Late Cambrian stratigraphy of the Heritage Range ...

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 1is 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 setting during the Middle Cambrian (Curtis et al. in press) In this paper, we address the issue of Late Cambrian tectonics and evolution of the Ellsworth Mountains by examining stratigraphic relationships and broad-scale facies distributions in the Upper Cambrian successions exposed throughout the Heritage Range. Observations made from such a large geographical area have allowed us to divide the stratigraphy into four sedimentary sequences, based on the identification of three sequence-bounding unconformities. Previously unreported carbonate talus breccia deposits in the Minaret Formation of the Independence Hills are also described. We discuss the sedimentary sequences and sequence-bounding unconformities in the light of recent endCambrian tectonic interpretations.

The Ellsworth Mountains lie along the northern periphery of the Ellsworth-Whitmore mountains (EWM) crustal block (Fig. I), a displaced terrane generally accepted to have originated from a position adjacent to the southern African / Weddell Sea sector of the palaeo-Pacific margin of Gondwana (Clarkson & Brook 1977, Watts & Bramall 1981, Grunow et al. 1991, Dalziel & Grunow 1992, Goldstrand et al. 1994, Curtis &Storey 1996, Dalziell997). This margin was the site of prolonged, complex tectonic activity from endNeoproterozoic to Early Ordovician times (Stump 1995). Tectonic style varied both spatially and temporally during this period, with regions of subduction-related compressional deformation coeval with carbonate-platform sedimentation andextensionaltectonics (Stump 1995).This complex tectonic period is represented in the Ellsworth Mountains by approximately 7.5 km of Cambrian age siliciclastic, volcaniclastic and volcanic rocks of the Heritage Group (Webers et al. 1992a) (Fig. 2). The nature of the tectonic regime during deposition of the Heritage Group is probably the most contentious geological aspect of the mountains. Tectonic interpretations vary from an active continental arc (Duebendorfer & Rees 1998), or back-arc basin setting (Curtis & Storey 1996), to an intracontinental rift environment (Vennum etal. 1992). New geochemical and isotopic analyses of volcanic and subvolcanic rocks exposed throughout the Heritage Range support the Vennum etal. (1992) suggestion

Post-Cambrian regional geological events Tectonic quiescence and gentle subsidence followed the Cambrian period, with deposition of the Crashsite Group (Upper Cambrian to Devonian), a 3-km thick succession of relatively mature siliciclastic rocks of predominant shallow marine origin (Sporli 1992). The Crashsite Group is conformably overlain by the Whiteout Conglomerate, a 1000 m thick glacial diamictite succession deposited during the Permo-Carboniferous glaciation of Gondwana. The youngest formation in the Ellsworth Mountains, the Middle to Upper

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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 arc provenance (Collinson etal. 1994). Inferred Andean-style convergence occurred along the palaeo-Pacific margin of Gondwana (Smellie 1981, Johnson 1991, Collinson et al. 1994)followeddepositionofthe PolarstarFormation,resulting in deformation of the entireEllsworthMountains stratigraphical succession (Sporli& Craddock 1992a)in a dextral transpressive regime (Curtis 1997, 1998a). This orogenic event, referred to on a Gondwana scale as the Gondwanian orogeny, imparted a well-developed NNW-SSE structural grain to the Ellsworth Mountains. Thepresence of two sets of contractional structures in the Middle Cambrian Springer Peak Formation has been presented as evidence for a Late Cambrian orogenic event in the Ellsworth Mountains (Duebendorfer & Rees 1998). However, Curtis (1998b) has identified two sets of structures in rocks from Cambrian to Permian in age, casting doubt on an early Palaeozoic event and suggesting that the Gondwanian orogeny was a polyphase event. During the final stages of Gondwanian deformation, structurelessand stratifiedpost-cleavagebreccia bodies formed in the carbonate lithologies of the Minaret Formation, due to cave-like dissolution processes and contemporaneous lowtemperature hydrothermal activity (Sporli etal. 1992). These



breccia bodies are predominantly exposed in the Independence and Marble hills, of the southern Heritage Range.

Stratigraphy TheLower Palaeozoic succession of the Ellsworth Mountains comprises the Heritage Group (Lower (?) / Middle to Uppcr Cambrian) and the Upper Cambrian to Devonian Crashsite Group (Fig. 2) (Webers et aL 1992a, 1992b). The Minarct Formation, which forms the focus of this study, has been regarded as the uppermost unit of the Heritage Group. Heritage Group

The Heritage Group crops out almost exclusively in the Heritage Range, and is composed of eight, predominantly siliciclastic, sedimentary and volcanic formations. For convenience we have split the group into the lower Heritagc Group, formed of the four lowest, lithologically diverse formations, and the upper Heritage Group that contains threc laterally equivalent formations, plus the overlying Minarct Formation.

