STRATIGRAPHY OF THE MIDCONTINENT RIFT SYSTEM IN IOWA
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UNIT H |
Unit H (depth 841 to 1085; m 2802 to 3615 ft) is dominated by two primary lithologies, a clear to light reddish brown, fine to very coarse grained arkosic sandstone, and a reddish brown, micaceous, shaley to sandy siltstone (McKay, 1990). The common occurrence of loose quartz and feldspar grains among the well samples implies that some of the sandstones are poorly indurated. Petrographic study of thin sections produced from cuttings collected during the drilling of the Eischeid well was conducted using a point counting stage that automatically advanced the thin section by regular increments under the petrographic microscope.
The study of Unit H yielded a mean framework composition (click here for QFL information) of Q=79 F=17 L=4 ( i.e. 79% of all sand grains were quartz, 17% were feldspars, and 4% were lithic or rock fragments). All of the feldspar grains encountered were potassium feldspar (Ludvigson and others, 1990). McKay (1990) also reported the presence of trace amounts of microbrecciated rock fragments, identical to those found down hole throughout a large part of the Red Clastics section. The presence of microbreccia chips probably implies that the uppermost preserved portion of the Red Clastics section was affected by structural movements similar to those which created the tectonized fabrics throughout much of the Eischeid drill hole sequence (Ludvigson and others, 1990). The presence of the microbreccia and other data led McKay (1990) to suggest a "Late Keweenawan" (Middle Proterozoic) age for Unit H. Anderson (1992) assigned Unit H to the Upper Red Clastic Group.
click for depositional model for Unit H
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