CLASTIC SEDIMENTARY ROCKS OF THE MIDDLE PROTEROZOIC MIDCONTINENT RIFT SYSTEM IN IOWA
Iowa Academy of Science
April 22-23, 1994, St. Ambrose University, Davenport, IA
1994 Program Abstracts, p. 17
The Middle Proterozoic (ca. 1.1 Ga) Midcontinent Rift System (MRS) is a failed rift that stretches from eastern Lake Superior to central Kansas. 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. 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. The MRS Iowa Horst is flanked by clastic rock-filled basins on the east, including (north to south) the Wellsburg, Ankeny, and Shenandoah basins, and on the west by 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. Study of MRS clastic rocks (informally called Red Clastics) from the Amoco #1 Eischeid deep oil test led to their subdivision into two groups and seven formations. 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 with a volume of about 64,700 km3. These clastic rocks reach an interpreted maximum thickness in excess of 11 km.