Late Precambrian and Early Paleozoic Tectonism and Associated Sedimentation in Northern Victoria Land, Antarctica

Late Precambrian and early Paleozoic tectonism and associated sedimentation in northern Victoria Land, Antarctica

THOMAS O. WRIGHT Earth Sciences Divistiti, National Science Foundation, Washington, D.C. 20550

ABSTRACT INTRODUCTION margin, between South Africa and Australia, was the site of active tectonism, as indicated by Northern Victoria Land, part of the Trans- During the late Precambrian and early widespread igneous activity, large volumes of antarctic Mountains, occupies a key position Paleozoic, northern Victoria Land was part of immature clastic sediments, and episodes of in the reconstructed Pacific margin of Gond- the continental margin of Gondwanaland. This intense compressive deformation. Attempts to wana, lying near the Oligocene rift between Antarctica, Tasmania, and Australia. Sedi- mentology, sedimentary petrology, and struc- South Pacific Ocean Figure L Sketch map of ^ tural history of the Robertson Bay Group and the Bowers Supergroup are here used to develop a regional interpretation of these rocks in plate-tectonic: terms. The Robertson Bay Group, a distal shale-turbidite sequence, is interpreted to have originated in a continent- continent collision environment and was com- pressionally deformed after the Cambrian- Ordovician boundary but before Devonian time. The Sledgers Group of the Bowers Supergroup, a proximal shale-turbidite and volcanic sequence, was derived from an undissected arc environment and was deformed during the same time interval as was the Robertson Day Group and possibly also during the Silurian-Devonian. The Cambrian Mariner and overlying Leap Year Groups represent shallow-marine and terrestrial deposition. Reworking and selective removal of unstable clasts obscure provenance, but, although these units may be derived from dissected arc sources, they are quartz-rich. They appear to have been deformed by Silurian- Devonian tectonism. The Robertson Bay Group and the Bowers Supergroup occur in large, fault-bounded terranes. The plate-tectonic interpretations developed for these units in this paper place the arc-related Bowers Supergroup between the Antarctic ciraton to the west and the recycled orogen-i elated Robertson Bay Group to the east. This geometry is not readily related to in situ tectonic environments; therefore, it is concluded that the possibility exists that these terranes are "suspect," that is, they may be entirely allochthonous to the Antarc- tic craton and have been added by later accretion.

Geological Society of America Bulletin, v. 96, p. 1332-1339, 7 figs., 3 tables, October 1985.

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develop a unified plate-tectonic model for this determined histories to try and find trends and East of the Leap Year fault, only Robertson Bay old continental margin face many problems. larger-scale similarities that may be useful in es- graywacke and slates occur, yet the contact- This paper addresses one facet of the over-all tablishing intercontinental correlations. metamorphosed graywacke and slate exposed in problem by examining the tectonic implications the Morozumi Range have all the characteristics of upper Precambrian and lower Paleozoic sed- GEOLOGY OF NORTHERN of the Robertson Bay Group, and it is a strong imentary rocks deposited in northern Victoria VICTORIA LAND possibility that these rocks are a part of that Land. group but that they are separated from the main Various reconstructions of Gondwanaland In northern Victoria Land, the work of the outcrop (Tessensohn and others, 1981). Sim- indicate that the Transantarctic Mountains ap- last decade has dramatically improved knowl- ilarly, Bowers Supergroup rocks occur only in a proximately mark the Pacific margin of the old edge of the local geology. The general picture narrow, fault-bounded area that extends from supercontinent—no known land blocks that that has emerged is shown in simplified form in the lower Rennick Glacier area on the South would have been adjacent to this part of Antarc- Figure 2. With a few important exceptions, the Pacific coast to the Ross Sea near the lower tica during the late Precambrian and early Pa- Wilson Group, Robertson Bay Group, and the Mariner Glacier (Stump and others, 1983). Im- leozoic have been identified, except for poorly Bowers Supergroup rocks occur in, and are re- portant possible exceptions to this assertion are understood plateaus and rises. Grindley and stricted to, large, fault-bounded areas or terranes. the presence of Bowers-like volcanics in the Davey (1982) have shown that reconstructions upper Tucker Glacier area east of the Leap Year between Antarctica, New Zealand, and Aus- fault (Findlay and Field, 1983) and Bowers-like tralia, made on the basis of sea-floor magnetic anomalies, polar wander paths, and bathymetric matching, produce reasonably consistent results. On the basis of present data, however, direct South Pacific matching of the pre-Mesozoic geology between Antarctica and Australia leads to rather unsatis- factory, or at least ambiguous, results. Within the Transantarctic Mountains (Fig. 1), it appears that the Precambrian and early Pa- leozoic tectonic history varies remarkably in style and timing. Stump (1980) indicated the considerable variations in age of orogeny along strike. From the Pensacola Mountains to the central Transantarctic Mountains, the Beard- more Orogeny, of late Precambrian age, strongly affected the rocks (Grindley and McDougall, 1969), whereas little, or no, evidence for this episode is present in Victoria Land. The Cambrian-Ordovician Ross Orogeny was felt from the Pensacola Mountains to northern Vic- toria Land, but the effects were more complex in southern Victoria Land than elsewhere. The so- called Borchgrevink Orogeny of Silurian (?)- Devonian age, which deformed both the Robert- son Bay Group and the Bowers Supergroup, was the terminal event in northern Victoria Land. In Tasmania and eastern Australia, compressive features of the Tasman Orogen (middle late Pa- leozoic) are conspicuous (Vandenberg, 1978). With this degree of lateral variation along the orogenic margin, it becomes less surprising that no easy match between the geology of Antarc- tica and its pre-drift neighbors exists. Indeed, if the rift had developed in southern Victoria Land, the geology on the two sides would not be easy to correlate either. Ross Sea It thus is probably unwise to force correlation of orogenic events over very broad areas. In- stead, it may be more productive to examine in detail the geologic and tectonic history of a Figure 2. Simplified geologic map of northern Victoria Land. Hallet Volcanics shown by v number of areas along the old Gondwanaland pattern; Robertson Bay Group, by wide-spaced lined pattern; Bowers Supergroup, by narrow margin in the manner of Stump (1976), for ex- lined pattern; Admiralty Intrusives, by + pattern; Wilson Group, by wavy pattern; and Granite ample, and then compare these independently Harbor Intrusives, by x pattern.

