UNSTABLE PROGRADATIONAL CLASTIC SHELF MARGINS

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Abstract
In some continental margin basins such as the northwestern Gulf of Mexico and the Niger Delta, large-scale slumping of the continental slope disturbs the topset-foreset geometry of the prograding shelf margin and thereby inhibits recognition of ancient shelfedges. As a result, concepts of shelf-margin dynamics have been underemphasized in explaining the structure and stratigraphy of such basins. Nonetheless, ancient unstable clastic shelf margins can be approximately located by criteria such as isopach maxima, timing of growth faulting, and the stratigraphic top of geopressure. Gravity sliding of the continental slope creates a strongly extensional regime along the shelf margin, resulting in growth faulting and greatly enhanced subsidence rates. The corresponding compressional regime along the lower slope is important in initiating salt and shale structures; if the shelf margin progrades over these structures, diapiric activity can greatly complicate the style of growth faulting. High subsidence rates result in greatly expanded progradational cycles, which serve to distinguish shelf-margin deltaic sequences from deltas of the more stable shelf platform. Rapid fault movement along the shelf margin can hydraulically isolate shallow-water sandstones and juxtapose them against dewatering slope shales, thus allowing the development and maintenance of excess fluid pressure. These deep-water shales are probably a major source of both hydrocarbons and brines instrumental in diagenesis of geopressured deltaic sandstones.

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