HYBRID PROGRADATIONAL-ANNEXATIONAL AVULSION SIGNATURES IN MODERN AND ANCIENT FORELAND BASINS

The process of fluvial avulsion produces a rapid and whole-sale shift in channel location across a floodplain. Avulsions control the position of channels and related overbank sedimentation, in turn determining stratigraphic architecture. Stratigraphic signatures of avulsions vary according to the behavior, or style, of the event. In annexational-style events, flow switches rapidly from a parent channel to a relict or peripheral channel with little to no overbank sedimentation. Stratigraphically, this produces single or multistory channel sand bodies that incise directly into fine-grained floodplain facies. In progradational-style events, crevassing leads to prograding distributary networks, which eventually coalesce into a new primary channel. This produces channel sand bodies that incise into heterogeneous packages of thinly bedded splay and minor channel deposits which preceded the new channel. Both progradational and annexational avulsions have been described in the stratigraphic record, and avulsion-prone fluvial systems are typically described as either annexational or progradational. However, in recent remote sensing analysis we found two important departures from the traditional binary avulsion style model. First, we find that all modern observations of “progradational” avulsion events exhibit both progradational and annexational behaviors. Second, we found that avulsion style progresses from annexation-dominated to progradation-dominated as fluvial systems progress from proximal (fan apex) to distal (fan toe) settings. We turned to the Salt Wash distributive fluvial system of the Jurassic Morrison Formation to investigate whether these avulsion style signatures are preserved and identifiable in the stratigraphic record. We find that Salt Wash outcrops that have been interpreted as proximal and medial, with respect to the paleo fan apex, in fact show evidence of both channel reoccupation (annexational processes) and significant overbank sedimentation (progradational processes). Comparing our modern data set with Salt Wash outcrops, we propose that outcrops that demonstrate both avulsion signatures indicate medial to distal system settings, and present evidence for hybrid progradational-annexation avulsion signatures in modern and ancient foreland basins.