Regional gravity survey, northern Marysvale , south-central

MARK E. HALLIDAY Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 KENNETH L. COOK

ABSTRACT the northern edge of a series of Tertiary intrusive bodies and are generally aligned with the trend of the Wah Wah—Tushar mineral New gravity data in south-central Utah reveal anomalies over belt of southwestern Utah. major structural and volcanic features in the northern . Gravity lows are associated with the Mount Belknap GEOLOGIC SETTING and Big John in the , and with the Red Hills in the Antelope Range. Steep gravity gradients mark The Marysvale volcanic field (Fig. 1) is a large region covered by the locations of the Sevier, Elsinore, Dry Wash, and Tushar faults; extrusive Tertiary volcanic rocks located in the southern High gravity lows are observed over the grabens dropped down between Plateaus of Utah and centered on Marysvale. The field is of consid- these major normal faults. The steep gravity gradient across the erable academic and economic interest due to its location with re- Basin and Range- transition zone is not necessar- spect to a number of regional trends. The east-northeast—trending ily the result of deepening of the Moho, but may simply reflect the Wah Wah-Tushar mineral belt of Hilpert and Roberts (1964) is thick low-density volcanics in the Marysvale field as well as broadly defined by a belt of Tertiary intrusive bodies, commonly changes in the density of sedimentary rocks across the Cordilleran with associated mineral deposits, which crosses the northern por- hingeline. East-northeast—trending gravity contours follow closely tion of the volcanic field. Physiographically, the Marysvale volcanic

114' 113° 112° JL _J

MARYSVALE VOLCANIC 39° FIELD

20 30 40 50 MILES I 1 I I 1 I !' Figure 1. Map of southwestern Utah 40 60 80 KM showing distribution of Tertiary volcanic rocks and outline of Wah Wah-Tushar mineral belt. Area of regional gravity survey outlined by rectangle around Marysvale.

Geological Society of America Bulletin, Part I, v. 91, p. 502-508, 9 figs., August 1980, Doc. no. 00810.

502

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Figure 2. Simple Bouguer gravity anomaly map of southwestern Utah, from Cook and others (1975). Area of regional gravity survey outlined by rectangle around Marysvale. Contour interval, 10 mgal.

38o-

37°

oo' Figure 3. Lithologie map of the survey area. (Symbols for rock units are explained in Figure 4 below.)

TERTIARY AND QUATERNARY ALLUVIUM -includes QTo I fluvial and lacustrine deposits of the formation with intercalated recent .

MOUNT BELKNAP VOLCAN ICS - r hyolit ic flows f m„ -, and tuffs of Miocene age.

Joe Lott Member of Mount Belknap •Tmj -V'; Volcanics - poorly to moderately welded silicic ash-flow tuff.

TERTIARY INTRUSIVE ROCK - granitic to monzonitic [Ti intrusive bodies of Oligocene and Miocene age.

BULLION CANYON VOLCANICS - intermediate lava flows, .Tb v welded tuffs, and volcanic breccios of Oligocene and Miocene age.

SEDIMENTARY ROCK -limestone, si Its tone , shale, sandstone, and conglomerate of Paleozoic, Mesozoic, and Tertiary age. Figure 4. Generalized stratigraphie column for rocks in the sur- 25 KILOMETERS vey area.

