depositional history of pre-devonian strata and timing of ross...

For permission to copy, contact [email protected] 2002 Geological Society of America1070

GSA Bulletin; September 2002; v. 114; no. 9; p. 1070–1088; 16 figures.

Depositional history of pre-Devonian strata and timingof Ross orogenic tectonism in the central

Transantarctic Mountains, Antarctica

Paul M. Myrow*Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA

Michael C. PopeDepartment of Geology, Washington State University, Pullman, Washington 99164, USA

John W. GoodgeDepartment of Geological Sciences, Southern Methodist University, Dallas, Texas 75275, USA

Woodward FischerDepartment of Geology, Colorado College, Colorado Springs, Colorado 80903, USA

Alison R. PalmerInstitute for Cambrian Studies, 445 North Cedarbrook Road, Boulder, Colorado 80304-0417, USA

ABSTRACT

A combination of field mapping, detailedsedimentology, carbon isotope chemostra-tigraphy, and new paleontological findsprovides a significantly improved under-standing of the depositional and tectonichistory of uppermost Neoproterozoic andlower Paleozoic strata of the central Trans-antarctic Mountains. On the basis of thesedata, we suggest revision of the existingstratigraphy, including introduction of newformations, as follows. The oldest rocks ap-pear to record late Neoproterozoic deposi-tion across a narrow marine margin under-lain by Precambrian basement. Siliciclasticdeposits of the Neoproterozoic BeardmoreGroup—here restricted to the CobhamFormation and those rocks of the GoldieFormation that contain no detrital compo-nents younger than ca. 600 Ma—occupiedan inboard zone to the west. They consistof shallow-marine deposits of an uncertaintectonic setting, although it was likely a riftto passive margin. Most rocks previouslymapped as Goldie Formation are in factCambrian in age or younger, and we reas-sign them to the Starshot Formation of theByrd Group; this change reduces the ex-posed area of the Goldie Formation to a

*E-mail: [email protected].

small fraction of its previous extent. Thebasal unit of the Byrd Group—the predom-inantly carbonate ramp deposits of theShackleton Limestone—rest with presumedunconformity on the restricted Goldie For-mation. Paleontological data and carbonisotope stratigraphy indicate that the Low-er Cambrian Shackleton Limestone rangesfrom lower Atdabanian through upperBotomian.

This study presents the first descriptionof a depositional contact between theShackleton Limestone and overlying clas-tic units of the upper Byrd Group. Thiscarbonate-to-clastic transition is of criticalimportance because it records a profoundshift in the tectonic and depositional historyof the region, namely from relatively pas-sive sedimentation to active uplift and ero-sion associated with the Ross orogeny. Theuppermost Shackleton Limestone is cappedby a set of archaeocyathan bioherms withup to 40 m of relief above the seafloor. Awidespread phosphatic crust on the bio-herms records the onset of orogenesis anddrowning of the carbonate ramp. A newlydefined transitional unit, the Holyoake For-mation, rests above this surface. It consistsof black shale followed by mixed nodularcarbonate and shale that fill in between,and just barely above, the tallest of thebioherms. This formation grades upward

into trilobite- and hyolithid-bearing calcar-eous siltstone of the Starshot Formationand alluvial-fan deposits of the DouglasConglomerate. Trilobite fauna from thelowermost siltstone deposits of the StarshotFormation date the onset of this transitionas being late Botomian.

