Depositional history of pre-Devonian strata and timing of Ross orogenic tectonism in the central Transantarctic Mountains, Antarctica

Paul M. Myrow* Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA Michael C. Pope Department of Geology, Washington State University, Pullman, Washington 99164, USA John W. Goodge Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275, USA Woodward Fischer Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA Alison R. Palmer Institute for Cambrian Studies, 445 North Cedarbrook Road, Boulder, Colorado 80304-0417, USA

ABSTRACT small fraction of its previous extent. The into trilobite- and hyolithid-bearing calcar- basal unit of the Byrd GroupÐthe predom- eous siltstone of the Starshot Formation A combination of ®eld mapping, detailed inantly carbonate ramp deposits of the and alluvial-fan deposits of the Douglas sedimentology, carbon isotope chemostra- Shackleton LimestoneÐrest with presumed Conglomerate. Trilobite fauna from the tigraphy, and new paleontological ®nds unconformity on the restricted Goldie For- lowermost siltstone deposits of the Starshot provides a signi®cantly improved under- mation. Paleontological data and carbon Formation date the onset of this transition standing of the depositional and tectonic isotope stratigraphy indicate that the Low- as being late Botomian. history of uppermost Neoproterozoic and er Cambrian Shackleton Limestone ranges The abrupt transition from the Shackle- lower Paleozoic strata of the central Trans- from lower Atdabanian through upper ton Limestone to a large-scale, upward- antarctic Mountains. On the basis of these Botomian. coarsening siliciclastic succession records data, we suggest revision of the existing This study presents the ®rst description deepening of the outer platform and then stratigraphy, including introduction of new of a depositional contact between the deposition of an eastward-prograding mo- formations, as follows. The oldest rocks ap- Shackleton Limestone and overlying clas- lassic wedge. The various formations of the pear to record late Neoproterozoic deposi- tic units of the upper Byrd Group. This upper Byrd Group show general strati- tion across a narrow marine margin under- carbonate-to-clastic transition is of critical graphic and age equivalence, such that lain by Precambrian basement. Siliciclastic importance because it records a profound coarse-grained alluvial-fan deposits of the deposits of the Neoproterozoic Beardmore shift in the tectonic and depositional history Douglas Conglomerate are proximal equiv- GroupÐhere restricted to the Cobham of the region, namely from relatively pas- alents of the marginal-marine to shelf de- Formation and those rocks of the Goldie sive sedimentation to active uplift and ero- posits of the Starshot Formation. Paleocur- Formation that contain no detrital compo- sion associated with the Ross orogeny. The rents and facies distributions from these nents younger than ca. 600 MaÐoccupied uppermost Shackleton Limestone is capped units indicate consistent west (or southwest) an inboard zone to the west. They consist by a set of archaeocyathan bioherms with to east (or northeast) transport of sediment. of shallow-marine deposits of an uncertain up to 40 m of relief above the sea¯oor. A Although the exact structural geometry is tectonic setting, although it was likely a rift widespread phosphatic crust on the bio- unknown, development of imbricate thrust to passive margin. Most rocks previously herms records the onset of orogenesis and sheets in the west likely caused depression mapped as Goldie Formation are in fact drowning of the carbonate ramp. A newly of the inner margin and rapid drowning of Cambrian in age or younger, and we reas- de®ned transitional unit, the Holyoake For- the Shackleton Formation carbonate ramp. sign them to the Starshot Formation of the mation, rests above this surface. It consists This tectonic activity also caused uplift of Byrd Group; this change reduces the ex- of black shale followed by mixed nodular the inboard units and their underlying posed area of the Goldie Formation to a carbonate and shale that ®ll in between, basement, unroo®ng, and widespread de- and just barely above, the tallest of the position of a thick, coarse clastic wedge. *E-mail: [email protected]. bioherms. This formation grades upward Continued deformation in the Early Ordo-

GSA Bulletin; September 2002; v. 114; no. 9; p. 1070±1088; 16 ®gures.

For permission to copy, contact [email protected] 1070 ᭧ 2002 Geological Society of America PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

Figure 1. Geologic map of central Transantarctic Mountains with inset (upper left) of Antarctica showing location of ®eld area. CPÐ Cotton Plateau, CRÐCobham Range, MUÐMount Ubique, PAGÐPrincess Anne Glacier, and PPÐPanorama Point. Close-up of Holy- oake Range on right shows basic geologic units. Details of thrust slices of transitional units cannot be shown at this scale, but rectangular blocks indicate locations where upper Shackleton through Douglas transitions occur, each with well-developed bioherms that all dip vertically and face west. S1 (section 1), S2 (section 2), and HR (High Ridge) are described in text. vician (younger than 480 Ma) in turn af- because of a lack of biostratigraphic con- Formation. Fossil-bearing strata of the Byrd fected these synorogenic deposits, causing straints, detailed sedimentology, and physical Group are generally considered to have been folding and thrust repetition of all pre- stratigraphy of these units. The traditional deposited unconformably upon the Goldie Devonian units. view of stratigraphic relationships in this re- Formation. The basal formation of the Byrd gion, as reviewed by Stump (1995) and dis- Group, the Lower Cambrian Shackleton Lime- Keywords: Archaeocyatha, Cambrian, Neo- cussed in more detail herein, is that basement stone, consists of dominantly carbonate rocks proterozoic, reef, Ross orogeny, Transant- rocks of the Nimrod Group (Archean and Ear- with a range of facies including archaeocyathan- arctic Mountains. ly Proterozoic) are directly overlain by Neo- bearing bioherms. Clasts of the Shackleton proterozoic rocks of the Beardmore Group, al- Limestone occur within overlying synorogenic INTRODUCTION though this relationship has not been conglomerate- and sandstone-rich units of the observed. The latter consists of unfossiliferous Douglas and Starshot Formations, respective- Stratigraphic relationships of pre-Devonian metamorphosed siliciclastic and carbonate ly. The stratigraphic relationships and ages of formations in the central Transantarctic Moun- rocks of the Cobham Formation and overlying these younger units were poorly de®ned; pos- tains (Fig. 1) have been the subject of much less metamorphosed siliciclastic rocks (mostly sible ages range from latest Early Cambrian conjecture and debate (Fig. 2), in large part sandstone with minor shale) of the Goldie to Devonian (Fig. 2).

Geological Society of America Bulletin, September 2002 1071 MYROW et al.

Figure 2. Interpretations of stratigraphic relationships of pre-Devonian Formations in the central Transantarctic Mountains. Right-hand column is revised view of these relationships. Formations: StÐStarshot Formation, ShÐShackleton Limestone, DoÐDouglas Conglom- erate, DiÐDick Formation, GoÐGoldie Formation. Authors: AÐLaird (1963), BÐSkinner (1964), CÐLaird et al. (1971), DÐLaird (1981), EÐRowell et al. (1986), FÐRowell et al. (1988b), GÐRees et al. (1989), HÐRowell and Rees (1989), IÐRees and Rowell (1991), JÐGoodge et al. (1993) and Goodge and Dallmeyer (1996), KÐGoodge (1997). For reports simply describing ``Lower Cambrian'' strata, base of formation arbitrarily chosen as upper Tommotian.

New geochronologic data recast the age Group, and these ``outboard'' Goldie strata are of the upper Byrd Group from the southern and stratigraphic relationships of these units. now assigned to that formation. A very nar- Holyoake Range. Sedimentological aspects of Detrital-zircon geochronology on sedimentary row exposure of enigmatic, generally more this transition from massive, archaeocyathan units of the region indicates that the bulk of metamorphosed, rocks in the vicinity of Cot- bioherms to overlying siliciclastic-dominated outcrops mapped as Goldie Formation are in ton Plateau and the eastern Cobham Range strata allow us to reconstruct the depositional fact younger than the Shackleton Limestone were also originally mapped as Goldie For- response to Ross orogenesis. In addition, a new (Goodge et al., 2000, 2002). Detrital zircons mation. These have older detrital-zircon sig- trilobite collection in strata above the bioherms from these sandstone deposits are as young as natures (1000 Ma or older) and probably rep- provides constraints on the ages of several for- ca. 520 Ma, making their depositional age resent bona ®de Neoproterozoic deposits mations and the timing of tectonic events. younger than the Atdabanian Stage of the (``inboard assemblage'' of Goodge et al., These new ®ndings, in conjunction with the Lower Cambrian (Landing et al., 1998; Bow- 2000, 2002). geochronological data already mentioned, have ring and Erwin, 1998; Fig. 3). These young This study provides new ®eld, fossil, and profound implications for the depositional and provenance ages come from strata in an are- isotopic data as the basis for additional strati- tectonic history of the Transantarctic Moun- ally widespread belt of ``Goldie'' rocks that graphic revisions to the pre-Devonian sedimen- tains. The ®ndings also help clarify a number occupied an outboard (eastern) position along tary succession in the region. Sedimentological, of unresolved ®rst-order stratigraphic issues of the continental margin at the time of deposi- physical stratigraphic, and chemostratigraphic correlation, and the relative ages, of nearly all of the pre-Devonian units in this region. tion. On the basis of detrital-zircon age spectra data set limits on the nature and timing of a and ®eld relationships, we interpret the ``out- Cambrian transgression onto the Antarctic cra- FIELD AREA board'' Goldie to be a temporal and strati- ton. We also describe the ®rst documented de- graphic equivalent of the younger siliciclastic positional contact between the Shackleton Geologic mapping, sampling, and measure- deposits of the Starshot Formation of the Byrd Limestone and younger synorogenic deposits ment of numerous stratigraphic sections were

1072 Geological Society of America Bulletin, September 2002 PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

BYRD GROUP

Deposits of the Byrd Group are widespread in the central Transantarctic Mountains. The stratigraphic units include the Shackleton Limestone and overlying clastic deposits of the Starshot, Douglas, and Dick Formations.