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lithologies in a similar structural position in the TABLE 1. PALEOCURRENT INDICATORS FROM THE N ROBERTSON BAY GROUP, DIP-CORRECTED Mariner Glacier area. Similarly, the Wilson

Group metamorphics in the Lanterman Range Location Type Inferred flow direction are apparently limited to a large, fault-bounded Robertson Bay-Murray block and are found to the coast west of the Glacier flute casts N60°W Mariner Glacier (Stump and others, 1983). cross-beds N30°E S60°W The presence of these large, fault-bounded Duke of York Island cross-beds NI0°E N30°E blocks naturally divides northern Victoria Land N40°E geology into distinctly different units or terranes. N50°E N10°E The task of developing a coherent plate-tectonic N5°W N10°E history of northern Victoria Land can be ap- N25°W proached in two ways: (1) working toward un- Lower Murray Glacier current lineation N20W°-S20°E ripple trains N80°W derstanding the geology within these terranes cross-beds N20°W N25°W and (2) understanding the structural, temporal, N35°W and facies relationships between them. Cape Klovstad ripples + cross-beds N40°E cross-beds N50°W N30°W flute-casts N25°W ROBERTSON BAY GROUP oscillation ripples N30°E-S30°E cross-beds N50°E Ackroyd Point cross-beds N The Robertson Bay Group is found in a large Mount Mulach cross-beds N25°W Figure 3. Paleocurrent directions (37 mea- asymmetric ripples N40°W triangular area between the Leap Year fault and Austin Peak cross-beds N25°W surements) from the Robertson Bay Group. flute-casts N5°W r Current lineations shown in inside ring. Di- its extensions, the Ross Sea, and the South Pa- ripples cross-beds N90°W cific (Fig. 2). From the time of Rastall and flute-casls N5°W rections derived from cross-beds are shown flute-casts N Priestley (1921), investigators have consistently N30°W in middle ring. Flute-cast orientations are Smithson-Graveson Glacier cross-beds N shown in outside ring. All orientation:; cor- described the unii; as a slate-graywacke series N60°W that is folded about northwest axes with strongly Everett Range flute-casts N10°W rected for dip but not for plunge. cross-beds N25°W developed axial-plane cleavage, and they have commonly interpreted most of the graywackes as being turbidites (Harrington and others, 1967; Crowder, 1968; Sturm and Carryer, 1970). The thickness of the Robertson Bay Group is plagioclase, orthoclase microcline, muscovite, More recently, Wright (1981) and Field and estimated to be a minimum of 2000 m (Wright, and minor lithic fragments of microg.ranite, Findlay (1983) again confirmed the ubiquitous 1981) or 3000 m (Field and Findlay, 1983) but quartzite, and muscovite schist and interpreted occurrence of turbidite beds in the Robertson is poorly constrained. Although it is now clear the provenance to be a granitic, cratonic source Bay Group and ha.ve used sedimentary features that the Robertson Bay Group represents a area. to develop a generalized depositional model. major turbidite basin, the distribution of sedi- To better document the provenance of the The majority of sandstone beds in the Robert- mentary facies studied so far only hints at the Robertson Bay Group, thin sections were made, son Bay Group are turbidites ranging from a few geometry of that basin. Lack of fossils and dis- and clasts were identified and point counted centimetres to a fe w metres in thickness. These tinctive horizons, coupled with structural com- (200 grains) from the coarser samples collected. beds are located in both fining-upward and plexity and the reconnaissance nature of studies, Figure 2 shows the localities, and Table 2 pre- coarsening-upward packets ranging from a few limits what can be interpreted about the lateral sents the results of the point counts. The devel- metres to hundreds, of metres thick. The beds are and vertical facies distribution within the ancient opment of cleavage in the rock has obscured laterally persistent and show extensive amalga- basin and its relationship to the Antarctic craton. some of the original clast texture; however, the mation only locally where the hemipelagic shale Some insight