KEWEENAWAN SUPERGROUP CLASTIC ROCKS IN THE MIDCONTINENT RIFT OF IOWA

by
R.R. Anderson

The Geological Society of America
Special Paper 312, 1997, p. 211-230
Middle Proterozoic through Cambrian Rifting in Central North America
R.W. Ojakangas, A.B. Dickas, and J.C. Green, eds.

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. In Iowa, the MRS is buried by about 660 to 1650 m of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. The structural configuration, nature of the major lithologic packages, and geologic history of the MRS in Iowa was investigated by examining and interpreting the limited data available. These data consisted of drill samples (including the M.G. Eischeid deep petroleum test), gravity and magnetic anomaly maps, and petroleum industry seismic reflection profiles. Investigations included petrographic examination of samples, mapping, and the production of a series of two-dimensional gravity profiles constrained by the seismic data. These studies reveal the MRS in Iowa to be characterized by a central horst (the Iowa Horst), dominated by mafic volcanic rocks, and thrust over thick sequences of younger Keweenawan Supergroup clastic rocks that fill flanking basins. At upper crustal depths, the MRS displays a relatively symmetrical structure, with sedimentary basins on both flanks of the central horst similar in depth and configuration. The clastic rocks that fill these basins appear to be dominated by two major sequences, the first apparently deposited shortly after the cessation of rift volcanism, and the second probably deposited during and shortly after the uplift of the Iowa Horst.