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© Terra Antartica Publication 2008-2009 Terra Antartica © 2008-2009,Terra 15(2), 239-253Antartica Publication 2008-2009 Geology of the Nilsen Plateau, Queen Maud Mountains, Transantarctic Mountains W.E. LONG1, D. MCLellAND2, J.W. COllINSON3 & D.H. EllIOT3 1Matanuska-Susitna College, University of Alaska Anchorage, Palmer, AK 99645 - USA 2 Present address unknown 3School of Earth Sciences and Byrd Polar Research Center, The Ohio State University, Columbus, Ohio 43210 - USA *Corresponding author ([email protected]) Abstract - The western escarpment of the Nilsen Plateau, Queen Maud Mountains, Antarctica, consists of basement igneous and low-grade metasedimentary and metavolcanic rocks overlain by a near-horizontal Permian-Triassic sedimentary sequence intruded by Jurassic diabase sills. The basement comprises metasedimentary rocks of Neoproterozoic to earliest Cambrian age and Lower Cambrian metavolcanic rocks intruded by Cambrian to Lower Ordovician granitoids. An erosion surface cut across the basement is overlain by 600+ m of strata comprising Lower Permian glacial beds (Scott Glacier Formation) and overlying post-glacial shales (Mackellar Formation) that are succeeded by fluvial-deltaic beds of the Fairchild Formation and the Upper Permian fluvial, coal-bearing Queen Maud Formation. Some of the glacial beds and the post-glacial shales were deposited in a large fresh to brackish water sea with probable marine connections. Deposition initially filled topographic lows on the basement surface. Increased subsidence and a reversal in paleoslope occurred during the deposition of the Queen Maud Formation as the region developed into a foreland basin resulting from tectonic activity along the Panthalassan margin of West Antarctica. The Queen Maud Formation is overlain disconformably by non-carbonaceous, fluvial beds of the Fremouw Formation. Silicified wood, probably Permian Glossopteris, in the lower part of the formation, and a report of Triassic vertebrate fossils, suggests that the Permian-Triassic boundary is within the lower Fremouw Formation. Ferrar diabase sills, cumulatively as much as 500 m in thickness, were intruded into the sedimentary sequence in the Jurassic. Basement rocks are cut by pre-Permian faults and the whole succession is displaced by post-Jurassic, probably Cenozoic, faults. INTRODUCTION This paper is based on fieldwork conducted by William E. Long and Douglas McLelland in the 1963-64 field season at the Nilsen Plateau in the Queen Maud Mountains of the Transantarctic Mountains (Fig. 1). A geological sketch map was compiled by Long and McLelland (1964). Subsequently a draft manuscript on the Permian-Triassic Beacon strata was prepared by Long (1965), and the basement geology was described in McLelland’s MS thesis (1967). Both the draft manuscript and the geological sketch map are lodged in The Ohio State University’s Knowledge Bank. There has been no comprehensive survey of the Nilsen Plateau since that time, and therefore this information fills a gap in the regional geology between the Scott Glacier and Shackleton Glacier regions. Like much of the Transantarctic Mountains south of the Byrd Glacier, the Nilsen Plateau exposes a pre-Devonian basement overlain by Permian-Triassic sedimentary strata that have been intruded by Jurassic sills (Fig. 2). Fig. 1 - Location map for the Nilsen Plateau, Transantarctic Mountains. CTM: central Transantarctic Mountains. NVL: north Victoria Land. SVL: south Victoria Land. ©240 Terra Antartica PublicationLong et al. 2008-2009 METAVOLCANIC ROCKS Metavolcanic rocks crop out principally in the central part of the map area around “Cougar Canyon” but also at the head of the Blackwall Glacier and at Crack Bluff (Fig. 3A, 3B). The rocks have a relict porphyritic texture with phenocrysts of quartz and feldspar in a very fine-grained matrix. They have been strongly altered and metamorphosed to the greenschist facies, but they were probably rhyodacite to quartz latite in composition. The rocks exhibit deformational textures and have acquired a NW-SE foliation. Metavolcanic rocks, locally, have an igneous (chilled, i.e. intrusive) contact with the quartzose sandstone unit. The volcanic rocks are intruded by the North quartz monzonite. Compositionally they correlate with the Wyatt Formation in the Scott Glacier region (Minshew, 1966, 1967). Fig. 2 - View to the northeast of Lindstrøm Peak (section 2, Fig. 3A). Photograph annotated to show rock units (symbols as in PLUTONIC ROCKS Fig. 3). (Photo: D.H. Elliot). Several plutonic bodies are recognized and BASEMENT ROCKS differentiated. Most constitute parts of a composite batholith, which is well exposed in the central and McLelland (1967) provided descriptions of the southern map area. various basement rock units. His information is