Shackleton Limestone

The Lower Cambrian Shackleton Lime- stone crops out between Beardmore and Byrd Glaciers (Fig. 1); its most complete exposures are in the Holyoake Range (Rowell et al., 1988b; Rees et al., 1989; Rowell and Rees, 1989) where Laird (1963) designated a type locality. Archaeocyathan-bearing rocks from this formation were collected during Ernest Shackleton's ill-fated 1908±1909 expedition, but extensive study did not begin on this for- mation until much later (e.g., Laird et al., 1971; Rees et al., 1989). The archaeocyathans Figure 3. Comparison of Lower Cambrian stages of Siberia, South China, and Laurentia. indicate a Botomian depositional age for at Age constraints from Landing et al. (1998). least part of the formation (Debrenne and Kruse, 1986). Trilobite fauna described by Rowell et al. (1988a) and Palmer and Rowell (1995) completed during the 1998±1999 and 1999± rocks near Cotton Plateau (Fig. 1), has been con®rmed a Botomian age but suggested that 2000 austral summers. Field camps were es- widely cited as evidence for rifting at that some deposits are Atdabanian and that youn- tablished at Moody Nunatak and Cotton Pla- time. A new U-Pb zircon age of 668 Ϯ 1Ma ger parts were possibly deposited in the To- teau in the western Queen Elizabeth Range, in for the gabbroic phase (Goodge et al., 2002) yonian (Fig. 3). Nearly all of the trilobites the Mount Ubique area of the Surveyors suggests that these rocks and their associated have close af®nities with Chinese forms of the Range in the Churchill Mountains, and in the sedimentary succession are signi®cantly youn- Tsanglangpuan Stage, which largely overlaps central Holyoake Range on Errant Glacier, ger than previously thought. the Botomian Stage. Carbonate units in the providing access to many key localities. De- The Goldie Formation is a thick succession Queen Maud Mountains that are considered tailed ®eld observations were made possible dominated by ®ne-grained sandstone. On the equivalents of the Shackleton Limestone also over a geographically large area by a combi- basis of the presence of graded beds, ¯ute contain Botomian and possible Atdabanian tri- nation of overland travel and helicopter transit. marks, and abundant climbing ripples, it was lobites and archaeocyathans (Rowell et al., Previous mapping and stratigraphic descrip- considered a deep-sea turbidite succession 1997). tions by several authors, cited subsequently, es- (Laird et al., 1971; Stump, 1995), but these The Shackleton Limestone is a thick car- tablished the distribution of sedimentary units observations may have been made on ``out- bonate deposit with a lower unit of unfossil- upon which our work is based. board'' Goldie rocks now included in the iferous interbedded quartzite and carbonate younger Starshot Formation. There is little di- that is exposed below Panorama Point (Fig. 1) BEARDMORE GROUP on the western slopes of Cotton Plateau, agnostic paleoenvironmental evidence in much where it is in contact with the Goldie For- of the ``inboard'' Goldie, in part because of low- The Beardmore Group, consisting of the mation. At other localities in the region, the grade metamorphism. However, one locality ϳ3 Cobham and Goldie Formations, is a thick as- contact was considered by Laird (1963) to be km southeast of Panorama Point above Prince semblage of graywacke, carbonate, and minor an unconformity, and at Cotton Plateau the Edward Glacier contains a well-developed suite volcanic units that have been correlated with same contact was interpreted similarly by similar siliciclastic deposits along the length of sedimentary structures, including large-scale Stump et al. (1991) and Stump (1995). How- of the Transantarctic Mountains (Stump, 1982; hummocky cross-strati®cation, which indicates ever, this contact at the base of a syncline in Stump et al., 1986). The inferred stratigraphic that at least part of this formation was depos- Shackleton Limestone at Cotton Plateau position of the Beardmore Group between ited above storm wave base. Coarse conglom- shows structural evidence for fault displace- basement rocks and Lower Cambrian carbon- erate beds were tentatively suggested to rep- ment (Goodge et al., 1999), including angular ate deposits led previous workers to suggest a resent Neoproterozoic diamictites (Stump et discordance, a mineral-elongation lineation late Precambrian age for the group (e.g., Laird al., 1988), but intense deformation of sections and stretched pebbles, asymmetric folds, et al., 1971), but the youngest constraint on it immediately below Panorama Point and at asymmetric microstructures, and mineraliza- is unclear (i.e., whether it extends into the other localities, including isoclinal folding of tion indicating a structural relationship. The Cambrian). A Sm-Nd isochron age of 762 Ϯ the deposits and stretching of pebbles to as- contact is also faulted at other localities that 24 Ma for basalt and gabbro (Borg et al., pect ratios up to 20:1, makes such a determi- have been rechecked (Rowell et al., 1986; 1990), interpreted as interlayered with Goldie nation tenuous. Rees et al., 1989; Palmer and Rowell, 1995).

Geological Society of America Bulletin, September 2002 1073 MYROW et al.

Therefore, although we disagree with previous pergroup. The Douglas Conglomerate at its acter, they are similar to the bulk of the Star- workers about the present nature of the Cotton type locality is purportedly in depositional shot Formation. Recent detailed analysis in- Plateau contact and although we have no- contact with ®ne-grained siliciclastic deposits dicates that the Starshot Formation was where observed the stratigraphic base of the of the Dick Formation (Skinner, 1964; Rees et deposited in environments that ranged from Shackleton Limestone, it is possible that it al., 1988; see subsequent discussion). shoreline and possible ¯uvial facies to deeper was originally deposited upon the currently Rees and Rowell (1991) provided the only shelf (Myrow et al., 2002). Trace fossils occur underlying ``inboard'' Goldie units. detailed sedimentological study of the Doug- in several sections, but they are not common. Detailed reconstruction of paleoenviron- las Conglomerate, but noted that no complete The vast majority of the formation was de- ments and depositional history of the forma- section of the formation exists and that fault- posited in inner-shelf to shoreline environ- tion has been hampered by locally severe de- ing has made stratigraphic correlations dif®- ments, in part as wave-modi®ed turbidites formation and numerous fault contacts. cult. Their facies analysis indicated deposition (Myrow et al., 2002). Abundant paleocurrent Formation thickness is also dif®cult to assess, mainly in proximal to distal alluvial-fan en- data indicate transport toward the east and and estimates of 1000±2000 m (e.g., Burgess vironments, but also in marine and lacustrine northeast. The presence of wave-modi®ed tur- and Lammerink, 1979) are not well con- settings. Clast compositions in the conglom- bidites and conglomeratic beds, the inferred strained. The depositional setting of the for- eratic alluvial-fan deposits are dominated by facies relationships with the Douglas Con- mation in our ®eld area was previously inter- quartzite and limestone, the latter eroded from glomerate, and the paleocurrent patterns all preted as a simple ramp with intertidal facies the Shackleton Limestone. However, the fol- suggest development of a high-relief proximal passing laterally into a high-energy oolitic shoal lowing clast types were also noted: argillite, hinterland adjacent to a storm-in¯uenced sea complex that contains many individual archaeo- sandstone, metaquartzite, diorite, two-mica (Myrow et al., 2002). cyathan bioherms; no basinal facies were re- granite, gneissic granite, amygdaloidal spilite, ported (Rees et al., 1989). Deposits of the oolitic rhyolite, basalt, chert, and dolomite (Skinner, Dick Formation shoal facies are laterally extensive and common- 1964, 1965; Rees et al., 1987; Rees et al., ly include small algal-archaeocyathan bioherms 1988). Clasts of Shackleton Limestone in the (Rees et al., 1989). The bioherms were locally Douglas Conglomerate are locally folded and The Dick Formation consists primarily of ecologically zoned as a function of depositional contain some calcite veins, which indicate de- ®ne siliciclastic deposits (shale through ®ne- energy and in cases coalesced to form com- formation prior to Douglas deposition (Rowell grained sandstone) exposed in the northern posite bioherm complexes up to 50 m thick. et al., 1986). The Douglas itself is folded and Churchill Mountains (Fig. 1). Its type locality Burrow-mottled mudstone and skeletal or pe- contains cleavage, indicating that Ross defor- is ϳ15 km east of Mount Dick, south of the loidal packstone were interpreted as shallow- mation outlasted deposition of the unit. outlet of Byrd Glacier at Stations P and N water, low-energy deposits formed on either (Skinner, 1964). At Station P, 150 m of this side of the oolite shoal complex (Rees et al., Starshot Formation unit is exposed below the Douglas Conglom- 1989). However, stratigraphic position, litho- erate. It is presumably conformably overlain logic characteristics, and facies relationships The Starshot Formation was de®ned by by Douglas Conglomerate at this locality suggest that these deposits are entirely deeper Laird (1963) to include shale, sandstone, and (Skinner, 1964; Rees et al., 1988), but little is subtidal in origin and formed basinward (east- minor conglomerate exposed along the east known about its age or relationship to other ward) of the oolite shoal complex. side of the lower Starshot Glacier (Fig. 1); its stratigraphic units. Burgess and Lammerink type section is at Mount Ubique. Clasts in the (1979) claimed that it conformably overlies Douglas Conglomerate conglomeratic units have compositions similar the Shackleton Limestone (no details provid- to those of the Douglas Conglomerate and ed), but this contact was considered a fault by Clastic rocks of the upper Byrd Group have also contain archaeocyathan fossils, thus in- Rees et al. (1988) and we agree. The forma- been assigned to several formations including dicating that the Starshot Formation was de- tion contains thin polymictic conglomerate the Douglas Conglomerate, a succession of posited contemporaneously with or after the beds with carbonate clasts whose character is coarse clastic deposits exposed throughout the Shackleton Limestone. No depositional con- similar to that of clasts of the Douglas and Churchill Mountains. Clasts derived from the tacts have ever been described between the Starshot Formations (Laird, 1963). Shackleton Limestone were recognized early Starshot Formation and any other formation in Rees et al. (1988) considered the Dick For- on within the Douglas Conglomerate (Skinner, the region. Likewise, no fossils have been re- mation to have been deposited after the initial 1964, 1965); these clasts indicate uplift of the covered from the Starshot Formation, other episode of early Paleozoic deformation, and Shackleton and older units prior to deposition than those inherited in clasts, so determina- they viewed it as genetically related to the of these conglomeratic strata. Laird (1981) tions of its minimum depositional age are gen- Douglas Conglomerate. They interpreted the made the case for a sharp but conformable erally unconstrained. unit to represent ¯uvial to marginal-marine de- contact between the Shackleton and Douglas The depositional environments of the Star- posits that were essentially the distal equivalent Formations. However, abundant faults in this shot Formation are poorly understood, and in- of the Douglas alluvial-fan facies. Rare trace region make this claim suspect, and locally at terpretations range from deep-water turbiditic fossils suggest that the deposits are at least part- sections along the Starshot Glacier, the Doug- to shallow-water shelf (Laird, 1963; Laird et ly marine. We did not study the Dick Forma- las Conglomerate rests unconformably on al., 1971). Laird et al. (1971) recognized prob- tion in detail, but our examination of the unit folded Shackleton Limestone (Fig. 1; Rees et able shoaling of the deposits toward the north at section N of Skinner (1965) indicates that it al., 1988; Rowell et al., 1988b). Its age has where conglomeratic facies occur. Descrip- contains most of the sedimentological aspects traditionally been bracketed as being younger tions of ®ne-grained parts of the Douglas Con- of the Starshot Formation. These include abun- than the Lower Cambrian Shackleton Lime- glomerate (Rees and Rowell, 1991) indicate dant ®ne-grained and very ®ne grained sand- stone and older than the Devonian Beacon Su- that in lithology and sedimentological char- stone beds, graded bedding, thick climbing-

