ARE CRETACEOUS ‘UNDIFFERENTIATED' ALLUVIAL SUCCESSIONS CANDIDATES FOR A COASTAL DISTRIBUTIVE FLUVIAL SYSTEM (DFS) MODEL?

Many authors have proposed eustatic vs tectonic controls on shallow marine and fluvial deposition in foreland settings, but only recent studies have applied a distributive fluvial system (DFS) alternative to explain vertical and spatial trends in coastal progradational fluvial deposits. We envision fluvial megafans draining the Sevier highlands that prograde toward the basin center, resulting in an upsection transition from marginal marine to coastal plain to alluvial plain deposition. A purely progradational succession includes: 1) basal shoreface strata divided by tidally-influenced channels that are capped by fine-grained distal DFS deposits, and 2) an upward increase in channel belt size, sandbody amalgamation and sandbody grain size and a corresponding decrease in overbank fines, coals and tidally-influenced channel strata.

This model is based on geomorphic observations of modern coastal DFSs of the Australia, India, and Africa passive margins. Observations on these progradational systems ultimately lead to sedimentologic and stratigraphic predictions on ancient regressive clastic wedges. Potential candidates for a DFS model include the Blackhawk and Williams Fork Formations of the Utah and Colorado Book Cliffs (respectively), the John Henry Member of the Straight Cliffs Formation of southern Utah, and the Menefee Formation of southwest Colorado and northwest New Mexico.

Coastal plain fluvial and marginal marine progradational successions have proven to be important hydrocarbon and carbon dioxide sequestration reservoirs, coal accumulations, and aquifers. However, existing fluvial facies models used to predict sandbody distribution and connectivity are typically based on incised valley fill successions in dominantly degradational settings. The DFS concept may explain common patterns (e.g. upsection changes in sand:mud, sandbody thickness and architecture) observed in Cretaceous rock record examples.