Late Jurassic Palaeogeography and Anaerobic-Dysaerobic Sedimentation in the Northern Antarctic Peninsula Region

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 suspension 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

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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 AntarcticPeninsula 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-arcbasin (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 basinwhich 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 thelocation of theAntarctic Peninsula

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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 was It comprises a distinctive sequence of radiolarian mudstones believed to be dueto the development of anoxygen and tuffs. The formation is exposed at five localities on the minimumzone, possibly related to high planktonic easternside of thenorthern Antarctic Peninsula; Mount productivity (Farquharson 1983~). Anoxiacaused by Alexander on JoinvilleIsland, northernDundee Island, restricted circulation within a barred basin was rejected, as Longing Gap, Sobral Peninsula and Cape Fairweather (see Farquharson (1983~)considered that a continuous emergent Fig. 1)(Farquharson 1982, 1983~).In addition, large arcterrane didnot develop until theEarly Cretaceous; Nordenskjold Formation glide blocks with dimensions of up hence in the Late Jurassic no barrier to oceanic circulation to 800 m X 200 m were redeposited within Cretaceous deep existed. The absence of coarse clastic sediment and the wide marine clastic sediments of theGustav Group on James distribution of fine-grainedlithologies was interpretedas Ross Island(Ineson 1985). Theseglide blocks are of indicating thatthere was no appreciable Late Jurassic stratigraphicalsignificance as they yield the only known subaerial arc terrane. The source for the airfall ashes was palynoflora from the Nordenskjold Formation (Snape 1992). considered to bea series of small,submarine or locally Stratigraphical studies by Whitham & Doyle (1989) revised subaerialvolcanoes in theAntarctic Peninsula region. theNordenskjold Formation, with the recognition at the Farquharson (1982, 19836)proposed that there was an type locality at Longing Gap of two members; the Longing abrupt palaeoenvironmental change in the Early Cretaceous Member and the Ameghino Member. A third member, the with the uplift and emergence of the magmatic arc terrane, Larsen Member, wasdescribed from the Sobral Peninsula moreor less onthe site of thepresent dayAntarctic (Whitham & Doyle1989). Macrofossil studies suggested a Peninsula. Terrestrial intra-arc sediments of the Botany Bay Kimmeridgian-Tithonian age for the Longing Member and Group were considered to be of Early Cretaceous age and aTithonian-Berriasian age for the Ameghino Member. related to this period of rapid arc uplift. These ages are supported by recent palynological investiga- Morerecent workers have questioned this palaeogeog- tions (Riding et al. 1992; Snape 1992). raphical model (Macdonald et al. 1988; Whitham & Storey Palaeoecologicaland sedimentological studies onthe 1989; Doyle & Whitham1991; Rees 1993a). In particular, NordenskjoldFormation suggest that it representsessen- re-investigation of theBotany Bay Group and overlying tially pelagicsedimentation in basinal a to slope Antarctic PeninsulaVolcanic Gp hassuggested that they environmentunder anaerobic to dysaerobicconditions may in fact be pre-Early Cretaceous in age. Sm-Nd dating (Doyle & Whitham 1991; Whitham1993). The Longing of garnets from an andesitic sill within strata overlying the Member wasdeposited in basinala setting with pre- Botany Bay Gp, and from detrital garnets within the Botany dominantlyanaerobic conditions with rare intervals of Bay Gp, plot together on a single isochron, yielding an age dysaerobicconditions (Whitham 1993). Incontrast, the of 156 f 6 Ma(Millar et al. 1990). Thissuggests that overlying Ameghino Member represents pelagic sedimenta- deposition of theBotany Bay Gp couldnot have been tionand synsedimentary deformation within slopea younger than Late Jurassic (Millar et al. 1990). Palaeobota- environment, under dysaerobic conditions with a progressive nicalstudies by Rees (1993a, b) ledhim to concludethat upwardincrease in oxygenation level (Doyle & Whitham fossil floras from the Botany Bay Gp are of Early Jurassic 1991; Whitham 1993). The progressiveshallowing and age, based on the presence of the fern genus Goeppertella. increase in oxygenation,combined with evidence for Rees (1993~)argues that an integral landmass existed in the prolonged synsedimentary deformation (Whitham & Storey Antarctic Peninsula region from the Early Jurassic onwards. 1989), was interpreted to represent tectonic uplift along the Although this is controversial, other recent authors support margin of theAntarctic Peninsula. Upliftwas thought to the view of a subaerial landmass by at least the Late Jurassic havebeen caused by change a from extension to (Whitham & Storey 1989; Doyle & Whitham 1991). compressionwithin an overall strike slip tectonicregime With the emergence of a sizeable topographic barrier, a (Whitham & Storey 1989). barred basin model for anoxia within the region has been Preliminary geochemical and mineralogical data from the proposed (Macdonald et al. 1988; Doyle & Whitham 1991). Nordenskjold Formation have been presented in Macdonald Anoxia was inferred tobe controlled by bottomwater et al. (1988)and Scasso et al. (1991). Theformation has stagnationwithin a silled basin(Doyle & Whitham 1991). TOC (total organic carbon) values between 0.32 and 3.50% However,as originally discussed by Farquharson(1983a), (mean l.81Y0) with Type I1 kerogens.One of the the current geographical juxtaposition of pelagic mudstones

