DISTRIBUTION OF KEWEENAWAN ORGANIC-RICH SHALES ASSOCIATED WITH THE MIDCONTINENT RIFT SYSTEM IN IOWA

Clastic rock sequences associated with the late graben stage of the Middle Proterozoic Midcontinent Rift System (MRS) include a gray, organic-rich shale unit. This shale is called the Nonesuch Fm in its northern Wisconsin and Michigan exposure belt and is informally known as “Unit C” in the Iowa subsurface. The shale in the exposure belt has been interpreted to have been deposited in a series of shallow lakes that developed within the MRS central graben, similar to the lakes in the East African Rift’s axial graben. This model explains the petrography and stratigraphic relationships where the shale is exposed, however drill data from Iowa suggest a much different model. “Unit C” was first encountered during the drilling of the Amoco M.G. Eischeid deep petroleum test well in west-central Iowa in 1987. The Eischeid well penetrated 450 m of gray shale near the base of a 4,400-m section of MRS clastics. This shale appears to correlate well with the Nonesuch Shale, but the two units were deposited in very different regions of the rift. Unlike the Nonesuch exposures, the Eischeid well sampled rocks about 8 km west of the bounding faults, outside of the MRS axial graben. So in Iowa, the rift-axial lakes in which the shale was deposited extended beyond the limits of the central graben. Dark shales identical to “Unit C” were also encountered in core on the uplifted central peak of the Manson Impact Structure, 20 km west of the rift. Since the “Unit C” shale was probably deposited along the axis of the subsiding MRS graben, and the Manson cores were 60 km from the rift axis, the water body in which it was deposited was at least 120 km wide and probably extended along most of the length of the MRS. This body of water would have been exceptionally large for a lake, and may have been linked to the Grenville Ocean.