Keweenawan Supergroup Clastic Rocks of the Midcontinent Rift System in Iowa

Abstract

The Midcontinent Rift System (MRS) of North America is a failed rift that apparently formed in response to region-wide stresses associated with the Grenville Orogeny about 1100 million years ago. The Iowa portion of the MRS is buried by about 660 m to 1650 m of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. The structural configuration, nature of the major clastic rock packages, and geologic history of the MRS in Iowa were investigated by examining and interpreting limited well samples (including the 5355 m-deep M.G. Eischeid deep petroleum test well), gravity and magnetic anomaly data, and petroleum industry reflection seismic profiles. These data were used to produce a series of two-dimensional gravity profiles coincident with the seismic profiles. These studies, described in detail by Anderson (1992) and reviewed by Anderson (1995), reveal the MRS in Iowa (Fig. 1) to be characterized by a central horst (the Iowa Horst) dominated by mafic volcanic rocks, thrust over its contacts with the thick sequences of younger Keweenawan Supergroup clastic rocks that fill flanking basins (Fig. 2). This investigation demonstrated that MRS rocks in Iowa are primarily restricted to middle and upper crustal depths. The MRS displays a relatively symmetrical structure, with sedimentary basins generally deepening toward the axis of the rift, on both flanks of the central horst. Opposing clastic basins are approximately similar in depth and configuration, overthrust by the central horst near the axis of the rift and displaying beveled erosional edges at their outboard limits (Fig. 2). Two basins were documented northwest of the Iowa Horst, the Duncan and Defiance basins (Fig. 1). Southeast of the horst three basins were identified, the Wellsburg, Ankeny, and Shenandoah basins. Three clastic rock-filled basins were identified on the Iowa Horst, the Stratford, Jewell, and Mineola basins.