Print this pageTide-Dominated Shelf-Edge Deltas Deliver and Store Sand from the Upper Slope: Columbus Basin, Trinidad
Analysis of the upper 1000 ms of 3-D seismic data at the present-day shelf edge of the Columbus Basin suggests that sand is highly partitioned from the shelf edge into fans on the upper slope when a canyon is not present, and are transported to deeper water only by fan collapse. Sigmoidal and oblique reflectors accreting basinwards are interpreted as late Pleistocene shelf-edge delta clinoforms with rising shoreline trajectories. The delta is situated within a down-to-the-basin growth-faulted compartment. Topset amplitudes are brightest within 1 km of the clinoform rollover and diminish landwards toward the growth fault. Reflectors also dim abruptly basinwards into poorly resolved foreset reflectors 2-3 km long and 150-200 m high. Bottomset reflectors exhibit the brightest amplitudes and onlap onto a mud ridge-cored rollover anticline created by the growth fault. Observed geometries scale with small-scale shelf-margin successions (i.e., Spitsbergen; Karoo Basin; Washakie Basin) and reflect the interaction of both shallow-marine and deepwater processes.
Amplitude and attribute analysis of shelf sands reveals planform morphologies similar to the modern-day Orinoco Delta (i.e., dip-elongate sand bars within broad, funnel-shaped distributary-channel complexes associated along strike with poorly developed, asymmetric coastal ridges) and suggests that tidal processes dominated the shelf-edge deltas. Long-shore drift and waves played a secondary role in shaping these deltas. This contrasts markedly with observations from Plio-Pleistocene outcrops on the south coast of Trinidad interpreted as wave-dominated deltas with minor tidal influence. Shelf-margin foresets exhibit narrow, dip-elongate patterns interpreted as gullies and channels caused by shelf-margin collapse and bypass of sediment-gravity flows generated from the deltas. Some of these features continue onto the bottomsets and form high-amplitude lobate features interpreted as sand-rich fans. The fan-associated channels originate in clusters around distributary mouths, implying that mouth-bar collapse and/or hyperpycnal flow were likely initiation mechanisms for the sediment-gravity flows. Fan deposition likely occurred due to deceleration of turbidity currents upon encountering a gradient break on the slope caused by the rollover anticline and associated mud-ridge uplift. An erosional surface above the fan complex is interpreted as a collapse scar due to remobilization of sand-rich sediments downslope.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009