Geologic Interpretation of Magnetic and Gravity Data in the Copper River Basin, Alaska

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Geologic Interpretation of Magnetic and Gravity Data in the Copper River Basin, Alaska Geologic Interpretation of Magnetic and Gravity Data in the Copper River Basin, Alaska By GORDON E. ANDREASEN, ARTHUR GRANTZ, ISIDORE ZIETZ, and DAVID F. BARNES GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGICAL SURVEY PROFESSIONAL PAPER 316-H UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library has cataloged this publication as follows: Andreasen, Gordon Ellsworth, 1924- Geologic interpretation of magnetic and gravity data in the Copper River Basin, Alaska, by Gordon E. Andreasen [and others]. Washington, U.S. Govt. Print. Off., 1964. 135-153 p. maps (2 fold. col. in pocket) profiles, table. 30 cm. (U.S. Geological Survey. Professional paper 316-H) Geophysical field investigations. Bibliography: p. 152-153. 1. Magnetism, Terrestrial Alaska Copper River Basin. 2. Geol­ ogy Alaska Copper River Basin. I. Title. (Series) For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Magnetic and gravity interpretation Continued Page Abstract_ ________________________________________ 135 Anomalies in the northern part of the surveyed Introduction __ ___________________________________ 135 area Continued Area covered.__________________________________ 135 West Fork feature...._______________ 142 Aeromagnetic survey.___________________________ 135 North Tyone low.____________ ____ 143 Gravity survey.________________________________ 136 Anomalies on the west and south sides of the surveyed Acknowledgments--.. __1________________________ 137 area_______________________________________ 144 Geology __ __ ____________ _ 137 Tyone Creek anomalies_____-______--_-______ 144 Rock units_____________________________________ 137 Horn Mountains feature_____________________ 144 Carboniferous and older rocks.______-_____-_- 137 Northern Chugach Mountains anomalies ____ 144 Permian and Triassic rocks_________________ 137 Twin Lakes anomaly_______________-___.____ 145 Jurassic volcanic and associated rocks.,._____ 138 Areas where Lower Jurassic volcanic rocks are Jurassic and Cretaceous sedimentary rocks_____ 138 buried by nonmagnetic rocks.______________ 145 Intrusive rocks_________________________ 138 Anomalies in the southeastern part of the surveyed Tertiary sedimentary rocks_________________ 138 area_____ __________________________________ 146 Cenozoic volcanic rocks____________________ 138 Cranberry Peak pattern.. _________________ 146 Physical-property measurements._________________ 139 Durham Creek anomaly.___________________ 146 Mount Drum pattern______________________ 147 Structure___ ___ _______ ____________ 139 Areas of thick sedimentary rocks____________ 147 Magnetic and gravity interpretation_________________ 140 Anomalies in the central part of the surveyed area__ 147 Techniques____________________ ___ _______ 140 Copper River Basin magnetic anomaly________ 148 Regional magnetic patterns and gravity anomalies_ 140 Old Man Lake gravity anomaly.____ ______ 148 Anomalies in the northern part of the surveyed area- 142 Glennallen and Gakona gravity lows_-_----___ 150 Excelsior Creek anomalies._________________ 142 Conclusions-__-______-_--__----------------------._ 152 Maclaren-Gulkana anomalies ________ ___ ____ 142 References cited_________-____-__-_-____-____-______ 152 ILLUSTRATIONS [Plates are in pocket] Paece PLATE 23. Aeromagnetic and generalized geologic map of FIGUBE 47. Index map_________ _. 136 the Copper River Basin area, Alaska. 48. Simplified aeromagnetic map. 141 24. Simple Bouguer gravity anomaly and gener­ 49. Three aeromagnetic profiles.. 149 alized geologic map of the Copper River 50. Upward continuation....... 151 Basin area, Alaska. TABLE TABLE 1. Densities of Copper River Basin rock units..______________________.______»________________.____.». 139 GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGIC INTERPRETATION OF MAGNETIC AND GRAVITY DATA IN THE COPPER RIVER BASIN, ALASKA By GORDON E. ANDREASEN, ARTHUR GRANTZ, ISIDORE ZIETZ, and DAVID F. BARNES ABSTRACT AREA COVERED Aeromagnetic and gravity surveys were made of approxi­ The Copper Eiver Basin is a broad topographic de­ mately 6,500 square miles of the Copper River Basin, Alaska. Magnetic data, compiled as a total-intensity contour map, show pression bounded by the Alaska Eange on the north, patterns that closely parallel the generally arcuate geologic the Wrangell Mountains on the east, the Chugaoh "grain" and seemingly can be correlated with lithology and Mountains on the south, and the Talkeetna Mountains geologic structure. Areas where volcanic rocks crop out are on the west (fig. 47). Wahrhaftig (1960) divided the indicated by a characteristic configuration of