1074 Geological Society of America Bulletin, September 2002 PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA ripple divisions, and minor carbonate-clast clic deposits into a carbonate platform repre- carbon isotope ratios were measured against conglomerate beds as thick as 1 m. sents a long-term transgression and establish- established standards (PDBÐPeedee belem- In summary, the age relationships of the ment of a long-standing carbonate ramp. nite) and plotted against stratigraphic height various formations just described are poorly As already mentioned, we consider the con- to produce a chemostratigraphic curve (Fig. known because of discontinuous regional ex- tact between the Shackleton Limestone and 6). This curve shows a prominent, long-term, posure, faulted formation contacts, lack of Goldie Formation to be a fault at all locations positive isotopic excursion that begins at ␦13C biostratigraphic control, and a near absence of we examined, including at Cambrian Bluff ratios of ϩ2½ and rises to nearly ϩ5½; the geochronologic data. Many stratigraphic and Cotton Plateau, where Laird et al. (1971) curve then gradually decreases to about ϩ2½. schemes have been suggested, as summarized and Stump (1995) described it as an uncon- The prominent positive excursion occurs over in Figure 2. We present data from several crit- formity. The faulted nature of the contact be- nearly 75 m of shallow-marine carbonate-rich ical sections that reveal important stratigraphic tween the lower Shackleton Limestone and section and therefore appears to represent a and age relationships among these formations. ``inboard'' Goldie Formation raises two pos- geologically signi®cant excursion of probable In combination with other stratigraphic and sible scenarios. First, faults may have devel- global signi®cance. A number of systematic sedimentological information, as well as new- oped at or near a possible unconformity be- isotopic excursions are documented in Lower ly acquired U-Pb detrital-zircon dates, a co- tween these units, owing to mechanical Cambrian rocks, particularly for the Siberian herent stratigraphic scheme is presented for all weakness coupled with a sharp rheological stages, and 10 (I±X) positive excursions are sub-Devonian sedimentary units in the central contrast, leaving the basal Shackleton relative- known (Kirschvink et al., 1991; Brasier et al., Transantarctic Mountains. ly intact. Second, some amount of the lower 1994; Brasier and Sukhov 1998). Six assem- Shackleton may be missing owing to fault blages of trilobites were noted in the Shack- LOWER SHACKLETON LIMESTONE truncation. Therefore, we maintain that a de- leton Limestone by Palmer and Rowell positional relationship between Shackleton (1995). The oldest assemblage contains forms, The sandstone-rich lower member of the and inboard Goldie has yet to be demonstrated including redlichiids, that overlap in age with Shackleton Limestone is exposed at Cotton and that an unknown amount of the lowermost fallotaspidid trilobites, which occur at the base Plateau beneath Panorama Point, where it con- Shackleton section may be missing regardless of the Atdabanian Stage (Fig. 3) and are as- sists of up to 133 m of interbedded white- to of its original relationship with the underlying sociated with carbon isotope excursion IV cream-weathering, vitreous, quartz sandstone Goldie rocks. Given the overall upward tran- (Brasier et al., 1994). The positive excursion and brown-weathering, white, ®ne-grained do- sition from sandstone to carbonate observed at the base of the Shackleton Limestone is lomitic grainstone (Figs. 4, 5A). These beds, at Cotton Plateau, one would predict that any considered to be excursion IV on the basis of in both the upright and overturned limbs of a older parts of the Shackleton would be dom- the following. First, the redlichiid trilobites large syncline, are in fault contact with adja- inated by quartz sandstone. A possible expo- were likely recovered from very low in the cent Goldie Formation (see previous descrip- sure of such a section occurs at the outlet of formation, presumably not much above the tion of the contact; Goodge et al., 1999). In the nearby Princess Anne Glacier. This out- mixed siliciclastic-carbonate deposits of the the less deformed Shackleton section above crop contains a thick section of white quartz lower Shackleton. Second, the upper half of the lower member, the percentage of carbonate sandstone and minor shale that Laird et al. the underlying Tommotian is globally char- increases up section, and the sandstone com- (1971) attributed to the Shackleton Limestone. acterized by a long-standing negative excur- ponent eventually disappears as the section The strata contain evidence for shelf and shore- sion. Finally, no faunas of clearly Tommotian shifts into a thick, massive boundstone unit. line deposition (e.g., mud cracks, paired mud- age have been recovered from the Shackleton A detailed section of the lower Shackleton stone drapes, hummocky cross-strati®cation). Limestone. Thus, identi®cation of the basal Limestone at Cotton Plateau was measured on An interval of coarse, ¯uvial cross-bedded con- Shackleton Limestone excursion as excursion the lower limb of the large overturned syn- glomerate, pebbly sandstone, and sandstone IV dates the timing of marine transgression cline (Fig. 4). This section contains meter- occurs near the top of the unit. The entire sec- onto the Antarctic craton in this region as be- scale, mixed siliciclastic-carbonate cycles tion, several hundred meters thick, is unrep- ing early Atdabanian. (Fig. 5B) that begin with 7±181-cm-thick, resented in the Cotton Plateau section just 17 ®ne- to medium-grained sandstone beds (av- km away. Unfortunately, neither outcrop is SHACKLETON±UPPER BYRD GROUP erage ϭ 60 cm) having sharp, commonly ero- fossiliferous, although given the facies char- TRANSITION sional basal contacts with underlying carbon- acteristics, it is unlikely that much time is rep- ate beds (Fig. 5C). These sandstone beds resented by these strata. An archaeocyathan Exposures of the Shackleton and Douglas locally show evidence for shallow-water de- fossil was collected from the Cotton Plateau Formations along the east side of the Holy- position including paired clay drapes diagnos- section at 127.95 m (Fig. 4), but this fossil is oake Range were examined by Rees and Row- tic of tidal deposition (Fig. 5D) and small- poorly preserved and only indicates that this ell (1991). They mapped several faults in the scale two-dimensional wave ripples. The part of the formation is Lower Cambrian. region and concluded that no contacts between sandstone beds have gradational upper con- To better determine the depositional age of these formations were depositional. Our study, tacts that include zones of carbonate-rich the lower Shackleton Limestone, a series of aided by helicopter support, revealed that sandstone and sandy carbonate. Carbonate samples for chemostratigraphic analysis was there is large-scale repetition of section in caps of these cycles consist of grainstone with taken approximately every 50±100 cm from thrust sheets carried by faults that trend sub- abundant hummocky cross-strati®cation (Fig. the top through the basal limb of the syncline parallel to the axis of the range. A deposition- 5E). These cycles appear (atypically) to record below Cotton Plateau (Fig. 4). Carbon isotope al transition between the Shackleton Lime- upward-deepening patterns. The upward de- ratios were determined by using a Finnegan stone and overlying siliciclastic units of the crease in sandstone from cycle to cycle and MAT Delta plus mass spectrometer housed at upper Byrd Group was discovered and eventual stratigraphic transition from these cy- the University of Tennessee, Knoxville. These mapped in successive thrust sheets on numer-

Geological Society of America Bulletin, September 2002 1075 MYROW et al.

Figure 4. Bed-by-bed measured section of mixed siliciclastic- carbonate deposits of the lowermost Shackleton Limestone, Cotton Pla- teau. Abbreviations: ShÐshale; SlsÐsiltstone; VFSÐvery ®ne grained sandstone; FSЮne- grained sandstone. Car- bonate: FGЮne-grained grainstone, CGÐcoarse- grained grainstone.