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CAMBRIAN STRATIGRAPHY OF THE HERITAGE RANGE

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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 equivalentformations: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 the Horseshoe Valley area (southern HeritageRange), whereas the Springer Peak Formation (SPF) crops out in central and northern Heritage Range. The Frazier Ridge Formation (FRF) is restricted to north-west Heritage Range. The basal contact of these laterally equivalent formations is generally unexposed. Contrary to the interpretation of Webers et al. (1992b), our observations indicate that the Springer Peak Formation overlies the lower Heritage Group along a tectonic contact, at Soholt Peaks. However, a possible stratigraphical contactbetween the SpringerPeak and Drake Icefall formations is indicated at the head of the Hyde Glacier on the geological map of Craddock et al. (1986). The SpringerPeak and Li berty Hills formationsare estimated each to be 1000m thick (Webers et al. 1992a, 1992b), intense folding and the incompletenature of exposuremake athickness estimate for the Springer Peak Formation difficult. The Frazier Ridge Formation is estimated to be approximately 500 m thick (Webers et al. 1992b). Trilobite faunas indicate a Middle Cambrian (Jago & Webers 1992) to earliest Late Cambrian (Shergold & Webers 1992) age for the Springer PeakFormation. Unfortunately, no fossils have been obtained from the Liberty Hills or Frazier Ridge formations. However, Late Cambrian fossils derived from the overlying formation suggest that the Liberty Hills and Frazier Ridge formations were deposited contemporaneously with the Springer Peak Formation (Jago & Webers 1992). The Liberty Hills Formation is a coarse-grained siliciclastic succession of conglomerates,quartzites, and argillites. Lateral



Fig. 2. Stratigraphical column for the Ellsworth Mountains (after Webers et al. 1992b). KPM = Kosco Peak Member, UGF = Union Glacier Formation, HGF = Hyde Glacier Formation, DIF = Drake Icefall Formation, CRF = Conglomerate Ridge Formation, LHF = Liberty Hills Formation, SPF = Springer Peak Formation, FRF = Frazier Ridge Formation, HNF = Howard Nunataks Formation, LPM = Linder Peak Member, MTM = Mount Twiss Member, LmPF = Landmark Peak Formation, MLF = Mount Liptak Formation, MWEF = Mount Wyatt Earp Formation.

facies variations reveal a northward gradational change from coarse-grained fluvial deposits, through transitional deltaic units to fine-grained shallow marine accumulations. At one of its most southerly exposures in the Patriot Hills, the Liberty Hills Formation forms an interbedded coarse-grained assemblage of reddish (hematitic) sandstones, conglomerates and pebbly sandstones. Textural maturity is low: most beds are poorly sorted with subrounded toangularclasts ofmoderate to low sphericity. Sedimentary structures include trough cross bedding, grading, imbrication, and pebble clusters. Individual depositional units are generally amalgamated with only rare argillaceous interbeds. Lateral facies variations are very marked over distances of metres or less, and channelization is recorded on a variety of scales. The hematitic grain coatings indicate strongly oxidising early diagenetic conditions, suggesting a terrestrial depositional environment. The association of sedimentary facies is suggestive of alluvial processes and probably represents alluvial fan to braided stream palaeoenvironments. In contrast, the central and

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northern Heritage Range exposes brown argillite, buff greywacke, and argillaceous limestone of the Springer Peak Formation, which possesses a varied fauna. These are interpreted as more distal fully marine deltaic sediments, deposited by low-concentration turbidity currents (Webers et al. 1992a). The distribution of facies in the Liberty Hills and Springer Peak formations suggests a sediment transport direction from the south (Webers et al. 1992a, 1992b). There are no previously published palaeocurrent data for these formations. However, in this study we present palaeocurrent direction data derived from flute casts within the Springer Peak Formation, and parting lineations from sandstones in the Liberty Hills Formation (Fig. 3). The polarity of palaeoflow for the parting lineations was derived from associated cross-laminations. Our data, albeit limited in extent, support the interpretation of Webers etal. (1992a, 1992b), revealing a palaeoflow toward the north-west. This change in depositional environment from terrestrial in the south-east to marine in the north-west is corroborated by the exposed volcanic rock successions. Massive lava flows at Moulder Peak (Liberty Hills Formation) display evid2ncc of subaerial weathering, whereas subaqueous pillow lava successions predominate in the Springer Peak Formation of the central and northern Heritage Range. Minaret formation

The Minaret Formation forms a discontinuous limestone unit exposed from Webers Peaks in the northern Heritage Range to the Independence Hills in Horseshoe Valley. It was Corrected palaeocurrent data for pre-Minaret Formation sediments

0

Parting lineation (Liberty Hills Fm.)

0 Flute cast (Springer Peak Fm.) Fig. 3. Stereogram of flute cast and parting lineation orientations

corrected for the effects of folding. Flute cast morphology and cross laminae associated with the parting lineations confirms the NW-directed sense of sediment transport.



