Incision of the Mississippi River through the Laurentide Ice Sheet Forebulge

Abstract:

The Upper Mississippi River dramatically incised and alluviated over Quaternary glacial cycles. Early Pleistocene till that crosses the present-day incised valley [Willman and Frye, 1969] indicates that at least one Pleistocene continental glaciation predated river incision; magnetically-reversed sediments in the Wisconsin River valley [Knox and Attig, 1988] record some amount of incision by 780 ka; and the modern Upper Mississippi valley is underlain by a thick alluvial fill [Horberg, 1950; Knox, 1999; Blumentritt et al., 2009]. Here we integrate geological data and geophysical modeling to provide insight into the ice sheet–fluvial system interactions that shaped the landscape of the modern Upper Mississippi valley. We show that buried ~50–95 m beneath the modern alluvial surface of the Mississippi lies a bedrock valley whose anomalously deep northern reach records fluvial incision through the ~1000 km wide and up to 40–50 m high Laurentide Ice Sheet forebulge. Subsequent forebulge collapse upon deglaciation led to thick alluviation that serves to preserve this fossil bedrock valley. Avulsions of the Mississippi during the last glacial cycle caused the remaining ~50 m of roughly uniform aggradation by trapping glacial-stage sediments behind bedrock sills along the new course of the river. Modeled rock uplift rates in the region over the last glacial cycle suggest that uplift should have peaked in the interval 16–12 ka, reaching ~10–40 mm yr⁻¹ across much of the Upper Mississippi valley. This is synchronous with deglaciation and the resultant enhanced meltwater discharge [Wickert et al., 2013] and proglacial lake outflow [Knox, 2007] into the Mississippi River. The presence of the bedrock trough implies that during deglacial phases lasting a few thousand years, the Upper Mississippi River—located in the center of the stable North American craton—was capable of keeping pace with rock uplift rates reaching 2–10 times that of the most active mountain belts on Earth [e.g., Leland et al., 2008].