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Stratigraphy and Holocene Evolution of Mobile Bay in Southwestern Alabama
John C. Mars (1), Albert W. Shultz (2), William W. Schroeder (3)
Mobile Bay is a large (1058 km2) estuarine system in southwestern Alabama. On the basis of borings, vibracores, radiocarbon dating, and high resolution seismic lines, Holocene inundation of the bay has been reconstructed. A paleotopographic map delineates an entrenched river valley that occupied the present-day bay area during the last Pleistocene lowstand. Vertical stacking of Holocene facies seen in vibracores and boring logs records the evolution of the bay during postglacial sea-level rise and the resumption of deposition above the Late Pleistocene exposure surface.
Two types of vertical sequences are present in Mobile Bay sediments. The first type is a fining-upward sequence that formed as beach, marsh, and near-shore sediments were covered by open-bay muds. Local progradation of the bay-head delta and the Dauphin Island-Morgan Peninsula barrier complex has produced the second type, a fining then coarsening-upward sequence. A disconformable contact with the pre-Holocene sediments is recognizable on seismic lines by erosional truncation and in cores by coloration, root mottling, and radiocarbon ages greater than 17,500 years before present (Y. B. P.). Bay inundation commenced approximately 7,500 Y. B. P. and proceeded in two phases.
The first phase, from 7,500 to 6,000 Y. B. P., was a time of rapid relative sea level rise in which 70% of the bay was inundated. Rapid submergence below normal wave base produced a low energy, open-bay setting in the central part of the bay. In this area, vertical sequences are characterized by thin near-shore and beach deposits (< 1 m) overlain by a thick layer of open-bay mud (> 5 m). By 6,000 Y. B. P., the bay extended farther to the north and was slightly deeper than the present-day bay.
The second phase, from 6,000 Y. B. P. to present, was a time of slow relative sea-level rise. Slow inundation resulted in more time for sediments to be reworked and to accumulate above normal wave base. Thus, vertical sequences from the slowly inundated bay margins contain thick sections of near-shore and beach facies (> 5 m) overlain by thin sections of open-bay mud (< 1 m). Both length and depth of the bay decreased as the bay-head delta prograded and the bay filled. Decreasing length and shallowing of the bay over this time resulted in increasing riverine dominance and a diminishing of the northward intrusion of high salinity Gulf of Mexico waters.
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