B1989I -- Clastic Rocks Associated with the Midcontinent Rift System in Iowa


By Raymond R. Anderson and Robert M. McKay

 

The Middle Proterozoic Midcontinent Rift System (MRS) of North America is a failed rift that formed in response to region-wide stresses about 1,100 Ma. In Iowa, the MRS is buried beneath 2,200-3,500 ft of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. An extremely large volume of sediments was deposited within basins associated with the rift at several stages during its development. Although the uplift of a rift-axial horst resulted in the erosional removal of most of these clastic rocks from the central region of the MRS in Iowa, thick sequences are preserved in a series of horst-bounding basins. Recent studies incorporating petrographic analysis, geophysical modeling, and other analytical procedures have led to the establishment of a preliminary stratigraphy for these clastic rocks and interpretations of basin geometries. This information has allowed the refinement of existing theories and history of MRS formation in Iowa. Additionally, drill samples previously interpreted as indicating the existence of early Paleozoic basins overlying the Proterozoic MRS basins were re-examined. Samples previously interpreted as deep-lying Paleozoic rocks are now known to have caved from upper levels of the drillhole and were out of stratigraphic position. No deep Paleozoic basins exist in this area. These investigations led to the development of petrographic parameters useful in differentiating the Proterozoic MRS Red clastics from Paleozoic clastic rocks having similar lithologies.

 


NOTE:

This is a cover page. To go to the actual document, click on the title.
 
The document is in PDF format (1M). Therefore, you will need Acrobat Reader 3.0 to view and print. If you do not have Acrobat Reader 3.0, you will need to download the program.
 
Click here to Download Adobe Acrobat 3.0 and follow the download and installation instructions provided. After downloading Acrobat, configure your WWW Browser to use Acrobat as a helper application.


USGS Bulletins

U.S. Department of the Interior
U.S. Geological Survey

This page is <URL: http://pubs.usgs.gov/bul/b1989i/b1989i.html>
Maintainers: Joan Nadeau and Gene Ellis
Last updated 15 Dec 2005