Abstract

The Midcontinent (USA) is on the North American craton and the heartland of the continent. As with other similar areas in the world, it is one of the classic examples of layer-cake stratigraphy. For the most part, flat-lying sediments are exposed from under the blanket of glacial material in the northern part of the Midcontinent with a bit of Precambrian basement poking through the cover here and there. Thin beds (~60 cm thick) are traceable for tens of thousands of square miles, essentially uniform and unchanged, and this has been recognized for more than a century. In addition, the cyclic nature, on both a large and small scale, has been mused upon for nearly as long. Why is the craton the geobakery of the layer cake? First of all, the cratonic area has been stable with only minor up and down movement during many million years and the foundation has been inundated by the rise and fall of sealevel more times than can be counted giving rise to the layering; secondly, the base is a firm Precambrian crystalline foundation on which the cake layers could be created; and thirdly, the Earth's heat flow and solar radiation through time has baked the cake. These factors, plus the world climate change, has created the cake as we see it today complete with ice-age glacial frosting.