examined at nine localities (Springer Peak, Bingham Peak, Yochelson Ridge, the north-east and east ridges of Mount Rosenthal, the north-east ridge of Moulder Peak, Marble Hills, Independence Hills, Patriot Hills, and Wilson Nunataks) (Figs 1 & 4). The formation displays a consistent increase in depositional thickness toward the south, from 8 mat Springcr Peak (Webers Peaks), to c. 500 m in the Patriot Hills. A considerably thicker carbonate succession is present in Independence Hills, to the west of Patriot Hills. However, approximately half of the 1400 m succession is formed of highly sheared, recrystallized white marble, commonly displaying evidence for tectonic thickening (see structural geology section later). Clearly, the succession does not represent the original stratigraphical thickness. Higher maximum thickness estimates of 750 m and up to 900 In (Sporli & Craddock 1992a,Webers etal. 1992~1,respectively) for theMinaret Formation at Marble Hills, may result from the: inclusion of the effects of tectonic thickening. The timing of deposition was late Middle Cambrian to Late Cambrian, as demonstrated by a generally well-preserved faunal assemblage including trilobites (Shergold & Webers 1992, Jago& Webers 1992), molluscs (Webers etal. 1992c), conodonts (Buggish etal. 1992), brachiopods, archaeocyathids and pelmatozoa (Henderson et al. 1992). The formation includes avariety ofrock types including oolitic and oncolithic limestones, as described by Buggish & Webers (1992) from areas north of Mount Dolence. In contrast, the most commotr lithologiesin Horseshoevalley, to the southofMount Dolence, are white orcream colouredrecrystallized limestone ormarblc, and grey, well-bedded, nodular limestone. The Minaret Formation displays a significant change i n facies from north (WebersPeaks) to south (southern Horseshoe Valley). From Webers Peaks to Mount Rosenthal, it is formcd of oolitic and oncolitic limestones, containing adiverse fauna. In contrast, southern Horseshoe Valley is dominated by two main lithologies: (1) well-bedded, grey nodular limestone, and (2) white structureless to banded marble. As detailcti above, many of the white structureless marbles of' thc Independence and Marble hills display structural and textural characteristics consistent with intense simple shear deformation, predominantly parallel to bedding, and consequently are no longer of stratigraphical significance. Buggisch & Webers (1992) provided detailed accounts of the oolitic, oncolithic and bioclastic limestone facies present in the Minaret Formation exposcd between Webers Peaks and Mount Dolence. At Mount Rosenthal, at the head of Horseshoe Valley, the Minaret Formation is formed predominantly of white to pale grey micritic limestone with thin, discrctc interbeds of oolitic and oncolithic grainstone. Unfortunately, the full succession is interrupted by c.100 m of bedding parallel, highly sheared white marble. This association clearly represents shallow water deposition with sea-bed agitation by tidal or wave action. The oolitic grainstone represent high-energy settings, while the oncolithic limestonerepresents periods of somewhat lowerenergy. These

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rocks areentirely consistent with the open marine, highenergy environments proposed by Buggisch & Webers (1992) for the Minaret Formation to the north of Horseshoe Valley. In southern Horseshoe Valley, at exposures where strain is low to moderate, grey nodular limestone (Fig. 5) is the dominant lithology in the Minaret Formation. The limestones are monotonous, pale grey, thinly bedded, heavily recrystallized, and contain irregular, buff-weathered, dark grey nodules up to 3-4 cm across. The nodules are dolomitic and the matrix is non-ferroan calcite. Fine-grained equant fabrics are preserved in the relatively competent nodules whereas the host calcite is thoroughly recrystallized and shows deformation textures even in relatively low-strain areas. Relict ghost allochems are discernible in some samples as (strongly flattened) ovoid, tabular and lenticular forms, but are generally unidentifiable. Discrete oncolithic and probable oolitic wackestone units are present at Patriot Hills. The most northerly occurrence of this lithology is found at theendoftheeastridgeofMountRosenthal,2kmalongstrike from the last locality dominated by oolitic and oncolithic limestone. These contrasting lithologies can be correlated by the presence of a concentrated fossil horizon exclusively made up of Orthothecida sp. indet., which was identified at Mount Rosenthal in a similar stratigraphical position by Webers et al. ( I 992c). We can suppose that the dolomitic nodules originated as intramicritic zones of early diagenetic marine cementation, probably by high-Mg calcite, facilitating selective conversion to dolomiteduring deep burial. Near-surface nodule formation would have been favoured by a low-energy depositional environment and relatively low rates of sediment accumulation. We interpret these nodular carbonates as hemipelagic slope carbonates or 'peri-platform ooze' (cf. Schlager & James

1978),deposited partly by turbid plumes from shallow water, perhaps augmented by dilute turbidity currents. Oncolithic and oolitic wackestonemayrepresent redeposition into deeper water by gravity flow processes. Laterally restricted, lenticulardepositional limestone breccia units are present in the upper levels of the Minaret Formation in Independence and Liberty hills (Figs 4 & 6). These breccia units are composed of unsorted, white carbonate blocks suspended in a grey limestone matrix and vary in thickness from approximately 5 m to an estimated 40-50 m. However, these thickness estimates may be exaggerated due to the effects of hinge zone thickening during Gondwanian deformation. In areas of relatively high strain, the blocks are reduced to flattened ellipsoids in the regional cleavage. The limestone blocks vary from a few centimetres to an estimated 40 m in length. Where strain is not too intense, the blocks are angular, andof low sphericity (Fig. 7). These brecciadeposits are distinct from the palaeo-cave breccias described Sporli &L Craddock (1992b) in three ways: 1) they are supported in amatrix of grey micrite, as opposed to being clast supported with a drusy calcite cement, 2) they pre-date deformation, unlike thecave-deposits which show a random cleavage orientation within their clasts, and 3 ) they form lenticular to planar sheets parallel to bedding, whereas the cave-deposits cross cut bedding and tectonic folds.

The lenticular shape of the depositional bodies, together with the lack of sorting, implies deposition by rockfall or debri,s flow processes. Chaotic, clast-supported examples with random fabric are likely to represent talus deposited by

Fig. 5. Typical appearance of the grey nodular limestone lithology (Marble Hills, Horseshoe Valley). A welldeveloped cleavage (S,) dominates the left side of the photograph, producing an elongation of the dolomite nodules. So = bedding.