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of the Nordenskjold Formation adjacent to the coarseclastic Mixed Marine members of Smellie et al. (1980) into three sediments on the arc terrane is problematic; why was coarse new formations: the Anchorage, Devils Point and President clastic sediment not transported into the basin? Whitham & Beaches formations. These threeunits, along with the newly Storey (1989) describedprolonged synsedimentary strike- defined Chester Cone Formation (which is at least in part slipdeformation within the Nordenskjold Formation, and equivalent to the Volcanic Member of Smellie et al. 1980), Millar et al. (1990)proposed that it had been displaced were included within the Byers Group (Crame et al. 1993). tectonically into its current location from an original site of Only the Anchorage Formation of the Byers Group will be deposition well away from the volcanicarc. However, coarse considered further in this paper. clastic sediment supply to both sides of the arc terrane may The Anchorage Formation is equivalent to the Mudstone in facthave occurred. Poorly known sequences onthe Member of Smellie et al. (1980). It is best exposed in the southeastern flanks of the Antarctic Peninsula (e.g. Sobral stream gully, low cliff and foreshore exposures approxim- Peninsula, Fig. 1) suggest that coarse clastic sediments are ately 1.25 km NNW of Ocoa Point (Fig. 3). The formation is associated with more typical Nordenskjold Formation facies in fault contact with the President Beaches Formation and (Scasso & Del Valle1989). Inaddition, coarse-grained the Agglomerate Member of Smellie et al. (1980), hence its turbiditicsandstones occur within the Anchorage Forma- true upper and lower boundaries have not been observed. tion,which is inturn overlain bycoarse grained clastic The formationcan be subdivided into four distinct units, sediments in the Berriasian on Livingston Island (Crame et which are themselvesin fault contact with one another; a al. 1993). minimum thickness of 105m has been determined (Fig. 4). Farquharson's (1983~) originalhypothesis of Late Although in fault contact detailed mapping has enabled the Jurassic anoxia linked to regional upwelling and expansion truestratigraphical relationships between each unit to be of the oxygen minimum zone was based on the absence of described. The formation has yielded a bivalve, ammonite anemergent arc, forming atopographic barrier, and the and belemnite assemblage which suggests a Kimmeridgian- widespreaddistribution of anoxicmarine facies oneither Tithonianage(Crame 1984; Crame et al. 1993). side of the current arc terrane. However, the evidence for an Palynological samples from the Anchorage Formation have uplifted arc terrane by at least the Late Jurassic, and the all yielded barren residues. restriction of the majority of outcrops of anoxic sediments to Smellie et al. (1980 p. 58) described a sequence of 'deep a rear-arc location, suggest that a barred basin model might water flysch-like mudstonesand thingraded sandstones' be more appropriate. Nevertheless, it should be emphasized fromthe Anchorage Formation. Interbedded bioturbated that this would not take into account the presence of the mudstones and thin sandstones, and pyroclastic tuffs, were AnchorageFormation anaerobic sediments onthe Pacific recognized. Abundantbioturbation was thought to be margin of the northern Antarctic Peninsula. The latter unit indicative of marine shelf conditions,with the normally would appear to be the key to understanding the controls graded sandstones interpreted as storm sands (Smellie et al. on, and distributionof, anaerobic conditions during the Late 1980). However, as discussed below, there are no data to Jurassic in this region. support a shelf depositional setting.

The Anchorage Formation Sedimentology of the Anchorage Formation Late Jurassic to Early Cretaceous sedimentary rocks crop On the basis of field mapping and logging, along with the out extensively on Byers Peninsula, Livingston Island (Fig. detaileddescription of polishedhand specimens and thin 3). Previousstratigraphical studies are summarised by sections, three main facies are defined Facies l-ungraded Crame et al. (1993), who revised the former Mudstone and to normally graded thick bedded sandstones; Facies 2-ash

82O 37's

Intrusions. agglomerates h lavas, o h Fig. 3. Sketch geological mapof Byers Peninsula, Livingston Island. Modified Younger Byers Group after Crame er nf. (1993, fig. 1). The 'younger Byers Group' includes the AnchorageFormation 4 ,;g": Devils Point, President Beachesand Chester Cone formations.

