The northeastern Gulf of Mexico, from the Mississippi River to Desoto Canyon, is a complex of interrelated depositional systems. Alluvial-deltaic, estuarine, barrier-island, and marine-shelf systems characterize this part of the Gulf. The Pearl, Pascagoula, and Mobile fluvial-deltaic systems are major sources of sediment to the area. This complex is similar to that of the Texas coastal zone, but specific facies, geometry, and spatial relations differ.

Mobile Bay and Mississippi Sound are the most striking of the several estuaries in the area. Mobile Bay is shallow (average depth, 11 ft) and elongate (31 by 10 mi). Salinity varies locally within the bay and at any particular location between periods of high and low stream flow. Most of the bay is floored by clay and silty clay, with the shallow periphery underlain by sand. Sedimentation rates of 1.7 ft per century have been calculated.

Mississippi Sound is a shallow, bar-built estuary approximately 85 mi long and 7-15 mi wide, bounded on the south by a chain of barrier islands. The mainland side of the sound is fronted by grassy tidelands and artificial beach. Current patterns are complex due to the influence of the tidal passes; however, there is a slow westward longshore current. Most of the sound is floored by silt and clay, with the shallow periphery underlain by fine sand.

The Mississippi-Alabama barrier-island system is part of a chain of small, low-relief, barrier islands and spits that extend from Cat Island, Mississippi, to Choctawhatchee Bay, Florida. Westward-flowing longshore currents accrete sediments to the western ends of the islands while eroding the eastern ends. The rate of accretion is greater than that of erosion so that the islands lengthen and migrate westward.

The Mississippi-Alabama shelf system is that part of the shelf between the Mississippi River delta and Desoto Canyon. The topography of the shelf bottom is relatively smooth and has uniform slope. Minor topographic irregularities occur at depths of 100-150 ft. Wave action is of moderate intensity and sediment transport mainly the result of longshore currents. Sediments

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on the shelf occur as 6 well-defined facies: (1) the Alabama-Mississippi sand facies; (2) The Alabama-Mississippi reef and interreef facies; (3) the St. Bernard prodelta facies; (4) the Chandeleur Islands sand facies; (5) a facies transitional between facies 1 and 3; and (6) an estuary-influenced fine-grained facies.

Recognition of these aspects of the Alabama-Mississippi coastal zone depositional systems is an important consideration in planning and developing a petroleum exploration program.

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