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field lies in the transition zone between the Basin and Range and II 2°30 1 112*00' Colorado Plateau provinces (Fenneman and Johnson, 1946; Stokes, 1977). A broad north-trending crustal lineament in this area, known as the Cordilleran hingeline (Stokes, 1976), is approx- imately coincident with the physiographic transition zone in th,e vicinity of the Marysvale volcanic field. The Cordilleran hingeline separates regions of thick deposition of Paleozoic sedimentary rocks to the west from Mesozoic sedimentary rocks to the east. The portion of the Marysvale volcanic field studied in this paper is outlined by the rectangle in Figures 1 and 2. The major lithologic, geographic, and structural elements of this area are shown in Fig- ures 3, 4, and 5, as compiled from the work of Hintze (1936), Ste- ven and others (1977, 1978), and Cunningham and Steven (1978). The structure of the Marysvale volcanic field is dominated by basin-range normal faults which began to form about 21 to 20 m.y. ago and continue to be active today (Rowley and others, 1978). The most important normal faults in the study area are the Sevier, Elsinore, Dry Wash, and Tushar faults (Fig. 5). Sevier Valley is a major graben dropped down between these faults, and the Clear Creek downwarp is an east-trending structural low between the Pavant Range and Tushar Mountains. Thrust faults located in sedimentary rocks along the northwestern edge of the Marysvale volcanic field were formed during the Sevier orogeny (Armstrong, 1968). Volcanic caldera structures in the study area have been de- scribed by Cunningham and Steven (1977); the largest is the Mount Belknap caldera in the Tushar Mountains. Eruptions from the Mount Belknap caldera spread the Joe Lott Tuff member of the Mount Belknap volcanics radially outward and caused the caldera to subside. Sedimentary rocks underlie volcanic rocks all around the margin Figure 5. Map of survey area showing major geographic and of the Marysvale field and are exposed in a few places within it; structural features. therefore it seems likely that sedimentary rocks underlie much of the Marysvale volcanic field. The extrusive Tertiary volcanics con- sist of two main phases: (1) the 31 to 21-m.y.-old Bullion Canyon volcanics, consisting of intermediate composition lava flows, (1972) over the San Juan volcanic field in southwestern Colorado. welded tuffs, and breccias; and (2) the 21- to 18-m.y.-old Mount In their case, a gravity low of about 25-mgal closure generally Belknap volcanics, consisting of silicic alkalic lava flows and tuffs. coincident with the extent of Tertiary volcanic rocks was inter- A series of intrusive bodies trend generally east-northeast across the preted to reflect an underlying batholith. study area from the Mount Belknap caldera to the northern Sevier Between Milford and Marysvale at about lat 38°30'N, a 40-km Plateau. right-lateral offset of the gravity contours corresponds with the Wah Wah-Tushar mineral belt and has been interpreted as an GEOPHYSICAL SETTING east-west structural lineament (Cook and Montgomery, 1975). Along this offset lies a north-northeast—trending belt of gravity Simple Bouguer gravity anomaly data in southwestern Utah contours with steep gradient which crosses the northwest corner of (Fig. 2) show a large region of low gravity generally coincident with the study area (Fig. 2). The alignment of this belt with the transition the Marysvale volcanic field. This region contains some of the low- between the Basin and Range and Colorado Plateau provinces est gravity values in Utah, reaching less than —250 mgal just south (Stokes, 1976) might suggest that the gravity gradient is due to of Marysvale. The extension of the regional gravity low to the changes in crustal thickness across the transition zone. Shuey and north of the volcanic field indicates that a portion of the low may others (1973), however, were unable to reconcile this hypothesis be due to a deep mass deficiency related to isostatic balance for the with aeromagnetic and other data which suggested that changes in high regional elevations throughout the High Plateaus section of crustal geophysical parameters across the transition zone are lo- the Colorado Plateau. The deep alluvial-filled grabens such as cated between 50 and 100 km east of the physiographic boundary. Sevier Valley also contribute distinct gravity lows within the larger Snow (1978) has suggested that the apparent conflict can be re- region of low values. However, the strong correlation between the solved if the gravity gradient is interpreted as due largely to density Marysvale volcanic field and the gravity low may be due to the changes in the upper crustal sedimentary section rather than thick accumulations of low-density extrusive volcanics and possi- reflecting any deep crustal change such as deepening of the Moho bly large masses of low-density intrusive rock beneath the area. across the transition zone. The Cordilleran hingeline of Stokes Similar regional relationships were observed by Plouff and Pakiser (1976) generally follows the physiographic transition zone in this