The abrupt transition from the Shackle-ton Limestone to a large-scale, upward-coarsening siliciclastic succession recordsdeepening of the outer platform and thendeposition of an eastward-prograding mo-lassic wedge. The various formations of theupper Byrd Group show general strati-graphic and age equivalence, such thatcoarse-grained alluvial-fan deposits of theDouglas Conglomerate are proximal equiv-alents of the marginal-marine to shelf de-posits of the Starshot Formation. Paleocur-rents and facies distributions from theseunits indicate consistent west (or southwest)to east (or northeast) transport of sediment.Although the exact structural geometry isunknown, development of imbricate thrustsheets in the west likely caused depressionof the inner margin and rapid drowning ofthe Shackleton Formation carbonate ramp.This tectonic activity also caused uplift ofthe inboard units and their underlyingbasement, unroofing, and widespread de-position of a thick, coarse clastic wedge.Continued deformation in the Early Ordo-

Geological Society of America Bulletin, September 2002 1071

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 field area. CP—Cotton Plateau, CR—Cobham Range, MU—Mount Ubique, PAG—Princess Anne Glacier, and PP—Panorama 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 rectangularblocks indicate locations where upper Shackleton through Douglas transitions occur, each with well-developed bioherms that all dipvertically 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-fected these synorogenic deposits, causingfolding and thrust repetition of all pre-Devonian units.

Keywords: Archaeocyatha, Cambrian, Neo-proterozoic, reef, Ross orogeny, Transant-arctic Mountains.

INTRODUCTION

Stratigraphic relationships of pre-Devonianformations in the central Transantarctic Moun-tains (Fig. 1) have been the subject of muchconjecture and debate (Fig. 2), in large part

because of a lack of biostratigraphic con-straints, detailed sedimentology, and physicalstratigraphy of these units. The traditionalview of stratigraphic relationships in this re-gion, as reviewed by Stump (1995) and dis-cussed in more detail herein, is that basementrocks of the Nimrod Group (Archean and Ear-ly Proterozoic) are directly overlain by Neo-proterozoic rocks of the Beardmore Group, al-though this relationship has not beenobserved. The latter consists of unfossiliferousmetamorphosed siliciclastic and carbonaterocks of the Cobham Formation and overlyingless metamorphosed siliciclastic rocks (mostlysandstone with minor shale) of the Goldie

Formation. Fossil-bearing strata of the ByrdGroup are generally considered to have beendeposited unconformably upon the GoldieFormation. The basal formation of the ByrdGroup, the Lower Cambrian Shackleton Lime-stone, consists of dominantly carbonate rockswith a range of facies including archaeocyathan-bearing bioherms. Clasts of the ShackletonLimestone occur within overlying synorogenicconglomerate- and sandstone-rich units of theDouglas and Starshot Formations, respective-ly. The stratigraphic relationships and ages ofthese younger units were poorly defined; pos-sible ages range from latest Early Cambrianto Devonian (Fig. 2).

1072 Geological Society of America Bulletin, September 2002

MYROW et al.

Figure 2. Interpretations of stratigraphic relationships of pre-Devonian Formations in the central Transantarctic Mountains. Right-handcolumn is revised view of these relationships. Formations: St—Starshot Formation, Sh—Shackleton Limestone, Do—Douglas Conglom-erate, Di—Dick Formation, Go—Goldie Formation. Authors: A—Laird (1963), B—Skinner (1964), C—Laird et al. (1971), D—Laird(1981), E—Rowell et al. (1986), F—Rowell et al. (1988b), G—Rees et al. (1989), H—Rowell and Rees (1989), I—Rees and Rowell (1991),J—Goodge et al. (1993) and Goodge and Dallmeyer (1996), K—Goodge (1997). For reports simply describing ‘‘Lower Cambrian’’ strata,base of formation arbitrarily chosen as upper Tommotian.

