Eustasy versus subsidence: Lower Paleocene depositional sequences from southern Alabama, eastern Gulf Coastal Plain
Four distinct sequences of rise and fall in relative sea level may be inferred from the lower Paleocene strata in southern Alabama. Southern Alabama is part of the eastern Gulf Coastal Plain, a passive continental margin. These third-order sequences are traceable from the western part of the study area, where a thick section of deltaic-influenced siliciclastic sediments was deposited in a subsiding salt withdrawal basin, to the eastern part of the study area, where a somewhat thinner sequence of carbonate and siliciclastic deposits accumulated on a stable platform formed over Paleozoic basement rocks. The fact that these sequences and their component systems tracts can be recognized and traced throughout southern Alabama, considering the significant differences in subsidence and sedimentation rates from the western part to the eastern part of the study area, indicates that some factor other than subsidence rates or sediment supply controls the cyclicity exhibited by these sequences. Eustasy is interpreted as that controlling factor. This interpretation is consistent with the observations of previous writers that the conditions of differential subsidence, climate, and sediment supply may overprint the effects of global sea-level change, but that these factors usually do not mask the impacts of eustasy.