R.R. Anderson

The Geological Society of America
28th Annual North-Central Section Meeting
Kalamazoo, Michigan, April 28-29, 1994
1994 Program with Abstracts, v. 26, no. 5, p. 2


The Midcontinent Rift System (MRS) is a Middle Proterozoic failed rift system that stretches from eastern Lake Superior to central Kansas. Over most of its length, it is characterized by a central horst (dominated by mafic volcanic rocks) flanked by a series of clastic-filled basins that thicken towards the horst in half-graben configurations. South of its Lake Superior outcrop belt, the MRS is buried beneath Phanerozoic marine and terrestrial sedimentary rocks and glacial drift. A recent investigation of the MRS in Iowa integrated all available drill samples with interpretation and modeling of gravity, magnetic, and seismic data to refine interpretations of the feature. In Iowa, the MRS is dominated by the Iowa Horst, flanked by clastic rock-filled basins, on the east (north to south) the Wellsburg, Ankeny, and Shenandoah basins, and on the west the Duncan and Defiance basins. Additionally, three basins (the Stratford, Jewell, and Mineola basins) and a detached segment of the horst (the Ames Block) preserve clastic rocks on the Iowa Horst. In 1987 the Amoco M.G. Eischeid #1 oil test penetrated almost 4500 m of these clastic rocks (informally referred to as the "Red Clastics"). The study of samples and logs from this well led to the erection of a preliminary stratigraphy, with two groups that are generally related to the Oronto and Bayfield groups in the Lake Superior region. Geophysical interpretation and modeling suggest that in Iowa the clastic rocks of the Lower Red Clastic Group cover an area of 33,600 km2 and total over 101,000 km3 in volume. The Upper Red Clastic Group covers an area of 38,840 km2 and a volume of about 64,700 km3. These clastic rocks reach an interpreted maximum thickness in excess of 11 km. A thick sequence of dark gray and black shale and siltstone informally designated Unit C may have been a rich petroleum source rock. Although it proved to have a low organic carbon content in its only sampled location, evidence suggests that Unit C may have produced large quantities of petroleum which may still be trapped within the Red Clastics.