Abstract: Fluvial Deposits and Leaky Aquitards of the Late Pleistocene Shallow Subsurface in Southwest Louisiana 

Abstract

ABSTRACT Shallow subsurface Pleistocene deposits underlying southwest Louisiana exhibit multiple cycles of fluvial deposition, each herein referred to as a fluvial system. The fluvial systems are discernible in a geologic model that reflects correlations and facies analysis on a scale of approximately 200 to 300 feet. The fluvial systems consist of point bar, channel-fill, levee, crevasse splay and flood plain facies. Channel sequences up to 60 feet thick are composed of several fining-up sandy sequences each on the scale of 10 feet thick. Lateral to the upper portions of the channel sequence are sediments interpreted as crevasse splay deposits. Marsh and estuarine deposits representing flooding surfaces cap these fluvial cycles. Relatively thin, clays deposited on aggraded floodplains overlie some fluvial cycles. Laterally persistent flooding surfaces are locally eroded by successive channels. Fluvial systems exhibit distinctive hydrologic characteristics with similar water levels in each system. These systems form a leaky aquitard overlying the Chicot Aquifer. Factors responsible for the leaky nature of the clay-rich section are: 1) fractures in the clay-rich flood plain deposits that increase the vertical permeability, 2) scour of fluvial systems that erodes thin, relatively impermeable, fine-grained organic-rich sections capping the sequences causing vertical connections between different systems and 3) stress caused by water withdrawal from the underlying aquifer. Well-developed, highly permeable point bar sequences act as reservoirs and connect thin permeable, silty layers ubiquitous in these fluvial and interfluvial deposits. This discussion will illustrate the model with data acquired from a roughly one square mile study area. End_of_Record - Last_Page 1-------