into the tectonic setting of the clasts do not show evidence of being well is thin and the sandstone beds are thickest. Robertson Bay basin can be gleaned from a rounded or well sorted. Instead, the original Wright (1981) concluded that the entire Robert- study of clast petrology of the turbidite material. grains seem to have been very angular and son Bay Group was deposited in a narrow range Wright (1981) reported a general continental crudely sorted. Although the petrology of the of environments (outer fan and basin plain) provenance with polycrystalline quartz grains original clasts has been altered near major faults within a very larj;e turbidite basin. Field and predominating but with a variety of rock frag- and later intrusions, elsewhere, the petralogic Findlay (1983) generally concurred with this ments of metasedimentary, igneous, and meta- characteristics of the clasts were well preserved, analysis but interpreted some sequences in the morphic fragments also present. Field and and the types of grains were relatively sisy to more southerly areis that they studied as belong- Findlay (1983) reported a generally high quartz identify. Figure 4 shows typical clasts found in ing to a variety of inner- to middle-fan environ- clast content and a mixture of sedimentary and Robertson Bay Group sandstones. ments as well. Puleocurrent directions deter- igneous rock fragments but suggested that high- Dickinson and Suczek (1979) and Dickinson mined from flute casts, grooves, and cross-beds grade metamorphic or basic igneous rock frag- and others (1983) compared sandstone compo- show northeast to northwest trends (Table 1; ments were rare or absent from the central and sitions with known or inferred tectonic settings Fig. 3). Rare sort-sediment slump horizons, southern parts of the outcrop area. In a narrow and produced a classification scheme that yields found at Robertson Bay and in the Everett strip of Robertson Bay Group rocks exposed a satisfactory predictions when used on a broad Range, show north vergence on slump folds, few kilometres east of the Leap Year fault, scale. Figures 5A, 6A, and 7A show plots of implying a northward-deepening paleoslope. Wodzicki and others (1982) reported quartz, Robertson Bay Group sandstone compositions

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TABLE 2. COARSE SANDSTONE PETROLOGY OF ROBERTSON BAY GROUP AND BOWERS SUPERGROUP on their triangular QFL, QmFLt, and QpLvLs compositional diagrams. On the QFL plot, all of Sample Single qtz Poly-qtz "Chert" Feldspar (plag.) Vole, rock frag. Phyllite Metasiltstone the Robertson Bay Group samples fall within

Robertson Bay area the Recycled Orogen field. All but two fall within the Recycled Orogen field on the QmFLt A-16 4(1 25 8 3 9 12 2 A-19 34 8 10 8 19 12 8 plot. The two points outside of the field plot in, X V! 17 17 1 6 7 34 A-21 52 18 3 7 5 14 0 or near, the Continental Block field. In the A-86 5: 28 4 9 4 4 0 QpLvLs diagram, the Robertson Bay Group A-86 5(1 28 4 5 5 8 A-84 61 6 6 8 20 plots in both the Collision Orogen and in the A-87 60 3 37 Subduction Complex fields. Dickinson anil Suc- Arkroyd Point zek (1979) divided all provenances into gsneral A-88 51 26 2 4 1 16 groups, each of which has a limited set of likely

Everett Range tectonic environments. The Recycled Orogen provenance has possible sources in deformed A-102 70 0 10 20 A-103 83 4 13 and uplifted parts of subduction zones, collision A-128 3:1 30 25 8 4 zones (such as arc-continent or conlinent- Mt. Mulach Graveson/Smithsan Glacier area continent), and in foreland fold-and-thrusi: belts. A-92 «i 15 8 8 Following the discussion in Dickinson and A-92 70 7 2 6 2 13 A-93 6:i 14 2 14 I 3 Suczek (1979), the Robertson Bay sandstones A-98 6:: 4 13 9 13 most closely match collision orogens where the A-98 49 22 2 5 10 12 sediment is derived primarily from sedimentary A-118 72 2 2 24 and metasedimentary rocks that were near both Lonely One nunatak continental margins just prior to collision. This A-125 47 25 10 7 8 3 source is characterized by moderate to high A-126 78 5 16 A-127 •15 33 4 10 4 4 quartz if the source was primarily cratonic, and

