Different correlation strategies must be applied to shoreface and shelf facies tracts of seaward- and landward-stepping genetic sequences in clastic wedges. This is a necessary consequence of differences in shelf depth over which shoreface progradation occurs. The Mesa Verde clastic wedge of the San Juan basin, which contains 15 genetic sequences arranged in seaward-stepping (lower Mesa Verde) and landward-stepping (upper Mesa Verde) stacking patterns, exemplifies these correlation strategies.

Shorefaces of seaward-stepping genetic sequences normally prograde over deep shelves below wave base and build their own shelf platform. Progradation of the shoreface cannot occur until the shelf immediately in front of it is brought into wave base by sediment aggradation. Correlations within these follow original topographic profiles from the flat coastal plain (0.01° to 0.001°), across the more steeply inclined (0.1° to 5°) shoreface or deltafront, onto and across the intermediate gradient shelf.

Shorefaces of landward-stepping genetic sequences prograde across shallow shelves within wave base. During progradation, sediment is winnowed from the shoreface and swept by the wave-base razor across the shallow shelf to distance sites below wave base. Normally the first site for sediment accumulation below wave base is beyond the depositional limit of the underlying shoreface. Correlations of landward-stepping genetic sequences follow the topographic profile of the coastal plain and shoreface or deltafront to a bypass surface on the top of the underlying genetic sequence, and then to an aggradational shelf cycle beyond the shoreface depositional limit of the underlying genetic sequence. Thus, she shelf sediment wedge lies at an elevation one stratigraphic cycle lower than its time-equivalent shoreface.

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