Journal ofthe Geological Society, London, Vol. 152, 1995, pp. 469-480, 10 figs. 1 table Printed in Northern Ireland Late Jurassic palaeogeography and anaerobic-dysaerobic sedimentation in the northern Antarctic Peninsula region D. PIRRIE' & J.A.CRAME' 'Camborne School of Mines, University of Exeter, Redruth, Cornwall TRI5 3SE, UK 2British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK Abstract: Late Jurassic anaerobic-dysaerobic mudstones crop out on both the Weddell Sea (back-arc) and Pacific (fore-arc) margins of the northern Antarctic Peninsula. The only known occurrence on the Pacificmargin of theAntarctic Peninsula is the Anchorage Formation of LivingstonIsland. This mudstone dominated unit comprises interbedded volcaniclastic sandstones, pyroclastic/epiclastic tuffs and radiolarian mudstones. The volcaniclastic sandstones are interpreted as representing deposition fromturbidity currents. The tuffsrepresent sedimentation by both primary airfall processes and resedimentation by low concentration turbidity currents. The radiolarian mudstones represent suspen- sion sedimentation, and reveal an upward increase in bioturbation with a transition from anaerobic- dysaerobic conditions to dysaerobic-aerobic conditions. These facies and the observed vertical change in oxygenation conditionsare similar to those seen in the Nordenskjold Formation on the Weddell Sea margin of the Antarctic Peninsula. However, biostratigraphical investigations show that the transition from dysaerobic to aerobic conditions occurred during the late Kimmeridgian-early Tithonian in the Anchorage Formation but late Tithonian or early Berriasian in the Nordenskjold Formation. This diachroneity is related to the palaeogeographical development of the Antarctic Peninsula magmatic arc. A wide epicontinental sea and subdued arc relief in the early Kimmeridgian was followed in the Tithonian by arc uplift,increasing oxygenation in the fore-arc basin, and the development of a restricted basin in the hack-arc region. In latest Tithonian-earliest Berriasian times a substantial arc had developed which supplied volcaniclastic sediment to the fore-arc basin; only then was the back-arc basin undergoing the transition from dysaerobic to aerobic conditions. Anaerobic conditions initiated by regional upwelling and expansion of the oxygen minimum zone were perpetuated in a silled basin in the back-arc area, formed by the emergent arc. Keywords: Jurassic, mudstone, anaerobic environment, Antartic Peninsula, palaeogeography. In recent years the presence of Late Jurassic mudstones to beassociated with the regionalpalaeogeography and from the eastern coastline of the Antarctic Peninsula region evolution of the Antarctic Peninsula magmatic arc, and two hasbeen well documented (Farquharson 1982, 1983a; main interpretations have been proposed: (a) that regional Whitham & Doyle 1989; Doyle & Whitham 1991; Whitham anoxiawas linked tothe development of anexpanded 1993). These mudstones are thought to be part of a regional oxygenminimum zone(Farquharson 1983a), or that(b) Late Jurassic anaerobic sequence known from both onshore anoxia was due to restricted circulation and water stagnation and offshorerecords throughout theWeddell Sea-proto withina barred basin(Doyle & Whitham 1991; Whitham South Atlantic region (Farquharson 1982; Stein et al. 1986; 1993). Recentstudies during the 1990/91 Antarctic field Doyle & Whitham1991). Anaerobic marine sediments of season onthe Anchorage Formation of LivingstonIsland Late Jurassic-Early Cretaceous agealong the eastern permitmore detailed documentation of LateJurassic margin of thenorthern Antarctic Peninsula have been anaerobic sediments from the western (i.e. Pacific) side of assigned tothe Nordenskjold Formation, a distinctive the magmatic arc. In this paper we aim to, (a) summarize sequence of interbeddedradiolarian mudstones and tuffs recentdevelopments in theunderstanding of thepalaeo- (Farquharson 1982, 1983a;Whitham & Doyle 1989). This geography of thenorthern Antarctic Peninsula,(b) formationhasbeen compared with similar onshore document in detail the sedimentology of Late Jurassic strata mudstonesequences on South Georgia(the Lower Tuff from Livingston Island (the Anchorage Formation), and (c) Member of theAnnenkov Island Formation; Pettigrew considerthe implications of thesenew data for both the 1981), Livingston Island (the Anchorage Formation; Crame regional palaeogeography and controls on the development et al. 1993), Low Island (Smellie 1980), and southern South of anoxia during the Late Jurassic inthis area. America (the Hardy and Zapata formations; Riccardi 1988) (Farquharson 1982; 1983a, -b) (Fig. 1).In addition, the NordenskjoldFormation is comparable to Late Jurassic