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The Laccolith-Stock Controversy: New Results from the Southern Henry Mountains, Utah

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The Laccolith-Stock Controversy: New Results from the Southern Henry Mountains, Utah The laccolith-stock controversy: New results from the southern Henry Mountains, Utah MARIE D. JACKSON* Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 DAVID D. POLLARD Departments of Applied Earth Sciences and Geology, Stanford University, Stanford, California 94305 ABSTRACT rule out the possibility of a stock at depth. At Mesa, Fig. 1). Gilbert inferred that the central Mount Hillers, paleomagnetic vectors indi- intrusions underlying the large domes are Domes of sedimentary strata at Mount cate that tongue-shaped sills and thin lacco- floored, mushroom-shaped laccoliths (Fig. 3). Holmes, Mount Ellsworth, and Mount Hillers liths overlying the central intrusion were More recently, C. B. Hunt (1953) inferred that in the southern Henry Mountains record suc- emplaced horizontally and were rotated dur- the central intrusions in the Henry Mountains cessive stages in the growth of shallow (3 to 4 ing doming through about 80° of dip. This are cylindrical stocks, surrounded by zones of km deep) magma chambers. Whether the in- sequence of events is not consistent with the shattered host rock. He postulated a process in trusions under these domes are laccoliths or emplacement of a stock and subsequent or which a narrow stock is injected vertically up- stocks has been the subject of controversy. contemporaneous lateral growth of sills and ward and then pushes aside and domes the sed- According to G. K. Gilbert, the central intru- minor laccoliths. Growth in diameter of a imentary strata as it grows in diameter. After the sions are direct analogues of much smaller, stock from about 300 m at Mount Holmes to stock is emplaced, tongue-shaped sills and lacco- floored intrusions, exposed on the flanks of nearly 3 km at Mount Hillers, as Hunt sug- liths are injected radially from the discordant the domes, that grew from sills by lifting and gested, should have been accompanied by sides of the stock (Fig. 4). Mount Holmes, bending of a largely concordant overburden. considerable radial shortening of the sedi- Mount Ellsworth, and Mount Hillers are taken According to C. B. Hunt, the central intru- mentary strata and a style of folding which is to represent successive stages in the development sions are cylindrical stocks, sheathed with a not observed. Geologic and geophysical data of this process. Although the inference that the zone of shattered sedimentary rocks, and the and mechanical models support a laccolithic central intrusions in the Henry Mountains are small flanking sills and laccoliths grew later- origin for the central magma chambers under- stocks has found acceptance in some textbooks ally as tongue-shaped masses from the dis- lying the domes. (Shelton, 1966, p. 16-17), the nature of these cordant sides of these stocks. New geologic bodies is recognized as problematic in others mapping demonstrates that the sedimentary INTRODUCTION (Billings, 1972, p. 341). Indeed, the present overburden, now partially eroded from the erosional level in the Henry Mountains makes domes, was uplifted about 1.2 km at Mount Toward the end of the last century, G. K. it difficult to distinguish Hunt's stocks from Holmes, 1.8 km at Mount Ellsworth, and at Gilbert (1877) originated the concept of a lacco- Gilbert's laccoliths as the principal constituents least 2.5 km at Mount Hillers. The radii of the lith, on the basis of his observations in the Henry of the large domes, on the basis of surficial domes are similar, between 5 and 7 km. The Mountains of Utah. He postulated a process in mapping alone. strata over the domes have a doubly hinged which magma rises vertically in a dike or nar- To clarify the differences between laccoliths shape, consisting of a concave-upward lower row pipe-like conduit until it spreads between and stocks as illustrated in Figures 3 and 4, the hinge and a concave-downward upper hinge. horizontal strata to form a sill. The sill propa- following points are emphasized. A limb of approximately constant dip joins gates laterally until it lifts the overburden and (1) Laccoliths may be low in height relative these two hinges and dips 20° at Mount causes the sedimentary strata to bend concor- to their horizontal dimensions, and they range Holmes, 50° to 55° at Mount Ellsworth, and dantly over the thickening laccolith. In the from circular to tongue-shaped in plan form. 75° to 85° at Mount Hillers. The distal por- southern Henry Mountains (Fig. 1), sedimentary Stocks have a great height relative to a roughly tion of each dome is composed of a gently domes forming Mount Holmes, Mount Ells- constant diameter, and they approximate a tall, dipping peripheral limb 3 to 4 km long, pre- worth, and Mount Hillers (Fig. 2) are approxi- upright, circular cylinder. sumably underlain by sills