RE-EVALUATION OF THE WILCOX AND CLAIBORNE OF CENTRAL TEXAS AS A MESOTIDAL COASTAL DEPOSITIONAL SYSTEM: SEDIMENTOLOGY, ICHNOLOGY AND PALYNOLOGY

In Central Texas the Wilcox Group (Paleocene-Eocene) consists of, in ascending order, the Seguin, Hooper, Simsboro, Calvert Bluff and Sabinetown formations. The lowest formation of the Claiborne Group (Eocene) is the Carrizo. Thick lignites in the Calvert Bluff are mined for electricity generation; the Simsboro and Carrizo are important aquifers. The shallow marine Seguin is capped by an oyster biostrome in Bastrop County. A change to tidal deposition occurs within the Hooper, its upper part being dominated by lenticular and wavy bedding, with some tidal inclined heterolithics. Fluvial delta and barrier bar models developed in the 1960’s still result in the Simsboro and Carrizo being treated as tabular fluvial to fluvio-deltaic deposits. Instead, the Simsboro was deposited as a tidal delta, indicated by consistent glauconite, carbonaceous drapes on foresets, and rare ichnofossils. Similarly, the Carrizo has tidal sedimentary structures with sporadic Ophiomorpha. The intervening Calvert Bluff is poorly exposed except in lignite mines. Older literature emphasizes non-marine deposition, based on the lignites and lack of invertebrate macrofossils, but at least the upper part definitely consists of tidal flats dissected by coarser grained tidal channels with Ophiomorpha, and some tidal inclined heterolithics. Dinocyst occurrences support a tidal interpretation, as do salt-marsh terrestrial palynomorphs. Sabinetown prograding muddy parasequences coarsen up to flaser and wavy-bedded fine-grained sandstone with rare dinocysts and small, sparse ichnofossils, the distal tidal delta-front deposits of a tidally-influenced delta.

Widespread tidal sedimentary structures, marine ichnofossils, and dinocysts, suggest a mesotidal coastline during Wilcox and lower Claiborne deposition. Until Miocene closure of Central America, a tidal bulge was unimpeded from the Pacific through the proto-Caribbean. Amplification across a broad, shallow continental shelf generated a mesotidal regime. Coastal peat swamps along the eastern, mesotidal coast of Sumatra are an analog for Calvert Bluff lignites. Meso- to macrotidal tidal flats along the western coast of South Korea are an analog for Calvert Bluff tidal flats.