La Sal Mountains Structure, Moab, Utah


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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 in the publication:

Geology of the Tertiary Intrusive Centers of the La Sal

Mountains, Utah—Influence of Preexisting Structural Features on Emplacement and Morphology


Michael L. Ross


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

Sal Mountains, 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 been emplaced at shallow levels

ranging between 1.9 and 6.0 kilometers. The intrusions are

holocrystalline and porphyritic and have a very fine- to finegrained




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). In general, 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 intrusive centers are commonly referred to as laccolithic centers.

Figure 1 Michael L. Ross


Figure 2 Michael L. Ross


Figure 3 Michael L. Ross

Figure 4 Michael L. Ross


Figure 5 Michael L. Ross


Figure 6 Michael L. Ross


Figure 7 Michael L. Rosss







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 the mountain 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 sedimentary rocks 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. Regional magnetic 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 La Sal Mountains indicates that no

such intrusion is present within 11–14 km beneath the mountains (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 trendingmonocline (fig. 5, sectionA–A'). Triassic strata are

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 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 have slickenside 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 described on 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 from 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 the level

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 margins of 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

plagioclase trachyte 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-rich fluids. 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 and Pollard, 1988a).



Macroscopic and microscopic evidence of shock metamorphism resulting from an extraterrestrial impact event



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.


Figure 8-a


Figure 8b

Figure 8-a and b - Quartz grain with planar microstructures (PM's) from a sandstone stringer within a Triassic? Redbed located at the 8000 foot level of the La Sal Mountains.


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.



Blob of melt with spherules adjacent to a quartz grain with two sets of PM's probably PDF's illustrating the propensity of a porus sandstone to concentrate a shock wave  causing melting and the formation of planar microstructures, This melt is on the same thin section as the photomicrograph above.



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.


Figure 11 - Photograph of a igneous clast with the above mentioned clastic dike.


Figure 12 -Photograph of a sandstone clast with the above mentioned clastic dike.


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,, show examples of clasts   rounded and polished in the excavation stage of an impact crater.


Figure 14 - Photomicrograph of high relief glass within the above clastic dike (the black circle is an ink mark on the thin section).


Figure 15 - Same view of the thinsection in figure 14 above illuminate with cross polarized light illustrating the glassy nature of the clastic dike matrix and in some cases clasts.


Figure 16 - Detail view of the high relief glass illustrated in figures 14 and 15 above.


Figure 17 - Photomicrograph of the matrix of the clastic showing flow structure and the formation of phenocrysts.


Figure 18 - Photomicrograph of a toasted area of melt within the same sandstone stringer in figures 8 and 9 above with what appear to be remnant planar microstructures.


Figure 19 - Photomicrograph of the same area of melt as in figure 18 above illuminated by cross polarized light.