THE MIDCONTINENT RIFT SYSTEM IN
IOWA
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EARLY (PRE-VOLCANIC) RIFT CLASTICS |
The history of the MRS itself began about 1100 Ma ago when an area from what is now eastern Lake Superior to Kansas was subjected to regional tensile stresses. This was probably related to the Grenville Orogeny, the closing of the Grenville Ocean to the east. As the stresses increased, the crust began to fail, thinning along an east/northeasterly trend from the area that is now the Lake Superior region and eastern Minnesota, southward across central Iowa, southeastern Nebraska, to southern Kansas. The trend of thinning generally paralleled the trend of the Grenville suture, but was locally influenced by pre- existing structural features. In the Lake Superior region, and probably to the southwest, the crustal thinning produced a linear depression into which rivers flowed, depositing clastic sediments and forming lakes in some areas of the depression. The Nopeming Formation (Minnesota) and equivalent units are the preserved products of this fluvial deposition. The Bessemer Quartzite (Wisconsin) and related strata are the lacustrine units. Clastic rocks related to these units have not yet been identified in Iowa, probably due in large part to the limited Iowa basement well data and the limited areas where these units crop out from beneath overlying Keweenawan volcanic rocks.
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