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SIMPLE BOUGUER GRA\7 L'fy and GENERALIZED GEOLOGIC MAP O'l 00 OJ 00 [TJ I I 0.. ::u 0 [T) 0.. z ~ 0_, ~ }> DEPARTMENT OF THE INTERIOR z 0 UNITED STATES GEOLOGICAL SURVEY 0 -l I [T] ::0 fJ) I OJ 0 PREPARED IN C08PERATION WITH THE c 0 c NATIONAL SCIENCE FOUNDATION [T] ::0 0 :::0 )> < ~ -< 7 0 SIMPLE BOUGUER GRA\ l'fY AND GENERALIZED GEOLOGIC MAP AND [T] 0 r AEROMAGNETIC PROFILES OF THE SCHMIDT HILLS QUADRANGLE AND 0 0 --< PART OF THE GAMBACORTA PEAK QUADRANGLE, ANTARCTICA }> [T) ::0 By 0 $ )> 0 John C. Behrendt9 William Rambo9 John R. Henderson, and z [11 Laurent Meister -l () '"0 :::0 0 'T1 r [T) GEOPHYSICAL INVESTIGATIONS fJ) fJ) MAP GP-889 () I $ 0 -l :c r r fJ) )> z 0 0 }> 3.': tD }> () 0 ::0_, )> '"0 [11 }> A )>z -l }> ::0 ()_, 0 )> - t\) Ul 0 0 0 0 3: )> PUBLISHED BY THE U.S. GEOLOGICAL SURVEY '1J WASHINGTON. D.C. 2024.4 0 "U I 1973 00 00 tD DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP GP-889 UNITED STATES GEOLOGICAL SURVEY SIMPLE BOUGUER GRAVITY AND GENERALIZED GEOLOGIC MAP AND AEROMAGNETIC PROFILES OF THE SCHMIDT HILLS QUADRANGLE AND PART OF THE GAMBACORTA PEAK QUADRANGLE, ANTARCTICA By John C. Behrendt, William Rambo, John R. Henderson, and Laurent Meister 1 INTRODUCTION to density increase in the upper ice shelf (Thiel and During the 1965-66 austral summer we made recon­ Behrendt, 1959); and absolute elevation accuracy was naissance gravity and magnetic surveys of the Pensacola estimated conservatively, at the reflection stations, as Mountains and the adjacent ice-covered area, Antarctica. ± 10 m. The reflection stations were tied, one by vertical The gravity survey consisted of 397 observations on angle and one by altimetry, to the Pensacola Mountains ice or rock and seismic reflection measurements of ice control net. The elevations at stations in the survey thkkness at 18 stations away from the mountains. were obtained from altimeter data corrected for tem­ Turbine helicopters were used and the work was coor­ perature and for barometric-pressure variations at a cen­ dinated with simultaneous geologic and topographic tral control station. In addition, 97 gravity stations were at work by the U.S. Geological Survey. Reconnaissance control points where vertical angle observations were aeromagnetic profiles were also flown perpendicular to made as part of the topographic mapping control. The the general northeast strike of the Pensacola Mountains. standard deviation of the unadjusted altimetry data at The objective of the geophysical surveys was to com­ the vertical angle stations is ± 12 m. All altimeter eleva­ plement the geologic mapping in an effort to under­ tions were adjusted by use of the 97 vertical angle stations stand the tectonic development of the Pensacola Moun­ as control. tains and their relationship to the transition from West Positions of gravity stations located at points in the to East Antarctica. Behrendt, Meister, and Henderson topographic control net are accurate to tenths of seconds; ( 1966) discussed preliminary results of the geophysical and stations away from the mountains where graphic work. solutions of astronomical observations were made are accurate to about a tenth of a minute. Therefore, lati­ GRAVITY SURVEY tude-correction errors are negligible. We used one Worden and two LaCoste and Romberg The largest source of error, and the most difficult to geodetic gravimeters. The Pensacola survey base station evaluate, is the terrain effect. Terrain corrections could at Camp Neptune (Schmidt Hills quadrangle) was tied not be made in the usual manner because of insufficient four times, over a period of several hours each, to the detail on the best maps ( 1 :250,000-scale, 200-m contour McMurdo base station (Behrendt and others, 1962). interval) available. A large unknown effect due to sub­ An additional check on the base station was a reoccu­ glacial terrain could not have been corrected for, even pation of one of the 1957 IGY (International Geophys­ if larger scale maps had been available. The corrections ical Year) traverse stations on bedrock in the Dufek could be as great as several tens of milligals in certain Massif (Behrendt, 1962). A difference of +0.9 mgal cases, but experience suggests they should be no more was observed when gravity at the base station was com­ than 10 mgal for most stations. We attempted to allow pared with that at the IGY traverse station; this differ­ for terrain effects in contouring the maps by assuming ence is acceptable considering that the older data were that all corrections for stations on rock would be posi­ all tied to North America by ship and land-surface tive and that complete Bouguer anomaly (unknown) vehicle (Behrendt and others, 1962) over a period of must be at least as positive as simple Bouguer anomaly. months and years. All gravity observations in this MAGNETIC SURVEY survey were tied to the Pensacola base station within a few hours, and errors in observed