Peter P. Flaig, Ph.D.
Bureau of Economic Geology - STARR
Jackson School of Geosciences
The University of Texas at Austin
Deposits of the Wilcox Group are important because they function as both oil-gas reservoirs and aquifers, and there are several ongoing Paleocene-Eocene source-to-sink investigations of broad interest to both the petroleum industry and climate scientists. Fluvial-distributive to shallow-marine deposits of the Wilcox Group are highly complex, in part because mixed tidal, fluvial-flood, and wave processes affected different parts of the depositional systems. The shifting locus of active sedimentation along with fluctuating base level, accommodation, and sediment discharge also contribute significantly to characteristics. I focus on two current studies of onshore Wilcox deposits 1) the Simsboro to Calvert Bluff transition and 2) the uppermost Wilcox to basal Claiborne Group. The advantage of incorporating drone photogrammetry and high resolution photomosaics (e.g., GigaPan robotic technology) into outcrop investigations is also discussed.
Deposits of the uppermost Simsboro and lowermost Calvert Bluff formations are exposed in pits at the Acme Brick Company quarry, Butler, Texas. These pits are actively excavated for brickmaking; hence it was critical to document the characteristics of these temporary exposures and preserve them digitally for posterity and additional study. Integrated sedimentology, palynology and drone photogrammetry reveals that Simsboro depositional environments include sand-rich channel thalweg, channel bar, and point bar deposits of meandering channels, along with silt-rich abandoned-channel deposits. A low-diversity and low-abundance trace fossil assemblage including diminutive Planolites and Teichichnus indicates some marine influence during deposition. Abundant mud-draped sedimentary structures, along with common double mud-drapes and mud rip-up clast lags suggest that these channels, although fluvial-distributive, repeatedly experienced tidal effects that drove mud out of suspension and forced mud deposition. High-resolution outcrop imagery reveals abundant erosional surfaces and complex cut-and-fill structures, point bars, and silt-rich channel-abandonment deposits. Abundant and recurrent incision and erosion, and the lack of preserved floodplain, indicates a low-accommodation setting. The Late Campanian fluvial–tidal channels of the Neslen Formation, Utah-Colorado are discussed as an analog for the Simsboro.
Although the contact between the Simsboro and the overlying Calvert Bluff is not exposed, the overlying Calvert Bluff contains sand-rich channel deposits isolated within floodplain deposits, with fewer erosion surfaces and cut-fill structures than the Simsboro. Lateral accretion surfaces are a prominent component of sandbodies, which are also interpreted as deposits of meandering channels. In contrast to the Simsboro, Calvert Bluff channel abandonment deposits are darker-colored, highly organic with abundant plant macrofossils, and are interbedded with paleosols containing rhizoliths and mottles. Paleosols appear gleyed and are likely immature (entisols and inceptisols) with some containing in-situ tree stumps. Calvert Bluff channel abandonment deposits suggest high-angle abandonment, and probably represent oxbow lakes. Pollen assemblage for both the Simsboro and Calvert Bluff along with sparse in-situ dinoflagellates suggest a “middle” Thanetian age. Assemblage composition is consistent with a coastal to delta plain paleoenvironment dominated by fluvial channels and riparian to floodplain-wetland deposition, with limited marine influence. The meandering fluvial and associated coastal plain deposits of the Prince Creek Formation of Alaska are discussed as an analog for the Calvert Bluff.
An ongoing, multidisciplinary study on outcrops that include the Wilcox to Claiborne Group transition in Bastrop is also showcased. A combination of sedimentology, ichnology, palynology, δ13Corg, and drone photogrammetry indicates that the Paleocene-Eocene Thermal Maximum (PETM) is likely preserved in these tidally modified deltaic deposits. Preliminary results from a related, expanded subsurface study across the same stratigraphic interval in Anderson County are discussed and compared.