--> Abstract: The Shoreface/Tidal Couplet—A Mechanism for Tapping into Coarse Sediment, by T. A. Cross; #90928 (1999).

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CROSS, TIMOTHY A.
Colorado School of Mines

Abstract: The Shoreface/Tidal Couplet—A Mechanism for Tapping into Coarse Sediment

An overlooked stratigraphic motif is the couplet formed by a fine-grained, wave-dominated shoaling-up shoreface facies succession followed by a coarse-grained tidally influenced sandy or heterolithic facies succession. The two facies successions are separated by a transgressive surface of erosion (“ravinement surface”) or a transgressive surface of nondeposition (“marine flooding surface”). The change from the wave-dominated to the tidal-current-dominated facies is abrupt, and the grain-size change usually is 1.0 to 1.5F. The couplet occurs in siliciclastic and carbonate strata; in the latter case the tidal portion of the couplet may be entirely siliciclastic, entirely carbonate or mixed lithology. Although tidally influenced facies are extremely common above progradational shoreface strata, they also occur above coastal aeolian dune and carbonate shoal to shallow shelf strata.

The nearly ubiquitous occurrence of this stratigraphic couplet in paralic strata irrespective of lithology ;begs a universal explanation. The wave-dominated, sand-rich shore face facies succession suggests accumulation on a straight, open-ocean-facing coast. Where mapped in the Mesaverde Group of the San Juan basin the shoreface facies tract is demonstrably straight and linear. By contrast, multiple sedimentary attributes of the overlying tidally influenced facies suggest accumulation within estuaries, bay and lagoons of a embayed coast. The change in coast geomorphology from straight to embayed, the accompanying change from wave- to tidal-dominated sediment transport, and the formation of the transgressive surface occurs during rising sea level after the base-level fall-to-rise turnaround. Coastal flooding and embayment, which accompany rising sea level, tap directly into the coarse sediment of the higher gradient alluvial plain. This coarse sediment has a direct conduit by fluvial and tidal currents to the flooded delta or shoreface platform.

The significance of the shoreface/tidal couplet to petroleum exploration and production is the uniformly coarser sand of the tidal portion. Where deltaic strata are deeply buried, the occurrence of coarse tidal sand above the fine-grained shoreface may be the difference between economic and non-economic reservoirs. Where an erosional surface separating a shoreface from overlying coarser current-laid sand is interpreted as a sequence-bounding subaerial unconformity at the base of a fluvial channel or incised valley, the search for a coarse-grained wedge of sediment downslope may be unwarranted, since all the coarse-grained facies are contained within the tidal wedge.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas