ABSTRACT

Ten major transgressive shales have been identified and named that bound and occur within the Wilcox depositional cycle in the northern portion of South Texas. This study covers the Wilcox area from the northern half of Webb and Duval Counties on the south side to the Yoakum channel on the northeast side. These shales contain maximum flooding surfaces associated with marine transgressions that define the top and base of the Wilcox cycle of deposition and subdivide this major cycle into lesser cycles or sequences. The ten shales are regionally or semi-regionally extensive on the shelf and growth faulted shelf margin and extend farther inland in relation to other shales.

There are two basic sequence stratigraphy models, the Exxon (Posamentier and Vail) model and the Galloway model. The boundaries of the Exxon model are regional unconformities that are difficult to identify and trace by electric logs. The boundaries of the Galloway model are maximum flooding surfaces represented by major transgressive shales that are readily identifiable and correlatable by electric logs. The ten shales of this study are boundaries in the Galloway model and their identification is the first step in making a sequence stratigraphy analysis of the Wilcox in the study area, using electric logs.

The ten shales in ascending order are: Poth shale (Midway), Dull shale, Tilden shale (Big Shale), Webb shale, Yoakum shale, Clayton shale, Kenedy shale, Runge shale, Hobson shale and "Reklaw 1" condensed section. The base of the Wilcox depositional cycle in the study area is defined by the Poth shale and the top by the "Reklaw 1" condensed section. In 1985 and 1986, Hargis proposed a stratigraphic classification of the Wilcox of South Texas and the stratigraphic position of the ten shales within the Hargis classification is summarized in Figure 1.

Electric logs on 20 wells scattered throughout the study area are used to identify the shales present at that particular location, thereby establishing a regional correlative framework for each shale. On the electric logs, the shales are divided into a shale core and a shale zone. The shale core is an interval of low to very low resistivity that is distinctive and persistent over the area covered by the core. The shale zone is an additional variable interval of shale closely associated and interconnected with the shale core but with slightly higher to higher resistivity. The shale core represents or contains the maximum flooding surface and is an approximate time horizon. The shale core fades into thick shale intervals in a downdip direction and wedges out, disappears into sand intervals or outcrops in an updip direction. None of the ten shales covers the entire study area.

For classification purposes, a shale core is a chronostratigraphic boundary. Where a shale core or a combination of shale core and shale zone separate sand sequences, the upper and lower contact with the sand sequences is a lithostratigraphic boundary. A brief discussion of the ten major transgressive shales follows.

Poth shale: The shale core of this Midway shale is the time-stratigraphic boundary for the base of the Wilcox Stage in the study area with two exceptions. In eastern Dewitt County and Zavala County, the Poth shale is absent and Wilcox sands occur below the stratigraphic position of the shale. Where sands are present, the Poth shale separates Poth sands below from basal Lower Wilcox sands of Unit 5 above. Where Midway fossils occur in shales above the Poth shale (Midway lithology), such section is included in the Lower Wilcox in the preferred chronostratigraphic classification. The base of the basal Lower Wilcox sand is the lithostratigraphic base of the Wilcox.

Dull shale: The Dull shale core separates the sands and shales of the Lower Wilcox in Unit 5 below from Unit 4 above. The Dull shale disappears into the sands of the Guadalupe Delta core in Gonzales County but is present in Dewitt County where it separates the Bunger sand below from the Tips sand above. The Dull shale separates stratigraphic sections with two different rates of deposition. The Unit 5 section thins rapidly along strike to the southwest, whereas, the Unit 4 section remains a fairly constant thickness.

Tilden shale (Big Shale): This shale contains two separate shale cores, the Tilden core above and the Big Shale core below. In part of the study area, the two shale cores are separated by a sand but beyond the wedge out of the sand the two shale cores are separate cores in the same shale interval. The Big Shale core is present downdip and the Tilden core updip with considerable overlap and represent two separate pulses of the transgressive sea. The Big Shale was traced from Louisiana into South Texas. The Tilden core is the time-stratigraphic boundary between the Lower Wilcox and the Middle Wilcox and separates the sands and shales of Unit 4 below from Unit 3 above. Over the area where the Tilden shale overlies the Guadalupe Delta, it separates a pronounced landward shift in depositional environments.

Webb shale: The Webb shale core separates the sands and the shales of the Middle Wilcox in Unit 3 below from Unit 2 above. The sands in Unit 2 prograde farther downdip than the sands in Unit 3 with the Webb shale separating this basinward shift. The Webb shale has been traced into Louisiana. The thick middle Wilcox shale fill of the Lobo basin is related to the Webb shale transgression.

Yoakum shale: The Yoakum shale core is the time-stratigraphic boundary between the Middle Wilcox and the Upper Wilcox. It separates the sands and shales of Unit 2 below from Unit 1 (Lower Member of the Upper Wilcox) above. Updip, it separates Unit 2 from the Carrizo and in southern Dimmit and northwest Webb Counties it separates the Indio from the Carrizo. The Yoakum shale also separates a pronounced basinward shift in depositional environments. The shale has been traced into Louisiana. The thick middle Wilcox shale fill of the Yoakum channel is related to the Yoakum shale transgression.

