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This dissertation consists of three chapters describing seismic stratigraphic studies in three very different continental margin settings. The first chapter discusses the northwestern Ross Sea, the second discusses the southeastern Weddell Sea and the third discusses the Alabama/west Florida margin. The purpose was to investigate stratigraphic relationships between deposition during glacial periods (i.e., eustatic lowstands) and interglacial periods (i.e., eustatic highstands) on high-latitude and low-latitude margins. the high-latitude Antarctic margin is of particular interest because it is over-deepened and surrounds a continent that is free of melt- water. Because of these factors, the shelf/shelf- edge were not subaerially exposed and fluvially incised during eustatic lowstands. Seismic-stratigraphic analysis of the southeastern Weddell and northwestern Ross Sea margins shows that sediments were sequestered in pre-existing glacial troughs or upper-slope fans, at the mouth of glacial troughs. Locations of trough fans and trough mouth fans were strongly controlled by the location of ice streams. This point-source component of stratal architecture contrasts with the implicit line-source assumption that dominates recent thinking of Antarctic stratal development. Point-sourced trough mouth fans are relatively stable features, but as evidenced in Weddell Sea, they have experienced intermittent large-volume collapse. Upper-slope collapse is predicted to have been related to a dramatic change in over- burden, initiated early in an interglacial period following a long-duration glacial period. Interglacial shedding contrasts with models of low/middle-latitude sedimentation, which suggest that the basin is sediment starved during the interglacial. Seismic stratigraphic analysis of the Alabama/west Florida low-subsidence, middle- latitude continental margin indicates that upper- slope shelf-margin deltas have remained intact throughout the glacial cycle, in spite of widesp

Undoubtedly, bypass of fluvial sediments through canyon-indented slopes is an important contributor to lowstand depositional systems. However, due to complex interactions with other mechanisms (sediment supply, antecedent topography, salt tectonics, etc.) the question of how the process of sediment bypass of the shelf margin-delta system is initiated has not been resolved. In this seismic-stratigraphic study of near-surface stratigraphy of the Alabama/west Florida shelf (an area with low subsidence, no faulting and no salt tectonics) the isolated effect of fluvial response to relative-sea-level fall is investigated. In the study area, shelf-margin-delta depositional units are primarily composed of discrete prograding wedges. Based on available age constraints, we infer that these depositional cycles are associated with failing limbs of 100 ka glacioeustatic relative-sea-level cycles of the Pleistocene. Seismic-stratigraphic analysis, contour mapping and paleogeographic reconstruction of shelf-margin-delta environments indicate that slope-canyon connections between the shelf margin delta and upper slope did not develop in spite of frequent and widespread subaerial exposure of the shelf. Slope canyons may be lacking in the study area due to the absence of salt tectonics and growth faulting. Another possibility is that the sandy nature of surface sediments offshore Alabama/west Florida inhibits organized drainage from the subaerially-exposed continental shelf during relatively short Pleistocene lowstands. Nonetheless, based on the absence of shelf-to-slope connections in the study area, we propose that, acting alone, lowstand fluvial incision of the shelf edge is not an important initiator of slope canyons and fluvial bypass to to the basin.

A study of the Apalachicola fluvial-deltaic system is part of a larger study covering the Northern Gulf of Mexico. The studies are being conducted to determine the response of different systems to changes in sea level and climate or fluvial geomorphology during the last glacial eustatic cycle. The Apalachicola system is unique for its ramp-type margin, relatively low subsidence, and drainage basin characteristics. The drainage basin of the Apalachicola system has significant relief with numerous perennial tributaries and a minimal distance separating Pleistocene uplands from the coast, compared with the Colorado and Brazos fluvial systems of Texas. As a result, the alluvial valley is deeply incised. Sediment delivered to the shelf is dominated by sands. During the summer of 1998, approximately 500 kilometers of high resolution seismic data was collected on the middle and outer shelf of west Florida. Delta lobes have been mapped along the shelf with relative ages constrained by the oxygen isotope/sea-level curve. Seismic facies analysis shows large (up to 500 square kilometers wide and 70 meters thick), sand dominated delta lobes on the shelf. Delta lobes occur within highstand, lowstand, and transgressive systems tracts. Progressive seaward shifts in the delta during the falling limb of sea level suggest a continuous sediment supply to the shelf. Sediment supply was more episodic during the transgression. A more precise history of delta evolution is currently under investigation

Approximately 2100 km of high-resolution seismic data from the Alabama/Florida shelf were interpreted to investigate the origin of a thick ( approximately 30 m) late Pleistocene shelf-perched seismic unit. In contrast to previous sequence stratigraphic studies, the eastern Alabama/northwest Florida study area is characterized by small coastal plain drainage systems, a narrow continental shelf, and very low subsidence rates. Moreover, the area of interest is beyond the influence of salt tectonics and growth faulting. Within a sequence stratigraphic framework, this shelf-perched unit has the appearance of a backstepped, drowned transgressive systems tract. However, seismic stratigraphic correlation indicates that locally the unit completely prograded the margin. Within our scenario, the majority of the shelf-perched unit is not transgressive, but rather part of the late highstand systems tract. Based on available age control, late-stage shelf-margin progradation is inferred to have occurred in the latest highstand of the last eustatic cycle (i.e. oxygen-isotope stage 2) when fluvial systems coalesced across the emergent coastal plain. The late Pleistocene transgressive systems tract was probably sourced by transgressive ravinement. Coastal plain sediment supply provided little new localized input during baselevel rise. The transgressive systems tract is therefore exceedingly thin with regional distribution.