Regional Geophysical Investigations in the Central Colorado Plateau GEOLOGICAL SURVEY PROFESSIONAL PAPER 736 Regional Geophysical Investigations in the Central Colorado Plateau By ]. E. CASE and H. R. JOESTING GEOLOGICAL SURVEY PROFESSIONAL PAPER 736 Gravity and magnetic anomalies indicate polygonization of the Precambrian basement complex and partial rejuvenation of basement structures during late Paleozoic and Laramide time UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600197 For sale by the Superintendent of Documents, U.S. Government Printing OfRce, Washington, D.C. 20402 Price: Paper Cover-$5.60, domestic postpaid; $5.25, GPO Bookstore Stock No. 2401-00222 CONTENTS Page Page Abstract..................................................................................... 1 Geophysical surveys Continued Introduction............................................................................. 2 Area west of the Green and Colorado Rivers. .... 21 Acknowledgments.-----.-.................................................... 4 San Rafael Swell. ..................................................... 21 Summary of regional geology...-----....-.---.-.....----..------.-----. 4 Monument uplift-------- .... ......................... 21 Stratigraphy.......--..-......... .................................................... 4 Area west of the Monument uplift . 22 Precambrian rocks----------.----. ................................ 4 Area east of the Monument uplift- ............ 23 Paleozoic rocks................................................................. 6 Defiance uplift.. .... ....... .- . .... 24 Mesozoic rocks..- ............................................................ 7 Geophysical anomalies over the laccolithic mountains.. 24 Tertiary and Quaternary rocks--..--------------- . 7 La Sal Mountains.. ----. --- -- ---.... ----- --------- 24 Younger igneous rocks-.- ..................................................... 7 Henry Mountains..... - .. 24 Summary of regional structure..-.-. .............................. 8 Aba jo Mountains---------.-- ...... ..... ......... 25 Geophysical surveys.. .. .. ............................................... 9 Sleeping Ute Mountain. 25 Interpretation of the geophysical maps- ---. .- 9 Carrizo Mountains - ... 26 Some major gravity anomalies in the northern part 26 of the region---.........-..-.... .................................... 11 Small intrusions------------------------........ ... Paradox basin.................................................................. 11 Deep-seated structural boundaries inferred from grav­ Gravity anomalies related to salt anticlines in ity data... - . .... 26 the Paradox basin..-.--.....-...--......-... ............... 11 Basement polygonization....... ... ....... ..... .. .. 26 Other geophysical anomalies in the Paradox Regional crustal relations . 27 basin.- . ..... ..................................................... 17 Summary.....-..-----.----..-.- ............................ .... ... ... 27 Uncompahgre uplift-.-- ............................................... 18 References cited . ------ 28 ILLUSTRATIONS Page PLATE 1. Gravity anomaly map of the central Colorado Plateau, Utah, Colorado, Arizona, and New Mexico........In pocket 2. Aeromagnetic map of the central Colorado Plateau.................................... ........ ............ In pocket 3. Basement map of the central Colorado Plateau..... - In pocket FIGURE 1. Index map of parts of Colorado, Utah, Arizona, and New Mexico, showing region covered by gravity and magnetic surveys and areas covered by published geophysical reports......- ........ .... ....... .. 3 2-8. Diagrams showing interpretation of gravity anomaly or anomalies: 2. Salt Valley, Fisher Valley, and Sinbad Valley anticlines. - . 12 3. Castle Valley anticline-- -.....-.. --. ............................................................ ......... ..... .-.. 13 4. Paradox Valley anticline.. ...... .... ................... .......... ... ..... .... ...... .... 14 5. Tenmile Wash and Pine Ridge anticlines--- ..-- .......................................... -... -... ......-.. .. .... 15 6. Moab Valley anticline- ---.--...--.-........-------- ..................... ........ ................... .. 16 7. Lisbon Valley anticline...................................-... -.. .. - . 17 8. Uncompahgre front...... ............ ......................... ................................................... 19 9. Diagram showing interpretation of magnetic profiles across the Uncompahgre front.---------...-----..------- 20 TABLES Page TABLE 1. Generalized stratigraphy of the central Colorado Plateau..........-.......-....-.....-. 