Abajo Mountain Structure

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Abajo Mountain Structure, Blanding, Utah

Tim McElvain

The La Sal Mountains are conventionally described

as Mid-Tertiary, shallow emplacement, laccolithic

structures and are more particularly described in the

following publication:

Geology of the Tertiary Intrusive Centers of the La SalMountains, UtahInfluence of

Preexisting Structural Features on Emplacement and Morphology

By Michael L. Ross

http://pubs.usgs.gov/bul/b2158/B2158-9.pd
http://pubs.usgs.gov/bul/b2158/B2158-9.pdfhttp://pubs.usgs.gov/bul/b2158/B2158-9.pdf


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The Abajo Mountains also known as the Blue Mountains are located

approximately 45 miles south southwest of the La Sal Mountains, west of

Monticello, Utah and North of Blanding, Utah. Along with the Henry

Mountains and the La Sal Mountains they have been mapped as Mid-

Tertiary laccolithic structures, but I have found the following evidence that

each of them may be the central uplift of a peak-ring impact crater or theresult of simultaneous impact of multiple bolides.

Shatter Cones


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The following three photos in increasing detail are of what appears to be a

shatter cone developed on a joint in the Dakota-Morrison sandstone at the

8300 foot level of the Abajo Mountains on Johnson Ridge near Johnson

Creek.


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The following three photos in increasing detail of what appears to be a

shatter cone development in a green shale in the Dakota-Morrison Formation

at the 8300 foot level on Johnson Ridge near Johnson Creek and adjacent to

the shatter cones illustrated above.


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Microscopic Evidence of Shock Metamorphism


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The above photomicrograph is of a quartz grain with one primary and

possibly a second set of planar microstructures (PMs) that fit thescale of planar deformation features (PDFs).


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The above photomicrograph of toasted, melted and plastically deformed

quartz grains with multiple sets of remnant PMs that fit the scale of PDFs.


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The above photomicrograph is of a toasted, partially melted and

plastically deformed quartz grain with multiple sets of what appear

to be remnant PMs that fit the scale of PDFs.

Burro Canyon Formation apparent forceful contact

with Dakota Sandstone


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The above photograph is of a boulder of Burro Canyon

Formation, pea sized conglomerate in which some of the

pea sized clasts seem to have been melted and quenched.

See following photos and photomicrographs.


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Burro Canyon Formation pea sized conglomerate intruding

or coming in contact with a Dakota-Morrison sand stringer,

in this and the following photo notice the turbulent nature

of the contact possibly caused by rapid forceful injection of

the conglomerate.


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THE FOLLOWING

PHOTOMICROGRAPHS ARE MADE

FROM THIN SECTIONS OF THE

CRETACEOUS DAKOTA-MORRISON

SANDSTONE FROM A SAMPLE

TAKEN OF THE ABOVE BOULDER.


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The above photomicrograph illustrates the shocked nature of the Dakota

Morrison sandstone in the vicinity of the injected Burro Canyon Formation

clasts. The quartz grains have been fractured, accelerated into one another,

and plastically deformed.


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THE FOLLOWING PHOTOMICROGRAPHS ARE

MADE FROM THIN SECTIONS OF THE BURRO

CANYON FORMATION PEA CONGLOMERATE

IN WHICH SOME OF THE CLASTS APPEAR TO

HAVE BEEN MELTED AND QUENCHED FROM A

SAMPLE TAKEN OF THE ABOVE BOULDER.


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The above two photomicrographs, the first illuminated by plane polarized

light and the second by cross polarized light, are of a melted and quenched

quartz spherule within the Burro Canyon Formation.


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The above two photomicrographs are of fractured and

plastically deformed quartz grains with 2 sets of PMs


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The above two photomicrographs, the first illuminated by plane polarized

light and the second by cross polarized light, illustrate the nature of the pea

sized melt clasts.


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The above photograph is of a clast of Dakota-Morrison clay caught up and

incased in the Burro Canyon conglomerate.

THE FOLLOWINF PHOTOMICROGRAPHSILLUSTRATE MORE EXAMPLES OF THENATURE OF THE SUEVITE CLASTS


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Photomicrograph of a clast in the melt clast in the Burro Canyon

conglomerate that has been melted and quenched with what appear to beplanar features, possibly remnant PMs that fit the scale ofPDFs, see the

detailed photomicrograph below of the same clast.


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Photomicrograph of a melted portion of one of the melt clasts that appears to

have formed quenched crystals that exhibit flow structure, see the more

detailed view below of the same melt in the photomicrograph.


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The above photomicrograph is of high definition melt in one of the melt

clasts in the Burro Canyon Formation, the following photomicrograph is of

the same view below illuminated by cross polarized light.


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Inner ring of the Abajo Mountain Impact Crater


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The above photograph is of a cross section in a road cut of the impact

craters inner ring, a pressure ridge where some of the Dakota-Morrison Formation's green clay has been squeezed up through one

of the same formations sandstone stringers.

Summary

All of the above evidence I believe confirms that the Abajo Mountains,

Blanding Utah are the central uplift of an impact crater.

Abajo Mountain Structure

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