Sand Bodies
Fluvial Channels

During regressions and transgressions, rivers construct channels on the shelf that have a higher preservation potential than many sequence stratigraphic models imply. The preservation of these highstand and transgressive channels depends on the rate of subsidence on the shelf and on the extent of fluvial erosion during regressions and on the depth of transgressive ravinement during transgressions. Highstand fluvial channels are widespread on the western Louisiana shelf (map 1) and offshore of the Brazos and Colorado rivers (map 2) and are an important potential reservoir within the highstand systems tract.

During transgressions, rivers avulse as they are backfilled with sediment. For example, the Brazos and Colorado rivers both avulsed a minimum of three times during the last transgression. Each time the river shifts its course it forms a new channel that adjusts to the base-level position at that time. These channels are eroded by transgressive ravinement and the sands in the channels reworked within the transgressive shoreline. Typically, the basal portions of the channels are preserved below the transgressive ravinment surface, depending on rates of subsidence and the depth of transgressive ravinement. Oyster Creek is an example of a channel formed during the recent transgression. Seismic records and cores collected offshore of Oyster Creek show that the transgressive ravinement surface has completely removed the channel. It is important to recognize that all fluvial channels on the shelf are not lowstand incised valleys. This is particularly true for transgressive valleys because they do not extend offshore into lowstand deltas and fans. Rather, they abruptly terminate down dip in marine muds resting above the transgressive ravinement surface.

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