Macalester College Geology Department

Comparative Sedimentology of Two Late Cretaceous Localities near New Ulm, Minnesota

Upper Cretaceous strata are preserved between Precambrian bedrock and Pleistocene glacial deposits in the Minnesota Valley Minerals (MVM) Courtland and Ridgley clay and gravel quarries near New Ulm, Minnesota. These exposures provide rare glimpses into the enigmatic eastern margin of the Western Interior Seaway. Field observations of outcrops were coupled with laboratory analyses (including X-ray Diffractometry, shale disaggregation, sandstone sieve analysis, and petrography) and were used to characterize the nature of the depositional environment for each locality.

Three distinct facies were identified in the MVM Courtland mine. The Lower Laminated Claystone (LLC) facies contains virtually no coarse-grained (>coarse silt) particles. Fine (mm- to cm- scale) color bands, scattered well-preserved leaves, and siderite concretions also characterize the LLC, which is interpreted to reflect deposition in a large, open intracratonic lake. A fine-grained, cross-bedded quartz arenite body comprises the Sandstone facies (SS) and caps the LLC in three of four measured sections. The SS facies includes a basal lag deposit with imprints of carbonaceous material and is interpreted to be a channel deposit. The Upper Massive Claystone facies (UMC) crops out above the SS facies in one of the four sections. Based on apparent soft-sediment deformation structures on the contact between the SS and UMC facies, the UMC is interpreted to be part of the same channel deposit as the SS facies. The juxtaposition of a high energy channel environment on an open lake environment is unconformable, and thus might reflect a channel avulsion event or a fall in sea level.

The MVM Ridgley mine is dominated by inclined heterolithic stratification (IHS) deposits characterized by gently dipping 5-10 cm-thick packages of alternating sandstone and claystone. Sandstone units are fine-grained and shows tabular cross-bed sets with tangential toes and carbonaceous drapes on foresets. Claystone units are massive. A dark, lignitic mudstone layer caps Cretaceous exposure at Ridgley. Ridgley deposits as a whole are likely the result of deposition in a tidally influenced meandering river system.

Detailed sedimentologic anlysis of limted Cretaceous nonmarine and marginal marine exposures in southern Minnesota afford a better understanding of the eastern margin of the Cretaceous Western Interior Seaway.