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Deltaic deposition during a eustatic sea level fall occurs in two phases, each controlled primarily by the morphology of the sea floor and the rate
of subsidence. The initial, or shelf phase of deposition, produces deposits similar to most modern deltas, although the rates of progradation are enhanced by the cumulative effects of sedimentation and absolute sea level fall. Such deposits are relatively thin and widespread, and internally are characterized by low angle clinoform reflections. The second, or shelf margin phase, results when sea level reaches the shelf edge and deposition occurs on the upper continental slope, where steeper sea floor gradients and more rapid subsidence produce a more localized deposit.
Interpretations of over 35,000 km of single channel high-resolution seismic profiles of the continental shelf and upper continental slope of the northwest Gulf of Mexico indicate the existence of 5 late Wisconsinan shelf margin deltas, including the ancient Rio Grande and Mississippi deltas. The deltas were recognized by geomorphic pattern, high angle clinoform seismic reflections, and association with buried river systems. Isopach patterns show that the deltas range in size up to 5,000 km2 (1,930 mi2) and reach thicknesses of over 160 m (525 ft). The deposits are elongate parallel to depositional strike, indicating subsidence of the shelf margin as a whole, as well as reworking by marine processes. Internal reflection patterns show the deltas to be fluvially dom nated. Multiple lobes can be recognized in most of the deltas studied, resulting both from short term eustatic sea level fluctuations and delta switching.
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