On August 3, 2012, Dr. Louis G. Zachos of the University of Mississippi, Department of Geology and Geological Engineering and Mr. Chap Brackett, graduate student working with the Mississippi Mineral Resources Institute (MMRI), organized a field trip to visit several of the classic Tertiary outcrops in the Jackson, Vicksburg, and Yazoo City area. Included in the agenda was the type section of the Oligocene Byram Formation exposed along the Pearl River (Figure 1) in Byram, Mississippi (Hinds County). The Byram Formation is included in the upper section of the Vicksburg Group and is underlain by the Glendon Limestone and overlain by the Bucatunna Formation, all of Oligocene age. The Byram is known for its fossil content and its bivalve fauna is well known. The type section consists of beds of fossiliferous marls containing bivalves as well as scaphopods, corals, and the echinoid Clypeaster rogersi. As indicated in Figure 1, some beds are more indurated that others, but all are fossiliferous.
Exposures in Vicksburg (Warren County) constitute the type section for the Vicksburg Group. The east bank of the Mississippi River at Vicksburg, near the I-20 bridge (Figure 2), contains exposures of the lower section of the Vicksburg Group. This exposed river section contains the Mint Springs Formation, Marianna Limestone, and the Glendon Limestone. On rare occasions when the Mississippi River is very low, the top of the Forest Hill Formation can also be observed. The type section of the Mint Springs Formation is located in the nearby Vicksburg National Military Park. The river exposures of the Mint Springs consisted of greenish-gray, fossiliferous clay. Overlying the Mint Springs is a Marianna Limestone section consisting of a thin, fosssiliferous, limestone and the younger Glendon consists of alternating beds of indurated limestone and softer, poorly indurated limestone. Although not as fossiliferous as the Byram; the Marianna and Glendon contain both a bivalve and echinoid fauna. The echinoid Clypeaster rogersi was identified in the basal bed of the Glendon Limestone. The Glendon Limestone was also examined in the road cuts in North Vicksburg along Washington Street.
North of Vicksburg near Redwood road cut exposures of Byram Formation are adjacent to Highway 61. These exposures consist of displaced blocks of Byram as well "in place" outcrop. These blocks are more weathered than the type section material, but contained an abundance of fossil material including crab claws, numerous bivalves and echinoid fragments.
The final stop of the trip was in Yazoo County on Techeva Creek just north of Yazoo City (Figure 3). Techeva Creek contains excellent exposures of the Moodys Branch Formation (basal component of the Jackson Group). Well preserved bivalve and echinoid fossils were collected. The bivalve Glycemeris idonea is an example of the bivalve fauna and Periarchus lyelli, an echinoid, was also present.
The purpose of field trip was to take advantage of the low levels of the Mississippi River to access exposures normally underwater. The information gathered on the field trip will also be used by Dr. Zachos in his paleontology class to be taught in the spring semester and to plan future fossil collection trips for UM students. The MMRI has traditionally supported higher education in the geological sciences at all of Mississippi’s Institutions of Higher Learning and is pleased to continue this tradition.
Selected references for further reading: