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UPPER RED CLASTICS GROUP

Red ball iconTHE MIDCONTINENT RIFT SYSTEM IN IOWA

 

UPPER RED CLASTICS GROUP


Introduction to Upper Red Clastics Group

The clastic rock sequence, encountered at depths from 841 to 3153 m (2802 to 10,510 ft) during drilling of the Amoco M.G. Eischeid #1 deep oil test in the Defiance Basinl, was assigned to the Upper Red Clastic Group by Anderson (1992). The lithologies of the sequence were described by Witzke (1990) from down hole logs and petrographic and other studies. He informally called them, from the top, Unit H, Unit G, Unit F, and Unit E. He assigned units G, F, and E to the Upper Red Clastic Sequence, but was unsure of the affinity of Unit H. McKay (1990) discussed compositional, grain size, and other petrologic features of Unit H that differed from basal Cambrian units in the region. He argued for a Keweenawan age for Unit H.

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UPPER RED CLASTICS GROUP STRATIGRAPHY
MIDCONTINENT RIFT SYSTEM ROCKS OF IOWA

Unit H

Unit G

Unit F

Unit E

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(click for map of thickness of Upper Red Clastics in flanking basins)


Discussion of the Upper Red Clastic Sequence.

The Upper Red Clastic Group occupies the same relative stratigraphic position as the Bayfield Group in the Lake Superior region. Both units are apparently dominated by fluvial deposition, with the possible exception of the Devils Island Formation of the Bayfield Group which was interpreted a lacustrine deposit by Ostrom in Mudrey and Ostrom (1986). There are, however, some compositional differences between the clastic rocks of the two groups. The Bayfield Group is more mature, both in texture and mineralogy, than the underlying Oronto Group (Ojakangas, 1986), whereas the rocks of the Upper Red Clastic Group are mineralogically less mature than the Lower Red Clastic Group. Volcanic rock fragments contribute less to Bayfield Group rocks than to the Oronto Group (Ojakangas and Morey, 1982b). However, in the Eischeid well volcanic rock fragments are more common in the Upper Red Clastic Group than the Lower Group. Finally, Eischeid drilling did not penetrate any Proterozoic quartz arenites, such as those seen in the Devils Island and Hinkley sandstones in the Lake Superior area.

The Bayfield Group and Upper Red Clastic Group do, however, share many characteristics. Both groups apparently overlie the initial Midcontinent Rift System clastic sequence (Oronto Group and Lower Red Clastic Group). The Bayfield Group, as described by Ojakangas and Morey (1982b), is dominated by fluvial deposition (excluding the possible lacustrine Devils Island Sandstone). The lithologies present in the Upper Red Clastic Group as described by Witzke (1990), and the petrology and sedimentary structures as established from the cored intervals by Ludvigson et al. (1990), also suggest a fluvial origin. The differences in the composition of the units of the Upper Red Clastic Group and the Bayfield Group can be explained by differences in the lithologies of source terranes.

Although no quartz arenites were encountered in the Eischeid well, Unit F of the Upper Red Clastic Group displays the highest content of siltstone and shale. These lower energy deposits may be related to the possible lacustrine deposits observed in the middle unit (Devils Island) of the Bayfield Group, perhaps deposited by a low gradient river that flowed into a nearby lake.

Several trends are evident in the composition of the sandstone component of the Upper Red Clastic Group. Quartzose grain content increases upward in the sequence, from 67% in Unit E to 79 % in Unit H. The QFL composition of the framework grains is comparable to compositions of units in the Lake Superior area (click to view QFL information). In addition, the average lithic rock fragment percentage decreased up section from 13% in Unit E to 4% in Unit H, with the volcanic rock component of the lithic rocks increasing from 22% in Unit E to 62% in Unit G and sedimentary and metamorphic rock component decreases from 65% in Unit E to 37% in Unit G. The compositions of the lithic fragments in the two intervals examined in Unit H were quite disparate and additional analyses are needed.

The relative increase in the concentration of volcanic rock fragments up section in the Upper Red Clastic Group in the Eischeid well may record the local proximal erosional unroofing of the basalts on the Iowa Horst. Well data from the trend of the MGA in Iowa and seismic interpretations indicate that Keweenawan sedimentary rocks were erosionally removed from most areas of the Iowa horst, exposing underlying volcanic rocks (Anderson, 1988).

click for references cited

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