Bowers Mountains by chert-rich sands if the source was mainly the uplifted oceanic terranes caught between the ap- A-106 59 2 20 8 11 A-107 in 6 6 7 proaching continental blocks. The data indicate A-123 16 12 2 43 27 A-136 71 3 6 7 4 9 a complete transition between firsl-cycle A-137 49 20 29 quartzose metamorphic source rocks and sucond- A-144 :i8 4 19 35 3 A-144 14 2 2 73 cycle quartz, as would be expected if miogeo- clinal units and their metamorphic equivalents Note: data are given in percentage of recognizable coarse detrital grains, excluding carbonate, heavy minerals, and muscovite. All samples except those from the Bowets Mountains are Robertson Bay Group. were the primary sources. Chert is present in Numbers shown between cMumns are combined values of single quartz and polyquartz and of volcanic rock fragments and phyllite. minor amounts, as are volcanic rock fragments and rare serpentinite grains; thus, there is also a small oceanic component in the sour«: area. This is entirely compatible with a continent-

Robertson Bay Group Bowers Supergroup

Robertson Bay area X Sledgers Akroyd Point Mariner Everett Range Mt. Mulach + Smithson/Graveson gl Lonely One Nunatak

F L F

Figure SA. Quartz (Q); feldspar (F); lithic fragments (L). Triangular Figure 5B. Quartz (Q); feldspar (F); lithic fragments (L). Triangu- diagrams of Robertson Bay Group rocks; tectonic fields modified from lar diagram of Bowers Supergroup rocks; tectonic fields modified Dickinson and Suczek (1979). from Dickinson and Suczek (1979).

continent collision orogen source, most of the cised in applying the models to areas of poorly these processes were the same as in more recent detritus coming from cratonic sources but minor understood tectonic history. Although these times. The mineralogic maturity of these sand- amounts coming from remnants of oceanic rocks are Precambrian, at least in part, the stones is poor, containing grains of phyllite and sources that were caught and uplifted by the occurrence of ichnofossils (Wright, 1981) does other rather unstable components, so that exces- collision. not allow them to be very far back in the Pre- sive selective removal of lithic grains and feld- Dickinson and Suczek (1979) and Dickinson cambrian, and the extension of their model thus spars has not occurred. The large size of the and others (1983) based these models on Phaner- seems reasonable. The underlying assumption, basin, the distal aspect of the facies, and the ozoic sequences and indicated that extension however, is that plate-tectonic processes oper- homogeneous distribution of clast types all sug- into older rocks is done at the user's risk. They ated in the late Precambrian in the same way as gest that these samples are a fair representation also caution that unusual source areas can invali- they did in the Phanerozoic and that the sedi- of the source area. It is concluded, therefore, that date the models and that care should be exer- ments that were derived as a consequence of the Robertson Bay Group was probably depos-

Robertson Bay Group Bowers Supergroup

X Sledgers • Robertson Bay area o Akroyd Point • Everett Range O Mt. Mulach + Smithson/Graveson gl A Lonely One Nunatak

Figure 6A. Monocrystalline quartz (Qm); feldspar (F); total poly- Figure 6B. Monocrystalline quartz (Qm); feldspar (F); total polycrystalline lithic fragments (Lt). Triangular diagram of Robertson Bay crystalline lithic fragments (Lt). Triangular diagram of Bowers Super- Group rocks; tectonic fields modified from Dickinson and Suczek group rocks; tectonic fields modified from Dickinson and Suczek (1979). (1979).

Figure 7A. Polycrystalline quartz (Qp); volcanic and metavolcanic Figure 7B. Polycrystalline quartz (Qp); volcanic and metavolcanic lithic fragments (Ly); sedimentary-metasedimentary lithic fragments lithic fragments (Lv); sedimentary-metasedimentary lithic fragments (Ls). Triangular diagrams of Robertson Bay Group rocks; tectonic (Ls). Triangular diagram of Bowers Supergroup rocks; tectonic fields fields modified from Dickinson and Suczek (1979). modified from Dickinson and Suczek (1979).

