ABSTRACT

Neogene and Quaternary lithostratigraphic units exposed at the surface in 35 quadrangles covering Jefferson Davis, Lawrence, and parts of adjoining counties in south-central Mississippi have been sampled and mapped as part of an effort to resolve uncertainty regarding criteria for recognition of mappable sedimentary units in this part of the Gulf Coastal Plain. Lithostratigraphic units designated as the Catahoula Formation (Miocene), Hattiesburg Formation (Miocene), Citronelle Formation (Plio-Pleistocene), Terrace Deposits (Quaternary), and Alluvial Deposits (Holocene) were distinguished on the basis of a combination of field and laboratory criteria.

Outcrops designated as belonging to the Catahoula were those observed to be more noticeably indurated than sediments typical of the Hattiesburg Formation, which is exposed to the south of the Catahoula outcrop belt. In addition to its induration, the Catahoula is characterized by a light-brownish-gray weathering color and conchoidal fracture of its fine-grained units.

The Hattiesburg and Catahoula Formations have a number of characteristics in common. The fine-grained sediments of both have a grayish color, although there may be real but subtle differences in both fresh and weathered coloration. Both the Catahoula and the Hattiesburg contain mica in their sandstones and abundant smectite in their fine-grained beds. The sandstones of both the Catahoula and the Hattiesburg are similar in grain size. Because of the similarity of many lithologic features, and the absence of a mappable contact between them, we suggest that the Catahoula and Hattiesburg Formations be considered a single mapping unit.

The Citronelle Formation occurs in every quadrangle mapped, occupying the highest elevations. The Hattiesburg-Citronelle contact is an easily recognized regional unconformity dipping generally southward. The contact is marked in numerous locations by a thin, dark-brown to black, iron-cemented sand. Numerous thin lenses of clay occur within the Citronelle, and some near the base of the formation have been reworked to form rip-up clasts. Citronelle clays consist dominantly of kaolinite and illite, and the clay lenses tend to be laminated.

In terms of sandstone and clay mineralogy, the Citronelle Formation shares characteristics with the Terrace Deposits, which presumably represent mostly reworked Citronelle sediments. On average, the sandstones of both have more straight monocrystalline quartz than either the Catahoula or Hattiesburg Formations, as well as more polycrystalline quartz than either. The Citronelle and Terrace Deposits tend to be coarser than the Catahoula or the Hattiesburg, but sandstones of the Terrace Deposits are unique in displaying a negative value of skewness. Like the Citronelle, clays of the Terrace Deposits consist of kaolinite and illite but also usually include vermiculite.