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m I P.2223 A B 19

A 18

cm 25------.::in m 5 20 0 0- 4

k3 2 P.1671

Fig. 4. (a) Schematic sedimentary log Facies 3a for the Anchorage Formation. (b) 1 Detailed logs showing the facies present Facies 2 within individual units in the Anchorage Facies 1 0 Formation.

layers,and Facies 3-radiolarianmudstones (Fig. 4; middle of the bed, and the erosion and resedimentation of summarized in Table 1). These facies can be grouped into theintraformational I clasts, arguesagainst transport and two distinct facies associations (see below). Throughout this depositionfrom debris flows; insteadrapid suspension paper the term ash layer is used in a non-genetic sense to sedimentation from high density turbidity currents is more describebeds orlaminae composed of fine grained likely (cf. Facies B1.l of Pickering et al. 1986). The presence pyroclastic material, both primary and reworked. The facies of intraclastsindicates that the transporting flows were recognized here are comparable to those recently described erosive at some stage. Postma et al. (1988) suggested that, by Whitham (1993) from the Nordenskjold Formation. formud clasts to survivewithin turbidity currents, the original flow mustlose turbulance rapidly flowby Facies 1: ungraded to normally graded thick-bedded transformation. Large intraclasts would be unable to settle sandstones throughthe resultant high concentration low turbulance flow. Facies 1 comprisestypically ungraded, or rarelynormally graded, fine to coarse grained sandstones. Beds are 8-60 cm thick (typically 30-50 cm), usually structureless, with sharp Facies 2: ash layers planarupper and lower bed contacts. Most bedscontain Ash layers ranging from less than 1mm to 15 cm thick occur angularintra-formational clasts of interlaminatedun- throughoutthe Anchorage Formation. Frequently, in- bioturbatedradiolarian mudstone (Facies 3a) and ashes dividualbeds andlaminae are laterally persistentover a (Facies2) (Fig. 5). Commonly,intraformational clasts are scale of tens of metres.Three discrete subfacies are concentratedtowards the bed centre. In thinsection, the recognized: (a) normally graded, massiveash layers, ,(b) sandstonesare predominantly composed of basaltic to ungradedmassive ash layers, and (c) parallelor ripple andesitic lithic volcanic grains, showing seriate to porphyritic cross-laminated ash layers (Table 1). textures,along with sericitised, euhedraltosubhedral Normally graded, but otherwise structureless ash layers plagioclase. Minor, clear, straight extinction monocrystalline (Subfacies2a) arethe dominant type throughout the quartzgrains and rare undulose extinction polycrystalline formation. Individual beds and laminae range from 3 mm to quartz grains also occur. Wood fragments were present in 15 cm thick, althoughthey are typically 5-15 mm thick. twosamples. Small intraformational mudstone clasts Individualunits have sharp, commonlyloaded lower bed occasionallyhave otherdetrital grains embedded in their contactsandeither sharp, discrete or occasionally margins. gradational,upper bed contacts. Some laminae within the This facies represents the resedimentation of pyroclastic upper part of the formation are disrupted by bioturbation, or epiclastic detritusand resultant transportation and andvery thin laminae, when traced laterally, become deposition by sediment gravity flow processes. The apparent homogenized with bioturbated mudstones of Subfacies 3b. internal bed organisation with clasts concentrated within the Thicker ash layers have discreteChondrites burrows towards

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Table 1. Summary of facies in the Anchorage Formation

Composition & Facies Interpretationgrain Structure size Boundaries Thickness l-ungraded to Fine- to coarse- 8-60 cm Sharp planar bed Structureless other High density turbidity normally graded grained, lithic- (typically contacts. than normal currents. thick bedded volcanic feldspar- 30-50 cm) grading. sandstones rich sandstones. intraformational mudstone clasts are common. Za-normally graded Very coarse sand to 3 mm-l5 cm Sharp loaded lower Normal grading. Probably pelagic ash layers siltstone/claystone. (typically contacts. Sharp, suspension Lithic volcanic + 5-15 mm) planar or sedimentation of plagioclase grains. gradational upper airfall ash. contacts. Zh-ungraded ash Very coarse sand to 1 mm-l0 cm Sharp planar or Structureless. Either primary layers siltstone/claystone. loaded basal suspension Lithic volcanic + contacts, sharp sedimentation of plagioclase grains. planar or airfall ashes or gradational bed dilute turbidity tops. currents. Zc-parallel/ripple Medium sandstone- Up to12 cm Sharp, weakly Ungraded or Tab,T, and 'L cross laminated siltstone. Lithic erosional bed normally graded divisions of low ash layers volcanic + bases. Sharp or parallel concentration sandy plagioclase gradational bed laminated/ripple turhidites. grains. tops. cross laminated. 3a--essentially Organic-rich 0.5 mm to Upper bed contacts Weakly laminated. Pelagic suspension unhioturbated radiolarian >l0 cm sharp, basal Minor (level 1-2) sedimentation radiolarian mudstones. contacts sharp or hioturbation. under anaerobic- mudstones gradational. dysaerobic conditions. 3b-hioturbated Less organic-rich 1 cm to Sharp upper and Bioturbated with Pelagic suspension radiolarian radiolarian >60 cm lower bed distinctive wavy sedimentation mudstones mudstones. contacts. bedding. under dysaerobic- anaerobic conditions.

