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Role of Incised Valley Systems in Source-to-Sink Sediment Routing and Storage: Examples from the Late Quaternary Northern Gulf of Mexico Margin

Blum, Mike 1; Garvin, Matt 2
1 Exxonmobil Upstream Research Company, Houston, TX.
2 ExxonMobil Production Company, Houston, TX.

Incised-valley systems form in response to sea-level fall, as fluvial systems extend across newly subaerial shelves to the lowstand shoreline and shelf margin. Recent work on Late Quaternary systems of the Gulf of Mexico passive margin illustrate how sediment supply might change over the course of a glacio-eustatic cycle, and how the evolution of incised-valley systems modulates source-to-sink sediment routing to deepwater environments through processes of storage and export of sediments on the coastal plain and inner shelf.

First, empirical data on links between sediment supply and climate suggests supply from the hinterlands should decrease during glacio-eustatic sea-level fall and lowstand due to temperature depression. Hence, total supply from the hinterland may be (a) at a maximum when river mouths reside in highstand positions, and sediment storage takes place on the coastal plain and inner shelf, and (b) at a minimum during time periods when river systems are extended to the shelf margin lowstand shoreline and directly feeding the slope and basin floor. Second, incised valleys form in a step-wise manner, with short periods of incision punctuated by extended periods of lateral channel migration and valley widening, and with contemporaneous deposition of channel-belt sands. The total volume of sediment exported during the period of incised-valley formation is a relatively small value compared to the ongoing flux from the hinterlands, and short periods of incision likely produce an insignificant amount. However, periods of lateral channel migration and valley widening significantly increase the export of sediment, perhaps by 25% or more, such that falling-stage fluvial deposition corresponds to increased sediment delivery to the shelf margin and beyond. Finally, for low-gradient continental margins with broad shelves, like those of the Late Quaternary Gulf of Mexico, drainage basins merge as channels extend across the shelf, which will in turn result in increases in the drainage areas that contribute to single point sources at the shelf margin. Apparent signals of increased or decreased flux of sediment to the shelf margin and beyond may therefore reflect geomorphic response to sea-level change - the merging of drainages as they transit a broad shelf - rather than changes in sediment supply from the hinterland.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009