1076 Geological Society of America Bulletin, September 2002 PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

Figure 6. Chemostratigraphic curve of ␦13C vs. stratigraphic height for lower Shackle- ton Limestone at Shackleton syncline sec- tion (Fig. 4).

quently). It consists of gray to dark gray, nod- ular beds of skeletal-pelletal wackestone and lime mudstone. This facies contains rare, thin, laterally discontinuous skeletal packstone and grainstone beds with bioclasts of trilobites, ar- chaeocyathans, and eocrinoids. A few thin in- tervals (commonly Ͻ2 m thick) of dark, thin- bedded lime mudstone occur within this nodular carbonate. The dark color and nodular bedding suggest that this facies developed in a deep subtidal setting colonized by burrowing organisms. The lack of interbedded archaeocyathan bioherms, ooids, and light-gray lime mud- stone (cf. Rees et al., 1989) suggests that it formed basinward of the ooid shoal facies. Figure 5. Shackleton syncline section, Cotton Plateau (Fig. 4). (A) Lower Goldie Formation The thin interbeds of skeletal grainstone/ (G) is overlain by interbedded quartz sandstone and dolostone of lower Shackleton Lime- packstone are interpreted to be storm depos- stone (S). Arrow points to contact. Upper part of section consists of massive archaeocyathid- its that infrequently affected this deeper sub- bearing dolostone cliffs. (B) Interbedded quartz sandstone and dolostone beds in 60±63 m tidal environment. zone (distances refer to measured section; Fig. 4) of lower Shackleton Limestone. Hammer for scale (arrow). (C) Deep-channel ®ll of quartz sandstone in cross section at 98.81 m. Archaeocyathan Bioherms (Shackleton Hammer for scale. (D) Paired clay drapes (arrows) in quartz sandstone bed at 74.15 m. Limestone) Pencil (at left) is 14 cm long. (E) Hummocky cross-strati®cation in carbonate grainstone at 27 m. Pencil is 14 cm long. Isolated bioherms at the top of the Shack- leton Limestone consist of massive-bedded, light gray to white algal boundstone with ar- ous spurs along an ϳ30 km north-south sec- assemblage of carbonate-clast conglomerate chaeocyathans. The bioherms range in thick- tion of the Holyoake Range. and ®ne-grained sandstone. The transition- ness from 0 to Ͼ38 m (Figs. 7, 10A). Most In the Holyoake Range, the upper Shack- zone strata were carefully measured in several of the bioherms are simple structures with leton Limestone consists of black to dark gray, localities, and two measured sections are pre- only one episode of growth. However, at a few burrow-mottled and skeletal wackestone that sented in Figures 8 and 9. A description and locations the bioherms are composite struc- grades upward into large, massive, isolated ar- interpretation of these facies follows in the tures with at least two separate growth phases, chaeocyathan bioherms (Fig. 7). Both the stratigraphic order in which they occur. each capped by an extensive phosphate- bioherms and the interbioherm strata on which encrusted surface (Fig. 11). The bioherms the bioherms rest are capped by an extensive, Nodular Carbonate (Shackleton form isolated bodies, and between them the multigeneration, black, phosphatic surface, Limestone) phosphate-encrusted surface occurs directly which is overlain by a unit of shale and nod- on top of the nodular carbonate unit described ular lime mudstone in a brown, calcareous The nodular carbonate facies makes up a earlier. shale matrix. This facies grades upward into tabular unit that is overlain by isolated ar- The upper surface of the lower growth fossiliferous siltstone and then into a complex chaeocyathan bioherms (described subse- phase of the composite bioherm at section 1

Geological Society of America Bulletin, September 2002 1077 MYROW et al.

cessation of bioherm growth and the succeed- ing development of the hardground were re- sponses to rapid drowning of the carbonate platform. The hardground records sediment starvation during ¯ooding and replacement of carbonate by phosphate that was likely intro- duced by upwelling. The borings indicate the hardgrounds were hard, cemented surfaces and that they acted as surfaces of sediment bypass (Bathurst, 1975; Mullins et al., 1980). The multiple-stage hardground on top of the archaeocyathan bioherms indicates terminal drowning of the downslope bioherms, which we interpret as likely caused by tectonic sub- sidence related to initiation of orogenesis in this region.

Shale and Nodular Carbonate/Shale (New Holyoake Formation)

Directly overlying the hardground surface, both between the bioherms and on their ¯anks, is dark gray to brown, calcareous ®ssile shale. This passes upward into nodular carbonate in a similar-colored calcareous shale that onlaps the uppermost parts of the taller bioherms (Fig. 10B). The nodular carbonate consists of lime mudstone nodules 2±10 cm thick and up to 40 cm wide in a brown shaly matrix. The nodules include rare trilobite and very rare hy- olithid bioclasts and up to 1% disseminated pyrite. Near the top of some bioherms, in shale just below the ®rst occurrence of the nodular carbonate facies, are rare, small (1±2 m wide, Ͻ1 m high) archaeocyathan bioherms that are capped by a thick phosphatic surface. Figure 7. Large light-colored archaeocyathan reefs (white arrows), tens of meters thick, The archaeocyathans in these bioherms are just south of section 1 of the Holyoake Range. Bedding is vertical, and stratigraphic up large and locally replaced by phosphate (Fig. is to right; dashed line represents bedding orientation. Reefs are surrounded by shale, 12D). nodular carbonate and shale, and ®ne-grained sandstone of Starshot and Holyoake (new This shale records deep-water deposition name) Formations. above the phosphate-encrusted bioherms in re- sponse to drowning of the carbonate platform. The nodular carbonate and shale also consti- has local small, irregular depressions with up of interbedded oolite shoals suggest that these tute a deep-water facies, and although the in- to 1.5 m of relief (Fig. 12A). These were ®lled algal bioherms developed in a deep subtidal crease in carbonate content may indicate a by wackestone, shale, and carbonate-clast con- setting basinward of the oolitic shoal complex. change in climate (Weedon, 1986), it more glomerate with boundstone fragments. A sec- The phosphatic surfaces formed during early likely represents a response to shoaling and a ond growth phase of microbial boundstone is cementation of the archaeocyathan bioherms stratigraphic shift into overlying calcareous capped with another thick composite phos- and represent a complex submarine hard- siltstone (described in the next section). Shale phatic surface (Fig. 12B). This composite sur- ground. Most of the bioherms grew during a and nodular shale within pockets in the upper face is composed of multiple irregular layers single phase of growth, although some of part of some bioherms (between lower and up- of phosphatic micrite cement incorporating a them show at least two phases of growth sep- per growth stages described earlier) and the thin (10±15 cm thick) skeletal packstone/ arated by hardground development and onlap grainstone with a ®ne-grained fauna of hy- of ®ne carbonate and shale. The carbonate olithid, eocrinoid, and trilobite bioclasts as conglomerate on the side of the bioherm Figure 8. Stratigraphic section 1 from east well as quartz silt and phosphatic fragments formed by submarine erosion of the growing side of Holyoake Range. Standard grain (Fig. 12C). Many of the phosphatic surfaces bioherm, possibly by storms. The irregular up- sizes from Sh (shale) to Bo (boulder). MÐ are bored with 1±5-mm-diameter, nearly cir- per surfaces of the bioherms were likely pro- micrite, BÐboundstone, TÐtrilobites. See cular borings. duced by nonuniform growth of the bioherm legend. Development of Ն40 m of relief and a lack or possibly by submarine erosion. Both the M

1078 Geological Society of America Bulletin, September 2002 PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

Geological Society of America Bulletin, September 2002 1079 MYROW et al.

small bioherm bodies within overlying shale and nodular carbonate facies re¯ect an interval of repeated drowning and attempted bioherm growth. Because of its distinction as a map- pable unit, we de®ne this facies of the tran- sition zone as the ``Holyoake Formation,'' as proposed later in this paper.

Calcareous Siltstone/Very Fine Sandstone (Starshot Formation)

This facies consists of brown-weathering, calcareous, siltstone to very ®ne grained sand- stone. A wide range of carbonate content ex- ists and locally exceeds 50%; thus the lithol- ogy is in cases sandy calcisiltite. The facies includes a few very widely spaced (several meters) decimeter-scale shaly intervals. The sandstone is blocky weathering and generally featureless in outcrop. This facies occurs above shale with nodular carbonate, as already described, and for less than1mofthis facies transition, the siltstone contains thin (Ͻ4 cm) nodules of gray micritic limestone (Fig. 13B). The lowermost 5±10 m of section above this nodular-facies transition contains abundant fossils of trilobites and hyolithids. This facies contains locally developed black phosphatic material that occurs both as disseminated grains in siltstone beds up to 1.1 m thick and as nodular beds up to 8 cm thick. The latter contain abundant Ͻ2-mm-diameter borings (Fig. 13C). Beds of pebbly ®ne- to medium-grained sandstone from 30 to 70 cm thick occur within this facies. These are nongraded and matrix

Figure 9. Stratigraphic section 2 from east side of Holyoake Range. For explanation, see Figure 8.

1080 Geological Society of America Bulletin, September 2002 PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

glomerate and sandstone facies (described subsequently) indicates that this siltstone/very ®ne sandstone was deposited in nearshore to shoreline environments. The pebbly sandstone beds have many of the characteristics of debris ¯ow deposits such as matrix support and lack of grading. These beds likely formed as shallow- marine debris ¯ows, possibly representing storm-generated deposits.

Trilobite Fauna The fauna from the calcareous siltstone/ very ®ne sandstone units is of Early Cambrian age. Intercontinental correlation of Early Cambrian trilobites is very dif®cult, and con- siderable uncertainties exist (Palmer and Row- ell, 1995; Palmer, 1998). However, the fauna reported here has closest af®nities to faunas from South Australia and south China. Al- though the trilobites are slightly deformed and indifferently preserved, there appear to be specimens of Redlichia, Megapaleolenus, and Hsuaspis cf. bilobata Pocock. The latter is probably conspeci®c with a trilobite described by Palmer and Rowell (1995) from a collec- tion (C-83±3) made directly below the Doug- las Conglomerate on the east side of the Holy- oake Range. Given that their samples came from a locality very close to ours, it is pos- sible that the very same interval was sampled. Hsuaspis occurs in China in rocks of late Chiungchussuan to medial Tsanglangpuan age (Zhang et al., 1995) and in Australia in beds considered to be Botomian equivalents (Jell in Bengtson et al., 1990). Megapaleolenus char- acterizes a zone at the top of the Tsanglang- puian (Chang, 1988). The late(?) Tsanglang- puian/Botomian age of this fauna suggests a need to reevaluate the age of assemblage 5 of Palmer and Rowell (1995) from the underly- ing Shackleton Limestone, determinations of which have been poorly constrained but thought to be a slightly younger Early Cam- brian age. Figure 10. (A) Onlap of shale (o) against ¯ank of large archaeocyathan reef (m). Small arrow indicates position where onlapping bed in foreground contacts the reef. The reef is Mixed Conglomerate, Sandstone, and Silty ϳ40 m thick. (B) Transition from uppermost Shackleton Limestone archaeocyathid Shale (Douglas Conglomerate) mound (m) into Starshot Formation at section 1 of the Holyoake Range. Nodular limestone and shale (n) directly overlie mound and are succeeded by fossiliferous calcareous siltstone This particularly complex facies consists of and very ®ne sandstone (s). Pencil (circled) is 14 cm long. interbedded conglomerate; sandstone beds with a wide range of grain sizes, bed thick- nesses, and sedimentary structures; and black supported with widely dispersed, well-rounded, below the ®rst stratigraphic occurrence of con- silty shale. Conglomerate beds range from pebbles that make up Ͻ20% of the lithology. glomerate beds. sandy pebble conglomerate to poorly sorted The pebbles consist mostly of quartzite, white Few diagnostic sedimentary structures oc- cobble/boulder conglomerate. Individual beds limestone, and intraclasts of ®ne sandstone cur in this facies, but the presence of abundant are generally Ͻ2 m thick, but amalgamated generally Ͻ7 cm in diameter, although one trilobite and hyolithid fossils as well as bored units of conglomerate range up to 17 m in bed contains a sandstone intraclast that is 45 phosphate nodules indicates a marine deposi- thickness. Beds range from massive, unorga- ϫ 5 cm in cross section. At section 1, this tional setting. The relative paucity of shaly nized and poorly sorted to well graded (Fig. facies occurs in the 10 m of strata directly beds and a stratigraphic transition into con- 14A) with well-organized transitions, e.g.,

