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Amalia Comanche Point Impact Structure
Tim McElvain 10/2011
Regional Map of the Amalia-Comanche Point Impact Structure
The impact structure is outlined in red the center of which is about 18
kilometers south of the Colorado state line and at the southern end of the
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Sangre de Cristo Mountains of Colorado. The center of the structure is
located N36.82967 and W105.31386 degrees, very close to the Comanche
Point Mountain and about 18 kilometers south east of Amalia, New Mexico.
The structure is approximately 20 kilometers in diameter and has multiple
rings.
The above map is a more detailed topographic map of the Amalia-ComanchePoint Impact Structure which lies within the outlined circle and centered
very close to Comanche Point Mountain.
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The center of the structure is slightly to the right of the satellite photograph,
and it extends out to bend the Tertiary Poison Canyon Formation hogback as
well as the Dakota and other older formation hogbacks.
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The Amalia-Comanche Point structure outlined in the satellite map above is
located on the east side of the San Louis Basin and on the west side of the
Raton Basin. The two basins are similar in name only the Raton Basin is
filled with the Cretaceous Intercontinental Seaway formations, and the San
Louis Basin is part of the Rio Grande Rift and filled with rift deposits.
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On the above more detailed map I have located where I have found some of
the evidence of shock metamorphism detailed below.
Shatter Cones
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The three photographs above are of shatter cones in granite, the flaring
horsetail striations and grooves are on a rather flat surface, but are similar to
shatter cones that I have found in other locations such as The Santa Fe
Structure and The Beaverhead Structure. As in the two other locations, there
are more conical shatter cones although more of an oval shape in cross
section.
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The above three photographs were taken of shatter cones in mica schist as in
the granite the horse tail striations of the shatter cone are either on a rather
flat surface or a flattened cone with an oval cross section.
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The three photographs above are of shatter cones and/or casts of shatter
cones in a hard, mafic, metamorphic rock.
Impact Melt
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The above photograph is of the Comanche Point Mountain which is entirely
impact melt with spectacular columnar jointing. The melt has been described
as follows Comanche Point, a 25-m.y.-old intrusion of low-silica biotite
ryolite at the junction of Costilla and Comanche Creeks. Irregular shape of
the intrusion is reflected by geometrically complex columnar (cordwood)joints that formed perpendicular to cooling surfaces.Day 2: SUPPLEMENTALROAD LOG 2C, FROM AMALIA TO COSTILY RESAVOIR, Lipman and Reed, 1984 New MexicoGeological Society Field Conference.
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The two photographs of vertically orientated columnar jointing of the melt
just 300 meters northwest of Comanche Point illustrate the geometrically
complex nature of the melt-body.
The above photograph is a close-up of the melt illustrating flow structure.
Planer Microstructures
The following three photomicrographs of quarts grains from the Tertiary
Poison Canyon Formation the youngest formation disturbed by the impact
event, which corroborates the 25 m. y. year old date of the impact melt. The
scale of the planar microstructures in the following photomicrographs of
quartz grains fit the 1 to 5 micron spacing the lamellae of PDFs. I have not
measured the angle of the pole of the planar microstructure to the c-axis and
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compared the result to the quartz index; however most of the grains have
two sets of planar microstructures or planar fractures and I have yet to see
evidence that low pressure lamellae can cause two or more straight planar
sets within one quartz crystal. In either case the quartz grains in the Poison
Canyon Formation and the Dakota Formation have never been buried deep
enough or subjected for enough time, heat and pressure to cause Bohm
lamellae to form within the crystal. I am therefore stating that the planar
features seen below are the result of shock metamorphism.
The above photomicrograph is of a shocked grain in the Tertiary Poison
Canyon Formation.
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The above photomicrograph is of a shocked grain in the Tertiary Poison
Canyon Formation.
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The above photomicrograph is of a shocked grain in the Tertiary Poison
Canyon Formation.
The above photomicrograph is of a shocked grain in the Cretaceous Dakota
Formation.
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The above photomicrograph is of a shocked grain in the Cretaceous Dakota
Formation.
Summary
I believe the above evidence is sufficient to prove that the multi-ring
Amalia-Comanche Point structure is the result of a bolide impact that took
place about 25 m. y. ago.
I have gleaned geological information from the two following New Mexico
Geological Society Field Conference Guide Books:
1984 Rio Grande Rift: Northern New Mexico especially Day 2:
SUPPLEMENTAL ROAD LOG 2C, FROM AMALIA TO COSTILY
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RESAVOIR, Lipman and Reed, 1984 New Mexico Geological Society Field
Conference.
1990 Tectonic Development of the Southern Sangre de Cristo Mountains,
New Mexico especially the supplement to the First Day Road Log:
PROTEROZOIC METAMORPHIC ROCKS NEAR COMANCHE POINT,NEW MEXICO, Jeffrey A. Grambling, Department of Geology, University
of New Mexico, Albuquerque New Mexico 87131
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