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A Sequence Stratigraphic Framework for Establishing the Chronostratigraphic Relationships Between the Woodbine and Eagle Ford Groups Across Texas


Within the geologic literature, the conventional belief is that the Woodbine and Eagle Ford Groups are in general coeval and in lateral facies relationship with each other across Texas. Sequence stratigraphic correlations of the Eagle Ford Group across Texas suggest that this is not the case. In Lozier Canyon, located in Terrell County (West Texas), the Eagle Ford Group occurs between the Buda and Austin. However, in North Texas near Dallas, both the Woodbine and overlying Eagle Ford Groups are present between the Buda and Austin. In Lozier Canyon, the Eagle Ford Group can be readily examined at numerous spectacular cut-bank outcrops each thousands of feet long and hundreds of feet high over a 50 square mile region. Detailed analysis of these outcrops, integrated with core and geophysical logs from a recent research borehole identified five distinct, surface-bounded, allostratigraphic members (depositional sequences) suitable for subsurface mapping across Texas. From the base up these allomembers are the K62 (Osman Canyon), K63 (Lozier Canyon), K64 (Antonio Creek), K65 (Lower Scott Ranch), and K70 (Langtry). Key to understanding the chronostratigraphic relationships between the Eagle Ford Group and Woodbine Groups across Texas are the identification and correlation of the organic-rich K63 (Lozier Canyon) and bentonite-rich K64 (Antonio Creek) Allomembers of the Eagle Ford. In the outcrops of West Texas, and the subsurface of South Texas, the thin (20 to 30 foot thick) grainstone-prone, low-resistivity K62 (Osman Canyon) Allomember occurs at the base of the Eagle Ford Group. However, in the western portion of the East Texas Salt Basin, the organic-rich, high-resistivity K63 (Lozier Canyon) Allomember marks the base of the Eagle Ford Group. Once this unit is defined and mapped, an angular unconformity is reveled between the overlying Eagle Ford Group and underlying Woodbine Group. Between Dallas and Waco, this angular unconformity sequentially truncates the Lewisville and then Dexter Members of the Woodbine leaving only tens of feet of the low-resistivity, mudstone-prone basal Pepper Shale as the sole erosional remnant of the Woodbine Group in the southern portions of the East Texas Salt Basin. These relationships suggest that if the thin basal K62 (Osman Canyon) Allomember in South and West Texas is excluded from the Eagle Ford, and included within the underlying Woodbine, the Eagle Ford Group would then actually be coeval across Texas.