gravity are negligible. Total magnetic-intensity measurements were made Absolute elevations are considered accurate within along the flight paths indicated on the magnetic map by ±25 m (±5 mgal) and relative elevation within± 10m means of an Elsec Wisconsin proton precession magne­ (±2 mgal). Two seismic reflections from the water-ice tometer (Wold, 1964), flown in an LC-117 aircraft at a contact at the base of the Ronne Ice Shelf northwest constant barometric elevation of 2,100 m. Trimetrigon of the Dufek Massif allowed a determination of sea­ photography provided position control in the area of level elevation based on hydrostatic equilibrium. Cor­ the mountains. Dead reckoning and solar observations, rections were made for changes in seismic velocity due 1 Geophysical Services International, London, England. adjusted to the photographically identified indicated and Ford, 1969) of sandstone, siltstone, shale, and a points, were the only controls at the northwest and mudstone bed containing some tillite. The younger rocks southeast ends of the lines over the featureless ice sheet. of this sequence are of Permian age. D. L. Schmidt Consequently, position errors are variable but are mini­ ( unpub. data) considers these rocks and the Neptune mal near the control points. Some lines crossed no Group to be generally correlative in age and lithology identifiable rock outcrops and these have greater posi­ with Beacon strata. These rocks seem to be of lower tion errors. A reliable estimate of quantitative error is density than the Cambrian and Precambrian sedimentary difficult to obtain but probably errors at the ends of rocks (as discussed below). After the Permian rocks the lines are several kilometers. Diurnal control was were deposited, tectonic activity deformed th0 sedimen­ obtained from a base line connecting the profiles. No tary rocks into broad and locally tight to overturned flights were made during magnetic storms. folds. Because of the widely spaced flight paths, position A fourth period of structural deformation folded and uncertainties, and relatively low amplitude and areal faulted the rocks over a broad region. extent of the anomalies in this quadrangle, profiles rather than contours provide the most geologic in­ DISCUSSION OF GRAVITY MAP formation. Because the flight lines approximately The most apparent feature of the gravity map is the parallel the main magnetic field contours, no regional existence of anomalies of greater amplitude than the gradient was removed. probable errors caused by terrain effects. The Bouguer values decrease regionally from northwest to southeast SUMMARY OF GEOLOGY from+20 to -60 mgal. Local anomalies over the Neptune Schmidt and Ford ( 1969) have described the geology Range are as low as -70 and -90 mgal. The regional in this quadrangle. A composite stratigraphic section gradient is the result of crustal thickening crossing the for the Pensacola Mountains, from Schmidt and Ford transition from West to East Antarctica, as discussed by ( 1969) is included here. Behrendt, Meister, and Henderson ( 1966). The contours The oldest rocks are those of the Precambrian separating the Schmidt Hills from the Neptune Range Patuxent Formation; these compose most of the ex­ are too closely spaced to be caused by deep crustal posed rock. The Patuxent is a thick section (at least effects alone; the gradient here is about 4 mgal/km as 10 km) of terrigenous turbidite sedimentary rock, compared with the general regional gradient of about interbedded, in the Williams Hills area, with pillow 2 mgal/km. D. L. Schmidt (unpub. data) concludes lavas and basalt flows. Large amounts of diabase and that a fault separates the Schmidt Hills from the Neptune minor amounts of felsic magma were injected into the Range, as indicated on our map. This fault is probably sedimentary rocks. The diabase, in the form of sills, associated with the source of the steep gravity gradient, is found mostly in the Schmidt Hills and Williams Hills but that is also partly due to crustal thickening. The where Schmidt and Ford ( 1969) indicated thicknesses diabase sills emplaced in the Patuxent Formation in the of 2-150 m. They gave a ratio of diabase to sedimen­ Schmidt Hills no doubt have increased the mean density tary rock in the Schmidt Hills of 1:2. of the upthrown western side of the fault. Probably After deposition of the Patuxent Formation and the Patuxent rocks in the Schmidt Hills are representa­ the intrusion of diabase, the rocks were uplifted and, tive of a deeper part of the crust. Allowing for the in Early Cambrian time, eroded. During subsequent regional gradient, there seems to be about 20-mgal dif­ marine transgression, Cambrian limestones, felsic ference across the fault. The elevation is about the same flows and pyroclastic rocks. silt and mud beds, and on both sides of the fault. The diabase intrusions in the volcanic rocks were deposited.
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