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Figure 1. The stratigraphic position of the ten major transgressive shales within the Hargis (1985, 1986) classification of the Wilcox. The classification is a combination lithostratigraphic - chronostratigraphic classification. The shale cores of the ten shales are time-stratigraphic boundaries representing or containing maximum flooding surfaces. In the northeastern part of the study area, the Clayton shale is not present and the base of the Massive sandstone unit (Massive Delta) occurs below the stratigraphic position of the Clayton shale where it is the lithostratigraphic top of Unit 1.

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Clayton shale: The Clayton shale core is the time-stratigraphic boundary between the Lower Member (Unit 1) and the Upper Member of the Upper Wilcox. Downdip, it separates the sands and shales of Unit 1 (Reagan and House sand series) below from the massive sandstone unit above. The shale extends into the Carrizo where it separates the Wilcox-Carrizo and Massive Carrizo members. The Clayton shale disappears into the sands of the Massive Delta in Bee County and is not present in the northeastern part of the study area. This is the least extensive of the ten shales but in the Wilcox Trend south of the study area, it separates the Fandango sand series from the sands of the Zapata Delta.

Kenedy shale: The Kenedy shale core separates the sands and shales of the Mackhank sandstone unit below from the Luling sandstone unit above. The shale extends into the Massive Carrizo member updip where it is a good semi-regional correlative marker in a stratigraphic section where correlations are limited to very short distances. In the Wilcox Trend, the Kenedy shale separates the Massive and Live Oak Deltas as well as a major lateral shift of the delta cores from Karnes-Dewitt Counties (Massive Delta core) to Live Oak-McMullen Counties (Live Oak Delta core).

Runge shale: Throughout the downdip Wilcox Trend, the base of the Runge shale core is the lithostratigraphic top of the Wilcox. In most of the Wilcox Trend the shale core separates the Slick sandstone unit below from the Second Reklaw sand or resistive remnant thereof, above. The "top of Wilcox" marker occurs at the top of the Second Reklaw and should be renamed the Reklaw 2 marker. In eastern Dewitt County, the Slick sandstone unit wedges out and the Runge shale overlies the Luling sandstone unit. Updip, the Runge shale extends into the upper Carrizo where it separates the Massive Carrizo and Carrizo-Bigford sands below from the Transitional Carrizo (Buehler and Weigang) sands above. The Runge shale represents the initial pulse of the Reklaw sea transgression.

Hobson shale: Downdip, the Hobson shale separates the Second Reklaw and First Reklaw sands or resistive remnants thereof. In eastern Dewitt County, all traces of both Reklaw sands disappear but the Hobson shale core is distinctive within the Reklaw shale section. In the McMullen-Duval area, two shale cores are present separated by the First Reklaw remnant. This remnant downlaps to the southwest wedging out the lower core while the upper core enters from the southwest and wedges out to the northeast. Updip on the San Marcos Arch, the base of the Hobson shale core is the lithostratigraphic top of the Carrizo over much of that area with the shale separating the Carrizo sands from the Atkinson sand (Reklaw). Updip in the Rio Grande Embayment beyond the updip limit of the Runge shale, the Hobson shale extends into the upper Carrizo and separates the Carrizo-Bigford sands from the Transitional Carrizo sands. The Hobson shale represents the second pulse of the Reklaw transgression.

"Reklaw 1" condensed section: The "Reklaw 1" condensed section (Rek. 1 c.s) designates an interval between the base of a shale core that occurs above the Atkinson ^TMFirst Reklaw sand and a semi-regional correlative marker named the Reklaw 1 marker. The marker is the maximum flooding surface of the Reklaw sea on the San Marcos Arch and the shale core is the maximum flooding surface in the Rio Grande Embayment. The Rek. 1 c.s. is the time-stratigraphic boundary for the top of the Wilcox Stage. Downdip, the marker is identifiable in many wells but is also often missing. The shale core fades out downdip in Dewitt, Karnes and Bee Counties but is identifiable in many wells in Live Oak, McMullen and Duval Counties. Updip where the Transitional Carrizo is above the Hobson and beyond the updip limit of the Hobson shale, the base of the Rek. 1 c.s. is the lithostratigraphic top of the Carrizo.

In conclusion, recognition of these ten major transgressive shales should be a valuable aid in using electric logs for sequence stratigraphy analysis of the Wilcox. They are good correlative reference surfaces that often separate different depositional systems, different depositional environments, different rates of deposition and major paleogeographic reorganization. The regional framework of 20 electric logs allows recognition of each shale throughout the area covered by that shale. These logs provide good correlative reference logs for identifying the shales and for defining the boundaries of the stratigraphic units in the Hargis classification. The full study, including the electric logs and maps showing the updip limits of each shale, will be published at a later date.

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