.
Wedepohl, K.H., 1991. The composition of the upper Earth's crust and the natural cycles of
selected metals. In: Merian, E. (Ed.), Metals and their Compounds in the Environment.
VCH Verlagsgesellschaft, Weinheim, pp. 3–17.
Wehner, M., Gardner R.D., Pope, M.C., Donovan, A.D., 2017, The Eagle Ford Group returns to
Big Bend National Park, Brewster County, Texas: Gulf Coast Association of Geological
Societies Transactions, v. 6, pp. 161-176
Williams, J. W., 2017, Fracture analysis of the Cretaceous Boquillas Formation, Black Gap
Wildlife Management Area, Brewster County, Texas. [Master’s Thesis] Texas Christian
University.
135
Wokasch, T., 2014, Elemental chemostratigraphy and depositional environment interpretation of
the Eagle Ford Shale, south Texas. [Master’s Thesis] Colorado School of Mines.
Udden, J. A., 1907, A Sketch of the Geology of the Chisos Country, Brewster County, Texas:
University of Texas-Austin, Bulletin of the University of Texas, no. 93, p. 29-33.
U.S. Energy Information Administration (EIA), 2016, Drilling productivity report. Last modified
August 18th. Accessed August 20th, 2017.
http://www.eia.gov/petroleum/drilling/archive/2017/08
136
VITA
Personal Robert Alan White Background Born March 25, 1993, Colorado Springs, CO Son of Jeffrey and Mary Ann White
Education Master of Science in Geology, 2019 Texas Christian University, Fort Worth, TX
Bachelor of Science in Geology, 2015 Western State Colorado University, Gunnison, CO
Experience Geology Intern, January 2017-April 2019 Cooper Oil & Gas, LLC. Fort Worth, TX
Graduate Teaching Assistant, 2016-2018 Texas Christian University, Fort Worth, TX
XRD Laboratory Technician, 2015-2016 KT Geoservices Inc., Gunnison, CO
Geology Intern, Summer 2015 Antero Resources, Denver, CO
ABSTRACT
STRATIGRAPHY OF THE ERNST MEMBER OF THE UPPER CREATACEOUS BOQUILLAS FORMATION, BLACK GAP WILDLIFE MANAGEMENT AREA, BREWSTER COUNTY, TEXAS
By Robert A. White, M.S., 2019 School of Geology, Energy, and the Environment Texas Christian University
Dr. Helge Alsleben – Associate Professor of Geology
The time-equivalent Boquillas Formation and Eagle Ford Shale (EFS) were deposited on the South Texas Shelf in the Late Cretaceous (Cenomanian-Turonian) during a time of widespread marine transgression. The two formations consist of organic-rich shales and marls interbedded with calcareous limestones that vary laterally and vertically in thickness. The organic-rich shales and marls in the lower part of the EFS have been major targets for unconventional production in south Texas and the East Texas Basin since 2008. The goal of this study was to characterize the depositional environment of the Ernst Member in the Black Gap area of Brewster County and potentially correlate the lithological, geochemical, and mechanical data gathered with the proximal and distal Ernst Member and Eagle Ford studies.
The Ernst Member of the Boquillas consists of alternating layers of low angle cross stratified to laminated grainstones, laminated argillaceous wackestones, and bentonites. The rocks were deposited below storm-wave base in a commonly anoxic/euxinic environment that contained rarely oxygenated bottom waters. Intermediate and surface-waters were commonly oxygenated and sustained life, which eventually settled as calcite and was deposited along with detrital clay material and lime-mud. Additionally, the system was influenced by bottom water currents that constantly reworked and deposited pelagic carbonate and clay material.
The Ernst Member in the Black Gap area is composed of four separate chemofacies. The section can further be divided into five zones based on lithostratigraphic data paired with major and trace elemental trends. Zones A and B in this study are correlated to the organic-rich zones being produced in south Texas. Detailed correlations can be made between the Ernst Member and other Boquillas/Eagle Ford sections over moderate distances, such as 30 to 50 miles (48.3 to
80.5 km), and units in Ernst Tinaja and Hot Springs in BBNP can be correlated northeastward to the Black Gap Wildlife Management area. A large sampling interval of every six inches to a foot
(15-30 cm) for obtaining XRF, spectral gamma ray (SGR), and mechanical data appears to be adequate to correlate between sections with moderate to large distances between them.
Additionally, broad geochemical correlations can be made from Black Gap Wildlife
Management area to the Langtry area in Val Verde County and to the northwest portion of the
Maverick Basin in Maverick County.