summarized first. Subsequent investigations at the North quartz monzonite Nilsen Plateau by Stump (1985) and Encarnación & These plutons or stocks, which crop out in the Grunow (1996) are discussed in a later section along vicinity of Blackwall Glacier, are porphyritic with a with correlation of the units in this general region. medium- to coarse-grained groundmass. K-feldspar phenocrysts are set in a groundmass of quartz, METASEDIMENTARY ROCKS plagioclase, biotite and accessory minerals. Rapakivi A greywacke-shale/slate sequence comprises most texture is present sporadically. Large angular blocks of the basement rock in the northwestern part of the of country rock and mafic inclusions are present. The map area (Fig. 3A) in the vicinity of the Blackwall rocks lack foliation and show only minor flow banding, Glacier. Individual greywacke beds are 0.3-3 m and on this basis are regarded as post-tectonic and (1-9 ft) thick whereas the fine-grained beds are 15 late in the intrusive history. cm (< 6 in) thick. The measured thickness is about 300 m (1,000 ft), but the true stratigraphic thickness Lonely Ridge granodiorite is probably greater given the outcrop distribution and Rocks assigned to this unit crop out at “Lonely the asymmetric folding. Fold axes trend north to Ridge” and adjacent to “Roaring Valley”. They are north-northwest and are approximately horizontal. medium-grained granodiorites comprising quartz, The greywacke-shale lithology occurs as a small plagioclase and microcline together with biotite and outcrop at the southwestern end of “Lonely Ridge” other accessory minerals. The granodiorite shows where it also is present as rafts in the Lonely Ridge cataclastic textures, and locally where the deformation granodiorite. is more intense exhibits a gneissic texture and locally The minor component of the sedimentary strata passes into mylonitic zones, which are up to 10 m is formed of a single outcrop, near the mouth (30 ft) wide. The foliation and mylonite fabrics are of the Blackwall Glacier, of a metasandstone- oriented vertically and trend 040±10°, and small metasiltstone-slate sequence about 60 m (180 ft) mafic inclusions are similarly aligned. thick. The sandstones are quartzose and exhibit cross The Lonely Ridge granodiorite is in fault contact stratification. The sequence clearly differs from the along its northern margin with the Cougar Canyon alternating greywackes and shales, but its relationship quartz monzonite, whereas along its southern margin to those rocks is unknown. it is intruded by apophyses of the South leuco quartz Lithologically the greywacke-shale sequence has monzonite (see below). These apophyses are generally been correlated with the La Gorce Formation in the oriented parallel to the foliation in the Lonely Ridge Scott Glacier region (Minshew, 1966) and the Goldie granodiorite, and large blocks and slabs of the latter Formation in the Beardmore Glacier region (Grindley, are incorporated in the South leuco quartz monzonite 1963). itself. These fabrics and the parallel orientation of © Terra GeologyAntartica of the Nilsen Plateau, PublicationQueen Maud Mountains, Transantarctic 2008-2009 Mountains 241 A B Fig. 3 - Geologic sketch maps of the western escarpment of the Nilsen Plateau. A. Northwestern outcrops. B. Southeastern outcrops. The two maps overlap fractionally. Slightly modified from geologic map of Long and McLelland (1964), and with basement age assignments updated. Names in parentheses are unofficial. The original map names “Black Rock Glacier” and “South Ridge” officially are now, respectively, Blackwall Glacier and Crack Bluff. ©242 Terra Antartica PublicationLong et al. 2008-2009 the metasedimentary rocks suggest the younger North quartz monzonite lacks deformational fabrics South leuco quartz monzonite was intruded along the and hence has been interpreted as post-tectonic. southern contact of the Lonely Ridge granodiorite. McLelland (1967) reported a Neoproterozoic Rb-Sr and an Ordovician K-Ar date for biotite from the Lonely South leuco quartz monzonite Ridge granodiorite From “Windy Ridge” southward, the plutonic rocks More recent field and geochronologic studies on are all assigned to the South leuco quartz monzonite. basement units from the Scott Glacier and elsewhere It is sparsely porphyritic with phenocrysts of K-feldspar have refined the correlations and age assignments. set in a medium- to coarse- grained groundmass Stump (1985) presented additional detail on the of feldspars, quartz, and accessory minerals. The stratified basement rocks at Nilsen Plateau, as well margin of the pluton contains xenoliths of Lonely as data on those units from other locations in this Ridge granodiorite and metasedimentary rocks. sector of the Transantarctic Mountains (see also The pluton intrudes
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