PLANKTONIC:BENTHONIC FORAMINIFERAL RATIOS FROM THE CROCKETT FORMATION (EOCENE; BARTONIAN) OF THE TEXAS GULF COASTAL PLAIN

Foraminifera were collected from the Stone City and Wheelock Members of the Crockett Formation in the Conover #1 Stone City core, Burleson County, Texas and from outcrops along the Little Brazos River, Brazos County, Texas. This interval represents Gulf Coast depositional sequence TE 2.4 in the Texas Gulf Coastal Plain and is correlated with the Cook Mountain Formation of Louisiana and Mississippi and the upper Lisbon Formation of Alabama. In Texas, the Crockett Formation is considered to represent the Transgressive Systems Tract and possibly the Highstand Systems Tract of this sequence. Foraminfera were picked from both the Stone City and Wheelock Members in order to calculate the planktonic:benthonic (P:B) ratio. The P:B ratio for each sample serves as a proxy for the distance of the shoreline during deposition. Determining a set of P:B values through a section can refine the sequence stratigraphic interpretation of an interval.

A low diversity planktonic foraminiferal assemblage was collected throughout the Crockett Formation. Common taxa include Acaranina bullbrooki, Acarinina topilensis, and Acarinina rohri among others. The benthonic foraminiferal fauna was much more diverse including genera such as Discorbis, Cibicidoides, Quinqueloculina, and Siphonina among others. The lower part of the Stone City Member (including the Cannonball bed and Main Sand bed) contains few planktonic foraminifera and shows signs of dissolution. In the upper part of the Stone City Member and lower Wheelock Member, P:B ratios fluctuate between 0.29 and 0.013. An apparent cyclicity in P:B values in the upper Stone City and Wheelock Members is considered to be related to stom bed sedimentation in the section.

The P:B ratios collected from the Crocket Formation are consistent with a stratigraphic progression through a transgressive systems tract. These results are similar to P:B ratios obtained from the Cook Mountain Formation in Louisiana and from the Potterchitto Member of the Cook Mountain Formation in Mississippi but differ somewhat from those obtained from the upper Lisbon Formationin at Little Stave Creek in southwestern Alabama. It is suggested that the section of the Crockett Formation studied here is in a shallower setting more proximal to the source of clastic sediment than the section studied at Little Stave Creek, Alabama.