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ited in a turbidite basin as continental blocks been folded at least twice and the Bowers Su- and bed amalgamation. The facies was inter- were closing an oc;an and were then deformed pergroup showing only a single deformation. preted as representing a slope environment and uplifted. As ths basement to the Robertson The Admiralty Intrusives of 360 Ma (Kreuzer rather than the much more distal environments Bay Group is not exposed, it is not known if this and others, 1981) are clearly post-tectonic, and of the Robertson Bay Group. Laird and others basin was floored by continental or oceanic subsequent uplift and erosion to form the Kukri (1982) reported that paleocurrent directions in- crust, but the "I" tj'pe granites of the Admiralty peneplain attest to a cessation of compressive dicate flow from the northwest to the southeast Intrusives (Wyborn, 1981) could be interpreted activity. Younger sediments above this major along the trend of the outcrop strip, nearly op- as implying a continental basement, assuming unconformity show only high-angle faulting. posite to the trend shown by the Robertson Bay that the granite source for the Admiralty Intru- From these lines of evidence, the convergent current indicators. sives was derived from the basement below the cycle from basin formation, filling of the basin, Clast petrology also shows considerable de- Robertson Bay Group. and compressive deformation all took place be- parture from the Robertson Bay sandstones. Some information exists that constrains the tween latest Precambrian and before the Late Wright (1981) noted the abundance of volcanic deformational everts implied by the model. The Devonian. rock fragments, and Laird and others (1982) age of the Robertson Bay Group places approx- listed a variety of redeposited rock fragments of imate temporal limits on the uplift of the source BOWERS SUPERGROUP igneous, sedimentary, and metamorphic rock terrane and the formation of the basin. Unfortu- types. From the Bowers Supergroup, seven nately, the lower age of deposition is not well The Bowers Supergroup rocks are found west samples were point counted: two from the constrained—estimates range from Vendian of the Robertson Bay rocks and east of the Lan- Mariner Group and five from the Molar based on acritarchi; (Cooper and others, 1982), terman (Wilson Group) and other metamorphic Formation. These data are displayed in Figures to "possible Cambrian," on the basis of tracks rocks in a long, narrow, fault-bounded strip 5B, 6B, and 7B, using the same triangular dia- and trails (Field arid Findlay, 1983). The pres- from the Ross Sea to the Pacific Ocean (Fig. 2). grams as before. The samples show a wider scat- ence of fossils in limestone clasts found in the The stratigraphy of these rocks is more varied, ter on the QFL and the QmFLt plots tut in upper part of the Robertson Bay Group (Wright and the presence of fossils helps constrain depo- general are more lithic-rich than are the Robert- and others, 1984) shows that sedimentation was sitional events for much of the section (Laird son Bay rocks. The QpLvLs plot, however, puts still active at the Cambrian-Ordovician bound- and others, 1982). The oldest rocks in the Bow- four of the five samples within the Arc Orogen ary. The age of deformation of the sediment ers Supergroup make up the Sledgers Group. field. The conclusion that follows, from a con- pile is also somewhat unclear (Bradshaw and This group contains basic to intermediate vol- sideration of these lines of evidence and from others, 1982). Oil the basis of whole-rock canic rocks and a turbidite and hemipelagic criteria in Dickinson and Suczek (1979), is that potassium-argon age dating of slates, Adams and shale sequence of probable Early to Middle the Sledgers Group most likely represents a fore- others (1982) proposed two low-grade meta- Cambrian age and more than 3,500 m thick. arc or back-arc basin within the undissected arc morphic events. The first, found in the Wilson Overlying these rocks, there is a +2,000-m-thick provenance. Volcanic lithic fragments with plagi- Group (451-483 Ma), Robertson Bay Group sequence of Middle and Upper Cambrian sand- oclase phenocrysts are characteristic constituents (447-502 Ma), and some parts of the Sledgers stones, mudstones, and limestones (Mariner of arc-derived sediments and predominate in Group (441-467 Ma), was assigned to the Group). In turn, these rocks are overlain by ter- undissected arc sources. As the arc complex be- Cambrian Ross Orogeny. The second event restrial quartz sandstones and conglomerates, comes eroded and metamorphic cores are ex- (384-421 Ma) was reported from other Sledgers the Leap Year Group which in places is thicker posed, quartz increases and volcaniclastic debris samples and from the Mariner and Leap Year than 3,000 m. continues. Several sedimentary features, espe- cially the presence of large, slumped ho:rizons Group rocks and was related to the Silurian- The Molar Formation of the Sledgers Group and large olistoliths reported by Laird and others Devonian Borchgrevink Orogeny. As Adams is of particular interest here because of the possi- (1982), may best be interpreted as fore-arc to and others (1982) pointed out, however, there is ble relationship between this turbidite sequence trench-fill transitional environments. The con- no field evidence for any important metamor- and the Robertson Bay Group. The Molar For- clusions of Weaver and others (1984) are com- phic or structural discordance between the mation contains graded and massive sandstone patible with this information. Sledgers and Mariner Groups, and the whole- beds, conglomerate layers, and thick chaotic rock potassium-argon age dates may not provide zones containing large angular blocks (Laird and After deposition of the Sledgers Group, the accurate deformation ages for the rocks in- others, 1982). The sequence also contains vol- more quartz- and lime-rich shelf sequence of the volved. Bradshaw and others (1982) interpreted canics which Weaver and others (1984) showed Mariner Group represents either differential re- structural data and concluded that a minimum as representing primitive island-arc tholeiites. working of clasts due to the shallower water or a of five tectonic events have occurred in northern Wright (1981) noted considerable variation in more dissected source terrane that exposed Victoria Land, the Robertson Group having bed thickness along strike, as well as channeling quartz-rich plutonic or sedimentary rocks. Finally, the clean sandstones and conglomerates of the Leap Year Group represent the terrestrial TABLE 3. SIMILARITIES AND DIFFERENCES BETWEEN THE ROBERTSON BAY GROUP AND THE BOWERS SUPERGROUP environments at the top of the exposed section. The deformation history of this sequence Depositional age Basement type Sedimentary facies starts with uplift in the source region for the

