Amalia-Comanche Point Impact Structure

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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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Canyon Formation.


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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

Documents

Amalia-Comanche Point Impact Structure