ABSTRACT

Neogene and Quaternary sediments cropping out in south-central Mississippi can be differentiated into several lithostratigraphic units on the basis of a number of field and laboratory criteria. A geologic map for the area has been constructed using these criteria within the framework of a simple model that can account for the present mapped distribution of the units.

Mapping assumptions that are the basis of the model include the following: (1) The Catahoula Formation (Oligo-Miocene?) is a regionally continuous deposit of sand, silt, and clay that lacks an easily identifiable contact with the overlying Pensacola Formation. (2) The Pensacola Formation (Miocene) is a regionally continuous deposit of greenish gray clays, silts, and fine sands. (3) There is an unconformable contact of regional extent between the Pensacola Formation and the overlying Citronelle Formation. (4) The original Pensacola-Citronelle contact has been incised by Quaternary streams in places; where this has occurred, Terrace Deposits and Alluvial Deposits now resting on the Pensacola Formation occupy a secondary unconformity, i.e., post-Citronelle stream incision has eroded additional material from the upper part of the Pensacola section. (5) The Pensacola-Citronelle unconformity surface may have been deformed by salt tectonics. (6) The Citronelle (Plio-Pleistocene) consists of coarse clastic sediments deposited by aggrading braided streams. (7) The Citronelle Formation was, prior to stream incision and attendant erosion, a laterally continuous blanket deposit. (8) Terrace Deposits (Quaternary) represent sediment derived primarily from the Citronelle, and they occupy valleys incised into the Citronelle, and, in places, older units. (9) Alluvial Deposits (Holocene) occupy valleys incised into Terrace Deposits, and, in places, older units.