CYCLOTHEMS: READING THE ROCK RECORD IN THE MIDCONTINENT

Cyclic deposition has long been recognized in the world’s rock record. Defining the nature and cause of cyclic deposition became a major focus in the midcontinent in the 20^th century, driven by petroleum exploration and improved knowledge of the region’s stratigraphy. Workers in the midcontinent developed the concept of cyclothems, consistent packages of rocks (generally limestones, shales, and coals) deposited by transgressive and regressive seas. They postulated mechanisms, including tectonic uplift and glacial waxing and waning (this was before the acceptance of plate tectonics, so neither explanation took plate movement into account). Workers in Illinois, particularly J.A. Udden, H.R. Wanless and J.M. Weller, are credited with advancing this conversation and developing the term “cyclothem.” R.C. Moore applied it to thin sedimentary units in Kansas. But no one agreed about a mechanism. In the 1980s, the concept of sequence stratigraphy became more widely applied, in some senses appearing to grow out of the idea of cyclothems. Both concepts, sequence stratigraphy and cyclothems, relate directly to worldwide sea-level change. Robert Dott’s Eustasy: The Historical Ups and Downs of a Major Geological Concept, published by GSA, includes papers from a 1990 GSA session on this topic. Several of these papers traced the evolution of the cyclothem idea and the players who developed and promoted its application, particularly in the midcontinent.