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8/4/2019 La Sal Mountain Impact Structure
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La Sal Mountain Impact Structure,
Moab, Utah
Tim McElvain
The La Sal Mountains are conventionally described as Mid-Tertiary,
shallow emplacement, laccolithic structures, on a north-south axis, on a
broad dome of about 600 meters of relief and 32 kilometers in diameter,
and are more particularly described, copied and pasted from the
following publication:
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Geology of the Tertiary Intrusive Centers of the La Sal Mountains, Utah—
Influence of Preexisting Structural Features on Emplacement and
Morphology
By
Michael L. Ross
http://pubs.usgs.gov/bul/b2158/B2158-9.pdf
ABSTRACT
The results of geologic mapping and subsurface data interpretation, combined with
previous regional gravity and magnetic surveys, define the structural setting and
emplacement history of the late Oligocene intrusive centers of the La SalMountains, Utah. The La Sal Mountains contain three intrusive centers: northern, middle,
and southern; they are located on a broad dome, which has about 600 meters of relief
across a diameter of about 32 kilometers. The intrusions are estimated to have beenemplaced at shallow levels ranging between 1.9 and 6.0 kilometers. The intrusions are
holocrystalline and porphyritic and have a very fine- to fine grained groundmass.
INTRODUCTION
The La Sal Mountains of southeastern Utah are one of several mountain ranges in the
central Colorado Plateau that contain hypabyssal intrusion-cored domes (fig. 1). Ingeneral, these intrusions have similar morphologies, lithologies, and chemical
compositions (Eckel and others, 1949; Hunt and others, 1953; Hunt, 1958; Witkind,1964; Ekren and Houser, 1965). A common form for these shallow intrusions is a
laccolith, as initially described by Gilbert (1877) in the Henry Mountains. Therefore, the
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intrusive centers are commonly referred to as laccolithic centers.
Figure 1 Michael L. Ross
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Figure 2 Michael L. Ross
Figure 3 Michael L. Ross
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Figure 4 Michael L. Ross
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Figure 5 Michael L. Ross
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Figure 6 Michael L. Ross
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Figure 7 Michael L. Ross
GEOLOGY OF THE LA SAL
MOUNTAINS
The La Sal Mountains consists of three distinct clusters of peaks separated by high
passes: the northern, middle, and southern La Sal Mountains (fig. 4). Each of themountain clusters is an intrusive center consisting of hypabyssal intrusions of trachyte
and rhyolite porphyries emplaced as laccoliths, plugs, sills, and dikes. The three intrusive
centers in the La Sal Mountains intruded upper Paleozoic and Mesozoic sedimentaryrocks and have a north-south alignment (fig. 2). As first recognized by Gould (1926),
both the northern and southern mountains are cored by large elliptical igneous intrusions
elongated northwest-southeast. The La Sal Mountains intrusive centers are on a broad
dome that has approximately 600 m of relief across a diameter of about 32 km. Regionalmagnetic data suggest there is no large intrusion in the subsurface below the La Sal
Mountains from which the intrusive centers were supplied. The magnetic low at the LaSal Mountains indicates that no such intrusion is present within 11–14 km beneath themountains (Case and others, 1963). I On the southwest flank of the northern mountains
dome, the sedimentary rocks are abruptly folded from a dip of 5° to dips of 60°–90°
southwest, forming a northwest trending monocline (fig. 5, section A–A' ). Triassic strataare in near-vertical contact with the hornblende plagioclase trachyte pluton along the
entire flank. A thin contact-metamorphic aureole of hornfels indicates minimal baking of
the country rock along the contact. At several locations, Triassic strata adjacent to the
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pluton form thin breccia zones. This clast-supported breccia has well-indurated clasts in a
matrix of calcite and crushed rock. Many of the frost-heaved Lower Jurassic Glen
Canyon Group sandstone blocks that cover the large flatiron ridge of the monocline haveslickenside surfaces and cataclastic shear bands. The breccia zones, slickensides, and
shear bands suggest near-bedding-plane faulting and stretching of the sedimentary rocks
as they were arched across the main igneous pluton. Similar features have been describedon the flanks of the laccolithic domes in the southern Henry Mountains (Johnson and
Pollard, 1973; Jackson and Pollard, 1988a). Along most of the northeast flank of the
northern mountains, Triassic and Jurassic strata dip about 45°–50° and 45°–60° northeast,respectively (fig. 5). The pluton– country rock contact along this flank appears to be
nearly vertical or to dip slightly northeast. At La Sal Peak, the structure is complex
because the hornblende plagioclase trachyte intrusion breached the flank of the anticline
and was injected as much as 1.7 km into the flanking rocks (fig. 5, section (A–A' ). The best inference for the elevation of the pluton’s floor may be one that derives fro m the
observation that the underlying salt diapir does not appear to have dissolved. If it had, it
would have caused collapse of the extended parts of the pluton. An elevation of 2,400–
2,700 m for the floor of the main pluton would be significantly higher (>500 m) than thelevel of collapse and the upper surface of the salt diapir in adjacent Castle Valley. The
breccias found in the intrusive pipes range from crackle breccia to matrix-supported breccia. The matrix is predominantly calcite and includes subordinate amounts of quartz,
crushed rock, and opaque grains. Quartz veins, stockworks, and pods are locally present.