Geological Society of America Bulletin, September 2002 1081 MYROW et al.

to marginal-marine deposits. Meter-scale grad- ed beds (Fig. 14A) are likely channel deposits that were either interbedded with ¯uvial sheet- ¯ood or shoreline sandstone beds. Similar fa- cies were described as part of a wider range of coarse clastic facies of the Douglas Con- glomerate by Rees and Rowell (1991). They interpreted the wider suite of facies as ranging from proximal to distal alluvial-fan deposits. The strata described from the Shackleton±up- per Byrd Group transition zone likely represent Figure 11. Drawing of upper part of composite reef from section 1, Holyoake Range. View a shift from clearly shallow-marine (trilobite- is to the west. Two distinctive phosphate-encrusted hardground layers near the top of the bearing) siltstone through marginal-marine and reef (heavy black lines) indicate that drowning occurred in at least two phases. The initial then alluvial-fan deposits. Much of the shallow- drowning event was followed by shale deposition and renewed growth of the archaeocy- marine deposits were deposited in fan-deltas. athan reef. The discontinuous carbonate conglomerate above the shale likely indicates The stratigraphic framework and implications of erosion of the reef, possibly by submarine currents (e.g., Schlager, 1998, 1999). Lateral these deposits are discussed next. expansion of the reef over the carbonate conglomerate was followed by the ®nal drowning event and formation of the laterally persistent phosphatic hardground at the top of the STRATIGRAPHIC REVISION reef. Nodular carbonate and shale then buried the reef following drowning. It is unclear whether the small archaeocyathan bioherm in the center of the drawing is a third phase The depositional ages and stratigraphic re- of reef development or is a three-dimensional extension of the second phase of reef growth. lationships of the Neoproterozoic to lower Pa- Small faults offsetting the hardground at the top of the reef cannot be traced into the leozoic formations in the central Transantarc- overlying shale or siltstone. tic Mountains have been dif®cult to discern because of structural complexity, abundant ice cover, and a general lack of fossils. Detailed stratigraphic and sedimentological data pre- clast-supported cobble conglomerate to pebbly bedding (Fig. 14D), and locally developed sented in this paper, in addition to ages of de- sandstone to cross-bedded sandstone. Beds are current ripple lamination. trital zircons from these units (Goodge et al., commonly lenticular with concave-up deeply Black shaly siltstone and silty shale units 2002), allow for a signi®cant stratigraphic re- scoured bases. Conglomerate beds have highly are widely dispersed in this facies. These are vision (Fig. 16). The depositional transition variable clast compositions from siliciclastic generally a few tens of centimeters thick, but described herein between the uppermost dominated to carbonate dominated. Large, range up to 14 m thick. In some cases, the Shackleton Limestone and the siliciclastic de- outsized intraclast blocks of sandstone range ®ne-grained units contain 1±3-cm-thick very posits of the upper Byrd Group is critical in up to 1 ϫ 0.4 m in cross section and are par- ®ne grained to ®ne-grained sandstone beds this regard. ticularly abundant in areas of intimate inter- that locally reach up to 15 cm in thickness. A multitude of stratigraphic schemes and bedding of conglomerate and sandstone (Fig. This diverse facies was almost certainly de- inferred depositional ages of the various for- 14B). Clasts clearly derived from the under- posited in a variety of subenvironments. It oc- mations have been proposed (Fig. 2). In many lying Shackleton Limestone are abundant and curs in a stratigraphic position above the pre- cases, previous workers had limited access to locally outsized, particularly in section 2 viously described facies such that it records particular formations and did not examine oth- (Figs. 9, 15, A and B), in which conglomerate continued shoaling and increased proximity of ers, which hindered stratigraphic synthesis. beds rest directly on the upper surface of a source regions. The bed thicknesses and For instance, Rees, Rowell, and various col- bioherm. Large white biohermal limestone coarse grain size of the conglomerate indicate leagues (Fig. 2, columns E through I) exam- clasts range up to 2 ϫ 0.8 m across and in- proximal deposition, particularly given the ined the Shackleton and Douglas Formations, dicate local erosion and transport of bioherm large outsized meter-scale blocks. These but did not study the Starshot Formation. Con- blocks. Large outsized blocks also locally oc- blocks, locally derived from the Shackleton versely, all previous workers uniformly placed cur stratigraphically well above the bioherms. Limestone, indicate local tectonic uplift and the Cobham and Goldie Formations in the One outsized block of fenestral mudstone, 1.5 erosion, particularly as strata from lower in Beardmore Group and generally considered ϫ 1.6 m in cross section, occurs in section 1, the formation are also represented as clasts. them to be Neoproterozoic in age because of 147 m above the high point of the underlying The conglomerate beds ®rst appear in section their apparent great thickness, lack of fossils, bioherm (Fig. 14C). 1 (Fig. 8) in close association with phosphate- and inferred position beneath the Lower Cam- Green light-red-weathering sandstone beds rich siltstone and farther up section with or- brian Shackleton Limestone. However, the (Fig. 14B) range from ®ne grained to coarse ganic-rich silty shale. Thus, at least some of bulk of rocks previously mapped as Goldie grained with ¯oating small pebbles. On aver- these beds were either deposited in, or in Formation are now known to be late Early age, these beds are medium to coarse grained proximity to, a subaqueous setting. Although Cambrian in age or younger. Thus, we suggest and moderately to poorly sorted. Bed thick- the phosphatic beds are likely marine in ori- that the Beardmore Group should be rede®ned nesses range from 4 cm to Ͼ1 m and average gin, the black silty shale beds could be either to include only those rocks that are known, or 30±40 cm. These beds make up amalgamated marine or marginal marine. Thicker units of reasonably inferred to be older than the Shack- units up to 12 m thick. Fine-grained sandstone mixed conglomerate and medium- to coarse- leton Limestone, namely, the Cobham For- beds show parallel lamination, trough cross- grained sandstone most likely represent ¯uvial mation and ``inboard'' Goldie assemblage

