Cerro Blanco Structure, Gallina, NM

 

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The Cerro Blanco Structure is located north of Gallina, New Mexico on the eastern edge of the San Juan Basin on the Archuleta Anticlinorium, and is bisected by the major Nacimiento-Gallina fault. It is a complex structure approximately seven kilometers in diameter. The central uplift is approximately two and one-half kilometers in diameter, surrounded by a mote. The eastern half of the structure has been destroyed. The Cerro Blanco suspected complex impact structure; disturbed sediments from the Triassic Cutler Formation the upper shale member of the Chinle Formation within the central uplift to Cretaceous Mesaverde Formation on the crater rim. The central uplift is surrounded by a Dakota Formation hogback resting on the Todilto Formation; the Morrison Formation has been faulted out. Some of the sand grains within the Dakota Formation have planar microstructures

Within the breached central uplift there are clastic dikes at the contact between the Entrada Formation and the Cutler Formation. The clastic dikes appear macroscopically as sand grains from the Entrada penetrating the Cutler Formation from above; however, microscopically the dike material contains cataclysmically shattered quartz grains, quartz grains with planar microstructures (PM’s), partially melted and quenched grains and clasts, and blobs of melt with quench crystals and spherules. The dikes vary in width from a few centimeters up to 1 meter, and they projected down into the Cutler Formation for at least 1.5 meters. Some of the larger dikes passed under the talus slope and I was unable to find out how deeply they penetrated..

I have found planar microstructures in some grains of porous and permeable sandstones ranging in age from Triassic to Cretaceous that do not show any evidence of either contact or regional metamorphism. The fact that the PM’s are found in formations of different ages and are localized in the structure would rule out the possibility that the grains are eroded from a metamorphosed rock.

I separated grains with PM’s and mounted them on a spindle stage and using methods developed by F. Donald Bloss (F. Donald Bloss (1981), The spindle stage: principles and practice. Cambridge University Press) I measured the orientation of the planar features and plotted those angles against a quartz template to determine the frequency of uniquely indexed PM’s (Figure 1.). The unusual frequency distribution of the measured PM’s is probably due to their being formed in a porous sedimentary rock at considerable depth. Even though these PM’s are aligned with the Miller indices expected for PDF’s there needs to be much more rigorous tests before they are confirmed to be PDF’s. (Grieve, R.A.F., Langenhorst, F., and Sto¨ffler, D., 1996, Shock metamorphism of quartz in nature and experiment: II. Significance in geosciences: Meteoritics and Planetary Science, v. 31, p. 6–35; Bevan M. French, William S. Cordua, J.B. Plescia, 2004, The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz: GSA Bulletin; January/February 2004; v. 116; no. 1/2; p. 200–218; DOI 10.1130/B25207.1).

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A.

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B.

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C.

Photomicrographs A., B., and C. above were taken of grains from a thin section of Cretaceous Dakota sandstone located on the flank of the central uplift. The planar microstructues (PM's) fit the scale of planar microstructures, and a histogram of indexed angles of the pole of the planar microstructue to the c-axis also resemble histograms of other known impact structures; however, the pressures indicated by the histogram are probably squed to higher pressures because of the tendency of porus sandstones to concentrate and focus the shock wave.

 

 

histogram

The following photomicrograph was taken of a spherule of melt in the same thin section as the grains with planar microstructures (PM's).  The spherule appears to be somewhat flattened and is either in the process of being disolved away or is porus and permeable, notice the blue epoxy penetrating the spherule.

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Clastic Dike

Other microscopic evidence includes what appear to be melted and quenched grains, partially melted grains, catastrophic brecciated sandstone with larger clasts of grains and material from several different formations disturbed by the impact that have been injected into clastic dikes at the base of the Triassic Entrada Formation in the Chinle formation.


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Clastic Dike at the contact between the red, clay in the Cutler Formation and the aeolian Entrada Formation the dike appears to be a dewatering soft sediment structure, but microscopically the dike material contains cataclysmically shattered quartz grains, quartz grains with planar microstructures (PM’s), partially melted and quenched grains and clasts, and blobs of melt with quench crystals and spherules.

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Quartz grain within one of the clastic dikes, with planar microstructures (PM’s) that fit the scale of planar deformation features (PDF’s). The photomicrograph’s field of view is 1 millimeter and illuminated by cross polarized light.

The following photomicrographs of a blob of melt within a clastic dike showing quench crystals and spherules within a glassy matrix, the melt blob is a clast within the dike.

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A. The field of view of this photomicrograph is 1.0 millimeters.

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B. This photomicrograph is a close up of a portion of A., the field of view is 0.20 millimeters.

 

The following photomicrographs are of spherical clasts within the shattered quartz grain matrix of the clastic dikes.

 

Cerro Blanco Impact Structure, Gallina, New Mexico

A. I have not identified the mineralogical composition of this spherical clast with the multiple sets of striations; however, it is very anomalously placed within this shattered quartz matrix. The field of view is 1.0 millimeter.

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B.

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C.

Photomicrographs B and C are of the same spherical clast under polarized and cross polarized light illustrating that it has been partially melted and quenched.

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Photomicrograph of a grain that has been melted and quenched and placed within the fine grained cataclastic quartz breccia. It is possible that the above spherical grains and the other melted and quenched grains were injected into the dike along with the fine grained quartz breccia.


I sent most of my thin sections made of rocks from this structure to Dr. French for his comment, but I am having some more made and hope to have better photomicrographs of quartz grains with planar microstructures PM's soon.

Quartz grain within one of the clastic dikes, with planar microstructures (PM’s) that fit the scale of planar deformation features (PDF’s). The photomicrograph’s field of view is 1 millimeter and illuminated by cross polarized light.