Facies Architecture, and Controls on Channel-belt Geometry: Cretaceous-Ferron Notom Delta, Utah, U.S.A.

Examination of fluvial deposits of the Late Cretaceous (Turonian) Ferron Sandstone Member, within the Mancos Shale Formation, in Central Utah (along Sweetwater Wash) was done to interpret fluvial style, characterize avulsion, and better constrain controls on channel-belt geometry. Cross sections were created based on measured sections and walking out beds, and showed two highly amalgamated sheet sands (Channel-belt A&C) separated by floodplain-encased channel-belts (isolated Channel-belt group B). The extensive sheet sandstones overlie distal floodplain deposits, and were subsequently characterized as regional avulsions. The isolated channel-belts overlie more proximal lacustrine and floodplain deposits, and were characterized as local avulsions. Detailed analysis of Channel-belt C showed a laterally extensive, unconfined channel-belt, formed by rivers 2.5-4 meters deep, with average storeys of 1.8 meters and bar lengths between 150-190 meters. Consistent paleo-current directions, evidence of downstream accretion, and a confluence zone, in highly scoured sheet-like deposits, suggest the deposition of Channel-belt C by a dominantly braided river. Correlation to a previously developed sequence stratigraphic framework showed that base level is an allocyclic control on alluvial architecture, with possible glacio-eustatic cycles controlling the regional stratigraphy. Channel-belt clusters were observed within the isolated channel-belts, but were not uniform across the basin, so they were interpreted as autocyclic or random, and not related to incised valleys below. Evidence for autocyclic controls, such as substrate erodability, was documented with laterally continuous indurated coals favoring extensive channel migration and the deposition of vi sheet sands. Sandy substrates favored channel reoccupation and vertical amalgamation. Comparing my data to net-to-gross models showed that the high net-to-gross and highly amalgamated deposits of the lower Channel-belt C are mainly associated with braided rivers, whereas meandering rivers are dominant in the low net-to-gross deposits of the isolated channel-belts; thus showing some similarities to previous net-to-gross models.