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Fig. 6. Photograph and line drawing of folded breccia deposits in the Independence Hills, southern Horseshoe

Valley. Lenticular breccia units are shaded grey, as are mega-clasts. Cliff exposure is estimated to be 250 m high.

rockfall and avalanche processes. Matrix-rich breccia units showing subtle internal organization are interpreted as debris flow deposits (cf. Krause & Oldershaw 1979). It is clear that the breccias represent proximity to steep bypass slopes of lithified limestone, but the nature of these steep slopes is uncertain. We consider two main possibilities. First, the breccia deposits may have formed on the foreslopes fringing the steep bypass slopes of a carbonate platform or



steep-edged shelf. Brecciadepositionwould have been episodic andlocalized,due to platformmargin instability. Suchplatformfringing foreslope talus deposits are well known from modern platforms (e.g. of the Caribbean: James & Ginsburg 1979) and well-exposedancientsystems (e.g.theTriassic of theDolomites, Italy: Bosellini 1984). The transition from shallow, highenergy deposits to low energy, hemipelagic slope deposits, suggests that thepalaeo-platform margin was located atMount

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Rosenthal. A thin breccia deposit lying at the top of a succession of grey nodular limestones at Moulder Peak, Liberty Hills, is only 4 km from this inferred platform margin, and would be consistent with its interpretation as a periplatform talus deposit. However, the main breccia deposits in Independence Hills are located over 50 km from Mount Rosenthal. Such a distance is inconsistent with the breccia units being peri-platform talus deposits, although this possibility can not be discounted as the three-dimensional geometry of the carbonate basin is not adequately constrained from the outcrop present in the Heritage Range. An alternative interpretation for the breccias exposed in Independence Hills is that they represent fault scarp-related talus deposits. The faults themselves may be related to partial disintegration of the carbonate deformation systems late in Minaret Formation time. Because the brecciaunits interdigitate with low energy, hemipelagic slope deposits, and are located 50 km from the inferred platform margin, we favour this interpretation.

Fig. 7. Breccia deposit from the Independence Hills. The white limestone clasts, contained within a matrix of fine, grey

recrystallized limestone, are slightly flattened and elongated within the regional cleavage (cleavage trace from bottom left to top right).



The basal contact of the Minaret Formation is conformable with the underlying strata at all localities where it was observed (Springer Peak, Bingham Peak, Yochelson Ridge, east ridge of Mount Rosenthal, north-east ridge of Moulder Peak, and Patriot Hills). In northern and central Heritage Range, thc Minaret Formation lies conformably upon Middle Cambrian fully marineargillites and thin sandstones ofthe Springer Peak Formation. In the Liberty Hills, the Minaret Formation conformably overlies the complex lithofacies relationships of the Liberty Hills Formation adjacent to the Moulder Peak volcanic centrc The bluff at the end of the north-east ridge of Mount Rosen thal exposes at least 300 m of limestone. Although the basal contact is not exposed, it is inferred to be concordant with the underlying green argillite and phyllite facies of the Liberty Hills Formation, which crop out in an adjacent rock exposure. At the endoftheeastridge ofMountRosentha1,thelimestoncs overlie a narrow exposure of basic lava flows with common peperite horizons. The north-east ridge of Moulder Peak reveals a conformable contact between basal calcarenites of the Minaret Formation, and an underlying 15-20 m thick, structureless unit of oligomictic conglomerates, composed of porphyritic mafic igneous clasts. The conglomerates lie at thc top of a 750-m thick basaltic lava pile. In the south-eastern corner of the Patriot Hills, the base of the Minaret Formation is well exposed, lying concordantly upon interbedded, coarsegrained reddish (hematitic) sandstone, conglomerate and pebbly sandstone of the Liberty Hills Formation. The upper contact of the Minaret Formation is exposed at five localities: Springer Peak and Bingham Peak (Webers Peaks), Soholt Peaks (Goldstrand et al. 1994), Mount Dolence area (Sporli 1992,Buggisch& Webers 1992),and Wilson Nunataks. This stratigraphical contact is variable in nature. The upper surface of the Minaret Formation displays karst dissolution features in exposures at Webers Peaks (Duebendorfer & Rees 1998) and Wilson Nunataks, suggesting emergence of this surface. However, the period of emergence was brief, as thc Minaret Formation is disconformably overlain at Webers Peaks and Wilson Nunataks by a predominantly argillaceous unit, up to 60 m thick, of Late Cambrian (Idamean) age (Shergold & Webers 1992). The argillites display agradational contact with the overlying basal Crashsite Group quartzites at Springer Peak and Bingham Peak (Sporli 1992, Goldstrand et at. 1994, respectively), and were therefore informally referred to as the 'transition beds' by Sporli (1992). At Wilson Nunataks, the Minaret Formation is disconformably overlain by approximately 20 m of silicified siltstone, which we correlate with the "transition beds". The siltstones are in turn conformably overlain along a distinct contact by coarsegrained quartzite of the basal Crashsite Group (Linder Peak Member). Erosional contacts have been identified between the Minaret Formation and the overlying Crashsite Group in the Mount Dolence area (Sporli 1992), and at Soholt Peaks (Goldstrand

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et al. 1994) where 62 m of conglomerate and sandstone of the basal Crashsite Group rest with concordant disconformity upon the limestone. At Pojeta Peak (south-east of Bingham Peak), the Minaret Formation is absent, leaving coarse-grained quartzite of the basal Crashsite Group resting with slight discordance upon the Springer PeakFormation (Duebendorfer & Rees 1998). Pojeta Peak exposes the only recognised angular unconformity along an otherwise conformable or concordant stratigraphical boundary between the Heritage and Crashsite groups. Basal Crashsite Group The depositional environments of the basal Crashsite Group have not been studied in great detail. However, the presence of oscillation ripple-marks and desiccation cracks in the Howard Nunataks Formation, particularly in the Mount Twiss Member, suggests very shallow water conditions with frequent periods of emergence (Sporli 1992). In the central Heritage Range, the subdivisions of the Howard Nanataks Formation (the LinderPeak, MountTwiss, andLandmarkPeakmembers: Fig. 2 ) , have variable thicknesses, with the lowermost Linder Peak Member being laterally restricted to the eastern side of the Heritage Range. The Linder Peak Member is composed of well-bedded, greenish-grey, coarse to medium grained, quartzite displaying dark-green laminations (parallel andcross) plus cross bedding. Conglomerate beds are common, formedfrom scattered, wellrounded, pebbles up to 4 cm in length, of vein quartz and dark quartzite. Sporli (1992) reported that the conglomerate beds

