Beaverhead Impact K-T Impact Boundary Crater?



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In my research in the literature concerning macroscopic evidence of terrestrial impact structures I read the following paper by Ernston et. al:

Ernstson, K., M. Rampino, and M. Hill, 2001: Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts. Geology, 29, 11-14, doi:10.1130/0091-7613(2001)029<0011:CCITBC>2.0.CO;2.

And in my travels looking for evidence of impact structures I came across the following articule in Roadside Geology of Montana: Roadside Geology of Montana, David D. Alt and Donald W. Hyndman, 1986. On page 160 the authors begin to describe the Beaverhead Conglomerate. In their description of the formation they state that it contains cratered, sheared and   crushed cobbles the significance of which has been gnawing at me ever since. The Beaverhead conglomerate is about 85 miles west of the center of the Beaverhead Impact Crater and approximately 25 miles west of the Montana Beaverhead Shatter Cone Site. I have puzzled for years whether or not the Beaverhead Impact could have been responsible for the unusual cratered cobbles and if so how could I prove it. The impact origin of cratered cobbles has been disputed by other researchers such as Chapman et. al. in the following paper:

TRIASSIC CRATERED COBBLES: SHOCK EFFECTS OR TECTONIC PRESSURE? M. G. Chapman1, M. A. Evans2, and J. F. McHone3; 1U.S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001, This e-mail address is being protected from spambots. You need JavaScript enabled to view it ; 2Dept. Geology and Planetary Sci., University of Pittsburgh, Pittsburgh, PA, 15260; 3Dept. Geology, Arizona State University, Tempe, AZ 85287.

After studying the Chapman et. al. document I realized that I probably would not find any evidence of shock metamorphism in the cobbles themselves, but in my experience elsewhere I thought I might find shocked quartz in the sand and sandstone associated with the cobbles. In August 2011 I did some fieldwork in the Lima, Montana to Beaver, Idaho area and at the Beaverhead Site in the  Medicine Lodge Valley. I took samples of the Beaverhead Formation and some of the cratered and shattered cobbles. I found the Beaverhead Formation around Lima MT has been subdivided into at least three groups: 

Kblc LIMA CONGLOMERATE (UPPER CRETACEOUS) — Limestone pebble to cobble conglomerate derived from erosion of Blacktail-Snowcrest uplift (Nichols and others, 1985). A-5 

Kbsn SNOWLINE SANDSTONE (UPPER CRETACEOUS) — Calcareous salt and pepper sandstone; contains limestone fragments (Ryder and Scholten, 1973). 

Kbm MONIDA SANDSTONE (UPPER CRETACEOUS) — Clean, calcareous quartz sandstone (Wilson, 1970; Ryder and Scholten, 1973).


And there are several Tertiary and Cretaceous Conglomerate formations in the area:


Tertiary and Cretaceous Formations



 TKb BEAVERHEAD GROUP UNDIVIDED (PALEOCENE AND UPPER CRETACEOUS) — Conglomerate, sandstone, mudstone, and limestone

 TKbq QUARTZITE CLAST CONGLOMERATE – Unit overlies the Red Butte Conglomerate in the Chute Canyon area. Clasts may be recycled from the Little Sheep Quartzite Conglomerate (Kblq).

 TKbr RED BUTTE CONGLOMERATE (PALEOCENE AND UPPER CRETACEOUS) — Limestone conglomerate contains clasts of recycled limestone conglomerate and well-rounded quartzite (Haley & Perry, 1991).


Cretaceous conglomerate formations



Quartzite roundstone conglomerate; Contains large limestone blocks of Madison

Group limestone (Mm) chiefly carried along thrust faults (Perry and others, 1988)

Kblc LIMA CONGLOMERATE (UPPER CRETACEOUS) — Limestone pebble to cobble conglomerate derived from erosion of Blacktail-Snowcrest uplift (Nichols and others, 1985).