5 2. Mean magnetic susceptibility and mean saturated bulk density of samples from the Colorado Plateau lac­ colithic mountain groups..-............---.-....-...-.............................----.-...--- .. - - 3. Key to anomaly classification..........--.---.....-.................................................................-.--....-..-..-..--... .. 10 III REGIONAL GEOPHYSICAL INVESTIGATIONS IN THE CENTRAL COLORADO PLATEAU By J. E. CASE and H. R. JoESTiNG1 ABSTRACT pahgre highland, and the salt anticlines; and (3) those Gravity and aeromagnetic surveys covering about 15,000 formed during the Laramide, including the Uncompahgre square miles in the central Colorado Plateau in Utah, Colo­ uplift, San Rafael Swell, Monument uplift, Defiance (De- rado, Arizona, and New Mexico were conducted to assist in Chelly) uplift, and reactivated salt anticlines. Some of these determining the regional subsurface geology as it may relate later structures were probably rejuvenated along lines of to uranium, oil, and potash exploration. weakness that were established during Precambrian and Precambrian rocks, either exposed on the Uncompahgre (or) Paleozoic times. Plateau or penetrated by drill holes, are granitic, dioritic, The gravity anomaly map shows several different types of and gabbroic bodies and complexly folded gneisses and anomalies. The dominant anomalies are conspicuous gravity schists. The densities of these rocks range from about 2.6 to lows over the salt anticlines. Both broad regional highs, 3.0 grams per cubic centimeter, and their magnetic suscepti­ which occur over the monoclinal uplifts where denser rocks bilities are as much as 5xlO~3 electromagnetic unit. are close to the surface, and regional lows, which occur over Paleozoic sedimentary rocks attain a maximum thickness the basin and platform areas, are caused by lateral density of about 17,000-18,000 feet in the deepest part of the Paradox contrasts related to structural relief. Superimposed on these basin, a concealed structural feature of late Paleozoic age. broad anomalies are conspicuous highs and lows that are Cambrian, Devonian, and Mississippian rocks consist of lime­ caused by rocks of different density within the Precambrian stone, sandstone, shale, and dolomite. Pennsylvanian rocks basement, by concealed major Paleozoic structures, and by consist of limestone, sandstone, shale, and, in the Paradox variations in thickness of Paleozoic sedimentary rocks. Small Member of the Hermosa Formation, evaporites. Permian rocks gravity highs are located over most of the laccolithic include sandstone, arkose, limestone, and siltstone. The lime­ mountains. stones and sandstones of low porosity probably have an On the basis of analysis of combined drill data and gravity average density close to that of parts of the Precambrian anomaly data, the amplitudes, or structural relief, of the basement, 2.6-2.7 grams per cubic centimeter. The evaporites salt cores of the anticlines were determined as follows: Salt are of low density, 2.2-2.35 grams per cubic centimeter. Valley, 8,000 feet; Fisher Valley, 10,000 feet; Sinbad Valley, Mesozoic sedimentary rocks are dominantly clastic and 9,000-11,000 feet; Castle Valley, 7,000 feet; Paradox Valley, attain a maximum thickness of about 5,000 feet. They are 10,000 feet; Tenmile Wash salt anticline, 3,500 feet; Moab less indurated than the Paleozoic rocks, and their densities Valley-Spanish Valley, 8,000 feet; Pine Ridge, 4,000 feet; probably range from 2.3 to 2.5 grams per cubic centimeter. Gypsum Valley, 9,000-10,000 feet; Cane Creek anticline, Thin Tertiary and Quaternary deposits, which consist of 1,500-3,000 feet; Lisbon Valley, 4,500-8,000 feet; and Dolores alluvium and glacial deposits, locally cover large areas, but anticline, 1,000-5,000 feet. Combined drill and gravity data they have little influence on the geophysical anomalies. indicate that grabenlike depressions underlie the salt at Laccolithic intrusions of latest Cretaceous or Tertiary age Paradox Valley, Moab Valley-Spanish Valley, and Lisbon form such prominent mountain groups as the Henry, La Sal, Valley salt anticlines. Abajo, and Carrizo Mountains, and the Sleeping Ute Moun­ Most of the magnetic anomalies arise from contrasts in tain. These igneous masses consist of one or more central magnetization of the Precambrian basement. The deeper part stocks from which radiate laccoliths, dikes, sills, and bys- of the Paradox basin is generally characterized by nearly maliths. They are composed of closely allied rocks which flat magnetic gradients, which indicate the greater depth range in composition from microgabbro to granite, but which to the Precambrian surface. However, the flat or gentle are predominantly