Robertson Bay Group Late Precambrian(?)/Ordovician Unknown Distal turbidites Molar Formation. Volcanic clasts are a large component; therefore, it is reasonable to think Bowers Supergroup Late Precambrian(?)/Oidovician Unknown Proximal turbidites overlain by shallow-water Cambrian marine that an uplifted volcanic arc is a major source deposits and Ordovician(?) terrestrial deposits for the detritus. Laird and others (1982) indicated that the Mariner Group units are inter- Current directions Major clast types Whole-rock K-Ar ages Other deformation age control rupted by unconformities and have transgressive- NW-NE Quartz-metamorphic rock fragments 447-502 m.y. B.P. 360 m.y. B.P. post-tectonic intrusion regressive cycles within shallow-marine units. SE Volcanic rock fragments Sledgere Group (441-467 m.y. B.P. Capped by Beacon and Ferrar The increase in the percentage of quartz grains is and younger) (Jurassic) thought to be due primarily to the increased

amount of winnowing and abrasion of more un- provenance that is characteristic of rift environ- REFERENCES CITED stable grains rather than to a change in the tec- ments? The available data do not allow resolu- Adams, C J.D., Gabiles, J.E., Wodzicki, A., Laird, M. G., and Bradshaw, J. D., 1982, Potassium-argon geochronology of the Precambrian-Cambrian tonic setting. For this reason, little significant tion of these various possibilities, yet resolution Wilson and Robertson Bay Groups and Bowers Supergroup, northern Victoria Land, Antarctica, in Craddock, Campbell, ed., Antarctic geo- tectonic information is likely from further dis- of these questions is critical. It is obvious that science: Madison, Wisconsin, University of Wisconsin Press, Interna- cussion of the clast petrology of the Mariner and forcing sedimentary facies and tectonic events to tional Union of Geological Sciences, Series B, no. 4, p. 543-548. Bradshaw, J. D„ Laird, M. G., and Wodzicki, A., 1982, Structural style and the Leap Year Groups. The Upper Cambrian conform to models that assume similar histories tectonic history in northern Victoria Land, in Craddock, Campbell, ed., Antarctic geoscience: Madison, Wisconsin, University of Wisconsin age of the Mariner plus the possible Ordovician for all rocks in the Transantarctic Mountains is Press, International Union of Geological Sciences, Series B, no. 4, trace fossils in the Leap Year Group do place a counterproductive approach if large alloch- p. 809-816. Cooper, R. A., Jago, J. B„ MacKinnon, D. 1., Shergold, J. H, and Vidal, G„ lower limits on the time of deformation of these thonous terranes are in fact present. 1982, Late Precambrian and Cambrian fossils from northern Victoria Land and their scratigraphic implications, in Craddock, Campbell, ed., beds. Adams and others (1982) reported Cam- Antarctic geoscience: Madison, Wisconsin, University of Wisconsin brian whole-rock phyllite ages for some samples Press, International Union of Geological Sciences, Series B, no. 4, SUMMARY p. 629-633. of the Sledgers Group, and Silurian-Devonian Crowder, D. F., 1968, Geology of a part of north Victoria Land, Antarctica: U.S. Geological Survey Professional Paper No. 600-D, p. D95-D107. ages for other Sledgers, Mariner, and Leap Year The Robertson Bay Group is a distal turbidite Dickinson, W. R., and Suczek, C. A., 1979, Plate tectonics and sandstone Group samples. compositions: American Association of Petroleum Geologists Bulletin, sequence with sources in continental recycled v.63, p. 2164-2182. From the preceding discussion, it is clear that orogenic terranes and with deposition beginning Dickinson, W. R., Beard, L. S„ Brakenridge, G. R., Erjavec, J. L., Ferguson, R. C., Inman, K. F., Knepp, R. A., Lindberg, F. A., and Ryberg, P. T„ some similarities exist between the Robertson in late Precambrian times in a large basin. On 1983, Provenance of North American Phanerozoic sandstones in rela- tion to tectonic setting: Geological Society of America Bulletin, v. 94, Bay Group and the Bowers Supergroup, al- the basis of presently available age control, these p. 222-235. though a number of important differences also strata were deformed, at least, by Ordovician Field, B. D., and Findlay, R. H., 1983, The sedimentology of the Robertson Bay Group, north