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The AAPG/Datapages Combined Publications Database
Utah Geological Association
Structural and Igneous Geology of the Henry Mountains, Utah
Sedimentary rocks exposed in the Henry Mountains region aggregate about 8,000 ft in thickness and include rocks of Permian, Triassic, Jurassic, Upper Cretaceous, and Quaternary age. The Permian and Mesozoic rocks are divided into 23 mappable units, classed as formations or members, five of which are Permian, three are Triassic, eight are Jurassic, and seven are Upper Cretaceous.
More than 80 percent of the pre-Cretaceous rocks are of continental origin, for the region is part of a large area that was marginal to the main Permian, Triassic, and Jurassic seaways. During these three periods the region was a low area, apparently a coastal lowland, but only three brief invasions by the marine waters of the main seaways are recorded. The Permian sea that lay to the west spread into the Henry Mountains region near the close of Permian time (as indicated by the Kaibab limestone) but it barely extended across the region; the Triassic sea that lay to the northwest failed to reach this region; the Jurassic sea that lay to the north spread southward twice (as indicated by the Curtis and part of the Carmel formations) to the site of the Henry Mountains but neither of these two invasions reached the southern part of the region.
During Upper Cretaceous time the conditions were reversed, at least during that part of the epoch represented by the rocks remaining in the region. The sea spread westward across this region early in Late Cretaceous time, except for two brief withdrawals (as indicated by the Ferron and Emery sandstone members of the Mancos shale) and the sea persisted over the area while 2,000 ft of marine sediments were being deposited in it.
Younger rocks, probably partly of late Tertiary age but mostly Quaternary, are poorly consolidated and relatively thin but widespread. They include many classes of deposits of which nearly a dozen have been distinguished and mapped.
The Henry Mountains are in a structural basin that is one of the major folds of the Colorado Plateaus. The basin is the counterpart of the adjoining Circle Cliffs Upwarp and San Rafael Swell, being of the same size and form, only inverted. The basin is sharply asymmetric and its trough is crowded against the steep west flank; the deepest part is 8,500 ft structurally lower than the neighboring uplifts.
Faults are uncommon, except a series of small en echelon faults that cross the north tip of the basin. Two principal sets of joints in the region trend respectively northeast and southeast.
The structural basin was formed near the close of late Cretaceous time or the beginning of Eocene time, as shown by the fact that the Eocene Wasatch formation lies undisturbed across folded Mesozoic rocks at Boulder Mountain and at Thousand Lake Mountains. The intrusions in the Henry Mountains are believed to be mid-Tertiary.
Each of the Henry Mountains is a structural dome several miles in diameter and a few thousand feet high. The big mountain domes are attributed to the deformation that accompanied physical injection of the stocks, because the sedimentary formations turned up around the stocks occupy the same amount of area that they did in their original horizontal position. In general the domes have smooth flanks but on most of them are superimposed many small anticlinal noses that were produced by the laccoliths. At the center of each of the domes is a stock, around which the laccoliths and other intrusive bodies are clustered. The stocks are of different width and the amount of uplift at the mountain domes seems to be a direct function of that width. The stocks are crosscutting intrusions, mostly surrounded by a zone of shattered rocks, which consists of highly indurated sedimentary rocks irregularly intruded by innumerable dikes, sills, and irregular masses of porphyry.
As Gilbert showed, the laccoliths are concordant injected masses that lifted their roofs by arching. Many of the laccoliths possess a very simple, linearly bulged, tongue-shaped form, but where the intrusions are crowded the forms are complex. Some of the intrusions have steep sides along which the roof rocks were faulted upward. These intrusions are bysmaliths, but they are like the laccoliths in all respects except this faulting.
Several lines of evidence indicate that the laccoliths and bysmaliths were injected radially from the stocks: the laccoliths are tongue-shaped in plan and make a radial pattern around the stocks; their roofs are bulged linearly and the axes of the bulges radiate from the stocks; dikelike ridges on the roofs of the laccoliths and bysmaliths trend away from the stocks. The laccoliths may have been injected as sills that later bulged and arched their roofs, or, they may have been injected at their full thickness and extended distally. Probably the growth was by a combination of these processes whereby the initial injection was wedgeshaped.
