Print this pageDelta Front Facies Architecture of Cretaceous Ferron Notom Delta from Caineville, South Central Utah
Terminal distributary channels represent the last channelized expression of a drainage system at the delta front and initiate from the last subaerial bifurcation of distributary channels. Based on previous studies, terminal distributary channels are very shallow, usually no more than a few meters deep, with width to depth ratios around a few hundred. Modern examples of river-dominated deltas show the density of the terminal distributary channel to be up to 6 channels per kilometer, but there are few ancient studies that are able to corroborate dimensions obtained from modern studies.
The Cretaceous Ferron Notom Delta in South-Central Utah contains superbly exposed examples of terminal distributary channels and associated mouth bars. Strike-oblique cliff cross-sections integrated with vertical measured sections enable an evaluation of bed-scale facies architecture and depositional facies within highstand, storm-, river- dominated deltaic parasequences.
Dune-scale cross bedded sandstones, group into low-angle accreting bar forms that fill shallow-incisions at the prodelta to delta front transition, within an overall coarsening upwards facies succession. Some bar-crests contain highly inclined cross-strata with poorly sorted mud clasts and abundant plant material, that indicates shallow channelized flow. The stratigraphic position, associated with architectural elements that show interaction of fluvial and basinal processes, and lenticular shape with erosive bases suggest that these are terminal distributary channels. Channels are filled by lateral migration of mouth bars, vertically stacked mouth bar complexes, or mud plugs following channel abandonment.
Erosional relief reaches 2 m deep and channel width averages 18m yielding a width to depth ratio of about 10. Channel density reaches 64 channels per kilometer, considerably higher than seen in many modern examples. The high channel density might be related to high sediment discharge versus low accommodation, which enhanced channel bifurcation. The oblique orientation of the outcrop enables us to observe lateral-, downstream- and upstream- migration of mouth bars, suggesting complex growth patterns, versus simple progradation. These mouth bar deposits show strong river influence with some storm/wave reworking, which results in facies including highly convoluted strata, mass transport complex, rip-up clasts, massive or graded beds with wave rippled tops, and local storm sheets.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009