CLASTIC ROCKS ASSOCIATED WITH THE MIDCONTINENT RIFT SYSTEM IN IOWA, MIDCONTINENT U.S.A.

by
R.R. Anderson and R.M. McKay

U.S. Geological Survey Bulletin 1989-I, 1997, 145 p.

ABSTRACT


The 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 660 to 1650 m (2,200-5,500 ft) of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. An extremely large volume of clastic rocks 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 reexamined. Samples previously interpreted as deep-lying Paleozoic rocks are now known to have caved from upper levels of the drill hole 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 with similar lithologies.