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II2°30' 112=15 II2°00' Tertiary volcanics of the Marysvale field, thereby suggesting that near-surface features are at least partly responsible for the gradient. The Tushar, Dry Wash, and Elsinore faults are clearly expressed in the gravity data as continuous gravity gradients, whereas the gradient along the Sevier fault is severely disrupted at about lat 38°30'N. There is a semi-continuous gravity gradient trending east-northeast along the northern edge of the Mount Belknap and Red Hills calderas, generally in alignment with the isolated out- crops of intrusive rock which define the Wah Wah-Tushar mineral belt. This gradient may mark the northern edge of a continuous belt of intrusive rock beneath the extrusive volcanic cover. This postu- lated belt of intrusive rock probably extends into the northern Sevier Plateau as evidenced by the disruption of the gravity gradient along the Sevier fault mentioned above. The three calderas indicated in Figure 5 are expressed as gravity lows, whereas the gravity signature over the Three Creeks cauldron is unclear. The gravity lows are probably due to low-density vol- canics accumulated within the calderas.

PROFILE ANALYSIS

Three gravity profiles were selected for analysis and are indicated in Figure 6. The profiles were selected to cross the major features of interest in such a way that the two-dimensional assumption to be used in modeling would be valid. Obviously this is not completely possible with such a complex gravity field, but in most cases, the error should be minimal. The gravity data nearest each profile were projected onto the profile and then reduced to a datum equal in ele- vation to the lowest station along the profile, assuming a density of

I 1 1 1 T-1 CONTOUR INTERVAL 2.4 gm/cc as a reasonable density for the volcanics based on density 0 5 10 15 KILOMETERS 2 MGAL measurements on 36 outcrop samples collected throughout the sur- Figure 6. Complete Bouguer gravity anomaly map of the survey vey area (Halliday and Cook, 1978). This additional reduction is area. Location of the three interpretive geologic cross sections is necessary for quantitative analysis of the gravity data, which would shown. otherwise assume improperly low gravity values in areas of high elevation due to the original assumption of 2.67 gm/cc for the Bouguer-reduction density. Finally, each profile was modeled using vicinity, and for that reason the gravity gradient may be reflecting a combination of two-dimensional forward and inverse techniques the high density of the predominantly carbonate Paleozoic section developed by Snow (1978) based on the method of Talwani and to the west of the hingeline in contrast to the low density of the others (1959). Mesozoic sedimentary rocks to the east. Profile A-A' (Fig. 7) crosses the Sevier Valley graben dropped down between the Sevier and Elsinore faults. The model shows the NEW GRAVITY DATA graben to be about 1,300 m in depth, assuming a density contrast of 0.4 gm/cc between the volcanics and alluvium. Part of the gravity During the summer of 1977, regional gravity data were collected gradient in the Pavant Range was modeled as due to lateral density throughout the area indicated by the rectangle in Figure 2. Assum- changes in the underlying sedimentary rocks. If the bodies shown ing a density of 2.67 gm/cc for both the Bouguer reduction and ter- with densities of 2.6 gm/cc and 2.8 gm/cc are considered to repre- rain correction, these data were reduced to a datum of sea level and sent Mesozic and Paleozoic rocks, respectively, the relationships terrain corrected out to a distance of 167 km from each station. are consistent with generalized changes in Paleozoic and Mesozoic Terrain corrections averaged slightly greater than 6 mgal, with a rock thicknesses across the Cordilleran hingeline. A large body of few corrections exceeding 50 mgal for stations located on very high 2.8 gm/cc density was modeled at the east end of the profile to ac- and rugged peaks in the Tushar Mountains. A total of 948 gravity count for the circular gravity high east of Monroe. A quartz station values were compiled into the complete Bouguer gravity monzonite intrusion exposed in the Sevier Plateau east of Monroe anomaly map shown in Figure 6. may be related to this gravity high, although 2.8 gm/cc would be an The new gravity data show in detail the many complex gravity unusually high density for such intrusive rocks. Sparse gravity data anomalies throughout the northern Marysvale volcanic field. One and lack of detailed geology prevent a better understanding of this major feature is the steep gravity gradient in the northwestern part feature. of the study area along the Pavant Range. The steepest part of the Profile B-B' (Fig. 8) crosses the steep gravity gradient in the Pav- gradient follows closely the contact between sedimentary rocks and ant Range, traverses the Clear Creek downwarp and Antelope

