| dc.description.abstract | The Late Cretaceous Austin Group, across the outcrops and subsurface of Texas, are faced with numerous, often provincial, terminologies based on different lithologic, faunal, and petrophysical markers used to define internal units by various geoscientists. In order to define and map a more regionally consistent chronostratigraphic-based framework for the Austin Chalk across a four-county area in the southwest portion of the East Texas Basin, an integrated petrophysical, sequence stratigraphic, chronostratigraphic, and chemostratigraphic study was conducted. This study revealed that the Austin Group is an approximately 12-million-year (2 nd - order) unconformity-bounded depositional sequence, bounded by the K720sb at its base, and the K800sb at its top. A major internal unconformity, herein termed the K750sb was used to subdivide the Austin Group into two, 3rd -order, depositional sequences. This surface separates a carbonate-rich, unconformity-bounded, Lower Austin Formation below, from a more terrigenous-rich, unconformity-bounded Upper Austin Formation above. Within the Lower Austin Formation, a regionally mappable high gamma ray marker, interpreted as a maximum flooding surface, was used to define a Lower Member below from an Upper Member above. Each of these two members also have distinct petrophysical and chemostratigraphic characteristics. Similarly, within the Upper Austin Formation, another regionally mappable high gamma ray marker, also interpreted as a maximum flooding surface, was used to define a Lower Member below from and Upper Member above. Each of the two members within the Upper Austin Formation also have distinct petrophysical and chemostratigraphic characteristics.
Across the study area, the K800sb, which also defines the base of the overlying Taylor Group is a major erosional surface. This surface forms a north to south incision, commonly referred to as the Waco Channel, which truncates into the Upper Austin Group across the study area. Interestingly, the thalweg of the Waco Channel, follows the present-day course of the Brazos River, suggesting a structural control to both. In terms of petroleum exploration, the primary target zone in the Austin Group is the lower member of the Lower Austin Formation, which is a carbonate-rich unit, distinctly devoid of clay rich partings, bounded by a bentonite bed. In general, this unit thickens from the northwest to the southeast of the study area, but only exceeds 40’ (13 m) in thickness in southeast Brazos County. Furthermore, a subunconformity play, targeting a potential carbonate-rich strata in the upper portions of the Upper Austin Formation, may also exist beneath the Waco Channel in the study area. | |
