THE MIDCONTINENT RIFT SYSTEM IN IOWA
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STRATIGRAPHY OF UNIT B |
The basal clastic unit that was deposited along the rift axis (in the axial graben that existed at that time) was a coarse, volcanic rock-dominated conglomerate. Although no drill holes penetrated this conglomerate in Iowa, it can be observed in the Lake Superior Region where it is called the Copper Harbor Conglomerate (click to view MRS stratigraphy).
click to view depositional model of rift axis deposition
As the central graben began to fill with coarse clastics and gradients were reduced, sedimentation was dominated by sandstones and siltstones deposited by rivers flowing down the axis of the MRS. This deposition expanded beyond the limits of the axial graben as shown by the Amoco M.G. Eischeid #1oil test. The basal clastic unit in the Eischeid hole (depth 4935 to 5310 m; 16,450 to 17,700 ft) was informally called Unit B by Witzke (1990), who described it as dominated by white to light gray and red, very fine- to fine-grained sandstone with possible siltstone and shale partings. Petrographic studies of Unit B sandstones by Ludvigson and others (1990) indicated that it contained the highest percentage of quartzose grains observed in the Red Clastic Group Q=87 F=12 L=1, with a small lithic component dominated by sedimentary and metamorphic clasts (74%) (click here for QFL information)..
click to view Unit B depositional model
return to Midcontinent Rift System Stratigraphy
