Distributary Channels, Fluvial Channels or Incised Valleys?
J. P. Bhattacharya, A. Robinson, C. Olariu, M. M. Adams, C. D. Howell, and R. M. Corbeanu
University of Texas at Dallas, Richardson, TX
Many deltas show several orders of branching resulting in a wide range of sizes and shapes of distributary channels. There is thus no such thing as “a distributary channel” in many deltas. At the largest scale, trunk rivers become distributive at the point where the river becomes unconfined (nodal avulsion). Intermediate-scale delta plain channels tend to be few in number, may be separated by wide interfluves, and may be exceedingly difficult to distinguish from fluvial channels. The smallest scale “terminal distributaries” lie in the delta front. Because discharge decreases downstream, terminal distributary channels tend to be narrow and shallow, rather than wide and deep.
In many mid-continent reservoirs, such as the Pennsylvanian Booch sandstone in Oklahoma, 100m thick channelized deposits cut into 10 m thick prodelta and delta front deposits, but have been historically interpreted as distributary channels. These interpretations were strongly driven by using the deep distributary channels of the Mississippi delta as a modern analog, which is probably not appropriate because it feeds into deep water, rather than an interior sea. These deeply incised channels might be better interpreted as multi-storey incised valleys rather than single-storey distributary channels.
Outcrop examples of terminal distributary channels in the Cretaceous Panther Tongue sandstone in Utah show multiple, shallow channelized sandstones, intimately associated with more extensive delta front clinoform beds. These better match modern shoal-water deltas such as the Atchafalaya delta in the Gulf Coast.
AAPG Search and Discovery Article #90905©2001 AAPG Southwest Section Meeting, Dallas, Texas