Abstract

Saltwater encroachment northward into freshwater sands of the Baton Rouge aquifer system, southeastern Louisiana, poses a serious environmental threat to this important municipal and industrial water resource. The aquifer system consists of a 2800-ft (850-m) thick succession of south-dipping siliciclastic sandy units and mudstones of Upper Miocene through Pleistocene age. A geology-based understanding of this aquifer system is necessary for developing strategies to control saltwater intrusion. Seventy-five digitized spontaneous potential-resistivity logs for boreholes in the area provided data for interpreting environments of deposition, for correlating sand-rich and mudstone-rich zones, and for identifying periods of low and high rates of sediment aggradation. The sands have complex geometries representing braided stream, meandering channel fill, floodplain, levee, and crevasse splay facies. Individual aquifer units are interpreted to be complex zones of amalgamated sand bodies that may have been deposited during times of low aggradation associated with sea-level falling-stages and lowstand system tracts. The amalgamation created a high degree of connectivity which results in these zones behaving as single hydrologic units. Numerous and major erosional unconformities are seen in the subsurface. Mudstone-rich confining sequences are interpreted to be flood-plain sediments that may have been deposited during times of high aggradation associated with sea-level highstands. Superimposed on this longer-term allogenic forcing caused by sea-level changes are shorter-term autogenic events, such as avulsion, which can be inferred from sand body geometries. Sequence stratigraphic characterization provides a process-oriented basis for understanding the details of the architecture and hydrologic behavior of this highly complex aquifer system and supports the spatial complexity of geostatistical models which are being developed for the aquifers.