Victoria Land, in Oliver, R. L., and others, eds., Antarctic exist. Table 3 summarizes the similarities and compression followed by Late Devonian, post- earth science: Canberra, Australia, Australian Academy of Sciences, tectonic granite intrusions. p. 102-106. differences between the various aspects of the Findlay, R. H., and Field, B. D., 1983, Tectonic significance of deformations two sequences. From these points, and with an The Bowers Supergroup has, at its base, a affecting the Robertson Bay Group and associated rocks, Northern Vic- toria Land, Antarctica, in Oliver, R. L., and others, eds., Antarctic earth allowance for the uncertainties involved, a more proximal turbidite sequence with sources science: Canberra, Australia, Australian Academy of Science, p. 107-112. number of plate-tectonic scenarios for the region in an arc orogen setting and with deposition in a Grindley, G. W., and Davey, F, J., 1982, The reconstruction of New Zealand, as a whole can be constructed. At one extreme, fore-arc slope basin environment. The deposi- Australia, and Antarctica, in Craddock, Campbell, ed., Antarctic geoscience: Madison, Wisconsin, University of Wisconsin Press, Interna- an argument can be made for little, or no, direct tion of arc-derived detritus apparently began in tional Union of Geological Sciences, Series B, no. 4, p. 15-30. Grindley, G. W„ McDougall, I., 1969, Age and correlation of the Nimrod relationship between the sequences during depo- the Early Cambrian, but deposition of more Group and other Precambrian rock units in the central Transantarctic sition. The terranes could have originated and quartz-rich sediments continued in shallow- Mountains, Antarctica: New Zealand Journal of Geology and Geophys- ics, v. 12, p. 391-411. developed in quite different places and then, marine environments through the Cambrian, Harrington, H. J., Wood, B. L„ McKellar, I. C„ and Lensen, G. J., 1967, Topography and geology of the Cape Hallett-Tucker Glacier district, later, juxtaposed by accretion and/or strike-slip ending in a thick terrestrial deposit of Ordovi- Antarctica: New Zealand Geological Survey Bulletin, Wellington, New faulting (Weaver and others, 1984) during the cian(?) age. Deformation history of these rocks Zealand Department of Scientific and Industrial Research, no. 80, 100 p. Ordovician-Devonian interval. This "suspect is uncertain but may involve both Cambrian- Kreuzer, H., Hohndorf, A„ Lenz, H., Vetter, U., Tessensohn, F., Muller, P., Jordan, H„ Harre, W„ and Besang, C„ 1981, K/Ar and Rb/Sr dating terrane" origin would be analogous to the his- Ordovician and Silurian-Devonian events be- of igneous rocks from north Victoria Land, Antarctica: Geologisches tory of the northwestern coast of North America fore Late Devonian intrusion of Admiralty Jahrbuch, B41, p. 267-273. Laird, M. G., Bradshaw, J. D., and Wodzicki, A., 1982, Stratigraphy of the where long-term subduction was also responsi- Granite and formation of the Kukri peneplain. upper Precambrian and lower Paleozoic Bowers Supergroup, northern Victoria Land, Antarctica, in Craddock, Campbell, ed., Antarctic geo- ble for an assembly of accreted terranes. The Relationships between these terranes are science: Madison, Wisconsin, University of Wisconsin Press, Interna- resulting area has a series of fault-bounded tional Union of Geological Sciences, Series B, no. 4, p. 535-542. equivocal with the present data. This is reflected Rastall, R. H., and Priestley, R. E-, 1921, The slate-greywacke formation of blocks, of different origins and histories, assem- by various models in which some of the terranes Robertson Bay: British Antarctica ("Terra Nova") Expedition, 1910: bled outboard of the former continental edge. London, Natural History Report, Geology 1, p. 121-129. are entirely allochthonous with respect to Stump, Edmund, 1976, On the late Precambrian-early Paleozoic metavolcanic The other extreme would be to assume that the and metasedimentary rocks of the Queen Maud Mountains, Antarctica, Antarctica and by other models in which all and comparison with rocks of similar age for southern Africa: Colum- rocks of northern Victoria Land are in approxi- rocks are essentially near their site of formation bus, Ohio, University of Ohio