GLOVER BLUFF IMPACT STRUCTURE
by: Charles O’Dale
- Type: Complex
- Age Ma: <500a
- Diameter: ~8 km
- Location: N 43°58.429’ W 089°32.081’
a Geologic estimate.
Glover Bluff Impact Site (Glover Bluff) near Coloma, Wisconsin has long been known as the site of an extraterrestrial (ET) impact. The bluff is a group of three low hills. All are about the same height, their tops rising a little over a hundred feet (30 M) above the valley bottom. There have been only four published geological studies at Glover Bluff; in 1930, 1956, 1983, and 1984.
A gravity and magnetic survey of the area was made by Koenen (1956). Part of his maps are reproduced in figure 4. The Glover Bluff structure is near the east end of a prominent west-trending positive magnetic anomaly and also is within the area of a positive Bouguer gravity anomaly. Koenen has calculated that both anomalies could be caused by a mafic plug at a depth of about 2 km (1.2 mi) below the surface. He proposed that the Glover Bluff structure could be caused by faulting and collapse during intrusion of the plug.
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. Based on the 1930 study, a Wiscah Geologic, LLC (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. 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.
Read reported shatter cones from dolostones in the north wall of the quarry. Since then more shatter cones have been found. Read described the structure as a crater 10 km in diameter and inferred that the impact postdates the deformed Early Ordovician strata.
The 1984 study put forward that the Glover Bluff site is 8 kilometers (5.0 miles) in diameter and the age is estimated to be less than 500 million years (Cambrian or younger). The crater is exposed at the surface.
At the quarry there are three major rock units. The Cambrian sandstone of the Jordan Fm. is overlain by a layer of conspicuous red breccia-conglomerate, which is overlain by Ordovician dolostone of the Oneota Fm. All are intensely deformed with several folds and faults.Read claimed that the red breccia-congomerate layer has clasts derived from both the overlying an underlying strata which he interpreted as a product of friction.
The Glover Bluff impact site is among the least studied in the world, in part because for years the uplifted central area has been actively quarried for dolomite beneath the moraine left by retreating glaciers. Ironically, the very evidence supporting a meteoritic impact uncovered in the quarrying process is systematically being destroyed. Such evidence includes a central bulge, dipped strata, the presence of “shatter cones” (although no longer as common as they may once have been), and, “impact breccia.” There are “red” and “yellow” varieties (more of a brick red and tan yellow) and sometimes a mixture of the two. The breccia contains many broken fragments (or shards) of quartzite and other rocks.
I took these images of the Glover Bluff impact structure flying in from the north and circling the quarry in a counter-clockwise direction.
GROUND EXPLORATION of GLOVER BLUFF – Kevin Evans
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Ekern, G. L., Thwaites, F., The Glover Bluff structure, a disturbed area in the Paleozoics of Wisconsin. Wisconsin Academy of Sciences, Transactions, v. 25, pp. 89-97. 1930.
Ernst, Bryan, THE GLOVERS BLUFF STRUCTURE, A POSSIBLE IMPACT FEATURE EXPOSED IN THE MID-CONTINENT CAMBRO-ORDOVICIAN STRATA OF CENTRAL WISCONSINDepartment of Geology, University of Wisconsin Oshkosh
Juday, Chancey Transactions of the Wisconsin Academy of Sciences, Arts and Letters volume XXV (1930)
Koenen, K.H. Geophysical studies in south central Wisconsin Master’s thesis Univ. of Wis. (Madison). 44pp
Read, W. F. Shatter Cones at Glover Bluff, Wisconsin Meteoritics, volume 18, number 3, page 241.
Renard, Gene M., EJECTA STRATA OF RECENT ORIGIN HAVE BEEN IDENTIFIED IN CENTRAL WISCONSIN AT GLOVER BLUFF IMPACT SITE Wiscah Geologic