Abstract

The Tallahatta Formation is a rock unit that was deposited in Mississippi during Eocene time. It is currently exposed along an arcuate outcrop belt extending from the north-central to the east-central part of the state. We investigated sedimentology, petrography, and mineralogy of an exposure of this formation in a road cut along the Interstate 20 Highway, west of the City of Meridian, Mississippi. The portion of the Tallahatta Formation that is exposed at this location displays a rhythmic sedimentation pattern consisting of the alternation of two lithofacies. Lithofacies 1 is the bioturbated and nodular mudstone which consists of centimeter-thick beds of unlithified argillaceous siltstone that alternates with thin layers of hard, lithified mudstone. Lithofacies 2 is composed of white, massive to thickly-bedded mudstone.

Petrographic studies of thin sections prepared from thirteen samples from this outcrop indicate that bioturbated and nodular siltstone beds of lithofacies 1 consist of a mixture of quartz silt and other very fine-grained particles. The lithified, hard beds are composed of volcanic ash and volcanic glass spherules. Lithofacies 2 consists of very fine-grained amorphous grains, suggesting that beds of white, massive mudstone lithofacies are volcanic ash layers.

The absence of shallow water indicators such as cross beds, the dominance of fine grain lithology, and the abundance of bioturbation indicate that the Tallahatta Formation was deposited in low energy waters below wave base in the study area. The abundance of amorphous grains points to the presence of volcanic ash as a major sediment source.