Institute of Polar Studies Report No. 62, mately the same relative position in which they 212 p. but are offset by vertical block faults. Analysis of 1980, Observations on the Ross Orogen, Antarctica, in Cresswell, originated and have only been offset by high- M. M., and Vella, P., eds., Gondwana five: Wellington, New Zealand, sedimentology and clast petrology has been use- Victoria University, p. 205-208. angle faults. In this case, the present data do not ful in the interpretation of plate-tectonic Stump, E., Laird, M. G., Bradshaw, J. D., Holloway, J. R., Borg, S. G., and Lapham, K. E., 1983, Bowers graben and associated tectonic features seem to allow a coherent plate-tectonic recon- environments and, when combined with local cross northern Victoria Land, Antarctica: Nature, v. 304, p. 334-336. struction. By reference to the sketch map (Fig. Sturm, A., and Carryer, S. J., 1970, Geology of the region between Matusevitch deformation history, has provided a method of and Tucker Glaciers, north Victoria Land, Antarctica: New Zealand 2), it is seen that the pattern of interpreted tec- independently determining the probable tectonic Journal of Geology and Geophysics, v. 13, p. 408-435. Tessensohn, F., Duphorn, K., Jordan, H., Kleinschmidt, G„ Skinner, D.N.B., tonic environments would place a singly de- history of individual terranes and larger geologic Vetter, U., Wright, T. O., and Wybom, D., 1981, Geological compari- formed, arc-derived sequence (Bowers Super- son of basement units in north Victoria Land, Antarctica: Geologisches units. The strong suggestion that accreted ter- Jahrbuch, B41, p. 31-88. group) between a polydeformed metamorphic ranes are present in northern Victoria Land is of Vandenberg, A.H.M., 1978, The Tasman Fold Belt System in Victoria: Tec- tonophysics, v. 48, p. 267-297. block (Wilson Group) on the craton side and a considerable importance to the larger questions Weaver, S. D., Bradshaw, J. D., and Laird, M. G„ 1984, Geochemistry of block derived from a continent-continent colli- Cambrian volcanics of the Bowers Supergroup and implications for the of the tectonic relationships of the Transantarc- early Palaeozoic tectonic evolution of northern Victoria Land, Antarc- sion (Robertson Bay Group) on the ocean side. tic Mountains and their predrift neighbors along tica: Earth and Planetary Science Letters, v. 68, p. 128-140. Wodzicki, Antoni, Bradshaw, J. D., and Laird, M. G., 1982, Petrology of the This pattern does not fit any simple plate- the Pacific Gondwanaland craton edge. Wilson and Robertson Bay Groups and Bowers Supergroup, northern tectonic model for in situ continental margins Victoria Land, Antarctica, in Craddock, Campbell, ed., Antarctic geoscience: Madison, Wisconsin, University of Wisconsin Press, Interna- undergoing compressional orogeny. Several tional Union of Geological Sciences, Series B, no. 4, p. 549-554. rather troublesome problems arise if it is as- Wright, T. O., 1981, Sedimentology of the Robertson Bay Group, north Victo- ACKNOWLEDGMENTS ria Land, Antarctica: Geologisches Jahrbuch, B41, p. 127-138. sumed that all rocks are near the place where Wright, T. O., Ross, R. J., Jr., and Repetski, J. E., 1984, Newly discovered youngest Cambrian or oldest Ordovician fossils from the Robertson Bay they formed. If the Bowers Supergroup repre- terrane (formerly Precambrian), northern Victoria Land, Antarctica: I would like to thank the U.S. National Geology, v. 12, p. 301-305. sents arc volcanism followed by uplift and cessa- Science Foundation and the West German Bun- Wybom, Doone, 1981, Granitoids of north Victoria Land, Antarctica—Field tion of activity, how did it form between the and petrographic observations: Geologisches Jahrbuch, B41, desanstaldt fur Geowissenschaften und Roh- p. 229-250. older (?) Robertson Bay Group and the Antarctic stoffe for their support of this research. Careful MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 1, 1983 craton? If it represents a rifting event, why does REVISED MANUSCRIPT RECEIVED APRIL 26, 1985 reviews by Edward Stump and Campbell Crad- MANUSCRIPT ACCEPTED MAY 7, 1985 the clast petrology not reflect continental-block dock were much appreciated. Printed in U.S.A.

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