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Sequence boundary

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are more common in the eastern side of the Heritage Range, with sorting improving toward the west. Near Mount Dolence, in the Enterprise Hills, the Linder Peak Member lies disconformably on theMinaret Formation, with the 'transition beds' being absent. "Erosional pockets" along this contact (Sporli 1992) may well be channel features. In the west and north-west of central Heritage Range, the Linder Peak Member is absent. In these areas, the distinctive Mount Twiss Member, composed of abundant maroon and red argillite interbedded with quartzite, lies with local unconformity upon the 'transition beds' and Minaret Formation. At Soholt Peaks, basal conglomerates of the Mount Twiss Member disconformably overlie the Minaret Formation with an erosional contact. The conglomerates are interpreted as having been deposited in a braided fluvial to tidal sand-flat environment (Goldstrand et a1. 1994). At Bingham Peak the conformable contact between the 'transition beds' and the Mount Twiss Member is clearly exposed. The quartzites of the Mount Twiss Member are interpreted as being shallow to near shore, marine, tidal dominated (Goldstrand et al. 1994). The question of whether the Howard Nunataks Formation was totally marine or partially freshwater remains unresolved. However, the few sections studied in any detail indicate that the basal Crashsite Group was deposited in a fluvial to nearshore, tide-dominated marine environment (Goldstrand et al. 1994), prone to repeated emergence (Sporli 1992). The variation in the degree of sorting in the Linder Peak Member possibly indicates that sediment was derived from the northeast or east (Sporli 1992). We speculate that the Linder Peak Member may have been deposited predominantly in a fluvial

.-'\,-,(no Uncertain sequence boundary relationship exposure)

Fig. 8. Schematic diagram depicting the sequence stratigraphical relationships between the formations of the Heritage Group, including Minaret Formation, and the basal Crashsite group. 0= Pipe Peak, 0 = Springer and Bingham Peak, 0= Pojeta Peak, @ = Yochelson Ridge, 0= Soholt Peaks, 8 = Mount Dolence area, 0 = Mount Rosenthal, @ = Wilson Nunataks, @ = Patriot Hills, @ = Ridge to east of Mount Geissel (Independence Hills).



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environment, which gave way to a near-shore marine environment toward the west, with localized overlap of these environments, e.g. Soholt Peaks conglomerate succession. This would explain the absence of the Linder Peak Member in the westerly areas of the Heritage Range. Later transgression would have resulted in the deposition of the near-shore marine sediments of the Mount Twiss Member upon the Linder Peak Member in the more easterly areas, i.e. Enterprise Hills and Thompson Escarpment. Nature of the stratigraphical boundaries The Minaret Formation displays essentially concordant basal contacts, butclearly represents agroup ofdepositional systems genetically unrelated to the underlying formations of the Upper Heritage Group. Regional transgression is required to explain the drowning of the earlier terrestrial deposystems, and could also account for exclusion of any significant terrigenous clastic material from the basin if low-lying sediment source areas were inundated. Hence, to the south, where offshore limestones overlie terrestrial clastic units, the base of the Minaret Formation is primarily aregional flooding surface. However, to the north very shallow water limestones overlie relatively deep-water ?prodelta shales, and there the base of the Minaret Formation is a regressive surface. Regionally therefore, this surface must be a sequence-boundary. Reported karst dissolution features along the upper contact of the Minaret Formation at Webers Peaks, supported by similar features observed at Wilson Nunataks, suggest a brief period of emergence prior to deposition of the "transition beds", and therefore the presence of a sequence boundary (Duebendorfer & Rees 1998). The Minaret Formation is therefore bounded top and bottom by sequence boundaries, making it a separate depositional sequence from under or overlying formations (Sequence 2 in Fig. 8). The 'transition beds' are overlain by the basal Crashsite Group along a localized erosional unconformity. Therefore, like the Minaret Formation, the "transition beds" form a separate sequence, bounded top and bottom by sequence boundaries (Sequence 3).

Structural geology of the Minaret Formation The structural geology of the Heritage Range has been described by Sporli & Craddock (1992a, 1992b),andrecently by Curtis (1997, 1998a). For the purposes of this paper, we concentrate on the structure and tectonic fabrics of the Minaret Formation, as they have significant implications for its stratigraphy. In the northern and central Heritage Range, the Minaret Formation crops out as isolated exposures along the steep, planar, north-east limb of the Landmark Peak syncline. The limestones are generally well cleaved with a pronounced stretching lineation. Analysis of deformed oncoliths indicates a strong flattening strain, k = 0.62-0.68 (Curtis 1997), with approximately 180-1 66%elongation parallel to the lineation