All of the above descriptions taken from the downloaded PDF: 




Jeffrey D. Lonn, Betty Skipp, Edward T. Ruppel, Susanne U. Janecke, William J. Perry, Jr.,

James W. Sears, Mervin J. Bartholomew, Michael C. Stickney, William J. Fritz, Hugh A. Hurlow,

and Robert C. Thomas

Montana Bureau of Mines and Geology

Open File Report MBMG 408



1/05 Correlation Chart symbols; Text, Part A, unit symbols 1/06 Map: faults with Quaternary movement; added Holocene faults in Centennial Valley north of Lima Reservoir; revised location and type of other faults; Text and Correlation Chart units

This report has had preliminary reviews for conformity with Montana Bureau of Mines and Geology’s technical and editorial standards.

Partial support has been provided by the STATEMAP component of the National Cooperative

Geologic Mapping Program of the U. S. Geological Survey under Contract Number 99-HQ-AG-



And Foemation discriptions in Idaho


Paleocene and Cretaceous sandstone and conglomerate (Beaverhead Formation)

Digital Atlas of Idaho, Nov. 2002 Compiled by Paul K. Link,

Idaho State University, Geosciences Dept.


Shock metamorphism in quartz


I made grain mounts of sand grains in the Ksbn and Kbm Formation collected near Monida, Montana and  I found a surprising high percentage of quartz grains with numerous sets of PDF's. I say they are PDF's because I have never heard of two or more sets of low pressure or tectonic planar microstructures in one quartz crystal, nor have I heard of low pressure planar structures that are straight and of the one to five micron scale that defines PDF’s. See the photomicrographs below. The PDF’s in quartz grains in this formation indicate that this Late Cretaceous Formation has been severely shocked. 



The above photomicrograph is of a quartz grains with two primary sets of PDF’s and at least two additional sets of PDF’s that are less prominent; the PDF’s are fresher than most I have found so far in this formation, and are different from any others I have ever found.


The above photomicrograph is of a quartz grain with at least two sets of PDF’s.




The above photomicrograph is of a quartz grain with at least 2 sets of highly decorated PDF’s, and the following photomicrograph is of the same grain illuminated with cross polarized light.





The PRELIMINARY GEOLOGIC MAP OF THE LIMA 30' x 60' QUADRANGLE, SOUTHWEST MONTANA (see above) indicates that the Kbn, Kbsn and Kblc Formations in the Lima, Montana area are severely folded, faulted and in places overturned. See a cropped section of the map illustrating the Lima Area of this map below.



The deformation of the Beaverhead Formation sandstones and conglomerates in this area could have been caused by thrust faulting and/or it could be caused by the Beaverhead Impact event.


ksbn fm



The above photograph illustrates the near vertical dip, possibly overturned, outcrop of one of the beds in the Kbsn Formation. The photograph was taken on the top of the ridge approximately 1.5 miles north of Monida, Montana. 



Tertiary and Cretaceous Quartzite Cobble Conglomerate



In the Lima Montana area and in the hills and valleys in Idaho south of Interstate Highway 15, I collected polished quartzite cobbles that were cratered and shattered with hair line fractures. The photographs of the cobbles below were collected in the Stoddard Creek canyon just west of the old railroad town of Beaver, Idaho. The Clark County of Idaho geologic map (see description above) classifies this sandstone and cobble conglomerate formation as a Paleocene and Cretaceous sandstone and conglomerate (Beaverhead Formation). The photos of the cratered, polished, quartzite cobbles illustrated below appear to have been struck by other cobbles which caused several small impact craters on the surface of the cobble. I believe these impacts happened while the cobbles were airborne, rounded and polished during the excavation phase of the Beaverhead Impact Crater (Of course there are competing explanations). The little impact craters on these cobbles would probably have had to occur during that phase of the excavation when all the ejecta was under tremendous confining pressure in order to keep the cobble from shattering and exploding. Kord Ernston and Fernando Claudin in their web site, Ernstson Cladin Impact Structures, The Perlarda Formation, propose that rocks can be rounded and highly polished in the excavation phase of the formation of an impact crater.


