ORIGIN OF MARIANNA GAP, A TRANSVERSE STREAM VALLEY, THROUGH CROWLEY’S RIDGE, NORTHERN MISSISSIPPI ALLUVIAL VALLEY, USA

Crowley’s Ridge is a fault-bounded remnant of unconsolidated sediment that splits the Upper Mississippi Alluvial Valley into separate east and west stream courses. There is a 13 km gap in Crowley’s Ridge near Marianna, Arkansas, occupied by a transverse drainage that hydrologically connects the two stream courses. We address how and when the Marianna gap and transverse drainage through Crowley’s Ridge were formed. Geomorphology, stratigraphy, and ages indicate formation of the gap at ≤ 17ka. It may have been the result of direct seismicity on one or more structures that cross Crowley’s Ridge near Marianna, or from fluvial erosion controlled by seismicity. Seismic events may have disrupted that part of Crowley’s Ridge, permitting streams that drain the ridge to incise it more easily. Contemporaneously, those events may have caused subsidence in that part of the ridge diverting nearby streams that eroded the ridge from the sides. Eventually, the L’Anguille River in the Western Lowlands was captured through the gap by a lower eastern channel course. Transverse drainage formation through fluvial erosion controlled by tectonic subsidence is similar to the headward erosion/stream piracy process described by previous researchers, but differs in being subsidence driven, and developing within a sedimentary basin rather than an uplifting mountain belt.