and 62% elongation in the cleavage plane. In thin section, extensive recrystallization and flattening of the allochems has left only remnant sedimentary textures. However, strain is heterogeneously distributed in these exposures, allowing, original sedimentary petrographic features to be describedl (Buggisch & Webers 1992) and the collection of a rich fossil fauna, as reported by several authors in Webers etal. (19923). The structural simplicity of the northern exposures is in contrast to those of Horseshoe Valley, where intense deformation is recorded. The earliest structures observed in the Horseshoe Valley area are bedding parallel, pre-cleavage, calc-mylonite zones. These mylonites have been folded, and crenulated on a smaller-scale, by the development of the main regional cleavage. The lineation direction associated with these mylonites is roughly coaxial to the local pervasive mineral stretching lineation, suggesting the mylonites arc probably the earliest recognisable phase of the Gondwanian orogeny in this area. The Patriot, Independence, and Marble hills of Horseshoe Valley are composed almost exclusively of carbonate rocks from the Minaret Formation, which are folded about a NNWSSE trending, sub-horizontal axis. The folds generally verge to the north-east, with axial planes dipping moderately toward the south-west. The Independence and Marble hills are dominated by a NE-verging first-order fold pair, the limbs of which show varying degrees of lower order parasitic fold development. Fold tightness varies from close to isoclinal, with apossible NE-verging recumbent fold exposed at Mount Geissel (Sporli & Craddock 1992a). Cleavage is generally well developed, oriented sub-parallel to the regional fold axial planes, except in the Patriot Hills (an area of relatively low strain), where cleavage fans about the vertical. Extensive units of structureless and banded white marble are exposed in the Marble and Independence hills. These units are commonly completely recrystallized, displaying an intense L/S fabric (formed sub-parallel to adjacent bedding), sheath folds, and intrafolial folds that obliterate original sedimentary structures (Fig. 9). Most of these white high-strain marble units remain parallel to bedding around major NNW-SSE trending regional folds, indicating that they formed prior to regional Gondwanian folding. In areas of lower strain, a bedding-obliquc anastomosing cleavage characterizes the grey, commonly nodular, limestones that dominate the unmetamorphosed rocks of the Independence and Patriot hills. Strain is concentrated locally into bedding-parallel reverse shear zones that display a well-developed S/C fabric around the relatively competent dolomite nodules (Fig. 10). Where deformation is more intense, the recrystallized and highly cleaved limestone surrounding the nodules is a pale grey to white colour. Sporli & Craddock (1992a) speculated on a tectonic origin for at least some of the white marble units in the Marble and Independence hills. However, it is clear from our study that the structural and textural characteristics of a significant proportion of the massive /banded white marble is the result of intense simple shear deformation. It remains unclear what

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73


Discussion Basin evolution

The upper Heritage Group and basal Crashsite Group record a history of rapid fluctuations in relative sea level during the

/

* * /

Fig. 9. Structural data from Independence and Marble hills. a.

Stereogram displaying contoured poles to regional cleavage,

and associated down-dip mineral stretching lineations. Dashed

line represents mean cleavage plane. b. Stereogram showing poles to bedding. Note that mean cleavage pole coincides with a tight group of bedding data, emphasising the bedding parallel nature of the tectonic fabrics developed in many of the structureless white marble units. n marks the statistical fold axis derived from bedding planes. percentage of white limestone on a regional scale is of a tectonic origin, as sedimentary contacts between white and dark limestone are present. However, as an example, approximately 48% of the observed stratigraphical section in the unnamed ridge to the east of Mount Geissel is composed ofwhitemarbledisplayingstructural andtexturalcharacteristics consistent with intense deformation and tectonic thickening.



Late Cambrian that resulted in the deposition of several distinct sedimentary sequences. At the top of Sequence 1 (Fig. 8), in the upper Heritage Group, lateral facies trends and palaeocurrent data in the Liberty Hills and Springer Peak formationsimply both basin deepening and sediment dispersal from south to north. It seems clear that, during the deposition of this siliciclastic assemblage,the basin configurationinvolved an emergent source to the south-east feeding a marine depocentre to the north. In contrast, the Minaret Formation (Sequence 2) thickens substantially from north to south and appears torecord relative deepening in the same sense, with the main locus of subsidence during this depositional phasein the south. Our interpretation of the facies distribution in the Minaret Formation is a shallow water platform to the north passing offshore to the south into fine-grained slope to basinal deposits. Comparison of the deposition facies aboveand below the basal sequenceb o u n d q reveals an apparent decrease in sea level in the north, and an apparent sea level rise in the south. Such contradictory relative sea level changes along the same sequence boundary suggest that reconfiguration of the deposition basin at the beginning of the Late Cambrian was tectonically induced. This conclusion is consistent with the interpretation of the carbonate breccias in the Independence Hills as fault scarp talus deposits. The presence of karst dissolution features along the upper surface of the Minaret Formation indicates a brief period of emergence prior to arapid relative sea level rise and deposition of the 'transition units' (Sequence 3) in storm-dominated shelf environments (Goldstrand et al. 1994). The base of Sequence 4 (lower Crashsite Group) is locally unconformable, with correlative conformity observed at three localities. (At this stage we do not attempt to define the top of Sequence 4 within the Crashsite Group.) In the northern Heritage Range, the depositional facies immediately above and below this boundary indicate a relative sea level fall, which is consistent with the presence of erosional contacts at several localities. Provenance studies of the basal conglomerate succession at Soholt Peaks (Goldstrand et al. 1994) suggest that significant local relief existed in the Ellsworth Mountains area at this time (CambroOrdovician)whichactedas aspatially and temporally restricted sediment source. However, in general the quartzites and sandstonesof the HowardNunataks Formation of the Crashsite Group were derived from the erosion of a rejuvenated mature cratonic source (Goldstrand et al. 1994) to the east of the EWM (relative to present day); probably East Antarctica. The presence of intra-basinal relief in the end-Cambrian basin suggests that tectonic uplift was coincident with the onset of Sequence 4 deposition. However, the absence of a regional angular or erosional unconformity at the base of the Crashsite

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74

Group suggests that uplift and formation of topographical relief must have been localized.