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5 -215-

> -220-

t -225 -

< -230 - % -235- -240 -

-245 J ELSINORE FAULT MONROE y—SURFACE ELSINORE SEVIER FAULT

SEVIER VALLEY • 0 5 -DATUM ELEVATION' 1.616 KM l 2.0 X 1 .0 QTol H Q. UJ 1 -5 o

2.0 VERTICAL EXAGGERATION = 2X

10 11 12 13 14 15 17 18 19 20 21 22 23 24 25 DISTANCE (KM) Figure 7. Interpretive geologic cross section along profile A-A'.

MARYSVALE

DISTANCE (KM) Figure 8. Interpretive geologic cross section along profile B-B'.

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DATUM ELEVATION = 2.270 KM

2.4

MOUNT BELKNAP CALDERA Z6

Tm and Tmj

S or Ti VERTICAL EXAGGERATION -2X

9 10 11 12 13 14 IS 16 17 18 19 20 21 22 23 24 25 26 27 DISTANCE (KM) Figure 9. Interpretive geologic cross section along profile C-C'.

Range, and terminates along the southern margin of the gravity low the assumptions made, the model appears reasonable and is not centered south of Marysvale. Although a portion of the total grav- unlike models developed for the Long Valley caldera in California ity relief of over 70 mgal along this profile may be due to changes in (Kane and others, 1976) and the Bonanza caldera in the San Juan depth to the Moho across the Basin and Range-Colorado Plateau volcanic field of Colorado (Karig, 1965). transition, the model shown was constructed using the shallowest bodies of reasonable density that could be made to fit the gravity CONCLUSIONS data. Therefore, although the model does not prove that the gravity gradient across the Pavant Range is due to upper-crustal features New gravity data in the northern Marysvale volcanic field reveal only, it does establish quantitatively that such a hypothesis is pos- important anomalies which provide information regarding at least sible. This model is appealing because it resolves the discrepancy gross aspects of the subsurface geology. Steep gravity gradients ob- observed by Shuey and others (1973) between gravity and served across normal faults and gravity lows observed over grabens aeromagnetic data across the transition zone by suggesting that the dropped down between the faults confirm these to be major gravity gradient is not related to the deep-crustal features which the upper-crustal features. Likewise, the pronounced gravity lows over aeromagnetic data are apparently delineating. Until additional sub- calderas in the study area are valuable confirming evidence for surface density information can be obtained in this area, the pro- these major volcanic source areas. An east-northeast—trending zone portion of the contribution to the gravity effect from the near- of gravity contours apparently delineates the northern edge of a surface rocks versus deepening of the Moho must still be consid- major belt of intrusive rocks related to the Wah Wah-Tushar min- ered uncertain. Other features shown in profile B—B' are (1) about eral belt of southwestern Utah. Gravity data along the steep gravity 500 m of low-density Joe Lott Tuff and alluvium in the Clear Creek gradient in the area of the Basin and Range-Colorado Plateau downwarp can account for the gravity low there; (2) an upwarp in transition zone reveal a strong correlation with surface geology, the sedimentary rocks, or alternatively intrusive rocks, beneath the thereby suggesting a near-surface source for at least part of this Antelope Range can account for the rise of gravity values there; and gradient. (3) the gravity low in Sevier Valley south of Marysvale may be due The large number of significant gravity anomalies observed to about 1,200 m of alluvial fill, assuming a density contrast of 0.4 throughout the study area provides encouragement that future gm/cc between the volcanics and alluvium. gravity studies could add to our knowledge of the Marysvale vol- Profile C-C' (Fig. 9) crosses the gravity low associated with the canic field and surrounding region. Mount Belknap caldera in the Tushar Mountains. Assuming a density contrast of 0.2 gm/cc between the volcanics in the caldera ACKNOWLEDGMENTS and surrounding sedimentary/underlying intrusive rocks, the low- density volcanics in the caldera are postulated to extend to a depth C. G. Cunningham, D. R. Mabey, and T. A. Steven reviewed this of about 2,500 m. Although this depth estimate is uncertain due to paper. Financial support for this work was provided under United

