GILGAI TOPOGRAPHY INFLUENCES AVULSION STYLE: EVIDENCE FROM THE DEVONIAN CATSKILL FORMATION AND COMPUTER MODELING

Microtopography on the floodplains of meandering rivers directly affects floodplain flow routing and sediment dispersal, but the degree to which floodplain topography impacts channel behavior is unknown. Here we hypothesize that floodplain hydrology can also influence the dynamics of channel avulsion, a major discharge diversion that results in the formation of new channels. There are two end-member styles of avulsion: progradational, characterized by heavy sedimentation of the floodplain, and incisional, characterized by rapid channelization of the floodplain. Previous work has shown that the grain size of sand and floodplain cohesion control avulsion style, but it is unclear what impact floodplain topography has on these events. Here we test the relationship between gilgai topography (regularly spaced mounds and depressions that can form in vertisol-dominated floodplains) and avulsion style using an ancient example and computer model.

We mapped paleo-gilgai features across ten outcrops of the Devonian Catskill Formation (north-central Pennsylvania, USA) based on the presence of 0.8-7.0-meter wide pedogenic slickensides in distal floodplain facies with a regular bowl-and-chimney arrangement. Results from avulsion style mapping at these same outcrops show five progradational avulsion sequences and ten incisional avulsion sequences; all incisional avulsion packages overlie paleogilgai deposits, while progradational packages overlie a range of floodplain facies. Overall, this suggests that gilgai topography is associated with incisional avulsions in the Catskill Formation.

To test the degree to which gilgai topography controls overbank flow and avulsion behavior, we use the morphodynamic modeling software Delft3D-FLOW to model channel avulsions with and without floodplain gilgai. Model results show that regularly spaced mound-and-depression topography limits sedimentation and increases incision of the floodplain, suggesting that gilgai increase the likelihood of an incisional avulsion. Overall, evidence from the Catskill Formation and modeling results show that gilgai topography can influence avulsion style in meandering river systems. This result suggests that floodplain processes can exert control on large-scale channel behaviors such as avulsion.