LARGE LAKES AND TIDALLY INFLUENCED RIVERS IN THE LATE CRETACEOUS OF SOUTHWESTERN MINNESOTA

Upper Cretaceous exposures in the Courtland and Ridgley clay and gravel quarries near New Ulm, Minnnesota provide rare glimpses into depositional systems on the eastern margin of the Western Interior Seaway. The Courtland quarry consists predominantly of claystone characterized by mm- to cm-scale laminae, scattered well-preserved leaves, and siderite concretions. The laminated claystone facies at Courtland is interpreted to have accumulated in the center of a large lake. Differential erosion of underlying Proterozoic and Paleozoic bedrock may have controlled development of the lacustrine basin. A cross-bedded quartz arenite caps the claystone facies, and the contact between the two units shows up to 50 cm of relief. The base of the sandstone body is marked by a lag deposit of claystone pebbles and plant debris, and the unit is interpreted to reflect fluvial deposition. The abrupt juxtaposition of a high-energy channel deposit upon open lake facies may reflect a major reorganization of depositional systems along the eastern margin. Exposures in the Ridgley quarry are dominated by inclined heterolithic stratification (IHS) characterized by gently dipping 5-10 cm-thick packages of alternating sandstone and claystone. Sandstone units are fine-grained and exhibit tabular cross bed sets with abundant carbonaceous drapes. Intercalated claystone beds preserve abundant carbonaceous debris and occasional well-preserved leaves. A 50 cm-thick bed of lignite caps the Cretaceous exposures at Ridgley quarry. The exposures at Ridgley are interpreted to reflect deposition in a tidally influenced meandering river system that presumably was in close proximity to the seaway. Detailed sedimentologic analyses of nonmarine and marginal marine deposits in southwestern Minnesota afford a better understanding of depositional systems along the eastern margin of the Cretaceous Western Interior Seaway.