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Continental margin architecture : sea level and climate


The stratigraphy and morphology of continental margins provide important insights into the evolution, history and paleoenvironments of these important regions that define the transition between continent and ocean. Research presented here illustrates how sea level fluctuations and climatic variability shape continental margin stratigraphy and morphology. Using CHIRP subbottom and side-scan sonar we imaged a series of large scale, elongated gas blowout features located on the outermost shelf-edge of the U.S. Atlantic margin. The stratal geometry suggests a composite formation, combining gas accumulation beneath a shelf-edge delta, down-slope creep of the deltaic strata and fluid expulsion. The abundance of gas on the margin may be related to hydrate dissociation from bottom water warming during interglacial periods and subsequent trapping by lowstand delta deposits. The location of the blowouts near the shelf break, as well as the proximity to other large slides on the slope indicates that these features may represent incipient large-scale failure of the outer shelf /upper slope. On the Chukchi shelf, offshore northwestern Alaska, CHIRP subbottom data imaged an extensive network of paleochannels infilled by a complex stratigraphy that records multiple sea level cycles as well as massive glacial discharge. Typically channels are incised as a result of base level lowering; however, the two most recent incisions on the shelf appear to have been downcut during the period of rapid sea level rise following the LGM; the downcutting triggered instead by climatic variations during deglaciation (i.e., catastrophic meltwater drainage). CHIRP subbottom data also imaged a regionally extensive heavily ice scoured surface on the outer Chukchi shelf. The stratigraphy is suggestive of discharge of a large number of icebergs in a single event, followed by largely iceberg free conditions. Both the iceberg scours and meltwater drainage appear to be sourced from the northwestern Alaskan margin during the most recent deglaciation and suggest a greater extent of continental glaciation than previously recognized. The drainage and ice discharge history across the shelf highlights the role of climatic variability and sea level change in shaping the Chukchi margin

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