ABSTRACT

Grain size analyses and chert to quartz ratios were studied on Pliocene and Quaternary gravel fractions from the Florida parishes of southeastern Louisiana in order to differentiate between gravels of different ages. Gravels are found in the following units: 1) Plio-Pleistocene Citronelle Formation; 2) the Prairie Formation (35,000 yrs. B.P.); 3) Deweyville terrace deposits (30,000-17,000 yrs. B.P.); and 4) Recent channel and bar deposits.

Petrologic studies show that the Citronelle gravels are easily distinguished from the younger terrace gravels by their coarser grain size, low granule content and high chert:quartz ratios. Prairie and Recent gravels have similar textures and composition indicating that they were deposited under similar conditions. Both Prairie and Recent gravels appear to have been recycled from the Citronelle. In some cases, Recent gravels were derived directly from the Citronelle (Bogue Chitto valley), while in others (Tangipahoa valley) they were cycled through the Prairie Formation.

In the Pearl River valley, Prairie and Deweyville gravels are significantly finer and have a higher granule content and lower chert:quartz ratios than other gravels in the Florida parishes, suggesting that the late Pleistocene Pearl River was mature and had high discharges, possibly due to an enlarged drainage basin that contained quartz-rich sources which are not available to the modern Pearl River. Post-Deweyville climatic and hydrologic changes have resulted in a smaller drainage basin and less discharge for the modern Pearl River.

Figure 1. Map showing location of the Florida Parishes and the major river valleys. Shaded portion of the index map shows study area.

Figure 2. Stratigraphic column for the late Tertiary and Quaternary of the Florida Parishes (Cullinan, 1969).

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