Middle Devonian depositional environments for Rapid Member (Cedar Valley Group) interpreted from exposures at the Coralville spillway and elsewhere in eastern Iowa
- Iowa Dept. of Natural Resources, Iowa City, IA (United States). Geological Survey Bureau
Ongoing studies of the Cedar Valley Group were fortuitously aided by the exhumation of Rapid Mbr (Little Cedar Fm) carbonate strata below the coralville spillway, Johnson Co., Iowa, during the summer floods of 1993. Extensive bedding-plane exposures provided an exceptional opportunity to document fine-scale lateral biotic and lithologic variations within the member, and to compare these with data from elsewhere in eastern Iowa. The base of the Rapid Mbr is drawn at an abrupt lithic change above packstones of the Solon Mbr, marking a regional transgressive event. The basal 2.6 m of the Rapid is dominated by argillaceous skeletal wackestones with common brachiopods and echinoderm debris, interspersed with thin mudstones. The overlying 4 m comprises a series of 50--100 cm thick couplets which display alternations of thin mudstones and thicker brachiopod-rich wackepackstones. The next unit (2.9 m) is dominated by sparse-skeletal argillaceous mudstones. The paucity of burrowing and skeletal benthos through much of the unit is interpreted to reflect bottom oxygen stress in a relatively deep, possibly stratified cratonic seaway. Nevertheless, skeletal horizons within the unit indicate episodically favorable benthic conditions. The mudstone unit shallows upward into a brachiopod-rich wackestone interval which is, in turn, capped by a condensed horizon of phosphatic and glauconitic enrichment (near base of subterminus Fauna). Above this, two regionally extensive coral-rich biostromes occur. Upper Rapid strata show a complex of wackestone and packstone facies, with glauconitic enrichment and hardgrounds noted. The member is capped by shoal-water grainstones in the Coralville area, and by peritidal facies in northern Iowa.
- OSTI ID:
- 7123421
- Report Number(s):
- CONF-9404217-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 26:5; Conference: 27. annual conference of the North-Central Section of the Geological Society of America (GSA), Kalamazoo, MI (United States), 28-29 Apr 1994; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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