Significance of the basal Crashsite Group sequence boundary Two recent interpretations of the basal Crashsite Group contact have suggested it either represents evidence for the presence of a Late Cambrian age deformation event, the Ross orogeny (Duebendorfer & Rees 1998), or that it is the represents the distal affects of the same orogenic event (Goldstrand et aZ. 1994). Our stratigraphical observations combined with structural data (Curtis 1997, 1998a, 1998b) are inconsistent with the existence of a Late Cambrian contractional deformation event in the Ellsworth Mountains. Two problems exist with such a model: 1) the timing of D, structures in the Ellsworth Mountains, and 2) the general concordance and local conformity of the Crashsite Group to underlying stratigraphicalunits. Duebendorfer& Rees (1998) based their interpretation on the presence of two sets of contractional structureswithin the Middle Cambrian Springer Peak Formation, and undescribed slate clasts at the base of the Crashsite Group at Webers Peaks that, it is argued, constraintheformationofearly D, structuresas Late Cambrian. However, two sets of contractional structures have now been reported from all stratigraphical levels of the Ellsworth Mountains, including the Permian age Polarstar Formation (Curtis 1998b),revealing that the Permo-TriassicGondwanian orogeny was a progressivepolyphase event. The Gondwanian age D, structures are locally developed, and may therefore explain the presence of stmctures previously assigned to the Ross orogeny. The reported slate clasts at the base of the Crashsite Group at Webers Peaks are unlikely to have been

derived from the underlying 'transition beds' and Springer PeakFormation, as suggestedby Duebendorfer &Rees (1998), for two reasons. Firstly, the formation of a slaty cleavage in a given body of rock requires a minimum of 30% shortening (Park 1983) at crustal depths probably 2 5-6 km (Price & Cosgrove 1990), with total shortening in excess of 30% when folding is takeninto account. Therefore, one would expect to find a significant regional angular unconfomity at the base of the Crashsite Group, representing approximately5-6km ofuplift and erosion,thus exhuming rocks possessing a slaty cleavage. However, as documented here, and by Goldstrand et al. (1998) and Sporli (1992), the basal contact of the Crashsite Group is essentiallyconcordant with the underlyingrockformations, with only slight angular discordance recognised at Pojeta Peak, where an estimated 80 m of erosion has occurred (Minaret Formation and 'transition beds' thickness). Secondly, the absence of cleaved clasts from the 62 mthick basal conglomerate succession at Soholt Peaks, which has a local provenance, further indicates the unlikelihood of a Late Cambrian contraction deformation event. We interpret the sedimentary sequences of the upper Heritage Group to the base ofthe Crashsite Group as recording aperiod of rapid, tectonically induced, relative sea level changes, associated with significant reconfiguration of the basin structure. Geochemical and isotopic analyses of Middle Cambrian volcanic and subvolcanic rocks from throughout theHeritageRange (Vennumet af. 1992,Curtis et al. in press) suggest that the sedimentary rocks of the upper Heritage Group (top of Sequence 1) were deposited in a continental rift

Fig. 10. Bedding-parallel reverse shear zone in grey nodular limestones, Independence Hills. S, =trace of regional Gondwanian cleavage deflected in bedding parallel sheatzone, displaying a heterogeneously distributed shear strain. 'S' and 'c' highlight the orientation of S/C planes, which are well developed in the centre of the photograph. In areas of high shear strain, mylonit isation and recrystallization have resulted in the formation of white to pale grey marble with a strong foliation approximately parallel to So.



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setting. Tectonic reconfiguration of the depositional basin followed, with tectonically induced rapid relative sea level changes associated with the deposition of Sequences 2-4. Although, the sandstones and quartzites of the Crashsite Group display a mature craton provenance, suggestive of hinterland rejuvenation, possibly in East Antarctica, we do not consider the base of Sequence 4 (lower Crashsite Group) as representing a discrete, distant tectonic event as suggested by Goldstrand et al. (1994). The continuity of tectonic activity from the eruption of the Middle Cambrian volcanic rocks to fiial uplift and the initiation of Sequence 4 deposition suggests that the tectonic events responsible for relative sea level changes, were probably genetically related and represent a prolonged continentalrifting event. Deposition ofthe Crashsite Group marked the cessation of major tectonic activity, and heralded a period of gentle regional subsidenceand deposition, from Late Cambrian to Devonian time. We interpret this sequence as the change fromrift to passive margin. Hence the sequence boundary at the base of Sequence 4 could be regarded as the 'break-up' unconformity. Comparison of the revised stratigraphical relationships of the EllsworthMountains successionwith other areas along the Antarctic sector of the palaeo-Pacific margin of Gondwana reveals significant differences. In the TransantarcticMountains (TAM), Proterozoic and Lower to Middle Cambrian sedimentary rocks of the Ross Supergroup were deformed, metamorphosed, and intruded by magmatic rocks during the Ross orogeny, and were overlain with marked unconformity by the Devonian-Triassic Beacon Supergroup (Elliot 1975). Likewise, in the Pensacola Mountains, at the Weddell Sea end of the TAM, episodic contractional deformation and the production ofan angular unconformity characterized the endCambrian to Ordovician (Storey et al. 1996, Rowel1 et al. 1997). In contrast, the Cambro-Ordovician succession of South Africa provides evidence for a mid-Cambrian rifting event, during which a successionof syn-rift,alluvial sandstones were deposited prior to the post-rift deposition of the Table Mountain Supergroup in a passive margin setting (Barnett et nl. 1997). We suggest that the Cambrian succession of the Ellsworth Mountains displays a much closer tectonic and stratigraphical similarity to those of the Cape Fold Belt of South Africa, than to the geological provinces of the TAM. Together with the previously noted post-Cambrian affinities (Craddock 1970, Sporli 1992) and Grenvillian crustal ages, these Cambrian correlations support the close spatial and geological association of the two provinces, independent of East Antarctica, since the mid Proterozoic (Dalziel 1997, fig. 11) to Jurassic. Conclusions