Re@ona' setting strataencountered in DSDP/ODP sitesthroughout the The Mesozoichistory of theAntarctic Peninsula region was southernSouth Atlantic region (Farquharson 1983a;Doyle dominated by theeasterly subduction of proto-Pacific W hitham 1991). oceanic lithosphere beneath Gondwanabeneathlithosphere oceanic &1991). Whitham (Fig. 2). The The development of anoxicmarine conditions is thoughtassociated magmatic arc is represented by apredominantly 469 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/3/469/4890278/gsjgs.152.3.0469.pdf by guest on 30 September 2021 470 D. PIRRIE & J. A.CRAME Fig. 1. Locality map for the northern Antarctic Peninsula region. Outcrops of Nordenskjold Formation are indicated by black circles; Anchorage Formation by black star. CF, Cape Fairweather; SP, Sobral Peninsula; LG, Longing Gap; JRI, James Ross Island. calc-alkalinebatholith together with thicka extrusive Antarctic Peninsula Volcanic Gp (Farquharson 1984). The volcanic cover (the Antarctic Peninsula Volcanic Group). In age of the Botany Bay Gp is controversial, with estimates the northern Antarctic Peninsula region, older accretionary ranging fromthe Early Jurassic to the Early Cretaceous complexrocks of theTrinity Peninsula Groupare (Farquharson 1984; Millar et al. 1990; Rees 1993~).This age unconforrnablyoverlain by intra-arccoarse alluvial fan assignment ,ofis considerablepalaeogeographical sig- sedirnents,the Botany Bay Group, which arein turn nificance and is considered in more detail below. interbeddedwith, and overlain by,volcanic rocks of the LateJurassic and Early Cretaceous sedimentary rocks crop out on both the northwestern and southeastern flanks of theAntarctic Peninsula.Sedimentary rocks tothe southeastare thought to havebeen deposited in amajor ensialic Mesozoic-Tertiary back-arc basin (Macdonald et al. 1988), whilstMesozoic sediments onthe South Shetland Islands are usually considered to represent deposition within either a intra-arc or fore-arc basin (Smellie et al. 1980 Elliot 1983; Macdonald & Butterworth 1990). Within the back-arc basin, theoldest marine strata are assigned tothe Late Jurassic-EarlyCretaceous Nordenskjold Formation. The currentplate tectonic setting of the Scotia arc region is complex. The SouthShetland Islands are located on the Shetland microplate which is bounded to the northwest by theSouth Shetland Trench and tothe northeast and southwest by theShackleton and Hero fracturezones, respectively. Theplate is separatedfrom the Antarctic Peninsula by Bransfield Strait which is an active extensional Late Jurassic 160 Ma basin which opened 4Ma ago. Most workers assume that, I during the Late Jurassic and Cretaceous, the South Shetland Fig. 2. Late Jurassic plate tectonic reconstruction of Gondwana. Islands were adjacent to the present day west coast of the AP, Antarctic Peninsula crustal block. Modified from Lawver et al. AntarcticPeninsula (e.g. Elliot 1983, fig. 1). Plate-tectonic (1 992). reconstructions of the location of theAntarctic Peninsula Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/3/469/4890278/gsjgs.152.3.0469.pdf by guest on 30 September 2021 LA TE JURASSIC, ANTARCTICJURASSIC,PENINSULA LATE 47 1 crustal block within West Antarctica and the development characteristic features of the Nordenskjold Formation is the of theproto-southern South Atlantic and WeddellSea abundance of bothprimary, and possibly redeposited, regionsare complex andcontroversial (e.g. Lawver et al. volcanic ash layers. Most authors assume a local Antarctic 1992; Grunow 1993). However, in all the published Peninsula source for these ashes (e.g. Farquharson 1983a), reconstructions, the South Shetland Islands are located on although Storey & Alabaster (1991) suggested that they may the fore-arc side of the Antarctic Peninsula magmatic arc, berelated to extensionalmagmatism in the WeddellSea, with an enclosed or partially enclosed ocean in the Weddell rather than subduction-related magmatism along the Pacific Sea-protoSouth Atlantic region (Fig. 2).Palaeocurrent margin. data from the Byers Group onLivingston Island consistently suggestssediment supply fromthe southwest towards the Previous palaeogeographical models northeast. Two major palaeogeographical models for the development of anoxia within the Antarctic Peninsula region have been Previous studies on the Nordenskjold Formation proposed.Farquharson (1983a, b) andThomson et al. The Nordenskjold Formation (also known as the Ameghino (1983) consideredthat, during the Late Jurassic, the area Formation in Argentinian literature; see Whitham & Doyle currently occupied by the Antarctic Peninsula was covered 1989) was originally defined by Farquharson (1982, 1983~). by a shelf sea with anoxic bottom conditions. Anoxia