Geology of the Northern Pensacola Ice Shelf. The age of the body is uncertain, but a post-Permian age is indicated by the metamorphic Mountains and Adjacent Areas effects on nearby carbonaceous, Glossopteris-bear- ing, sedimentary rocks. DWIGHT L. SCHMIDT and A. B. FORD Three areas outside of the Pensacola Mountains, U. S. Geological Survey not previously visited by man, were mapped. A group of nunataks, 120 kilometers (75 miles) south- The northern Pensacola Mountains are composed west of the Neptune Range, consists of intensely of folded sedimentary rocks and stratiform gabbroic folded, rhythmically interbedded graywacke and rock. About half of the northern Pensacolas con- slate (Patuxent Formation) of Precambrian age and sists of folded, interbedded graywacke and slate folded quartz sandstone that unconformably over- (Patuxent Formation) of Precambrian age that is lies the graywacke. The Mount Ferrara area, 95 unconformably overlain by folded limestone (Nelson kilometers (60 miles) east of the Forrestal Range, Limestone), volcanic rocks, and shale of Cambrian consists of folded Nelson Limestone containing age. This, in turn, is unconformably overlain by abundant arch acocyathids. The Mount Spann area, folded sandstone of middle Paleozoic age (Schmidt 120 kilometers (75 miles) northeast of the Forrestal et al., 1965). A gentle regional plunge to the north Range, consists of interbedded, metasedimentary results in prominent outcrops of pebbly mudstone quartzite and siltstone that probably underlies the (Gale Mudstone) of Permian (?) age and overlaying Cambrian Nelson Limestone and is probably much coal- and Glossopteris-bearing siltstone and shale of thicker than the estimated 1,000 meters (3,300 feet) Permian age. The Permian rocks in the northern of exposed section. This quartzitic formation is not Pensacolas were broadly folded at the same time that exposed in the Pensacola Mountains, and its relation the middle Paleozoic rocks were folded, but before to the Patuxent Formation is not known. the intrusion of stratiform gabbroic rock. The Gale Mudstone, a diamictite containing clasts of all the References older sedimentary and igneous rocks of the area, is Frakes, L. A., .1. L. Matthews, I. R. Neder, and J. C. Crowell. (in press). Movement directions in Late Paleo- a tillite occurring within an area 50 by 100 kilo- zoic glacial rocks of the Horlick and Pensacola Moun- meters (30 by 60 miles) (Frakes et al., in press). tains. Antarctica. Science. Stratiform gabbroic rock in the Dufek Massif and Schmidt, D. L., P. U Williams. W. H. Nelson, and J. R. F-e. 1965. Upper Precambrian and Paleozoic stratig. Forrestal Range of the northern Pensacola Moun- raphy and structure of the Neptune Range, Antarctica. tains has been mapped and sampled in detail. If U.S. Geological Surve y Prof. Paper 525-D, p. 112-119. the mafic rock of these two ranges is part of a single intrusion, as seems likely, it forms one of the largest stratiform bodies in the world; its exposed part underlies a minimum area of 8,000 square kilometers (3,000 square miles) and is a mini- mum of 2,000 meters (6,600 feet) thick. Wide- spread feldspathic pyroxenite, in layers I to 3 Geophysical Reconnaissance in meters (3 to 10 feet) thick, and less widespread the Pensacola Mountains magnetite layers and lenses lie with sharp contact on thick layers of anorthosite or leucogabbro. They JOHN C. BEHRENDT, JOHN R. HENDERSON grade upward into thick layers of gray gabbro. and Centimeter-scale, rhythmic layering is characteris- LAURENT J. MEISTER tic. Numerous channel-like structures, tens of Regional Geophysics Branch meters wide, are filled with layered pyroxenites, iron U.S. Geological Survey oxides. anorthosite, and leucogabbro, with sharp Denver, Colorado basal contacts. The rock which fills the channels A seismic reflection, gravity, and aeromagnetic grades across an upper contact into normally inter- reconnaissance was made in the Pensacola Moun- layered gabbroic rock. These channels may have tains, Antarctica, during the 1965-1966 austral sum- been formed by currents of probably convective mer. Prominent ice streams between the Neptune origin. Neither the base nor the top of the strati- and Patuxent Ranges and east of the Forrestal Range form pile is exposed, but fine-grained, chilled phases overlie channels, probably of glacial origin, in the border locally contact-metamorphosed Paleozoic rock surface 2,000 meters (6,600 feet) below sea quartz sandstone. The layers dip about 0SE. The level. Seismic reflections show the Filchner Ice Shelf body has been uplifted along high-angle faults to be 1,250 meters (4,100 feet) thick near its south- bordering the southeastern margin of the Filchner ern margin.

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