The upper Middle Cambrian-Upper Cambrian strata of the Ellsworth Mountains can be divided into four sedimentary sequences.



75

Sequence 1 (upper Heritage Group) is a fluvial to marine deltaic siliciclastic succession, with sediment transport toward the north-west. Sequence2 (MinaretFormation) is a carbonatesuccession displaying a southerly transition from shallow-water platform carbonates in the north to deeper water basinal limestones in the south. Sequence 3 ('transition beds') is a thin succession ofouter to inner shelf argillites and sandstones. Sequence 4 represents the Upper Cambrian (and younger) lower Crashsite Group, a shallow marine to fluviatile succession. The boundarybetween sequences 1 and2 represents aprimary flooding surface in the south and a regressive surface in the north. This boundary must therefore record a significant structural reconfiguration of the Late Cambrian depositional basin. A rapid relative sea level fluctuation during the Idamean caused brief emergence of the upper surface of the Minaret Formation (Sequence 3) prior to the disconformable deposition of Sequence 3 ('transition beds') in fully marine conditions. Sequence4 (lower CrashsiteGroup) was deposited following localized uplift, and lies with local unconformity upon Sequence 3 and 2. However, a correlative conformity is present between Sequence 3 and 4 at several localities. Despite the presence of a slight angular unconformity at Pojeta Peak, the Crashsite Group is concordant with the underlying stratigraphical units on a regional scale. The absence of a regional angular unconformity along any of the sequence boundaries identified appears to preclude the possibility that the Cambrian succession of the Ellsworth Mountains was subjected to a Late Cambrian deformation event. The tectonic setting of the Upper Cambrian succession in the EllsworthMountains remains uncertain. The continuity of a tectonic influence on the Upper Cambrian stratigraphy of the Ellsworth Mountains, following a period of mid-Cambrian continental rifting (Vennum etal. 1992, Curtis etal. in press), suggests that both mid- and Late Cambrian tectonics were genetically related. We interpret the depositionofthe Crashsite Group (Sequence 4 and above) as representing the transition from continental rift to passive margin. Acknowledgements

B. Hull and S. Garrod are thanked for their assistance and companionship during the field seasons of 1993-94 and 1995-96, respectively. The Air Unit and staff of British Antarctic Survey research station, Rothera, are gratefully acknowledged for logistical support. We are grateful to K.B. Sporli andG.F. Webers forthoroughreviewsofthemanuscript.

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76 References

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VENNUM, W.R., GIZYCKI, P., SAMSONOV, V.V., MARKOVICH, A.G. & PANKHURST, R.J. 1992. Igneous petrology and geochemistry of the southern Heritage Range, Ellsworth Mountains, West Antarctica. In WEBERS, G.F., CRADDOCK, C. & SPLETTSTOESSER, J.F., eds. Geology and paleuntology uf the Ellsworth Mountuins, West Antarcticu. Geological Society of America Memoir, No. 170. 295-324. A.M. 1981. Palaeomagnetic evidence for a WATTS,D.R. & BRAMALL, displaced terrane in Western Antarctica. Nuture, 293,638-641. J.J., BUGCISCH, W., OJAKANGAS, WEBERS, G.F., BAUER,R.L., ANDERSON, K.B. 1992a. The Heritage Group of the Ellsworth R.W. & SPORLI, C. & Mountains, West Antarctica. In WEBERS,G.F., CRADDOCK, SPLETTSTOESSER, J.F., eds. Geology andpaleontology uf the Elisworth Muuntuins, WestAntarcticu. Geological Society of America Memoir, No. 170, 9-19. J.F. 1992b. Geological WEBERS, G.F., CRADDOCK, C. & SPLETTSTOESSER, history of the Ellsworth Mountains, West Antarctica. In WEBERS, G.F., CRADDOCK, C. & SPLETTSTOESSER, J.F., eds. Geology und paleontology uf the Ellsworth Mountuins, West Anturcticu. Geological Society of America Memoir, No. 170, 1-8. E.L. 1992c. Cambrian WEBERS,G.F., POJETA,J. & YOCHELSON, Mollusca from the Minaret Formation, Ellsworth Mountains, West G.F., CRADDOCK, C. & SPLETTSTOESSER, J.F., Antarctica. In WEBERS, eds. Geology und paleontology of the Ellsworth Mountains, West Antarctica. Geological Society of America Memoir, No. 170, 18 1-248. C. & SPLETTSTOESSER, J.F., eds. 1992d. WEBERS,G.F., CRADDOCK, Geology and poleontology uf the Ellsworth Mountuins, West Antarctica. Geological Society of America Memoir, No. 170,459 pp.

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