Facies partitioning and trajectory of shoreline migration in fourth-order sandstone tongues: Upper Cretaceous lies Formation (Mesaverde Group), Sand Wash Basin, Northwestern Colorado

CRABAUGH, JEFF P., and RONALD J. STEEL

Fourth-order sandstone tongues within the Iles Formation (Upper Cretaceous, Mesaverde Group) of northwestern Colorado extend basinwards for a few hundred kilometers, and are separated by relatively fine- grained intervals representing coastal-plain deposition in the west and marine environments in the east. Although sand-tongue progradation over these distances may be possible under relative sea-level rise, such extended progradation is favored under conditions of relative fall or stillstand, considering the increase in sediment supply which would be required to fill the increased space associated with a climbing shoreline trajectory during sea-level rise.

The significant rise in relative sea-level during the deposition of the Mesaverde Group inferred from its thickness (approx. 1 km), and the improbability that extended Mesaverde sandstone tongues were deposited during sea-level rise suggests that most of the rise is recorded in the relatively fine-grained facies lying between sandstone tongues. This is suppported by field evidence of climbing, transgressive surfaces which are commonly mantled with deposits rich in coal rip-ups or shell hash. These transgressive surfaces rise steeply landwards in an east-to-west direction and are commonly in erosive contact with underlying coal-bearing facies. This relationship between transgressive sands rising steeply landwards across significant thicknesses of coastal-plain successions, combined with the conceptual arguments made above, indicate that sediment was stored predominantly on the coastal plain during fourth-order, relative sea-level rise, but was bypassed mainly to the prograding shoreface during stillstand or fall. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.