Last year I discovered that the Geological Society of America (GSA) has annual regional meetings with really great field trips to local geological locations of interest. So I joined up, and last year paid a visit to the Serpent Mound Disturbance, a fairly ancient meteor impact site in southeastern Ohio.
This year a trip was offered to the Kentland, Indiana impact site. The trip was ably led by Dr John C. Weber, Professor of Geology at Grand Valley State University, Allendale, Michigan.
The site is located in the flat Indiana farm land typical of the north western part of the state, and the only clue about the area’s unusual geologic history is a limestone quarry, now called the Newton County Stone Quarry, operated by the Rogers Group. The surrounding landscape is covered by Pleistocene glacial deposits to considerable depth, which is the source of the rich soil that makes this such productive farm land.
Beneath the soil and glacial till, I’m told, are tidy, virtually horizontal layers of Middle-Lower Paleozoic bedrock that slope very gently from the Kankakee-Cincinnati Arch southwest to the Illinois basin.
Apparently there was a limestone outcrop that was obvious to local residents early on; the site has been a quarry since the 1880’s.
Geologists recognized that explanation was required both for the raised bedrock, and for the very un-tidy, not-horizontal placement of the strata within the formation.
Why is Kentland important? The impact site currently has no surface signs of a crater. However the impact uplifted the strata at the center of the site, providing a handy outcrop of limestone for local use. The resulting quarry is exactly why the impact effects are so accessible; the site is a laboratory for studying impact-deformed strata. Geologist love roadcuts because they expose strata that are normally covered by earth or vegetation. The quarry is like a continually expanding roadcut that regularly reveals new windows on the tortured layers of the site.
R.S. Dietz, one of the early proponents of impact geology, studied Kentland’s impact structures in place, especially shatter cones, and found the orientation of the cones were invariably normal (perpendicular) to the bedding:
“The orientation of the shattercones suggests that, assuming that the beds were essentially horizontal prior to deformation, the shock force resulted from some type of explosion directly above the beds rather than from a crypto-volcanic explosion below the beds.” (Science 10 January 1947: 42-43.)
Along with Meteor Crater in Arizona, Kentland provided the essential clues to put “crypto-volcanic” theories to rest.
The site was carefully described and mapped by R.C. Gutshick from the 1960’s to late 1980s. He showed that the quarry is at the apex of a structural dome, the central uplift area of a complex crater.
The crater itself has long since eroded away (hence, “impact structure”), along with evidence of exactly when the impact occurred, and how large the crater was. The glacial till that covers the neighborhood is about 50,000 years old; the upper layers of bedrock are of Silurian age, about 300 million years old. At some time in this 300 million year gap, an object made a sudden stop in Indiana – from maybe 20 km/sec to 0 in less than a second- and created a 6-13 km (3.7 to 8 miles) diameter crater. The center of the impact was raised by 600 meters by the rebound, and created a chaos of the formerly tidy Silurian and Ordovician layer cake. Then all evidence of the crater, the outer rim and glassy melts were eroded away, both here and over the surrounding region. Much later, glaciers covered the area, leaving behind a layer of debris that has become home to corn and soybeans.
How do you figure out the size of a crater that has long since eroded away? The Earth Impact Database gives a diameter of 13 km probably based on Gutschick’s mapping. However Gutshick also measured the uplifted Ordovician Shakopee dolomite to be about 600 meters above its un-deformed counterparts. Applying impact models developed by HJ Melosh, GS Collins, and RA Marcus (http://impact.ese.ic.ac.uk/ImpactEffects/effects.pdf), a 13 km crater should show a central uplift of 1 to 1.3 km. A crater of 6 to 7 km would have a central uplift that more closely matches Gutschick’s measurement. You can play with the various impact parameters with the online Impact Effects Program at http://impact.ese.ic.ac.uk/ImpactEffects/
For me, the highlights of seeing this place first hand, were the incredible, and obvious distortions of the rock layers; the numerous shattercones, now considered an icon of impact geology; and the impact breccia, another hallmark of impacts, but not at all unlike volcanic breccia to my eyes.
I think I expected to get some understanding of a “system” that would describe the disturbed strata. Nope. I liked Weber’s description from his field guide: “It is a steeply dipping, bedding sub-parallel, folded fault that juxtaposes the St Peter Sandstone with the Middle Ordovician Platteville Group” But I must admit this doesn’t seem to do the chaos justice. Here’s a St Peter sandstone – Platteville sequence that is turned on its side, and then repeats. The St Peter sandstone is the distinctive white layer. At its base, it’s pulverized to a flour consistency, which is characteristic of other impact sites. I don’t know if that accounts for this occurrence, or if it has just weathered.
Here’s a detail of a section with Platteville and St Peter sandstone. There are shatter features, breccia dikes with hefty clasts, and a general sense of craziness.
Similarly, Maquoketa shale and Galena Platteville. The Ordovician black Maquoketa shale is, I believe, a member of the Richmond formation. It’s another distinctive marker bed, and quickly weathers into talus slopes.
More Maquoketa shale and Galena showing the MFS (Marine Flooding Surface):
The north-east pit is working Silurian carbonates. I don’t believe it’s been mapped; I don’t which Silurian formations are presented.
Some really big clasts thrown into Maquoketa shale:
Shakopee dolomite is the oldest exposed layer.
One of the definitive, but not exclusive, indicators of impact are breccias. At Kentland there are dikes filled with polymict breccias that were apparently forced into fissures and voids opened by the rearrangement of massive blocks by the impact.
Inside, the breccias have a fairly fine grained matrix with clasts that are familiar from the neighborhood: St Peter Sandstone and various carbonate chunks, with conspicuous voids.
I haven’t gone into the studies of microscopic features of this site. Weber authored an interesting study (“Kentland Impact Carater, Indiana: An Apatite Fission-Track Age Determination Attempt” ; Weber, et al) using fission tracks in apatite grains in the St Peter SS to attempt to find an impact date. The fission tracks would show a thermal reset, possibly at the time of impact. Unfortunately, the reset seems to occur regionally, so presumably local evidence has eroded away. There are several GSA Field Guides available from http://fieldguides.gsapubs.org/ (by subscription).