THE MARIANNA CREVASSE SPLAY: A 20 KY MISSISSPPI RIVER SPLAY THROUGH THE MARIANNA GAP OF CROWLEY'S RIDGE, EASTERN ARKANSAS

The northern Mississippi Alluvial Valley is divided into the Eastern and Western Lowlands by a linear topographic high, known as Crowley's Ridge. The ridge extends for approximately 350 km (215 miles) from Thebes Gap in Missouri south to Helena, Arkansas. Thirty km (20 miles) north of the southern terminus is a gap in the ridge which is 13 km (8 mi) wide. This gap is the path through which the Mississippi River temporarily reinvaded the Western Lowlands during Wisconsin glaciation, ca. 20 kya (Blum et al., 2000, Geol. Soc. Am. Bull.). Immediately west of the Marianna gap there is a tremendous crevasse splay, likely formed when the Mississippi River broke through the Ridge into the Western Lowlands. Based on geomorphology, the landform is 20 km (12 miles) long and 10 km (6 miles) wide, which is substantially larger than the Advance crevasse splay of similar age to the north, as described by Blum et al. Multiple sinuous channels appear to be cross-cut by the oblate bars of the splay. These bars form ridges and swales approximately perpendicular to the regional gradient and appear to emanate from Mariana gap.