the bed top (Fig. 6a). Grain size ranges from very coarse normallygraded interval fining up into siltstone-claystone sandstone to siltstone/claystone,with medium to fine with abundantmatrix supported verycoarse sand to grained sandstones typically fining upto siltstones. The style pebble-sized pumice clasts which coarsen upwards (Fig. 6d). of gradingwithin individual laminae varies from almost Subfacies2b comprises ungraded massive ash layers perfectdistribution grading toabrupt grading only in the ranging from less than 1 mm to 10 mm thick. Individual upper part of the laminae (Fig. 6b). Some laminae comprise medium sand-sized ash layers 1-2 grains thick .also occur. separatecouplets of gradedsandstones and siltstone- Theungraded ash layers typicallyhave sharpplanar or claystonelaminae (Fig. 6c). One ashlayer shows a lower loaded basal contacts and sharp discrete or bioturbated bed tops.Within the upper part of theformation individual laminaeare in some casespartially homogenized by burrowing with the interbedded bioturbated mudstones of Subfacies 3b. Rare ungraded and normally graded ash layers displaying eitherparallel or occasionalripple cross lamination are assigned to Subfacies 2c. Individual beds are up to 12 cm thick, andrange from medium sandstone to siltstone in grain size. Bed bases are sharp, rarely weakly erosional, and have either sharp or gradational upper bed contacts. In the absence of bioturbation, most Facies 2 ash layers arelaterally persistent. However, in crosssection some laminae arebroken, having abruptterminations (see Fig. 6b). In addition, small scale reverse and normal microfaults occur,with minor displacement (mm-cm) usually only affecting 10 cm or less of vertical sediment thickness. In thinsection, the ashlayers are predominantly Fig. 5. Field photograph of Facies 1 sandstones (Section P.2223a) composed of basaltic-andesiticlithic volcanic grains and overlying interbedded unbioturbated radiolarian mudstones (Facies plagioclasefeldspar. The lithic volcanicgrains commonly 3a) and ash layers (Facies 2). Note concentration of havevery irregular grain outlines (modified during intraformational mudstone clasts towards bed centre (arrowed). diagenesis) and show a wide variety of textures from highly Divisions on scale bar are 10 cm. vesiculated to porphyritic. The phenocrysts arepre-

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Flg. 6. (a) Polished hand specimen photograph showing interbedded unbioturbated radiolarian mudstones (Facies 3a) and ash layers (Facies 2) with minor Chondrites type bioturbation at the top of the ash layers (arrowed). Scale bar = 2 cm, sample number P.2225.4. (b) Polished hand specimen photograph of interbedded unbioturbated radiolarian mudstones (Facies 3a) and normally graded ash layers (Facies 2a). Note that some ash layers have been disrupted into fragments by synsedimentary deformation (arrowed). Scale bar= 2 cm, sample number P.2223.36. (6c)Polished hand specimen photograph showing a distinctive siltstone-claystone ash layer (arrowed) overlain by a sandstone ash layer. Scale bar = 2 cm, sample number P.2225.1. (a) Polished hand specimen photograph showing a silstone-claystone ash layer with outsized clastsof pumice weakly coarsening upwards (Facies 2a). Scale bar= 2 cm, sample number P.2223.3.

