LATE PLEISTOCENE-HOLOCENE GEOLOGIC HISTORY AND FRAMEWORK OF THE MISSISSIPPI-ALABAMA COASTS AND SHELF, NORTHERN GULF OF MEXICO

A variety of geophysical and sedimentological studies of Late Pleistocene-Holocene geologic framework and coastal processes over the past several decades demonstrate that the Mississippi-Alabama-Florida panhandle coasts and shelf result from a history of low to moderate sediment supply and micotidal (<2m) range. The primary sediment source is the Mobile, Pascagoula, Pearl and Mississippi Rivers. Interactions among riverine, coastal, and shelf processes have been superimposed resulting in distinctive geology and sedimentary sequences.

The Mississippi-Alabama shelf province encompasses the eastern Louisiana barrier islands, the Mississippi-Alabama and Florida western panhandle barrier islands and shelf, Mississippi Sound, and Mobile Bay. The topography and shallow subsurface characteristics of the Mississippi-Alabama shelf were formed by the late Wisconsin regression and Holocene transgression. These characteristics are the result of highstand shelf-phase and lowstand shelf-edge deltaic deposition and erosion during the lowstand. The main topographic features on the shelf are sand ridges and shoals formed by marine processes and a belt of limestone pinnacles near the shelf break. The present shelf can be divided into distinct regions on the basis of surficial grain morphology and age. The Mississippi-Alabama outer shelf was covered by a thin layer of relict well-sorted fine to medium quartzose sand of late Pleistocene and early Holocene age.

The stratigraphy and morphology of the eastern Louisiana barrier islands and adjacent shelf are dominated by the multiple, stacked, delta complex of the Mississippi River. Mississippi Sound and Mobile Bay are estuarine systems that have evolved differently. Mississippi Sound, initially a shallow open-marine coast, was created by westward-migrating barrier islands. Mobile Bay is a wave-dominated estuary. During the most recent lowstand, the Mobile River incised into Pleistocene deposits. Since the lowstand, the valley has been progressively flooded and partially closed by a westward-prograding Holocene spit. The spit has restricted the circulation of marine waters and has allowed the valley to fill with primarily lagoonal deposits.