Abstract

At Grayson Bluff in Denton County, Texas, the Grayson formation (Cretaceous) is 78 feet thick. It is divisible into four lithologic units: a lower marl, lower clay, upper marl, and upper clay. Each unit contains tests of Foraminifera in astonishing numbers that range from 1500 to more than 100,000/cm3.

Twenty-two samples collected from as many horizons in the Grayson were analyzed according to their content of foraminiferal tests, classified by genera and families. The Lituolidae are the predominant benthonic family in the upper clay. In the lower clay the Bulminidae generally outnumber the combined Anomalinidae, Rotaliidae, and Lagenidae, while in both marl units the converse is the case.

Lowman has shown that different families of Foraminifera predominate at different depths beneath the present Gulf of Mexico. Comparing the microfaunal facies of the Grayson with those described by him, it may be inferred that the lower two units of the Grayson were deposited in deepening waters, while the upper two were deposited in shoaling waters. This is compatible with the chronicle for the late Comanchean as known from studies of lithology, megafossils, and geologic structure. Similar analyses in other parts of the Coastal Plain might establish the order of reliability of foraminiferal populations as indicators of fluctuations in depth around the borders of the ancient Gulf of Mexico.

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