RI-05 The Cretaceous System in Minnesota

Abstract

The Cretaceous rocks of Minnesota are flat-lying, loosely consolidated sediments of variable lithology. Two formations, both showing wide facies variation are recognized, the predominantly marine Coleraine Formation in northern Minnesota and the predominantly nonmarine Windrow Formation in southern Minnesota. Elsewhere in the State the strata are poorly exposed and knowledge of their lithology, thickness, and correlation is fragmentary. The strata rest unconformably and with profound hiatus on a surface with a maximum relief of 1,400 feet, which developed during a long interval of erosion and weathering that extended from sometime after the Devonian into the Cretaceous. In general, marine sandstones and shales deposited in the western and northern parts of the State grade eastward into estuarine, paludal, deltaic, and lacustrine sediments. The sediments lie. on rocks ranging in age from Precambrian to Devonian. The distribution and character of the sediments are interpreted to indicate that they formed in and adjacent to the Late Cretaceous sea that invaded Minnesota from the west and continued to advance eastward over an irregular terrain. In general, the vertical succession at any particular locality consists of a basal regolith developed on pre-Cretaceous bedrock, an unconformity, basal nonmarine stream deposits, and finally marine clastic sediments; but the full succession is not present at all localities. The fossil record indicate s that the strata can be correlated with the entire Colorado Group of the western interior United State s. Sediments that were deposited at lower present- day altitudes are Cenomanian in age, whereas those deposited on bedrock at higher altitudes are Turonian and later in age.