1082 Geological Society of America Bulletin, September 2002 PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA of Goodge et al. (2002). The ``inboard'' Gol- Limestone into the younger siliciclastic units ence of young 500±490 Ma detrital zircons in die is best de®ned as that part of the Beard- of the upper Byrd Group described in this pa- the Dick Formation at co-type section N in- more Group lacking detrital minerals having per indicates that, in this area, the latter en- dicates that the strata at this locality are po- ages younger than 600 Ma. This criterion re- tirely postdate Shackleton carbonate deposi- tentially younger by ϳ20 m.y. than at its other stricts the mapped extent of the group to the tion (i.e., none is even partly coeval). The co-type section P, including part or all of the eastern Cobham Range and the vicinity of siltstone/sandstone facies within this upward- Douglas Conglomerate. If the Dick Formation Cotton Plateau (Fig. 1). The available detrital- coarsening transition (Fig. 8, 23.8±63.5 m) is is at different locations both older than, and zircon ages only indicate that this restricted similar in grain size, texture, color, and weath- younger than, a (presumably) large part of the Beardmore Group is younger than ca. 1000 ering to the bulk of the Starshot Formation Douglas Conglomerate, then the latter likely Ma, although a new zircon U-Pb age of 668 and is thus included in that formation. Over- forms a wedge within the sandy Dick For- Ma from gabbro associated with pillow basalts lying coarse-grained units are assigned to the mation, as described for the Starshot Forma- at Cotton Plateau (Goodge et al., 2002) indi- Douglas Conglomerate. It is apparent from tion. The base of the Dick Formation is not cates that deposition is at least in part late spatial distributions of conglomeratic facies exposed, but we would predict that it overlies Neoproterozoic. Although the same geochro- and paleocurrents that the coarse-grained units a shaly unit that itself directly overlies the nological constraints allow the ``inboard'' of the Douglas Conglomerate make up a thick Shackleton Limestone, as described for the Goldie to be as young as the lowermost sedimentary wedge of limited lateral extent. Holyoake Range in this paper. Shackleton Limestone, it is unlikely to be This is a logical geometry because this unit We see no bene®t in continuing to use the Cambrian. Sandstone samples with a young represents the deposits of an alluvial-fan com- name ``Dick Formation'' because this unit detrital signature (younger than 520 Ma) from plex (Rees and Rowell, 1991). The distal ®n- appears to be identical to the Starshot For- the Lowery and Beardmore Glacier areas, as gers of this wedge make up the conglomerate mation in lithologic and sedimentologic char- well as the northern Churchill Mountains, sug- beds of the Starshot in the Mount Ubique area. acter, detrital-zircon geochronology, and gest that the outer ranges contain younger The result of progradation of this alluvial stratigraphic relationship with the Douglas ``outboard'' Goldie deposits with a different wedge into a marine setting is that sandstone Conglomerate. We thus suggest that the name provenance (Goodge et al., 2002). These facies both underlie and are laterally equiva- ``Dick Formation'' be abandoned and that ex- should hereafter be mapped as part of the Star- lent to this conglomeratic body (Fig. 16). In posures previously mapped as Dick Formation shot Formation (Figs. 1, 16). fact, this wedge is also overlain by a thick be placed in the Starshot Formation (Fig. 16). Early workers recognized that the presence section of sandstone (Fig. 16), as seen at High The name ``Starshot'' is preferred given its of Shackleton Limestone clasts in both the Ridge (Fig. 1). Here, an upward-coarsening earlier introduction (Laird, 1963). In summa- Douglas and Starshot Formations indicated succession identical to that in sections 1 and ry, we suggest that the Starshot Formation that these units were equivalent to and/or 2 is followed by hundreds of meters of sand- should include all rocks originally mapped younger than that formation. Most subsequent stone and minor shale facies typical of the within this formation plus rocks of the ``out- workers considered the Douglas to be younger Starshot Formation. board'' Goldie Formation and all of those than, and locally in unconformable contact The presence of sandstone below, adjacent rocks previously mapped as Dick Formation. with, the underlying Shackleton Limestone to, and above a conglomeratic wedge helps We suggest continued use of the term (Fig. 2), and we concur. Several lines of evi- explain a number of other stratigraphic com- ``Douglas Formation'' for those rocks domi- dence indicate age equivalence between the plexities, including the relationship of these nated by conglomerate. Starshot and Douglas Formations (Fig. 16). units with the Dick Formation. As described First, ``Douglas''-like conglomeratic units oc- earlier, our brief study of the Dick Formation Holyoake Formation cur in rocks mapped as Starshot Formation in at co-type section N of Skinner (1964, 1965) the Mount Ubique area. Second, these con- revealed a remarkably similar suite of bedding The depositional transition between the glomeratic parts of the Starshot occur spatially characteristics and internal sedimentary struc- Shackleton Limestone and overlying clastic between more inboard regions mapped as tures to those of the Starshot Formation. In deposits of the upper Byrd Group marks a ma- Douglas Conglomerate that are dominated by addition, detrital-zircon ages from the Dick jor lithologic break that re¯ects profound coarse conglomerate and more outboard re- Formation at section N include populations at changes in depositional systems, depositional gions of the Starshot Formation that are al- 495, 510, 555, 1020, and 1130 Ma and a range history, and tectonics. This transition occurs most exclusively sandstone and minor shale. of older Proterozoic and Archean ages; these at the contact between the phosphatized upper Third, paleocurrent patterns in these sandy ages are very similar to zircon-age distribu- surface of the Shackleton Limestone and the units are uniformly oriented from inboard ar- tions from the Starshot Formation elsewhere overlying ®ne-grained shale and mixed nod- eas toward more outboard regions (i.e., ¯ow (J. Goodge, unpubl. data). These data and oth- ular carbonate and shale deposits. The shaly from west to east and southwest to northeast) er geologic relationships lead us to consider deposits form an extensive stratigraphic unit (Myrow et al., 2002). Finally, the age distri- these units as equivalents. According to Skin- that can be mapped throughout the Holyoake butions of detrital zircons from samples of ner (1964), Burgess and Lammerink (1979), Range and may be present within adjacent these formations are similar (J. Goodge, un- and Rowell and Rees (1989), the Dick For- ranges as well. Previous workers did not rec- publ. data). mation is overlain by, and in conformable ognize this unit as a separate mappable litho- Rees et al. (1988) and Rowell et al. (1988b) contact with, the Douglas Conglomerate at co- some and therefore would have likely includ- clearly demonstrated an unconformity be- type section P. This is therefore analogous to ed these deposits as a facies within the tween the Shackleton and Douglas Formations the transition from the Starshot Formation Douglas Conglomerate or Starshot Formation. at one locality, but noted fault contacts be- (with Botomian fossils, ca. 515 Ma) to the This lithologically distinct unit, which con- tween these units at other locations. The de- Douglas Conglomerate in the Holyoake sec- sists of ®ne-grained dominantly siliciclastic positional transition from the Shackleton tions described previously herein. The pres- facies sandwiched between carbonate-

Geological Society of America Bulletin, September 2002 1083 MYROW et al.

Figure 12. Section 1 of the Holyoake Range. (A) Irregular relief on top of lowermost ar- chaeocyathan reef (m); bedding is vertical. White arrow points to composite phosphat- ic hardground surface that caps the reef. Shale and nodular limestone (n) overlie and onlap the hardground surface. Pen is 14 cm long (circled). (B) Phosphatic hardground surface (thin arrow on right edge) near top of composite archaeocyathan reef. Bedding is vertical. Pencil is 14 cm long. (C) Close- up of phosphatic grainstone ®ll (g) and multiple hardground surfaces (white ar- row) on top of composite archaeocyathan mound (m). Tip of pencil is 2 cm in length. (D) Large phosphatized archaeocyathid (a) in small reef within shale and nodular car- bonate facies. Bedding-plane view. Pencil is 14 cm long.

the Shackleton Limestone. The top of the for- mation is de®ned as the top of the extensive nodular carbonate and shale unit that underlies calcareous siltstone to very ®ne grained sand- stone of the basal Starshot Formation. The for- mation reaches a maximum thickness of ϳ45 m and thins appreciably over the bioherms of platform deposits (Shackleton Formation) and cluding sections 1 and 2 (Figs. 8 and 9), but the upper Shackleton Limestone. Locally, in coarse, synorogenic molasse (Douglas and is not present where uplift and erosion of the section 2 and possibly elsewhere, conglom- Starshot Formations), is herein assigned to a Shackleton Limestone caused angular uncon- eratic channels of the overlying Douglas separate formation. Because this unit occurs formity with overlying conglomerate units of Conglomerate cut out both the Starshot For- within the Holyoake Range, we suggest that the Douglas Conglomerate (Rees et al., 1988; mation and the thinned Holyoake Formation it be designated the ``Holyoake Formation.'' Rowell et al., 1988b). The base of this for- at the high point of a bioherm (Fig. 9). The This formation is exposed at numerous sec- mation is de®ned as the lowermost shale that type section of the Holyoake Formation is tions along the eastern Holyoake Range, in- rests on the phosphatized surface at the top of designated at the section 1 locality (Fig. 1; GPS [Global Positioning System] location 82Њ13.185ЈS, 160Њ16.122ЈE). The Holyoake Formation probably also ex- ists in more outboard regions, between the Shackleton and Starshot Formations, but this stratigraphic relationship has not been ob- served. Such a transition would be similar to those at inboard localities except that the over- lying clastic deposits would be mostly sand-

Figure 13. Section 1 of the Holyoake Range. (A) Shale and nodular carbonate of Holy- oake Formation. (B) Transition from inter- bedded nodular limestone (n; circled) and shale of upper Holyoake Formation to very ®ne calcareous sandstone (s) w/limestone nodules of basal Starshot Formation. Pencil (vertical; lower center) is 14 cm long. (C) Borings in nodular, black phosphate car- bonate in Starshot Formation at 59.17 m (Fig. 9). Arrow points to small borings on bedding plane. Pencil tip is 2 cm long.

1084 Geological Society of America Bulletin, September 2002 PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

Figure 15. Section 2 of the Holyoake Range. (A) Conglomerate (c) resting directly on eroded upper surface of large archaeocy- athan mound (m). Hammer (lower center) for scale. Large blocks of archaeocyathan bioherm (b) occur in basal conglomerate Figure 14. Douglas Conglomerate at section 1 of Holyoake Range. (A) Graded conglom- beds in Douglas Conglomerate. (B) Top of erate bed 1.8 m thick at 70.2 m (Fig. 9). Yellow, size 9 men's boot on left for scale (ϳ30 large archaeocyathan mound at section 2 of cm). Arrow points in direction of stratigraphic top. (B) Interbedded conglomerate and the Holyoake Range. Both the mound (m) sandstone. Camera case for scale. (C) Large block of black fenestral mudstone with calcite and the ¯anking shale/nodular limestone ®ll .veins (b) within conglomeratic bed at 166 m (Fig. 9). Block is 1.5 ؋ 1.6 m in cross section (sh) are truncated by boulder conglomerate Interbedded sandstone and shale on right. Hammer for scale. (D) Trough cross-bedded (c). Large block (b) of archaeocyathan sandstone and pebbly sandstone near the top of section. Pencil is 14 cm long. bioherm in conglomerate. Hammer for scale. stone of the Starshot Formation with minor, if DISCUSSION any, conglomerate (i.e., no Douglas Conglom- erate). In fact, the High Ridge section (Fig. 1) New data outlined in this paper on Byrd the lower sandstone-rich part of the formation records such a transition. Here, the Holyoake Group strata provide multiple constraints on to be early Atdabanian in age (isotopic excur- Formation is overlain by deposits with only the timing of deposition of various units and sion IV of Brasier et al., 1994; Brasier and one 95-cm-thick conglomerate bed, and the the onset of Ross orogenesis. Previously pub- Sukhov, 1998). Although this section, which rest is almost entirely sandstone and minor lished trilobite and archaeocyathan biostrati- we interpret to be in fault contact with the shale of the Starshot Formation. The Holy- graphic data from the Shackleton Limestone Goldie Formation at Cotton Plateau, may not oake Formation is therefore considered an ex- generally constrained determinations of the be the absolute base of the formation, lower tensive unit of the middle Byrd Group that age of this unit, but the unfossiliferous basal parts may not be much older. This quartz-rich everywhere overlies Shackleton Limestone in unit of the formation was never dated, and the sandstone of the basal Shackleton has ana- the study area and underlies sandy and con- stratigraphic top of the formation was never logues with relatively thin Cambrian basal glomeratic deposits of the Starshot and Doug- recognized, let alone dated. Chemostratigraph- transgressive sandstone units across Laurentia las Formations. ic data presented in this paper strongly suggest and elsewhere. Thus, Cambrian seas appar-

Geological Society of America Bulletin, September 2002 1085 MYROW et al.