coble 1

Cratered polished quartzite cobble – all the circular white marks are little impact craters that shattered the cobble with fine hair line fractures.



 coble 2


 The above photo is of a polished, quartzite cobble that has three relatively larger impact craters and several lesser impact scars.



coble 3


The above photograph is of a small cobble illustrating the numerous intersecting hairline fractures.


coble 4


PThe above hotograph is of a broken cobble illustrating how the rock fractured along hairline fractures caused by the various impacts; notice the curved surface where the rock separated along one of the ring fractures, and other flat surfaces where the rock separated along shear fractures almost like cleavage.



coble 5


 The above photograph is of a sheared, polished, quartzite cobble.



The fact that this formation with the cratered cobbles has been dated at or near the K-T Boundary raises all sorts possibilities for wild speculation one of which is that this impact structure could be coeval with the Chicxulub Impact Crater.




Highly Folded, Faulted, Paleozoic Formations possible Megabreccia

 I drove up the Big Sheep Creek Road in order to go to the Beaverhead shatter cone site and at the mouth of the canyon I encountered the extremely deformed formations in the Snowcrest Range Group. 


 Mississippian and Pennsylvanian


Pl Msr SNOWCREST RANGE GROUP (PENNSYLVANIAN? AND UPPER MISSISSIPPIAN) — Includes Conover Ranch and Lombard Formations. Dark gray, black, and reddish-brown limestone, calcareous mudstone, sandstone, siltstone, and minor gypsum (Wardlaw and Pecora, 1985).) 


 The photographs below illustrate how intensely these formations have been deformed. 



The above photo is of broken, rotated and displaced blocks of the Snow Crest Range Group at the mouth of Big Sheep Canyon that look like some mega-breccias I have seen.


 deformation 1


The above photo is a bed of the Snow Crest Range Group in Big Sheep Canyon exhibiting intense deformation.



I suppose thrust faulting could have caused this deformation but so could a large bolide impact. The fact that the Beaverhead Impact Crater overprints the Sevier Thrust Belt complicates the investigation and interpretation of the impact crater, but may help date the impact as post Sevier. No shatter cones have been found in the Mississippian – Pennsylvanian limestone formations in the area but this does not necessarily mean that the limestone formations were not present when the Beaverhead bolide arrived. There are examples of large impact craters such as the Lockney Impact Crater in Sweden where the limestone target rock does not have any shatter cones. If the Beaverhead Impact Structure is K-T Boundary in age then it would have been a wet impact. The Alamo Breccia and Lockney are both wet impact structures and I do not think shatter cones have been found in either which begs the question, does a wet impact inhibit the formation of shatter cones in limestone?


 K-T Boundary Impact Event ?


The possibility that the Beaverhead Impact could have happened at the K-T Boundary invites me to indulge myself in wild speculation. The date of this impact at the K-T seems likely to me at this time, but a lot more work needs to be done. One interesting possibility is that the Beaverhead bolide could have been a companion to the Chicxulub bolide, and/or the Beaverhead impact could have been instrumental in the formation of the Yellowstone Hot Spot and/or the Challis Magmatic Episode. The Bouguer gravity map of the region surrounding the proposed thrust detached root to the Beaverhead structure, East Idaho (copied, cut and pasted below from the Beaverhead page in the Earth Impact Database ( is very interesting to me. The Snake River Plane looks remarkably integrated into a giant circular structure some 460 kilometers in diameter. If the Snake River Plane turns out to be the moat of a giant Beaverhead Impact Structure the structure would be of the same size class or a lot bigger than the Chicxulub Impact Structure.



It has been my experience researching the Santa Fe Impact Structure, that due to the controversial nature of impact structures on earth, researchers after establishing the fact that the basement rocks are part of an impact structure further geological investigation is stopped and the whole structure swept under the rug so to speak – into the Precambrian – removing the necessity of looking for evidence of the impact in younger rocks. Hopefully if this new evidence proves to be part of the Beaverhead Impact Structure it will help dispel that tendency.