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dominantly plagioclase, whilst the groundmass usually shows stones is placed at the recognition of level 3 bioturbation extensivediagenetic alteration and replacement by clays, (Droser & Bottjer 1986) within Subfacies 3b, (i.e. level 1-2 carbonates and zeolites. Opaque minerals are also common bioturbation can occur within Subfacies 3a). Macdonald et within the groundmass. Feldspar grains are usually euhedral al. (1988) reportTOC values of up to 1.0% for these to subhedral in shape and are sericitized. Angular pumice mudstones,although which subfacies was sampled is not grains can also be recognized, although they are oftentotally known. Subfacies 3a is considerably more organic rich than replaced by calcite. Minorframework grains include clear Subfacies 3b. straight extinction monocrystalline quartz and rare undulose Subfacies 3a comprises brown-black, 0.5 mm to >l0 cm extinction polycrystalline quartz. thickbeds of organic-richradiolarian mudstones (Fig. 7a) The characteristics of Subfacies 2a suggest deposition by whichlack pervasive bioturbation. Rarely, level 1-2 pelagic settling from suspension. The relatively coarse grain bioturbation(Droser & Bottjer 1986)with Chondrites is size of the ash layers indicates that they were not deposited observedat bed contacts with Facies 2. Radiolariaare by turbid plumes of sediment suspended by wave or current always present, varying in abundance from approximately 5 activity (Whitham1993). The absence of tractional to 20%. They are circular to oval in shape and are replaced bedforms,lack of erosivebedbases andlack of by either micro-crystalline quartzsparryor calcite. non-volcaniclastic detritus implies that they were probably Dispersedamorphous organic matter is common and not deposited by turbidity currents, although this cannot be partiallydefines weak lamination. Sand-sized wood frag- totally discounted.This subfacies is interpretedas ments, partially replaced by calcite, are also observed and suspensionsedimentation of pyroclastic airfall ashes (Ledbetter & Sparks 1979; Sigurdsson et al. 1980; Whitham 1993). The development of two distinct intervalsin subaqueous airfall deposits is typical of more proximal facies (Ninkovitch et al. 1978). The single graded ash layer with floating pumice clasts within a siltstone-claystone matrix is comparable to Facies lb of Whitham (1993). He interpreted this subfacies as indicating deposition from floating rafts of pumice following either subaqueous or explosive subaerial eruptions. The concentration of coarsepumice at the bed top is a function of the amount of time taken for the pumice to becomewaterlogged andeventually sink (Whitham & Sparks 1986). The ungradedash layers of Subfacies 2bcannot be assigned confidently to any single depositional mechanism, althoughthey also probably represent primary suspension sedimentation of pyroclastic airfall ashes through the water column.Most of the ungraded ash layers are thinner and finer grained, implying that they may represent more distal airfall deposits. If they do represent suspension settling of airfall ash, then the absence of normal grading may be: (a) a function of an initially limitedgrain size rangebeing deposited, or(b) insufficient waterdepth to allow differential settlingrates through the water column to generatenormal grading.Some of thethin fine-grained (very fine sandto claystone) ash layers mayhave been transportedin turbulent plumes of suspendedsediment within the water column (Thornton 1984; Whitham 1993). In addition,it is possible that this facieswas transported by dilutelow concentration turbidity currents, although no diagnostic features of turbidites are observed. In contrast, Subfacies 2c is interpreted to represent resedimentation of pyroclastic detritus byturbidity currents. The presence of erosional scours at bed bases andprimary parallel and ripple cross lamination suggests deposition of the TahrTh and T, divisions of low concentration sandy turbidites (Pickering et al. 1986).

Facies 3: radiolarian mudstones Brown-blackmudstones containing 5-20% recrystallized Fig. 7. (a) Thin section photomicrograph (plane polarized light) Radiolariaoccur throughout the Anchorage Formation. showing an unbioturbated radiolarian rnudstone of Facies 3a. Scale Two distinct subfaciesare recognized: (a) Subfacies3a, bar = l mm, sample number P.2223.9. (b) Polished hand specimen unbioturbated weakly laminated radiolarian mudstones, and photograph of bioturbated radiolarian rnudstone (Facies 3b) (b) Subfacies3b, bioturbatedradiolarian mudstones. The overlain by an ash layer (Facies 2). Scale bar = 2 cm, sample distinction between bioturbated and non-bioturbated mud- number P.2223.23.

