ABSTRACT

The distribution of Holocene marginal marine lithofacies from Alabama State waters and the inner shelf was determined from vibracores, surface sediment samples, and shallow seismic surveys. Facies range from silty clay (bay muds) to orthoquartzitic sand (shelf sand ridges) and shell gravel (transgressive lag). A type 1 sequence boundary separates the Holocene transgressive tract from the eroded pre-Holocene; the surface locally exhibits oxidized paleosols, change in seismic velocity, or underlying stiff clay. The irregular surface preserves Wisconsinan paleotopography, including dip-trending paleochannels up to tens of meters deep (e.g., the Mobile River system) and shore-parallel lows.

Early transgressive systems tract deposits include marsh deposits in low-energy lagoons in the paleolows; muddy channel fills; onlapping muddy shallow shelf deposits; and coarse-graded shelly transgressive lags in some open-shelf settings. Carbon-14 dates from oyster and wood samples from Mobile Bay and Mississippi Sound indicate that the present shelf and paleochannels were inundated ca. 7,000 years before present (YBP). By 4,000 YBP the present bays and lagoon areas were largely inundated.

The early Holocene lithofacies configuration of more open circulation in the bays evolved to that of the present latest transgressive to highstand systems tract as sea-level rise decelerated. Pleistocene paleohighs acted as nuclei for the development of the present barrier complexes, which restricted water energy and salinity in the bays. These bays acted as sinks for aggrading bay/lagoonal muds and muddy sands from Mobile River, with a bayhead delta prograding rapidly in the incised Mobile River valley. The coastal Escatawpa River exhibited significant delta destruction. Development of extensive wetlands and downbay migration of oyster reefs reflected this slowing of sea-level rise. On the inner shelf, quartz sand ridges and hardbottoms developed updrift from Mobile Bay outfall; downdrift the shelf continued to receive fine-grained sediments. This lithofacies pattern represents a classic sequence evolution due to climatic-induced transgression.