Coherence and competency of the invaded rocks appear to have been an important factor controlling the stratigraphic distribution of the laccoliths. The pre-Jurassic formations, about 5,000 ft thick, which consist of well-bedded, relatively coherent, alternating competent and incompetent units contain very few laccoliths. The overlying competent and highly coherent sandstones of the Glen Canyon group (Wingate, Kayenta and Navajo formations), 1,200 ft thick, contain still fewer laccoliths. The next high formations, the San Rafael group and lower half of the Morrison formation, about 1,000 ft thick, consisting of incoherent, incompetent, poorly bedded rocks and interbedded competent layers, contain about 15 percent of the total volume of the laccoliths. By far the greatest number of laccoliths and bysmaliths (at least 70 percent by volume) are in the highest rocks, the upper half of the Morrison and the Cretaceous formations which have a total thickness of about 2,500-3,000 ft and consist largely in incoherent, incompetent shale in very thick units separated by thin competent layers. In these incompetent rocks the concordant intrusions are concentrated along the thin competent layers.
Among the more important factors controlling the form of intrusions are the volume and viscosity of the magma and its rate of injection. Volume affects the form only because very large volume, in general, leads to irregular form. Viscosity controls intrusive forms because a liquid magma tends to transmit the pressure readily and can readily enter all cracks in the strata, whereas a viscous magma tends to spread less widely and merely bulge. Progressive increase of viscosity during intrusion tends to restrict the spreading of a magma and cause it to bulge. Rate of intrusion is a factor in controlling intrusive form because a rapid increased rate has the effect of increasing the viscosity.
Probably the domal curvature of a laccolith is greater under greater load, but in the Henry Mountains the range of load—the overburden—was not sufficient to produce very different intrusive forms. The other factors apparently were much more important because sheetlike and bulbous intrusions occur side by side. It seems probably that the Henry Mountains intrusions formed beneath something like a mile of overburden. None of the laccoliths or bysmaliths breached the surface rocks but the stocks may have penetrated to the surface and erupted.
The several laccolithic mountains in the Colorado Plateaus are believed to represent a series of examples of one igneous process that was arrested at various stages of completion. The several mountains are rather alike in regard to the form of the intrusions, their general structure, and igneous rock types; and the stratigraphy and structure of the host rocks is fairly uniform at the mountains. the differences between the mountains seem best explained by differences in the stage reached by the process at the different places.
All igneous rocks in the Henry Mountains are intrusive. These rocks, which include diorite porphyry, monzonite porphyry, aplite, and basalt, have a total volume of about 16 cu mi. Diorite porphyry makes up about 95 percent of the total, and monzonite porphyry most of the remaining 5 percent. The aplite and basalt form only very thin sills and dikes near the stocks.
The diorite porphyry is composed mostly of oligoclase, hornblende, and magnetite phenocrysts in a fine-grained feldspathic groundmass. A few intrusions contain augite or biotite in addition. The monzonite porphyry contains the same phenocrysts as the diorite porphyry plus small quantities of aegirine-augite and very large crystals of soda orthoclase. These rocks resemble others in the Colorado Plateaus in containing more than average soda and alumina.
Exceedingly fine-grained feldspathic material and sericite have partly replaced the plagioclase phenocrysts along cleavage cracks and irregular fractures along composition zones, or in irregular areas.
Inclusions constitute a small percent of the total volume of the igneous rocks. Ninety-seven percent of the inclusions are composed of the same hornblende and plagioclase that occur in the porphyry. These inclusions may be fragments of the wall rocks that were altered to produce minerals in equilibrium with the magma, or they may be derived from early intrusive differentiates of the magma. Whatever their origin, they probably were derived at great depth.
The metamorphic effect of the intrusions is everywhere slight. Baking tests of the shale, alteration of coal xenoliths, and theoretical consideration of the fusion temperature of the magma indicate that it contained only small quantities of volatile constituents and that the temperature at the time of intrusion was of the order of 600° C.
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