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there are occasionalsmall oval pellets of radiolarian-poor mudstone.Gradational or sharpcontacts were observed with underlying ash layers although upper bed contacts are alwayssharp. Thin, millimetre-scale ash laminae load into the underlying mudstones, implying that seafloor substrate conditions were soft. Some mudstones directly overlying ash layersare Radiolaria-poor, but then show increasing proportions of Radiolaria vertically above the bed base. In some beds, very fine to very coarse sand-sized lithic volcanic and plagioclasegrains occur dispersed throughout the mudstone.Authigenic, usually framboidal, pyrite is very abundant.Macrofossils are rarelyobserved (Crame et al. 1993), although this may partially be due to the nature of the outcrop. Subfacies 3b comprises light brown to grey, bioturbated radiolarian mudstones. Bed thickness ranges from 1cm to >60cm, with distinctive wavy bedding defined by thin ash layers. Upperand lower bed contacts are sharp. The mudstonestypically have a bioturbatedfabric (Fig. 7b), althoughdiscrete Chondrites and Helminthoida are recog- nised. Thin ash layers are often partially homogenized and dispersed within the mudstones, giving an apparent poorly sortedtexture. Rare pumicegrains also occur scattered throughout.Bedding-parallel oval carbonate concretions occurwithin the mudstones; a pre-compactional originfor the concretions is suggested by the deflection of lamination within the mudstones around the concretions. In comparison withSubfacies 3a, the bioturbated mudstones have less abundantRadiolaria (typically less than 5%), andalso appearto have less pyrite. Asingle well-preserved leaf impressionwas observed on one bedding plane and small wood fragments also occur. Macrofossils collected from this facies include both pelagic (ammonites and belemnites) and Fig. 8. Thin section photomicrograph (plane polarized light) rare benthic (bivalve) organisms (Crame et al. 1993). Within showing vertical stylolite marked by concentration of opaques both subfacies,vertical andbedding-parallel dissolution within radiolarian mudstone. Scale bar = 0.5 mm, sample number seams or stylolites occur (Fig. 8). They are defined by the P.2223.9. concentration of opaques. Usually the extent of the vertical dissolutionseams is restricted by the thickness of the mudstonebed in whichthey are developed,with the Facies associations dissolution seam stopping at bed contacts with ash layers. The AnchorageFormation is divided into four distinct Beddingparallel dissolution seams commonly occur as lithostratigraphic units (Crame & Farquharson 1984; Crame anastamosing vein networks. et al. 1993), which can in turn be combined into two facies The radiolarianmudstones of Subfaciesare3a associations (see Fig. 4). Although the units occur only in interpretedto represent pelagicsettling from suspension faultcontact with one another, their stratigraphical (Pickering et al. 1986; Whitham 1993), based on the absence relationshipswith one another is based on detailed field of current related structures and the abundance of biogenic mapping. The basal unit, which is of unknown thickness, is silica. The restrictedbioturbation and absence of benthic seen onlyin intermittent exposures adjacent to asmall faunas suggests that deposition occurred under anaerobic to intrusion at locality P.1671 (Fig. 3). It comprises interbedded dysaerobicconditions (Savrda & Bottjer 1991; Doyle & thick, pale ash layers (Facies 2) and baked, unbioturbated Whitham 1991). Subfacies 3bis also interpreted to represent mudstones(Facies 3a), and has yielded poorly preserved pelagicsettling from suspension, but under dysaerobic to bivalves referable to the Retroceramus haasti (Hochstetter) aerobic oxygenation conditions, allowing benthic coloniza- group (Crame & Farquharson 1984). The second unit, which tion and consequent bioturbation (cf. Pickering et al. 1986; is at least 24 m thick, comprisesfinely interbedded ash layers Savrda & Bottjer 1991; Whitham 1993). A decrease in the (Facies2) andunbioturbated mudstones (Facies3a), sedimentation rate would also allow increased colonization interbedded with sandstones of Facies 1. This unit is fault if sufficient oxygen was available. The poor sorting seen in bounded and is overlain by at least 30 m of interbedded, some of the mudstones of Subfacies 3b is a function of the unbioturbated mudstones (Facies 3a) and ash layers (Facies intermixing of thinash layers and pelagicsuspension 2), witha mudstone:ash ratio of 3:2 (Crame et al. 1993). sedimentationmudstones. The decreasedabundance of These three units are combined herein into a single facies organic matter and authigenic pyrite within Subfacies 3b is association. Based purely on lithological considerations, this also consistent with increased oxygenation conditions, and facies association is similar to the Longing Member of the eitherdecreased primary organic matter production or Nordenskjold Formation (Whitham & Doyle 1989; Doyle & increaseddegradation of theorganic matter theat Whitham1991). Whitham (1993) interpreted the Longing sediment-water interface prior to burial. Member as representing a basinal association, with pelagic