Hematite pseudomorphs after various sulfides are common. At the northwest end of the
northern mountains, other discrete areas of similar breccias are present near the marginsof the main pluton and adjacent to the quartz plagioclase trachyte body. In these breccias
clast types are variable: some contain only fragments of either Triassic rock and
(or) Glen Canyon Group sandstone, some have only fragments of hornblende plagioclasetrachyte or quartz plagioclase trachyte, and at least one contains a mixture of both
sedimentary and igneous rock fragments. The breccia formed at the sedimentary-igneous
contact because of either forceful emplacement of magma or the release of volatile-richfluids. Intrusions of the La Sal Mountains were probably emplaced at depths ranging
between 1.9 and 6.0 km. These depths of emplacement are consistent with emplacement
depths for the Henry and Abajo Mountains laccoliths (Witkind, 1964; Jackson andPollard, 1988a).
MACROSCOPIC AND MICROSCOPIC EVIDENCE
OF SHOCK METAMORPHISM RESULTING
FROM AN EXTRATERRESTRIAL IMPACT EVENT
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The above photomicrograph is of a quartz grain with at least two sets of
planar microstructures (PM's) that fit the scale of planar deformation
features (PDF's) from a sandstone stringer within a Triassic? Redbed located
at the 8000 foot level of the La Sal Mountains.
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Figure 8-a
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Figure 8b
Figure 8-a and 8b – are photomicrographs of a quartz grain with planar microstructures
(PM's) from a sandstone stringer within a Triassic? Redbed located at the 8000 foot levelof the La Sal Mountains.
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The above photomicrograph is of a quartz grain with at least two sets of
planar microstructures (PM's) that fit the scale of planar deformation
features (PDF's)from a sandstone stringer within a Triassic? Redbed located
at the 8000 foot level of the La Sal Mountains.
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Blob of melt with spherules adjacent to a quartz grain with two sets of PM's
probably PDF's illustrating the propensity of porous sandstone to
concentrate a shock wave causing melting and the formation of planar
microstructures. This melt is on the same thin section as the hotomicrograph
above.
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Figure 10 - Photograph of the breccia zone mapped in figure 1 and 3 above,
exhibiting what appear to be vertical beds probably formed by flow of the
clastic, melt dike as it was inserted between the mote and central uplift of the
La Sal Mountain Impact Structure.
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Figure 11 - Photograph of an igneous clast with the above mentioned clastic
dike.
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Figure 12 -Photograph of a sandstone clast with the above mentioned clastic
dike.
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Figure 13 -Photograph of a rounded cobble clast that was rounded and
polished in the excavation stage of the impact crater a clast in the above
mentioned clastic dike. Kord Ernston and Fernando Claudin in their web
site, Ernstson Cladin Impact Structures, The Perlarda Formation,
http://www.impact-structures.com/., show examples of clasts rounded
and polished in the excavation stage of an impact crater.
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Figure 14 - Photomicrograph of high relief glass within the above clastic
dike (the black circle is an ink mark on the thin section).
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Figure 15 - Same view of the thin section in figure 14 above illuminated
with cross polarized light illustrating the glassy nature of the clastic dike
matrix and in some cases clasts.
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Figure 16 - Detail view of the high relief glass illustrated in figures 14 and
15 above.
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Figure 17 - Photomicrograph of the melt matrix of the clastic dike
illustrating flow structure and the formation of phenocrysts.
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Figure 18 - Photomicrograph of a toasted and partially melted quartz grain
within the same sandstone stringer in figures 8 and 9 above with what appear
to be remnant planar microstructures.
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Figure 19 - Photomicrograph of the same grain in figure 18 above
illuminated by cross polarized light.
Conclusion
All of the above evidence I believe confirms that the La Sal Mountains,
Moab Utah are the remnant of the central uplift of a multiringed or possibly
peak ringed impact structure.