Figure 16. Inferred age rela- tionships of previously de®ned units in the central Transant- arctic Mountains. ``Inboard'' and ``outboard'' components of the Goldie Formation were distinguished by Goodge et al. (2002). Revised stratigraphy includes newly de®ned Holy- oake Formation and Goldie Formation that is restricted to deposits older than the Shack- leton Limestone (Beardmore Group).

ently transgressed this part of the Antarctic from initial deep-water deposition (shale and timing constraints on clastic units of poorly margin during the early Atdabanian. nodular carbonate encased in shale) through known age. The stratigraphic relationships The ensuing carbonate platform existed shallow-marine to subaerial deposits (con- discussed here not only provide precise con- throughout the rest of the Atdabanian and sub- glomerate facies). This large-scale upward- straints on the timing of supracrustal defor- sequent Botomian stages. The ®rst document- coarsening succession records a regionally mation, but they also help to clarify the re- ed depositional contact at the top of the signi®cant shift from passive carbonate-ramp gional tectonic patterns (discussed next). Shackleton Limestone is described herein and deposition to a major in¯ux of terrigenous Assigning the onset of supracrustal defor- records a major depositional shift in the geo- sediment. This was triggered by a major tec- mation to within the Botomian Stage is con- logic history of the Transantarctic Mountains. tonic event, the structurally active phase of the sistent with stratigraphic relationships in other Archaeocyathan bioherms developed during Ross orogeny, which resulted in faulting, up- areas. Rowell et al. (1992) concluded that the the last stage of deposition of the formation lift, progressive unroo®ng, and widespread de- earliest Ross deformation was likely to be are encrusted with multiple thick, phosphatic position of an enormous clastic wedge. The Middle Cambrian in age, on the basis of (1) hardgrounds. The preserved growth morphol- trilobite fauna from the lowermost siltstone the presence of folded rocks beneath the Mid- ogy of these bioherms and lack of evidence deposits of this transition zone (basal beds of dle Cambrian Nelson Limestone in the Nep- for subaerial exposure indicates the bioherms Starshot Formation) date the onset of this tune Range of the Pensacola Mountains, and were killed as a result of rapid drowning of event as being within the Botomian Stage (ca. (2) the Douglas unconformity above the Low- the carbonate ramp. Multiple hardgrounds 515±510 Ma). This age is consistent with ca. er Cambrian Shackleton Limestone (de®ned at near the apex of the bioherms suggest that de- 490 Ma and older ages of detrital zircons in that time to be Toyonian and older). The new mise of these structures was a multiphase the overlying Starshot and Douglas Forma- fossil control on the basal Starshot Formation event. The phosphatic crusts capping the bio- tions (J. Goodge, unpublished data). It is mea- described herein indicates that deformation herms developed at, or near, the sediment- surably younger, however, than the onset of commenced somewhat earlier in the late Early water interface in a deep-marine setting basement deformation in this area, dated to an Cambrian. New evidence shows that the Pen- (Schlager, 1998), indicating that they formed Early Cambrian interval between ca. 540 and sacola Mountains underwent an initial major over a considerable amount of time during 520 Ma (Goodge et al., 1993). phase of late Early to early Middle Cambrian which there was very little or no other sedimen- Inception of Ross orogenesis within the su- deformation and subsequent episodes of late tation. Rapid initial drowning led to deposition pracrustal successions of the central Transant- Middle Cambrian to possible Ordovician ac- of the regionally extensive phosphatic crust and arctic Mountains is therefore recorded by the tivity (Rowell et al., 2001). In the Queen subsequent deposition of the Holyoake Forma- drowning and ®nal stratigraphic occurrence of Maud Mountains, deformation of Middle tion, a unit of black shale followed by mixed archaeocyathan reefs. It is also coincident with Cambrian limestone of the Taylor Formation nodular carbonate and shale facies. The latter a dramatic change in composition, prove- (Rowell et al., 1997; EncarnacioÂn et al., 1999) grades upward into trilobite-bearing calcare- nance, and depositional setting of marginal- is inferred to have occurred in the Middle ous siltstone of the Starshot Formation and basin siliciclastic deposits. Detrital-zircon Cambrian to Ordovician, well after the initial then into mixed conglomerate and sandstone ages from the Starshot and Douglas Forma- phase of tectonism described here. Together, facies of the Douglas Conglomerate. Onlap- tions show that they were deposited well into these results support the idea of protracted ping relationships between the overlying deeper- the Late Cambrian and probably into the Early and/or temporally punctuated deformation water siliciclastic rocks and the phosphate- Ordovician. Given that the relatively thin during the Ross orogenic cycle (Rowell et al., encrusted bioherm further indicate the Holyoake Formation is Botomian in age, the 1992, 2001; Goodge et al., 1993; Goodge, carbonates were drowned prior to deposition overlying clastic units of the Byrd Group span 1997), which can be inferred on the basis of of the siliciclastic sediments. The siliciclastic at least the rest of the Cambrian (Ͼ20 m.y.). radiometric age control to span the period facies transitions record progressive shoaling These relationships provide important new from 540 to 480 Ma.

1086 Geological Society of America Bulletin, September 2002 PRE-DEVONIAN STRATA AND TIMING OF ROSS OROGENY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

TECTONIC IMPLICATIONS and unroo®ng. Farther outboard, rapid subsi- tica: Implications for the Cambrian time scale: Journal of Geology, v. 107, p. 497±504. dence occurred as a result of thrust loading of Goodge, J.W., 1997, Latest Neoproterozoic basin inversion Although the transition zone between the the inboard crust. The stratigraphic response of the Beardmore Group, central Transantarctic Moun- Shackleton and upper Byrd Group clastic de- to thrusting was drowning of the bioherms and tains, Antarctica: Tectonics, v. 16, p. 682±701. Goodge, J.W., and Dallmeyer, R.D., 1996, Contrasting ther- posits is apparently recorded only within a rel- development of a phosphatic hardground, fol- mal evolution within the Ross Orogen, Antarctica: Ev- atively small outcrop area, this interval pro- lowed by deposition of shale and then progra- idence from mineral 40Ar/39Ar ages: Journal of Geol- ogy, v. 104, p. 435±458. vides keys to understanding the nature and dation of a thick clastic wedge. Although the Goodge, J.W., Walker, N.W., and Hansen, V.L., 1993, Neo- scale of crustal deformation. Our revised speci®c geometry of thrusting and ¯exural proterozoic±Cambrian basement-involved orogenesis stratigraphic framework suggests that low- loading remains to be established, the strati- within the Antarctic margin of Gondwana: Geology, v. 21, p. 7±40. angle faults within the Shackleton Limestone graphic transitions reported here offer a broad Goodge, J.W., Paulsen, T., Deering, S.K., EncarnacioÂn, J., and between the older Shackleton Limestone framework in which to understand the inter- and Watkeys, M., 1999, Progressive(?) deformation of and the upper Byrd Group clastic units are of play of tectonism and sedimentation. In ad- supracrustal rocks in the Ross orogen, central Trans- antarctic Mountains: Wellington, New Zealand, 8th In- reverse geometry. Kinematic displacement dition, the implied vertical and lateral strati- ternational Symposium on Antarctic Earth Sciences, may vary from one area to another, but rec- graphic changes re¯ect the shift from Abstracts with Programs, p. 121. Goodge, J.W., Myrow, P.M., and Williams, I.S., 2000, Age ognition that ``outboard'' Goldie Formation preorogenic passive carbonate sedimentation and provenance of the Beardmore Group, Antarctica: rocks are in fact part of the Starshot Forma- to synorogenic deposition of molassic strata Constraints on Rodinia supercontinent breakup: Geo- tion, and therefore younger than the Shackle- spanning proximal alluvial-fan to shelf logical Society of America Abstracts with Programs, v. 32, no. 7, p. 9. ton Limestone, refutes earlier assumptions that environments. Goodge, J.W., Myrow, P.M., Williams, I.S., and Bowring, for all Shackleton-``Goldie'' contacts, the S., 2002, Age and provenance of the Beardmore Shackleton rests either unconformably or ACKNOWLEDGMENTS Group, Antarctica: Constraints on Rodinia supercon- tinent breakup: Journal of Geology, v. 110, no. 4, structurally upon older strata. Those exposures p. 393±406. This work was supported by National Science with Shackleton above newly recognized Star- Kirschvink, J.L., Magaritz, M., Ripperdan, R.L., and Zjurav- Foundation (NSF) award OPP-9725426 to Goodge. lev, A.Yu., 1991, The Precambrian/Cambrian boundary: shot Formation rocks (``outboard'' Goldie) are We are grateful for the tremendous logistical and Magnetostratigraphy and carbon isotopes resolve cor- now viewed as an older-over-younger struc- helicopter support provided by the NSF Polar Op- relation problems between Siberia, Morocco, and South tural juxtaposition. Intraformational faults, erations section, Antarctic Support Associates, and China: GSA Today, v. 1, p. 69±71, 87, 91. Laird, M.G., 1963, Geomorphology and stratigraphy of the which are well exposed at Cambrian Bluff Petroleum Helicopters, Inc. Shaun Norman provid- ed unsurpassed assistance in all aspects of the ®eld Nimrod Glacier±Beaumont Bay region, southern Vic- north of Nimrod Glacier (Laird et al., 1971), work. Discussions with Malcolm Laird, Bert Row- toria Land, Antarctica: New Zealand Journal of Ge- probably represent initial structural repetition ology and Geophysics, v. 6, p. 465±484. ell, and Peg Rees have been exceptionally helpful Laird, M.G., 1981, Lower Paleozoic rocks of Antarctica, in of the Shackleton carbonate ramp. in guiding our ideas. Reviews by Bert Rowell and Holland, C.H., ed., Lower Paleozoic of the Middle East, The consistent west to east and southwest Ed Stump were extremely helpful. eastern and southern Africa, and Antarctica: Chichester, UK, John Wiley and Sons Ltd., p. 257±314. to northeast (outboard) paleocurrent data from Laird, M.G., Mansergh, G.D., and Chappell, J.M.A., 1971, the Starshot Formation (Myrow et al., 2002) REFERENCES CITED Geology of the central Nimrod Glacier area, Antarc- agrees with the spatial distribution of con- tica: New Zealand Journal of Geology and Geophys- ics, v. 14, p. 427±468. glomerate within the Byrd Group. Rapid Bathurst, R.G.C., 1975, Carbonate sediments and their diagen- esis: Amsterdam, Elsevier Science Publishers, 658 p. Landing, E., Bowring, S.A., Davidek, K.L., Westrop, S.R., drowning of the outboard carbonate platform Bengtson, S., Conway Morris, S., Cooper, B.J., Jell, P.A., Geyer, G., and Heldmaier, W., 1998, Duration of the was likely caused by ¯exural loading of crust and Runnegar, B.N., 1990, Early Cambrian fossils Early Cambrian: U-Pb ages of volcanic ashes from Avalon and Gondwana: Canadian Journal of Earth underlying the inner margin to the west (near from South Australia: Brisbane, Association of Aus- tralasian Paleontologists Memoir 9, 364 p. Sciences, v. 35, p. 329±338. the present-day axial zone of the orogen). We Borg, S.G., DePaolo, D.J., and Smith, B.M., 1990, Isotopic Mullins, H.T., Neumann, A.C., Wilber, R.J., and Boardman, speculate that loading was initially caused by structure and tectonics of the central Transantarctic M.R., 1980, Nodular carbonate sediment on Bahamian slopes: Possible precursors to nodular limestones: the development of imbricate east-vergent Mountains: Journal of Geophysical Research, v. 95, p. 6647±6669. Journal of Sedimentary Petrology, v. 51, p. 117±131. thrust sheets farther to the west, but further Bowring, S.A., and Erwin, D.H., 1998, A new look at evo- Myrow, P.M., Fischer, W., and Goodge, J.W., 2002, Wave- subsidence may have resulted from the nega- lutionary rates in deep time: Uniting paleontology and modi®ed turbidites: Combined-¯ow shoreline and high-precision geochronology: GSA Today, v. 8, shelf deposits, Cambrian, Antarctica: Journal of Sed- tive buoyancy of subducting oceanic litho- p. 1±8. imentary Research (in press). sphere (Dickinson, 1995). Nonetheless, the Brasier, M.D., and Sukhov, S.S., 1998, The falling ampli- Palmer, A.R., 1998, Why is intercontinental correlation within the Lower Cambrian so dif®cult?: Revista Es- timing of the stratigraphic record relative to tude of carbon isotopic oscillations through the Lower to Middle Cambrian: Northern Siberia data: Canadian panÄola de Paleontologia nЊ extr. Homenaje al Prof. known deformation events in the region indi- Journal of Earth Sciences, v. 35, p. 353±373. Gonzalo Vidal, p. 17±21. cates tectonic rather than global eustatic con- Brasier, M.D., Cor®eld, R.M., Derry, L.A., Rozanov, A.Y., Palmer, A.R., and Rowell, A.J., 1995, Early Cambrian tri- 13 lobites from the Shackleton Limestone of the central trol. As deformation progressed, the carbonate and Zhuraleva, A.Y., 1994, Multiple ␦ C excursions spanning the Cambrian explosion to the Botomian cri- Transantarctic Mountains: Journal of Paleontology, platform was uplifted and eroded, so that in sis in Siberia: Geology, v. 22, p. 455±458. v. 69, Part II, supplement to no. 6, p. 1±28. some places it was entirely removed and older Burgess, C.J., and Lammerink, W., 1979, Geology of the Rees, M.N., and Rowell, A.J., 1991, The pre-Devonian Pa- Shackleton Limestone (Cambrian) in the Byrd Glacier leozoic clastics of the central Transantarctic Moun- Neoproterozoic rocks were exposed and erod- area: New Zealand Antarctic Record, v. 2, p. 12±16. tains: Stratigraphy and depositional settings, in Thom- ed to become clasts in the coarser molasse fa- Chang, W.T., 1988, The Cambrian System in Eastern Asia: son, M.R.A., Crame, J.A., and Thomson, J.W., eds., cies. Locally, incomplete removal of the car- Correlation chart and explanatory notes: International Geological evolution of Antarctica: Cambridge, UK, Union of Geological Sciences Publication 24, 81 p., 4 Cambridge University Press, p. 187±192. bonate led to an unconformity of folded foldout charts. Rees, M.N., Girty, G.H., Panttaja, S.K., and Braddock, P., Shackleton Limestone overlain by Douglas Debrenne, F., and Kruse, P.D., 1986, Shackleton Limestone 1987, Multiple phases of early Paleozoic deformation Conglomerate. Differences in the composition archaeocyaths: Alcheringa, v. 10, p. 235±278. in the central Transantarctic Mountains: Antarctic Dickinson, W.R., 1995, Forearc basins, in Busby, C.J., Journal of the United States, v. 22, p. 33±35. of the Douglas ConglomerateÐrelative abun- and Ingersoll, R.V., eds., Tectonics of sedimentary Rees, M.N., Rowell, A.J., and Cole, E.D., 1988, Aspects dance of carbonate versus siliciclastic clastsÐ basins: Cambridge, Massachusetts, Blackwell Sci- of the Late Proterozoic and Paleozoic geology of the ence, p. 221±261. Churchill Mountains, southern Victoria Land: Antarc- resulted from spatial and temporal changes in EncarnacioÂn, J., Rowell, A.J., and Grunow, A.M., 1999, A tic Journal of the United States, v. 22, p. 23±25. source rocks during progressive deformation U-Pb age for the Cambrian Taylor Formation, Antarc- Rees, M.N., Pratt, B.R., and Rowell, A.J., 1989, Early

