Lithostratigraphy and Petrology of the Bertram Member of the Wapsipinicon Formation (Devonian) in East-central Iowa

Rock samples were collected from the Bertram Member of the Wapsipinicon Formation in six quarries in Linn County, Iowa. Polished sections and thin sections were made of selected samples and were studied by use of binocular and petrographic microscopes. Sample cuttings from selected wells in east-central Iowa were studied for the presence of Bertram lithologies. An isopachous and paleogeologic map of Bertram strata was prepared from the data obtained from surface and subsurface samples. Prior to Bertram deposition east-central Iowa was exposed to subaerial erosion. The first sediments of the Middle Devonian transgressive sea were deposited on a supratidal mudflat which was occasionally flooded by abnormally high tides, as evidenced by the presence of very fine-grained dolomites, dessication features, traces of gypsum, intraclasts and scattered sand grains