New geochronologic data recast the ageand stratigraphic relationships of these units.Detrital-zircon geochronology on sedimentaryunits of the region indicates that the bulk ofoutcrops mapped as Goldie Formation are infact younger than the Shackleton Limestone(Goodge et al., 2000, 2002). Detrital zirconsfrom these sandstone deposits are as young asca. 520 Ma, making their depositional ageyounger than the Atdabanian Stage of theLower Cambrian (Landing et al., 1998; Bow-ring and Erwin, 1998; Fig. 3). These youngprovenance ages come from strata in an are-ally widespread belt of ‘‘Goldie’’ rocks thatoccupied an outboard (eastern) position alongthe continental margin at the time of deposi-tion. On the basis of detrital-zircon age spectraand field relationships, we interpret the ‘‘out-board’’ Goldie to be a temporal and strati-graphic equivalent of the younger siliciclasticdeposits of the Starshot Formation of the Byrd

Group, and these ‘‘outboard’’ Goldie strata arenow assigned to that formation. A very nar-row exposure of enigmatic, generally moremetamorphosed, rocks in the vicinity of Cot-ton Plateau and the eastern Cobham Rangewere also originally mapped as Goldie For-mation. These have older detrital-zircon sig-natures (1000 Ma or older) and probably rep-resent bona fide Neoproterozoic deposits(‘‘inboard assemblage’’ of Goodge et al.,2000, 2002).

This study provides new field, fossil, andisotopic data as the basis for additional strati-graphic revisions to the pre-Devonian sedimen-tary succession in the region. Sedimentological,physical stratigraphic, and chemostratigraphicdata set limits on the nature and timing of aCambrian transgression onto the Antarctic cra-ton. We also describe the first documented de-positional contact between the ShackletonLimestone and younger synorogenic deposits

of the upper Byrd Group from the southernHolyoake Range. Sedimentological aspects ofthis transition from massive, archaeocyathanbioherms to overlying siliciclastic-dominatedstrata allow us to reconstruct the depositionalresponse to Ross orogenesis. In addition, a newtrilobite collection in strata above the biohermsprovides constraints on the ages of several for-mations and the timing of tectonic events.These new findings, in conjunction with thegeochronological data already mentioned, haveprofound implications for the depositional andtectonic history of the Transantarctic Moun-tains. The findings also help clarify a numberof unresolved first-order stratigraphic issues ofcorrelation, and the relative ages, of nearly allof the pre-Devonian units in this region.

FIELD AREA

Geologic mapping, sampling, and measure-ment of numerous stratigraphic sections were



Figure 3. Comparison of Lower Cambrian stages of Siberia, South China, and Laurentia.Age constraints from Landing et al. (1998).

completed during the 1998–1999 and 1999–2000 austral summers. Field camps were es-tablished at Moody Nunatak and Cotton Pla-teau in the western Queen Elizabeth Range, inthe Mount Ubique area of the SurveyorsRange in the Churchill Mountains, and in thecentral Holyoake Range on Errant Glacier,providing access to many key localities. De-tailed field observations were made possibleover a geographically large area by a combi-nation of overland travel and helicopter transit.Previous mapping and stratigraphic descrip-tions by several authors, cited subsequently, es-tablished the distribution of sedimentary unitsupon which our work is based.

BEARDMORE GROUP

The Beardmore Group, consisting of theCobham and Goldie Formations, is a thick as-semblage of graywacke, carbonate, and minorvolcanic units that have been correlated withsimilar siliciclastic deposits along the lengthof the Transantarctic Mountains (Stump, 1982;Stump et al., 1986). The inferred stratigraphicposition of the Beardmore Group betweenbasement rocks and Lower Cambrian carbon-ate deposits led previous workers to suggest alate Precambrian age for the group (e.g., Lairdet al., 1971), but the youngest constraint on itis unclear (i.e., whether it extends into theCambrian). A Sm-Nd isochron age of 762 624 Ma for basalt and gabbro (Borg et al.,1990), interpreted as interlayered with Goldie

rocks near Cotton Plateau (Fig. 1), has beenwidely cited as evidence for rifting at thattime. A new U-Pb zircon age of 668 6 1 Mafor the gabbroic phase (Goodge et al., 2002)suggests that these rocks and their associatedsedimentary succession are significantly youn-ger than previously thought.