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Cambrian reefs, reef complexes, and associated lith- Rowell, A.J., Van Schmus, W.R., Storey, B.C., Fetter, A.H., Mountains, in Craddock, C., ed., Antarctic geoscience: ofacies of the Shackleton Limestone, Transantarctic and Evans, K.R., 2001, Latest Neoproterozoic to Mid- Madison, University of Wisconsin Press, p. 565±569. Mountains: Sedimentology, v. 36, p. 341±361. dle Cambrian age for the main deformation phases of Stump, E., 1995, The Ross orogen of the Transantarctic Rowell, A.J., Rees, M.N., and Braddock, P., 1986, Pre-De- the Transantarctic Mountains: New stratigraphic and Mountains: Cambridge, UK, Cambridge University vonian Paleozoic rocks of the central Transantarctic isotopic constraints from the Pensacola Mountains, Press, 284 p. Mountains: Antarctic Journal of the United States, Antarctica: Journal of the Geological Society of Lon- Stump, E., Smit, J.H., and Self, S., 1986, Timing of events v. 21, p. 48±50. don, v. 158, p. 293±308. of the Late Proterozoic Beardmore orogeny, Antarc- tica: Evidence from the La Gorce Mountains: Geolog- Rowell, A.J., Evans, K.R., and Rees, M.N., 1988a, Fauna Schlager, W., 1998, Exposure, drowning and sequence ical Society of America Bulletin, v. 97, p. 953±965. of the Shackleton Limestone: Antarctic Journal of the boundaries on carbonate platforms, in Camoin, G.F., Stump, E., Miller, J.M.G., Korsch, R.J., and Edgerton, United States, v. 23, p. 13±14. and Davies, P.J., eds., Reefs and carbonate platforms Rowell, A.J., Rees, M.N., Cooper, R.A., and Pratt, B.R., D.G., 1988, Diamictite from Nimrod Glacier area, in the Paci®c and Indian Oceans: Oxford, UK, Black- Antarctica: Possible Proterozoic glaciation on the sev- 1988b, Early Paleozoic history of the central Transan- well Science, International Association of Sedimen- tarctic Mountains: Evidence from the Holyoake enth continent: Geology, v. 16, p. 225±228. tologists Special Publication 25, p. 3±21. Range, Antarctica: New Zealand Journal of Geology Stump, E., Korsch, R.J., and Edgerton, D.G., 1991, The Schlager, W., 1999, Type 3 sequence boundaries, in Harris, myth of the Nimrod and Beardmore orogenies, in and Geophysics, v. 31, p. 397±404. P.M., Saller, A.H., and Simo, J.A., eds., Advances in Thomson, M.R.A., Crame, J.A., and Thomson, J.W., Rowell, A.J., and Rees, M.N., 1989, Early Paleozoic his- carbonate sequence stratigraphy: Application to res- eds., Geological evolution of Antarctica: Cambridge, tory of the upper Beardmore Glacier area: Implica- ervoirs, outcrops, and models: Tulsa, Oklahoma, UK, Cambridge University Press, p. 143±147. tions for a major Antarctic structural boundary within Weedon, G.P., 1986, Hemipelagic shelf sedimentation and the Transantarctic Mountains: Antarctic Science, v. 1, SEPM (Society for Sedimentary Geology) Special Publication 63, p. 35±45. climatic cycles: The basal Jurassic (Blue Lias) of p. 249±260. south Britain: Earth and Planetary Science Letters, Skinner, D.N.B., 1964, A summary of the geology of the region Rowell, A.J., Rees, M.N., and Evans, K.R., 1992, Evidence v. 76, p. 321±335. of major Middle Cambrian deformation in the Ross between Byrd and Starshot Glaciers, South Victoria Land, Zhang, W.T., Xiang, L.W., Liu, Y.H., and Meng, X.S., 1995, orogen, Antarctica: Geology, v. 20, p. 31±34. in Adie, R.J., ed., Antarctic geology: Amsterdam, North Cambrian stratigraphy and trilobites from Henan: Pa- Rowell, A.J., Gonzalez, D.A., McKenna, L.W., Evans, K.R., Holland Publishing Company, p. 284±292. laeontologia Cathayana, v. 6, p. 1±165. Stump, E., and Van Schmus, W.R., 1997, Lower Paleo- Skinner, D.N.B., 1965, Petrographic criteria of the rock zoic rocks in the Queen Maud Mountains: Revised ages units between the Byrd and Starshot Glaciers, south MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 15, 2001 REVISED MANUSCRIPT RECEIVED DECEMBER 14, 2001 and signi®cance, in Ricci, C.A., ed., The Antarctic Re- Victoria Land, Antarctica: New Zealand Journal of MANUSCRIPT ACCEPTED JANUARY 23, 2002 gion: Geological Evolution and Processes: Siena, Italy, Geology and Geophysics, v. 8, p. 292±303. Terra Antarctica Publication, p. 201±207. Stump, E., 1982, The Ross Supergroup in the Queen Maud Printed in the USA

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