Foraminiferal Distribution in the Saratoga Chalk of Arkansas

Abstract

ABSTRACT The Saratoga (early Navarroan) is a variably sandy and argillaceous chalk, which crops out only in southwestern Arkansas. It thins from 60 feet (18 m) to pinchout both northeasterly and southwesterly. It is the basal unit of the Navarro Group in Arkansas, lying unconformably on the Marlbrook Marl and reportedly unconformably beneath the Nacatosh Sand. The Saratogan foraminiferal fauna has been well established, but the sampling has been at or near the type locality, and no observations of relative abundance have been made. The objectives of the present study are to update the benthic taxonomy, extend the sampling laterally and vertically, gather populational data by statistical counts, and seek information on faunal distribution, paleobiofacies, and depositional environment. Of these only the statistical counts, paleobiofacies, and depositional environment are reported here. Species distribution was determined by statistical counts of 14 samples at three localities along the outcrop. The fauna of all samples was dominated by Cibicidoides alleni (Plummer), Rugoglobigerina rugosa (Plummer), and Anomalinoides pinguis Jennings, suggesting a Cibicidoides-Rugoglobigerina-Anomalinoides paleobiofacies for the Saratoga. The combined frequency of these taxa averages 42% for the 14 samples. A secondarily prominent group of two to ten, changing species combine to average 22% of the sample populations. Taxonomic changes in this group suggest secondary paleobiofacies indicative of geographic separations. The remaining portion of the sample populations is distributed among approximately 40 species, which change from sample to sample so that more than 100 taxa are involved. The species distribution is strongly skewed. On average, 82% of the species encountered in a sample have a frequency of less than 3%. The arithmetic mean of frequencies encountered in sample populations ranges from 1.49 to 2.71, and the average of these averages is 1.9. The standard deviation ranges from 2.75 to 5.32. Plankton-benthos ratio, and comparable biofacies indicate a middle to lower neritic paleoenvironment. Simple diversity measures were disappointing. Statistical induction measures were not attempted. The extreme rarity of miliolids suggests medial paleolatitude for the depositional environment.