2011 GSA Annual Meeting in Minneapolis (912 October 2011)
Paper No. 117-9
Presentation Time: 9:00 AM-6:00 PM

EJECTA STRATA OF RECENT ORIGIN HAVE BEEN IDENTIFIED IN CENTRAL WISCONSIN AT GLOVER BLUFF IMPACT SITE

RENARD, Gene M., Wiscah Geologic, LLC, 3224 Memorial Drive, Two Rivers, WI 54241, gene.renard@wiscah.com

Wiscah Geologic, LLC (WISCAH), a Wisconsin limited liability company, has completed a geological evaluation of the natural history of the Glover Bluff Impact Site (Glover Bluff) near Coloma, Wisconsin. The site has long been known as the site of an extraterrestrial (ET) impact. There have been only four published geological studies at Glover Bluff; in 1930, 1956, 1983, and 1984. The 1984 study put forward that the Glover Bluff site is positioned on the rim of a crater having a 7 mile diameter. The WISCAH evaluation was unable to confirm this.

Based on the 1930 study, the WISCAH evaluation noted that the three main non-contiguous Ordovician fault blocks at Glover Bluff were once a contiguous Magnesian outlier, similar to Bald Bluff just 2 miles to the southwest. This former outlier was ruptured into distinct fault blocks which were subsided by as much as 200 feet in vertical elevation into the underlying Cambrian formations. Two abutted fault blocks were steeply tilted from 65-80 and 45-100 degrees, respectively. The WISCAH evaluation concluded that the fault blocks record a central impact ring and the outlier was ruptured by a continental airburst.

The 1930 study interpreted geological float at Glover Bluff as drift. In contrast to the expected undifferentiated clastics, however, float are exclusively native, random, angular, Ordovician dolomite. Clast sizes vary from two feet in maximum length to pebble size, suspended in a sandy matrix. No glacial erratics are noted. Fluvial and outwash stratification are absent. In matrix organic detritus are exhibiting. Three distinct observable strata are: A light lower stratum (5 feet); a black mat stratum (3 inches); and a dark upper stratum (4 feet). The WISCAH evaluation reinterpreted float as ejecta and concluded that the loose assemblages were deposited recently, less than ~15.0 ka ybp, after Late Wisconsinan ablation of the outlier.

The WISCAH evaluation presents evidence that the black mat stratum at Glover Bluff is the depositional result of an ET event due to its clear association with ejecta strata. Additional geological analysis is recommended at Glover Bluff to establish the exact timing, and to determine the relationship, if any, to known phenomena.

2011 GSA Annual Meeting in Minneapolis (912 October 2011)
General Information for this Meeting
Session No. 117--Booth# 339
Impact Cratering on the Earth, Moon, and Planets: Remote, Field, and Lab Studies (Posters)
Minneapolis Convention Center: Hall C
9:00 AM-6:00 PM, Monday, 10 October 2011

Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 306

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