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North-Central Section - 37th Annual Meeting (March 24–25, 2003)

Paper No. 2
Presentation Time: 8:20 AM

SEDIMENTOLOGY AND CORRELATION WITHIN THE ORDOVICIAN GALENA GROUP, NORTHEAST IOWA


BEYER, S.R.1, BYERS, C.W.2 and SIMO, J.A.2, (1)Dept. of Geology & Geophysics, Univ. of Wisconsin, 1215 W. Dayton St, Madison, WI 53706, (2)Dept. of Geology & Geophysics, Univ of Wisconsin, 1215 W. Dayton St, Madison, WI 53706, srbeyer@geology.wisc.edu

The Ordovician (Trentonian) Galena Group is widely distributed in the Upper Mississippi River Valley (UMV). Geologists at the Illinois Survey established the currently-accepted stratigraphic nomenclature for the Galena Group strata in northern Illinois. This study provides a sedimentological description and interpretation of the Dunlieth and Wise Lake Formations of the Galena Group in northeast Iowa. The carbonate-dominated strata of the Galena Group in northeast Iowa have escaped the pervasive dolomitization that is widespread in the UMV. Textural heterogeneity is present at various scales, and cyclic sedimentation can be recognized, associated with hardground surfaces. Depositional environmental interpretations are proposed based on sedimentologic and petrographic observations.

Lateral correlation within the Galena Group is complicated by a shift from carbonate to shaly carbonate lithofacies along a transect from southeast to northwest. Some paleontological event horizons or zones can be recognized in both lithofacies. A zone characterized by ubiquitous prasoporan bryozoa is present at both ends of the transect, at the Guttenberg and Decorah localities. The zone is approximately 0.5 m thick in carbonate strata at Guttenberg and increases northward to 1.5–2.0 m in shale at Decorah. Delgado (1983) recognized two Receptaculites zones in the Dunlieth at Guttenberg, encompassing 1) the Eagle Point/Fairplay/basal Mortimer Members, and 2) the Rivoli/Sherwood Members. Zone 1 is also recognizable at Decorah, but zone 2 is less evident.

Ordovician strata in the UMV, including the Galena Group, contain thin, but laterally extensive, altered volcanic ash beds, or bentonites. Bentonites were sampled from the Galena Group in northeast Iowa. The trace element contents of apatite crystals were analyzed via electron microprobe, to determine chemical fingerprints for individual bentonites. Correlation along the transect is possible via recognition of specific bentonites.

Additional stratigraphic control was established using spectral gamma ray spectrometry. Sections were recorded with an Exploranium GR-320 portable spectrometer to reveal variations in gamma radiation emitted by potassium, uranium, and thorium. Vertical stratigraphic variations are consistent among sections, providing another means of correlation.

Session No. 1

S1. Stratigraphic Correlation and Nomenclature of the Upper Midwest Paleozoic
Monday, 24 March 2003: 8:00 AM-9:50 PM
Geological Society of America Abstracts with Programs. Vol. 35, No. 2, February 2003, p.3
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