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States Department of Energy/Division of Geothermal Energy Con- investigation of the Long Valley caldera, Mono County, California: tract No. EY-76-S-07-1601. Additional information regarding this Journal of Geophysical Research, v. 81, no. 5, p. 754-762. Karig, D. E., 1965, Geophysical evidence of a caldera at Bonanza, Col- research may be found in Halliday and Cook (1978). orado: U.S. Geological Survey Professional Paper 525-B, p. B9-B12. Plouff, D., and Pakiser, L. C., 1972, Gravity study of the San Juan REFERENCES CITED Mountains, Colorado: U.S. Geological Survey Professional Paper 800-B, p. B183-B190. Armstrong, R. L., 1968, Sevier orogenic belt in Nevada and Utah: Geologi- Rowley, P. D., and others, 1978, Age of structural differentiation between cal Society of America Bulletin, v. 79, p. 429-458. the Colorado Plateaus and Basin and Range provinces: Geology, v. 6, Cook, K. L., and Montgomery, J. R., 1975, Structural trends in Utah as p. 51-55. indicated by gravity data: Geological Society of America Abstracts Shuey, R. T., and others, 1973, Aeromagnetics and the transition between with Programs, v. 7, p. 598. the Colorado Plateau and the : Geology, Cook, K. L., and others, 1975, Simple Bouguer gravity anomaly map of v. 1, p. 107-112. Utah: Utah Geological and Mineral Survey Map 37. Snow, J. H., 1978, A study of structural and tectonic patterns in south- Cunningham, C. G., and Steven, T. A., 1977, Mount Belknap and Red Hills central Utah as interpreted from gravity and aeromagnetic data [M.S. Calderas and associated rocks, Marysvale volcanic field, west-central thesis]: Salt Lake City, Utah, University of Utah, 206 p. Utah: U.S. Geological Survey Open-File Report 77-568. Steven, T. A., and others, 1977, Revised stratigraphy and radiometric ages 1978, Geologic map of the NW quadrangle, west-central of volcanic rocks and mineral deposits in the Marysvale area, west- Utah: U.S. Geological Survey Miscellaneous Field Studies Map MF- central Utah: U.S. Geological Survey Open-File Report 77-569, 45 p. 967. Steven, T. A., Rowley, P. D., and Cunningham, C. G., 1978, Geology of the Fenneman, N. M., and Johnson, D. W., (in cooperation with Physiographic Marysvale volcanic field, west central Utah: Brigham Young Univer- Committee, U.S. Geological Survey), 1946, Physical divisions of the sity Geology Studies, v. 25, part 1, p. 67-70. United States: U.S. Geological Survey, scale 1:7,000,000. Stokes, W. L., 1976, What is the Wasatch Line?, in Rocky Mountain As- sociation of Geologists — 1976 symposium, p. 11—25. Halliday, M. E., and Cook, K. L., 1978, Gravity and ground magnetic sur- 1977, Subdivision of the major physiographic provinces in Utah: Utah veys in the Monroe and Joseph KGRA's and surrounding region, Geology, v. 4, no. 1, p. 1-17. south central Utah: University of Utah Final Technical Report 77-7, Talwani, M., Worzel, J. L., and Landisman, M., 1959, Rapid gravity com- DOE/DGE Contract EY-76-S-07-1601, 163 p. putations for two-dimensional bodies with application to the Men- Hilpert, L. S., and Roberts, R. J., 1964, Geology-economic geology, in U.S. docino submarine fracture zone: Journal of Geophysical Research, Geological Survey, Mineral and water resources of Utah: U.S. 88th v. 64, p. 49-59. Congress, 2nd session, p. 28-38. Hintze, L. F., 1963, Geologic map of southwestern Utah: Utah Geological and Mineral Survey, scale 1-250,000. MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 25, 1980 Kane, M. F., Mäbey, D. R., and Brace, R., 1976, A gravity and magnetic MANUSCRIPT ACCEPTED MARCH 3, 1980

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