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Late Cambrian Stratigraphy of the Heritage Range, Ellsworth Mountains: Implications for Basin Evolution

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Late Cambrian Stratigraphy of the Heritage Range, Ellsworth Mountains: Implications for Basin Evolution Antarctic Science 11 (1): 63-77 (1999) Late Cambrian stratigraphy of the Heritage Range, Ellsworth Mountains: implications for basin evolution MICHAEL L. CURTIS and SIMON A. LOMAS British Antarctic Survey, Natural Environmental Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK E-mail: [email protected] Abstract: Deposition of the Upper Cambrian succession of the Ellsworth Mountains was influenced by major, episodic tectonically-driven changes to the depositional basin geometry. We subdivide the succession into four stratigraphical sequences based on the recognition of three sequence-bounding unconformities. The upper part of Sequence 1 is composed of the laterally equivalent Liberty Hills, Springer Peak and Frazier Ridge formations, a siliciclastic fluvial to marine deltaic association displaying NW-directed palaeocurrents. A switch in the position of the Late Cambrian depocentre from the north-west to the south coincided with cessation of terrigenous clastic deposition and accumulation of Sequence 2, the limestones of the Minaret Formation. Previously unreported talus breccias from the Independence Hills provide important clues to basin configuration at this time. A brief period of emergence of the Minaret Formation is inferred, prior to rapid subsidence and disconformable deposition of Sequence 3 (the 'transition beds') in outer-inner shelf environments. Localized intra-basinal uplift occurred prior to the deposition of Sequence 4 (the lower Crashsite Group), the base of which is locally an erosive unconfonnity , with a correlative conformity exposed elsewhere. We interpret the Upper Cambrian succession as representing the 'rift-drift' transition from initial rifting (preceded by Middle Cambrian volcanism) to thermal subsidence along the South African sector of the palaeo-Pacific margin of Gondwana. Received 10 June 1998, accepted 9 November 1998 Key words: continental rifting, Ellsworth Mountains, Late Cambrian, sequence stratigraphy Introduction that theEllsworthMountains were situated in acontinental rift The Ellsworth Mountains lie along the northern periphery of setting during the Middle Cambrian (Curtis et al. in press) the Ellsworth-Whitmore mountains (EWM) crustal block In this paper, we address the issue of Late Cambrian (Fig. I), a displaced terrane generally accepted to have tectonics and evolution of the Ellsworth Mountains by originated from a position adjacent to the southern African / examining stratigraphic relationships and broad-scale facies Weddell Sea sector of the palaeo-Pacific margin of Gondwana distributions in the Upper Cambrian successions exposed (Clarkson & Brook 1977, Watts & Bramall 1981, Grunow throughout the Heritage Range. Observations made from et al. 1991, Dalziel & Grunow 1992, Goldstrand et al. 1994, such a large geographical area have allowed us to divide the Curtis &Storey 1996, Dalziell997). This margin was the site stratigraphy into four sedimentary sequences, based on the of prolonged, complex tectonic activity from end- identification of three sequence-bounding unconformities. Neoproterozoic to Early Ordovician times (Stump 1995). Previously unreported carbonate talus breccia deposits in the Tectonic style varied both spatially and temporally during this Minaret Formation of the Independence Hills are also period, with regions of subduction-related compressional described. We discuss the sedimentary sequences and deformation coeval with carbonate-platform sedimentation sequence-bounding unconformities in the light of recent end- andextensionaltectonics (Stump 1995). This complex tectonic Cambrian tectonic interpretations. period is represented in the Ellsworth Mountains by approximately 7.5 km of Cambrian age siliciclastic, Post-Cambrian regional geological events volcaniclastic and volcanic rocks of the Heritage Group (Webers et al. 1992a) (Fig. 2). The nature of the tectonic Tectonic quiescence and gentle subsidence followed the regime during deposition of the Heritage Group is probably Cambrian period, with deposition of the Crashsite Group the most contentious geological aspect of the mountains. (Upper Cambrian to Devonian), a 3-km thick succession of Tectonic interpretations vary from an active continental arc relatively mature siliciclastic rocks of predominant shallow (Duebendorfer & Rees 1998), or back-arc basin setting (Curtis marine origin (Sporli 1992). The Crashsite Group is & Storey 1996), to an intracontinental rift environment conformably overlain by the Whiteout Conglomerate, a 1000 (Vennum etal. 1992). New geochemical and isotopic analyses m thick glacial diamictite succession deposited during the of volcanic and subvolcanic rocks exposed throughout the Permo-Carboniferous glaciation of Gondwana. The youngest Heritage Range support the Vennum etal. (1992) suggestion formation in the Ellsworth Mountains, the Middle to Upper 63 Downloaded from https://www.cambridge.org/core. Open University Library, on 18 Jan 2020 at 12:33:13, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0954102099000103 64 M.L. CURTIS & S.A. LOMAS Fig. 1. Simplified geology map of the Heritage Range, Ellsworth Mountains. 