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suspensionsedimentation under anaerobic to dysaerobic Discussion and regional implications conditions at water depths of between 500 and 1OOOm. The presence of unbioturbated pelagicmudstones, suspension Despitesome differences, thesedimentology of the sedimentation airfall ashes,and relatively rare reworked Anchorageand Nordenskjold formations is comparable. volcaniclastic turbidites within the lower facies associationin Similar facies occur in both,and each has lowera the AnchorageFormation is consistentwith a relatively anaerobic-dysaerobic unit overlain by sediments deposited deep marine depositional environment at least below storm under dysaerobic to aerobic conditions, although anoxia is wavebase, underanaerobic to dysaerobic oxygenation more pronounced in the Nordenskjold Formation. However, conditions. on the basis of biostratigraphical studies this facies change Thislower facies association is overlain by a was stronglydiachronous, with theonset of better- mudstone-dominated interval, at least 40 m thick, composed oxygenated bottom water conditions occurring earlier on the of interbeddedbioturbated radiolarian mudstones (Facies northwestern(Pacific-margin) flank of the peninsula. The 3b) and ash layers (Facies 2). The number and thickness of Anchorage Formation is the only documented anoxic Late ashlayers decreases upsection. The transitionfrom Jurassicsequence on the Pacific-margin of theAntarctic anaerobic-dysaerobicconditions in thelower facies Peninsulamagmatic arc. Farquharson(1983a) originally association to dysaerobic-aerobicconditions in theupper correlatedthe Nordenskjold Formation with mudrocks on faciesassociation is comparable tothe transition seen SouthGeorgia, Low Island and southern South America. betweenthe Longing and Ameghino members of the The units on South Georgia and southern South America NordenskjoldFormation (Doyle & Whitham1991). In can be shown to have been deposited in a retro-arc position addition, both the Ameghino Member and the upper facies (e.g. Macdonald et al. 1987). Onthe basis of recent associationin theAnchorage Formation have prominent reinvestigation by theauthors, the mudstones on Low wavy bedding. However, unlike the Ameghino Member, the Island,which are on the Pacific margin, are no longer upper faciesassociation doesnot show anincreased correlatedwith either the Anchorage or Nordenskjold abundance of other syn-sedimentary deformational features formations on either lithological or palaeontological grounds andreveals a decreased abundance of airfall ashesrather (D. Pirrie and J.A. Crame unpublished field notes). than an increase. The upper facies association is dominated Even though systematic palaeontological studies of both by suspensionsedimentation mudstones and airfall ashes. the Anchorage and Nordenskjold formations have yet to be The decreasing number and thickness of airfall ashes either completed, it is possible to makea number of biostratig- representschangea to more distal airfall deposits,a raphical correlations between the two units (Fig. 9). Poorly decrease in the scale of the eruption columns, or, possibly, a preserved specimens of the Retroceramus haasti group from change in theatmospheric wind circulation patterns (cf. unit 1 of theAnchorage Formation can be matched with Ninkovitch et al. 1978; Sigurdsson et al. 1980).Deposition manyloose specimens obtained from clasts of Longing wasbelow storm wavebase in lowa energy marine Member lithologies within Early Cretaceous conglomerates dysaerobic to aerobic environment. on James Ross Island (Crame 1982; Whitham & Doyle 1989,

Byers Penmsula NE Antarctic Peninsula T L m

l berriasellids

m 100- Fig. 9. Biostratigraphical correlation between the Anchorage and Nordenskjold formations. Biostratig- 0- raphical ranges for the Nordenskjold Formation are based on Whitham & Doyle (1989).

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p. 382). Such retroceramids do not occur in-situ at Longing Gap, but are assumed to come from a stratigraphicalBerriasian level larly q? C beneath the lowest exposure (Fig. 9). The next macrofossil assemblagefrom theAnchorage Formation comes from approximatelythe uppermost 11 m of unit 4 (i.e. Facies Association 2). This association has yielded a sparse fauna of finely ribbedperisphinctid ammonites with affinities to coarse-grained-aerobicdysaerobicsignificant genera such as Subplanites and Pectinatites. Thesesedimentation resemble uplift arcvolcaniclastic perisphinctidtaxa occurring (back-arc)between approximately 80- sedimentalion 122m in the Longing Memberat its type locality (Fig. 9; (fore-arc) Whitham & Doyle1989; Doyle & Whitham1991). The latter forms are frequently encrustedTithonian by oxytomid bivalves, early B at least some of which can be referred to Arctotis australis Crame (1985). Theseencrusters are not seen on the Anchorage Formation specimens, but it is interesting to note that twolarge (i.e probably fully adult) specimens of A. anaerobicdysaerobic-aerobic arcprominent australis (one of which is the holotype) were collected from Sedimentationsedimentation terrane this level in the Anchorage Formation (Crame 1985). These (back-arc)(lore-arc) uplifted correlationsare strengthened by theoccurrence of small, continuous barrier smoothhaploceratid ammonites in theuppermost Ancho-

