A New View of Aggradational Fluvial Systems - the Distributary Fluvial System (Dfs) Paradigm
When we think of fluvial systems and their preservation in the rock record, we typically draw upon our experiences with tributary rivers most commonly observed in the world. Indeed, most facies models that we use to interpret the ancient rock record have been developed on tributary rivers that exist outside active sedimentary basins or, if the river studied lies within a sedimentary basin, the models developed typically do not place the studied reach into the context of the basin. A review of approximately 700 modern continental sedimentary basins around the world showed that aggradational rivers in these basins are not typically tributary in nature; rather they form either distributary fluvial systems (DFS), commonly called megafans, fluvial fans, and alluvial fans in the literature, or axial stream systems that parallel the basin trend, with the vast majority of sedimentation in the basin occurring on the DFS (up to 95%). Thus, most continental sedimentary basins undergoing aggradation do not contain tributary fluvial systems. Most facies models currently used to develop models of fluvial stratigraphy, however, were developed through studies of degradational, tributary fluvial systems! Rivers on DFS differ from tributary rivers in many, potentially significant ways, including 1 a radial pattern of channels away from an apex (or intersection point), though many of the DFS rivers do curve to become sub-parallel to the basin strike distally;
2 channel systems commonly decrease in width and discharge (and thus cross-sectional area) distally, while tributary systems tend to increase in size downstream;
3 meanderbelts on DFS tend to have few chute and neck cutoff avulsions unless they are somehow confined (though exceptions to this trend exist), creating a different distribution of facies stacking patterns in resulting sandstone bodies;
4 floodplain deposits on DFS may be dominated by avulsion successions; (5) floodplain deposits may be more readily preserved in braided systems on DFS than in tributary systems; and (6) axial stream character may contrast with DFS stream character (these tend to be more similar to the tributary models of rivers). In order to reasonably model these aggradational fluvial systems, new facies models that consider fluvial position and DFS form in the sedimentary basin are needed.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009