INTERACTION BETWEEN TOPOGRAPHIC, THERMOHALINE, AND OVERPRESSURED FLOW REGIMES IN THE SOUTH LOUISIANA GULF COAST

The discharge areas of the regional topographically-driven ground water systems in the U.S. Atlantic coastal plain are situated at or near the Atlantic coast. In some areas, fresh groundwater lenses extend far out into the continental shelf. In contrast, the principal pre-development discharge areas of the regional fresh water aquifer system in the Louisiana coastal plain are located as much as 100 km inland from the Gulf of Mexico. Possible explanations for the far inland location of the discharge areas include the presence of growth faults and/or a Gulfward change in sediment facies, which may act as barriers to lateral fluid migration to the south toward the Gulf. However, with the exception of the Baton Rouge fault, which appears to be a leaky barrier to lateral fluid flow in the Baton Rouge area, the zones of pre-development freshwater discharge do not correspond spatially to the location of faults. Examination of well logs indicates there is no significant Gulfward facies change south of the discharge zones. We note instead that the distal end of the topographically-driven aquifer system corresponds spatially to the northern limit of the South Louisiana salt dome province. The subsurface dissolution of salt has produced a regional mass of dense brine at shallow depth. We propose that topographically-driven fresh waters are forced to ride up and over these much denser waters, resulting in a pronounced thinning of the zone of freshwater and the development of groundwater discharge areas located far from the coast. The rates and mechanics of fluid flow within the mass of saline water are not completely known. Potential driving mechanisms include: 1) density differences induced by spatial variations in temperature and salinity, i.e., thermohaline circulation, and 2) the episodic upward expulsion of overpressured fluids from below. A possible example of the latter process may occur at the St. Gabriel salt dome, where saline water extends from the top of salt at a depth of 4 km all the way up to the land surface. The presence of salt dome-derived brines at shallow depth south of Baton Rouge makes it unnecessary to invoke vertical transport of brine up the Baton Rouge fault to account for saline intrusion of freshwater aquifers in the Baton Rouge area, as has been suggested by some workers.