rage Formation and the 122-150 mlevel at Longing Gap earlyKimmeridgian (Fig. 9). It couldbe argued thatthe Anchorage Formation Anchorage e Nordenskjald A Formation Formation Formation v perisphinctids correlate with forms known from higherlevels of the Longing Member (Fig. 9). Nevertheless, it is apparent that the latter occur in close association with bivalves such as Retroceramus everesti, Buchia (=Australobuchia) sp., Aulacomyella willeyi Kelly and Anopaea sp. ? (Whitham & anaerobicrelief anaerobic subduedarc sedimentaticnvolcanismminorarc sedimentation Doyle 1989; Doyle & Whitham 1991; Kelly & Doyle 1991). (tore-arc) (back-arc) (tore-arc) None of thesetaxa is known fromthe Anchorage W E Formation. The AmeghinoMember, which lithologically is com- Fig. 10. Simplified cartoon geological evolution of the northern parable withFacies Association 2 of theAnchorage Antarctic Peninsula during the Kimmeridgian to Berriasian. Not to Formation, is characterised initially by a distinctive scale. berriasellid-Bochianites-'Lytoceras' ammonite assemblage, rather than a continuous land barrier.By approximately late andsubsequently by olcostephanidsreferable to Spiticeras Kimmeridgian-earlyTithonian times, theemergence of a (Spiticeras) (Whitham & Doyle 1989). On Byers Peninsula, substantial arc terrane led to diachroneity between the fore- suchammonites are much more typical of the succeeding and back-arc regions (Fig. lob). In the fore-arc basin, the DevilsPoint andPresident Beaches formations (Fig. 3; upper facies association of theAnchorage Formation Crame et al. 1993);such acorrelation implies thatthe heraldedthe onset of better-oxygenatedbottom water Ameghino Member must be laterally equivalent to at least conditions, whilst true anaerobic conditions prevailed in the part of the thick sedimentary sequence which post-dated the back-arc basin. By latest Tithonian-earliest Berriasian times phase of anaerobic sedimentation on Byers Peninsula. asubstantial arc edifice waspresent with coarse-grained Whereasthe transition between parallel-bedded, un- volcaniclasticsediment supply intothe ByersPeninsula bioturbated mudstones and tuffs (Facies Association 1) and sedimentary basin (Fig. 10c). However, in the James Ross the overlying wavy bedded bioturbated mudstones and tuffs Basin the change from anaerobic-dysaerobic to dysaerobic- (Facies Association 2) within the Anchorage Formation can aerobic sedimentation was only just occurring. be datedbe approximatelyas lateKimmeridgian-early Themodel presented here is consistent with late Tithonian,that between the Longing andAmeghino Oxfordian-early Kimmeridgian anoxia due to upwelling and members may beas young aslate Tithonian or early expansion of theoxygen minimum zone in thenorthern Berriasian (Fig. 9). It is possible thatthe diachroneity Antarctic Peninsula region. It supports the initial hypothesis between the western and easternflanks of the peninsula may of Farquharson (1983a),established largely on geological spanthegreater part of theTithonian stage (i.e. grounds, and theoretical predictions of regional upwelling in approximately 7 millionyears). Such aconclusion is of theLate Jurassic.However, as arc uplift began,anoxia considerable importance as it suggests that the Anchorage ceased within the fore-arc with the uplift of the basin in the Formationcannot be an exotic block,displaced froman ByersPeninsula region above the oxygenminimum zone. original back-arc setting. Upon the establishment of a substantial topographic barrier, It is envisaged thatin approximately late Oxfordian- the back-arcregion effectively becameabarred basin. early Kimmeridgian times a widespread epicontinental sea Restrictedcirculation within this basin undoubtedly with anoxic bottom conditions covered muchof the northern prolonged anoxic marine conditions. AntarcticPeninsula region (Fig. 10a). It is assumedthat there was a low emergent volcanic arc present at this time, We are grateful to the officers and crew of the RRS John Biscoe for but that it probably represented a discontinuous archipelago logistical support during the 1991 Antarctic field season. We would

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also like to acknowledge the contributions made by BAS geologists MILLAR,I.L., MILNE,A.J. & WHITHAM,A.G. 1990. Implications of Sm-Nd overa number of yearstowards developing ourunderstanding of garnet ages for the stratigraphy of northern Graham Land, Antarctica. ByersPeninsula geology. The manuscriptwas improved by the Zentralblatt fir Geologie und Palaontologie, 1,97-104. constructivecomments of A. G. Whithamand B.Wignall. L. NINKOVITCH,D., SPARKS,R.S.J. & LEDBEITER,M.T. 1978. Theexceptional P. magnitude and intensity of the Toba eruption, Sumatra: an example of Ankers (CSM) and C. Gilbert (BAS) are thanked for photographic the use of deep-seatephra layers as a geological tool. Bulletin of work. Volcanology, 41,286-298. PETTIGREW,T.H. 1981. The geology of Annenkov Island. British Antarctic Survey Bulletin, 53,213-254. PICKERING,K.T., STOW, D.A.V., WATSON,M.P. & HISCOIT. R.N. 1986. 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Received 3 November 1993; revised typescript accepted 22 July 1994.

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