Structural and tectonic implications of pre-Mt. Simon strata -- or a lack of such -- in the western part of the Illinois basin
- Illinois State Geological Survey, Champaign, IL (United States)
The discovery of a pre-Mt. Simon lithic arenite (arkose) in southwestern Ohio has lead to reevaluation of many basement tests in the region. Several boreholes in adjacent states have been reexamined by others and are now believed to bottom in the Middle Run Formation. Seismic-reflection sections in western Ohio and Indiana have indicated pre-Mt. Simon basins filled with layered rocks that are interpreted to be Middle Run, however, the pre-Mt. Simon basins and east of Illinois. Samples from Illinois basement tests were reexamined to determine whether they had encountered similar strata. All reported crystalline-basement tests in Illinois show diagnostic igneous textures and mineralogical associations. Coarsely crystalline samples in cores show intergrown subhedral grains of quartz, microcline, and sodic plagioclase. Medium-crystalline rocks in cuttings samples show numerous examples of micrographic intergrowths of quartz and K-feldspar. This texture cannot be authigenically grown in a sediment and probably could not have survived a single cycle of erosion and deposition. Aphanitic rocks show porphyritic and spherulitic textures that are distinctly igneous and would be destroyed by weathering. Substantial relief on the Precambrian crystalline surface in Illinois is postulated for major structural features like the LaSalle Anticlinorium, the Sparta Shelf, the Ste. Genevieve Fault zone, etc. Paleotopographic relief up to 300 m (1,000 feet) is documented from drilling on the western flank of the basin.
- OSTI ID:
- 7069853
- Report Number(s):
- CONF-9303210-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:3; Conference: 27. annual Geological Society of America (GSA) North-Central Section meeting, Rolla, MO (United States), 29-30 Mar 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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