and minor lacco- mately equidimensional in plan form and have (2) Laccoliths have a local feeder, such as a liths. Although geologic cross sections and radii of 5 to 7 km. Near the summits of Mount dike or stock, which has a very different size and regional aeromagnetic data for the three Ellsworth and Mount Hillers, large masses of mechanism of formation from those of the lacco- domes are consistent with floored, laccolithic diorite porphyry are exposed and are interpreted lith. Stocks do not have a local feeder; they are central intrusions, these data alone do not as the tops of central intrusions. Numerous dikes continuous to great depth, perhaps extending to and sills of diorite porphyry crop out in and a deep magma reservoir. adjacent to the domes, as do small laccoliths that •Present address: U.S. Geological Survey, Hawaii (3) Stocks grow upward, perhaps by stoping, National Park, Hawaii 96718. have well-exposed floors (for example, Black zone melting, and/or diapiric piercement as dis- Geological Society of America Bulletin, v. 100, p. 117-139, 19 figs., 2 tables, January 1988. 117 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/1/117/3379075/i0016-7606-100-1-117.pdf by guest on 24 September 2021 118 JACKSON AND POLLARD Figure 1. Simplified geologic map of the southern Henry Mountains (after Hunt, 1953, and this study). Younger rocks crop out pro- gressively from east to west, indicating a gen- tle 1° to 2° dip to the west-southwest. Open triangles indicate mountain summits. Out- lined areas indicate maps shown in Figures 7, 8, and 9. cussed by Daly (1933) and Marsh (1982), and so they are not floored and they may be largely discordant. Laccoliths grow from a thin sill that thickens into a floored body, and so they are largely concordant. Distinguishing between lac- coliths and stocks is made more difficult because laccoliths can attain greater height by peripheral faulting. This produces bysmaliths, bodies that have discordant sides but are floored. The purpose of this paper is to introduce new evidence that bears on the laccolith-stock con- troversy and on the origins of epizonal magma chambers. We describe the structure of the three southern domes in the Henry Mountains on the basis of new geologic mapping. We give detailed descriptions of the host-rock flexures, using cross sections through each dome. Models of the shapes and volumes of the central intrusions based on regional aeromagnetic surveys are summarized, and the sequence of intrusion is interpreted using paleomagnetic data from sills and thin laccoliths at Mount Hillers. Finally, some of the mechanical differences that might characterize laccoliths and stocks are reviewed. From this work, a sequential progression of magmatic and deformational events is inferred and compared with the previous hypotheses of Gilbert (1877) and Hunt (1953). GEOLOGIC SETTING The Henry Mountains, in southeastern Utah (Fig. 1), are one of more than ten mountain ranges on the central and eastern Colorado Pla- teau composed of sedimentary domes with ig- neous cores, surrounded by dikes, sills, and minor laccoliths. The ages of these ranges cluster 110°45' 110° about the Cretaceous-Tertiary boundary and the mid-Tertiary (Armstrong, 1969; Cunningham and others, 1977). The Henry Mountains form a range that is about 60 km long, trends roughly 48 Ma. Sullivan (1987), however, has found The mountains are situated on the gently dip- north-south, and is composed of 5 peaks sepa- younger ages for these rocks, 20 to 29 Ma, ping (about 2° west) eastern limb of a north- rated by low passes. From north to south, the using fission-track methods. In the southern south-trending structural basin that is bounded peaks are Mount Ellen, Mount Pennell, Mount Henry Mountains, all of the intrusive rock on the west by the Waterpocket monocline Hillers, Mount Holmes, and Mount Ellsworth. is diorite or quartz diorite porphyry. The (Hunt, 1953, Plate 1). A stratigraphic section K-Ar dating of the diorite porphyry from flank- petrology of these rocks is described by Engel (Fig. 5) compiled from data of Hunt (1953), ing laccoliths at Mount Ellen and Mount Hillers (1959), and Kilinc (1979) has studied their Hintze (1963), Peterson and others (1980), and by Armstrong (1969) gives Eocene ages, 44 to crystallization temperatures. Stokes (1980) shows estimated thicknesses of Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/1/117/3379075/i0016-7606-100-1-117.pdf by guest on 24 September 2021 NEW LACCOLITH-STOCK RESULTS, HENRY MOUNTAINS, UTAH 119 the formations. About 2.7 km of strata, from the p. 51) gives a maximum of 1 km of lower Ter- about 3.7 km of sedimentary rock overlay the base of the Permian Cutler Formation to the tiary sediments presently overlying the Mesa Permian Cutler Formation during the early to Cretaceous (Campanian) Mesa Verde Forma- Verde Formation about 100 km west of the mid-Tertiary, when the intrusions were em- tion, is exposed in the Henry Mountains.
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