Paleoecology of Eocene Wheelock Member of Cook Mountain Formation in Western Houston County, Texas

Barbara A. Gaskell

Sixty-nine foraminifera species and 49 mollusk species were recovered from the Wheelock Member of the Cook Mountain Formation, in the Porter's Springs area of Houston County, Texas, and along the Little Brazos River in Brazos County, Texas.

The Wheelock Member is primarily a regressive unit. A thin shell lag deposit at the base of the member, consisting of abraded mollusk fragments and subrounded pebbles of impure fossiliferous limestone, is all that remains of the transgressive part of the sequence.

Above the shell lag deposit, the lower part of the member consists dominantly of bioturbated, glauconitic, fossiliferous mudstone. The species composition and diversity of benthic foraminifera and the planktonic/benthic ratio, suggest deposition in a normal marine inner or inner-middle shelf environment. Dominant benthic foraminifera include: Ceratobulimina eximia, Cibicides mauricensis, Eponides mexicanus, Gyroidina octocamerata, Melonis planatum, Siphonina claibornensis, and Spiroplectammina mississippiensis. The mollusk population has a low diversity and is dominated by only a few species. These include: Buccitriton texanum, Mesalia claibornensis, Polinices aratus, Notocorbula texana, Vokesula petropolitana, and Cadulus subcoarevatus. The low diversity and high dominance of the mol usk population suggest deposition in an unstable environment. This instability, however, had little or no effect on the foraminifera.

The upper part of the Wheelock Member consists dominantly of regularly layered mudstone and siltstone and contains only a low-diversity assemblage of agglutinated foraminifera. This part of the member was probably deposited in a shallow brackish-water environment with a high sedimentation rate. Dominant agglutinated species include: Haplophragmoides mauricensis, Trochammina claibornensis, and Verneuilina cushmanii.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.