0= Springer Peak, 0 = Bingham Peak. 0= Yochelson Ridge, @ = Soholt Peaks, 0 = Mount Rosenthal, 8 = Moulder Peak, 8 = Wilson Nunataks, @ = Patriot Hills, (3 = Mount Geissel. L.P.S. = Landmark Peak syncline. Inset map shows the location of the Ellsworth Mountains in Antarctica. Permian Polarstar Formation, has a dissected or transitional breccia bodies are predominantly exposed in the Independence arc provenance (Collinson etal. 1994). Inferred Andean-style and Marble hills, of the southern Heritage Range. convergence occurred along the palaeo-Pacific margin of Gondwana (Smellie 1981, Johnson 1991, Collinson et al. Stratigraphy 1994)followeddeposition ofthe PolarstarFormation,resulting in deformation of the entireEllsworthMountains stratigraphical TheLower Palaeozoic succession of the Ellsworth Mountains succession (Sporli & Craddock 1992a)in a dextral transpressive comprises the Heritage Group (Lower (?) / Middle to Uppcr regime (Curtis 1997, 1998a). This orogenic event, referred to Cambrian) and the Upper Cambrian to Devonian Crashsite on a Gondwana scale as the Gondwanian orogeny, imparted a Group (Fig. 2) (Webers et aL 1992a, 1992b). The Minarct well-developed NNW-SSE structural grain to the Ellsworth Formation, which forms the focus of this study, has been Mountains. The presence of two sets of contractional structures regarded as the uppermost unit of the Heritage Group. in the Middle Cambrian Springer Peak Formation has been presented as evidence for a Late Cambrian orogenic event in Heritage Group the Ellsworth Mountains (Duebendorfer & Rees 1998). However, Curtis (1998b) has identified two sets of structures The Heritage Group crops out almost exclusively in the in rocks from Cambrian to Permian in age, casting doubt on an Heritage Range, and is composed of eight, predominantly early Palaeozoic event and suggesting that the Gondwanian siliciclastic, sedimentary and volcanic formations. For orogeny was a polyphase event. convenience we have split the group into the lower Heritagc During the final stages of Gondwanian deformation, Group, formed of the four lowest, lithologically diverse structureless and stratified post-cleavagebreccia bodies formed formations, and the upper Heritage Group that contains threc in the carbonate lithologies of the Minaret Formation, due to laterally equivalent formations, plus the overlying Minarct cave-like dissolution processes and contemporaneous low- Formation. temperature hydrothermal activity (Sporli etal. 1992). These Downloaded from https://www.cambridge.org/core. Open University Library, on 18 Jan 2020 at 12:33:13, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0954102099000103 CAMBRIAN STRATIGRAPHY OF THE HERITAGE RANGE 65 The lower Heritage Group is composed of four formations: the Union Glacier, Hyde Glacier, Drake Icefall and ConglomerateRidge formations,which are restricted in outcrop to the Edson Hills, central Heritage Range. The Union Glacier Formation, ofpossibleEarly Cambrian age (Rees etal. 1998), consists of 3000 m of terrestrial volcaniclastic diamictites, which are locally overlain by laterally discontinuous fluvial to shallow marine deltaic deposits (up to 1700 m thick) of the Hyde Glacier Formation (Webers et al. 1992b). Black shales and limestones, which have yielded early Middle Cambrian trilobites (Jago & Webers 1992, Rees & Duebendorfer 1997), form the overlying Drake Icefall Formation, interpreted as recording shallow marine sedimentation in arestrictedeuxinic environment (Webers et al. 1992b). Uplift and emergence followed with deposition of laterally discontinuous fluvial to shallow marine polymict conglomerates of the Conglomerate Ridge Formation. Despite the presence of tectonic contacts between these formations, all the radiometric and palaeontological data are consistent with them being in stratigraphical order (Curtis 1998a). The upper Heritage Group is composed of three laterally equivalent formations: SpringerPeak, Liberty Hills, and Frazier Ridge, overlain for themost part by the laterally discontinuous Minaret Formation. Together, these formations form a significant proportion of the rock exposure in the Heritage Range. The Liberty Hills Formation (LHF) is exposed only in Fig. 2. Stratigraphical column for the Ellsworth Mountains the
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