On September 7, 1996, unexperienced geology students set out to study the Kentland Quarry. On September 28, 1996, they set out again, this time to map the northeastern section at the quarry. Many questions were raised and many speculations were made.
The Geologic Setting
The Newton County Stone Company, Inc. is approximately four meters east fo Kentland, Indiana. The Newton County Stone Company will be referred to as the Kentland Quarry in this report. The Kentland Structure is the fourth largest known impact site in the United States. The disturbance is 12.5 kilometer in the diameter. The is a total of 700 meters of Ordovician through Pennsylvanian Strata involved. The regional bedrock is mostly flat lying. The disturbed rock is covered with glacial till. The approximate age of the structure is between early Pennsylvanian the beginning of the Pleistocene (Laney, 1978). Figures 1 and 2 show the stratigraphic column of the Kentland structure.
Our planet has always been hit by asteroidal objects.
The moon shows craters, some as large as 95 kilometer in diameter
(Silver, 1982). The total number of impact structures on the
(and that are visible at the surface) is unknown because it depends on
the criteria for defining impact structures (Grieve, 1982) (see figure
3). According to the Diery (1959), there are several
of impact structures. It is completely formed
should show evidence of great shock and the deformation should decrease
the farther down or out you go from the center. It should exhibit
radial and concentric fracturing as well as normal faults at high
angles. The explosion crater should be approximately circular,
regardless of the impact angle. Figure 4 is a diagram of an
Shatter cones are small, striated, cup and cone structures that are commonly associated with limestones and dolomites in cryptoexplosion deformations. They are associated with “mechanical shock” rather than constant stress and there is no evidence to show otherwise. Figure 5 is a picture of shatter cones.
The Kentland structure contains a large percentage of dolomite, as well as snadstone and shale. The lowest stratum is the Shakopee dolomite and is lower Ordovician in age. This dolomite contains chert and green shale. Agal Stromatolite moujnds are present. This is the oldest recognized formation in the quarry. Between the Shakopee and the ST. Peter Sandstone, there lies an unconformity. The St. Peter Sandstone is pure white quartz and is usually quite soft. Next is the Joachim dolomite. This light grey dolomite is pure and thick bedded. It also contains some shale. In the middle Ordovician, we find the Platteville Group. These rocks go from thickly bedded dolomite to fine-grained limestone and back to thick dolomite. The very top formation of this group is a dolomite with shale. The Platteville Group is followed by the Galena Dolomite. This is a light grey, medium to thickly bedded dolomite with characteristic rusty weathering. Finally, we come to the Magvoketa Shale which is a grey silty shale in continuity with the Galena (Gutschick, 1986).
Measurements and Results
On our first trip to the Kentland quarry, we measured
faults and a syncline. The most prominent fault was seen best by
offset of a red dolomit bed. This fault was a left leteral
Figure 6 is a diagram of the fault as well as detailed measurements
taken there. The second fault we measured was a right lateral
Figure 7 illustrates the second fault. We also measured a large
syncline. Figure 8 is a sketch of this syncline.
On our second trip to the Kentland quarry, we mapped faults and contact. In the northeastern corner, the dolomite block moved up, this is shown by the striations on the the fault face. Also, this movement bent the beds on the right side of the block upwards. The other faults appeared to be a series where the left block moved up. It can be seen by the V-shaped sections of shale on top of the dolomite.
The Kentland structure has been called an anomoly by many authors. Just a drive through Indiana will reveal flat lying rock. Our trips to Portland Arch and High Bridge also showed horizontal bedding. The Kentland structure is isolated and complex and is a notable contrast to Indiana’s relatively simple geologic setting. “The Kentland Dome is an uplifted, complexly folded and faulted, truncated dome.” The Shakopee Dolomite was lifted more than 2000 feet. The folding preceded faulting. From west to east, the quarry follows the plunging anticline-syncline trend. Brittle fracture caused by mechanical stress during folding resulted in faulting. The quarry diplays “localized catastrophic disruption of strata in an area” of relative simplicity. The faulting is very obvious just by looking at the dramatic changes in lithology. The shatter cones, unique conical fractures, have been found, some a few meters in length. These shatter cones have been found in almost all the rocks in the quarry. The orientations of these cones range from almost normal to “highly oblique.” To support a meteorite impact theory, they should be oriented toward the center of the dome (Gutschick, 1983).
According to several authors, the surrounding rock at Kentland quarry is flat lying. I feel it is safe to say that before any deformation took place, the beds at Kentland were flat lying as well. The shatter cones indicate cryptoexplosion deformation. There is a trace amount of coesite in the rocks. The St. Peter Sandstone has been turned to rock flour. All of this suggests that the impact must have come from above rather than from a cryptovolcanic explosion. That would have created the high temperature deformation we see, but not the shatter cones (Dietz, 1959). Also, the shatter cones are normal to the bedding. If you reconstructed this structure so all the beds were flat again, the shatter cones would point to the center of the impact. This structure could have been formed very quickly. With an impact, the pressure and temperature would have been extremely high.
I like the meteorite impact theory. Having seen the shatter cones and reading about them, I find it very hard to suggest another means for their formation. There is otehr evidence of shock metamorphism at Kentland quarry, besides the cones. There are high pressure silica polymorphs coesite and stishovite (French 1972), as well as isotropic modification of feldspars. Also, there are high temperature decompositions. There is central uplist as well. All of which indicates some type of cryptoexplosion. In other places of meteor impact, all of these pieces of evidence serve to show meteor impact, so why not here? I can not, to my saticfaction, show how this structure could have been formed by any kind of stress. Dietz (1959) suggests taht a “violent shock is indicated by the jumbling of strata, the shattering of the limestones, and the pulverization of the St. Peter Sandstone into a rock flour” like at Meteor Crater. The magnitude of this impact would have been equivalent to an 8.0 (if not greater) earthquake. Its intensity would have been XII on the Mercalli Intensity Scale (the damage is total, waves can be seen on ground surfaces, and objects are thrown into the air).
I believe that the Kentland sturcture formed relatively quickly. Being hit by a meteorite would have deformed the flat lying rock very quickly. The faulting occured either contemporaneously or very shortly after the impact. If these structures had formed slowly, the St. Peter Sandstone would not be rock flour, the beds would not be so mixed up and rebounded, and, also, there would be no shatter cones. If this had formed by stress alone, we would have to work across all the faults in the northeast part of the quarry from right to left, and watch as the left blocks move up in relation to the right blocks.
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French, B.M., 1972. Shock-metamorphic features in the Sudbury structure, Ontario: A review.
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