magnetic con­ Basin into two physiographic subdivisions the Lake tours. The magnetic data suggest that Lower Jurassic volcanic rocks exposed in the Talkeetna and northern Chugach Moun­ Louise Plateau on the west, entirely included witlin tains underlie sedimentary rocks in the southwest quadrant of the surveyed area, and Copper Eiver Valley on the ea^t, the surveyed area. The data also indicate that Tertiary and most of which is included in the surveyed area. The Quaternary lavas of the Wrangell Mountains occur at shallow north side of the surveyed area also includes almost all depths in the eastern part of the Copper River Basin in the of another physiographic division, the Gulkana Uplard, vicinity of Mount Drum. Alternate bands of high and low magnetic values characterize much of the northern third of the which comprises the southern foothills of the AlasVa surveyed area and are interpreted as products of the plutonic Eange. Fringes of four other physiographic divisions, rocks and metamorphosed volcanic and sedimentary rocks that the Talkeetna Mountains, the Matanuska Valley, the crop out at many places. Kenai-Chugach Mountains, and the Wrangell Moun­ Large negative Bouguer gravity anomalies were observed in tains, complete the area covered by the survey. two areas where the magnetic gradients are low and where the sedimentary rocks may be thick. One of these areas is near AEROMAGNETIC SURVEY Old Man Lake in the southwestern part of the surveyed area, and there the lowest gravity values were measured; the other The aeromagnetic survey covered approximately area occupies most of the southeast quadrant of the Copper 6,500 square miles and consisted of 75 north-south River Basin, where the gravity values decrease toward the traverses approximately 80 miles long and spaced 1 Wrangell Mountains. The largest Bouguer anomalies are asso­ mile apart. The flight lines extend north from the ciated with outcrops of lower Mesozoic volcanic rocks and Paleozoic basement rocks north and south of these gravity northern Chugach Mountains to lat 63°00' N". in the lows. The 60-milligaI difference between the low- and high- southern foothills of the Alaska Eange. The eastern­ gravity values probably represents, at least in part, an increased most line, at long 145°00' W., skirts the 2,600-fcot thickness of post-Lower Jurassic sedimentary rocks. contour of the western Wrangell Mountains; the west­ INTRODUCTION ernmost line, at long 147°20' W., crosses the eastern foothills of the Talkeetna Mountains. An aeromagnetic survey of the Copper Eiver Basin Aeromagnetic traverses were flown at a barometric was made by the U.S. Geological Survey in 1954 and flight elevation of 4,000 feet, except locally where 1955, and more than TOO gravity stations wore obtained topography required higher flight elevations. Contin­ in approximately the same area between 1958 and 1960. uous total-intensity magnetic data along flight traverses Both surveys supported geologic field mapping projects were obtained from a modified AN/ASQ-3A airborne in and adjacent to the Copper River Basin and were magnetometer from which a fluxgate-type detecting planned so that the geophysical data would assist the element was towed about 75 feet below the aircraft. geologic mapping and the interpretation of the sub­ Flight lines plotted on topographic maps at a scale of surface geology. 1:63,360 were used for flight guidance. The actual 135 136 GEOPHYSICAL FIELD INVESTIGATIONS 150° 148° 146" - 144° 142" Base from U.S. Geological Survey Alaska Map E, 1946 FIGURE 47. Index map showing location of the Copper River Basin area. Area of aeromagnetic survey stippled; area of gravity survey diagonally ruled. flight paths were recorded by a gyro-stabilized contin­ magnetic-field intensity in the 'Copper Elver region in­ uous-strip camera. These data were compiled and creases approximately 5 gammas per mile in a north­ published as a total-intensity magnetic-contour map easterly direction. (Andreasen and others, 1958). This map is essentially the same as the one presented in this report as plate 23. GRAVITY SURVEY The aeromagnetic-contour map has not been corrected Gravity data were obtained throughout the area of for the earth's normal magnetic gradient. According the aeromagnetic survey, but the Bouguer contour map to Vestine and others (1947), the earth's normal total does not cover the northern fifth of the aeromagnetic- MAGNETIC AND GRAVITY DATA, COPPER RIVER BASIN, ALASKA 1,37 survey area where the station density was too low to Chugach Mountains and the northern half of the permit contouring. The data were obtained with a Copper Eiver Basin, respectively. Between the geanti­ Worden gravimeter having a scale constant of 0.242 clines lies a belt of marine sedimentary rocks of Middle
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