MIDDLE DEVONIAN FAULTS AND JOINTS, CORALVILLE SPILLWAY, EAST-CENTRAL IOWA: A BRITTLE MANIFESTATION OF THE ACADIAN OROGENY IN THE MIDCONTINENT

CZECK, Dyanna, Dept. of Geology & Geophysics, University of Minnesota, 310 Pillsbury Dr., Minneapolis, MN 55455, and FAULDS, James E., Dept. of Geology, University of Iowa, Iowa City, IA 52242.

The 1993 flood waters on the Iowa River beautifully exposed Middle Devonian strata of the Cedar Valley Group in the spillway of the Coralville Dam, Iowa. Although best known for its fossils, the spillway also contains excellent exposures of closely-spaced faults and joints. The faults are roughly constrained to ~375 Ma, because they appear to terminate upward within the 375 Ma Little Cedar Formation. Two prominent sets of faults and joints occur within the spillway: N35E, 70°NW and N75W, 35°NE. Striae and corrugations are well developed on many faults and generally suggest dip-slip movement. Where displacement can be inferred, the majority of the faults accommodated normal offset. However, reverse displacement characterizes those faults that accommodated the greatest amount of offset. The major question considered in this initial study is whether the faults originated through tectonic processes or through dissolution and collapse of Middle Devonian evaporites at depth.

The presence of the reverse faults and consistency in orientation of all types of faults favor a tectonic origin. The two dominant orientations of fractures roughly parallel the Appalachian and Ouachita fold and thrust belts. Furthermore, the apparent age of the fractures is contemporaneous with the Acadian Orogeny. We therefore suggest that minor faults and joints within the Midcontinent may provide a record of major tectonism along the eastern and southern margins of North America, particularly the collisional events within the Appalachian orogen. In a study of calcite twins in Paleozoic carbonates, Cradddock et al. (Tectonics, 1993) noted ductile strains of up to 6% within the Midcontinent and attributed them to the Appalachian-Ouachita orogens. The minor faults and joints within the Middle Devonian carbonates of the Coralville Spillway may indicate that brittle deformation of similar magnitude and origin locally affected the Midcontinent.




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