The Goldie Formation is a thick successiondominated by fine-grained sandstone. On thebasis of the presence of graded beds, flutemarks, and abundant climbing ripples, it wasconsidered a deep-sea turbidite succession(Laird et al., 1971; Stump, 1995), but theseobservations may have been made on ‘‘out-board’’ Goldie rocks now included in theyounger Starshot Formation. There is little di-agnostic paleoenvironmental evidence in muchof the ‘‘inboard’’ Goldie, in part because of low-grade metamorphism. However, one locality ;3km southeast of Panorama Point above PrinceEdward Glacier contains a well-developed suiteof sedimentary structures, including large-scalehummocky cross-stratification, which indicatesthat at least part of this formation was depos-ited above storm wave base. Coarse conglom-erate beds were tentatively suggested to rep-resent Neoproterozoic diamictites (Stump etal., 1988), but intense deformation of sectionsimmediately below Panorama Point and atother localities, including isoclinal folding ofthe deposits and stretching of pebbles to as-pect ratios up to 20:1, makes such a determi-nation tenuous.

BYRD GROUP

Deposits of the Byrd Group are widespreadin the central Transantarctic Mountains. Thestratigraphic units include the ShackletonLimestone and overlying clastic deposits ofthe Starshot, Douglas, and Dick Formations.

Shackleton Limestone

The Lower Cambrian Shackleton Lime-stone crops out between Beardmore and ByrdGlaciers (Fig. 1); its most complete exposuresare in the Holyoake Range (Rowell et al.,1988b; Rees et al., 1989; Rowell and Rees,1989) where Laird (1963) designated a typelocality. Archaeocyathan-bearing rocks fromthis formation were collected during ErnestShackleton’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 archaeocyathansindicate a Botomian depositional age for atleast part of the formation (Debrenne and Kruse,1986). Trilobite fauna described by Rowell etal. (1988a) and Palmer and Rowell (1995)confirmed a Botomian age but suggested thatsome deposits are Atdabanian and that youn-ger parts were possibly deposited in the To-yonian (Fig. 3). Nearly all of the trilobiteshave close affinities with Chinese forms of theTsanglangpuan Stage, which largely overlapsthe Botomian Stage. Carbonate units in theQueen Maud Mountains that are consideredequivalents of the Shackleton Limestone alsocontain Botomian and possible Atdabanian tri-lobites and archaeocyathans (Rowell et al.,1997).

The Shackleton Limestone is a thick car-bonate deposit with a lower unit of unfossil-iferous interbedded quartzite and carbonatethat is exposed below Panorama Point (Fig. 1)on the western slopes of Cotton Plateau,where it is in contact with the Goldie For-mation. At other localities in the region, thecontact was considered by Laird (1963) to bean unconformity, and at Cotton Plateau thesame contact was interpreted similarly byStump et al. (1991) and Stump (1995). How-ever, this contact at the base of a syncline inShackleton Limestone at Cotton Plateaushows structural evidence for fault displace-ment (Goodge et al., 1999), including angulardiscordance, a mineral-elongation lineationand stretched pebbles, asymmetric folds,asymmetric microstructures, and mineraliza-tion indicating a structural relationship. Thecontact is also faulted at other localities thathave been rechecked (Rowell et al., 1986;Rees et al., 1989; Palmer and Rowell, 1995).


MYROW et al.

Therefore, although we disagree with previousworkers about the present nature of the CottonPlateau contact and although we have no-where observed the stratigraphic base of theShackleton Limestone, it is possible that itwas originally deposited upon the currentlyunderlying ‘‘inboard’’ Goldie units.

Detailed reconstruction of paleoenviron-ments and depositional history of the forma-tion has been hampered by locally severe de-formation and numerous fault contacts.Formation thickness is also difficult to assess,and estimates of 1000–2000 m (e.g., Burgessand Lammerink, 1979) are not well con-strained. The depositional setting of the for-mation in our field area was previously inter-preted as a simple ramp with intertidal faciespassing laterally into a high-energy oolitic shoalcomplex that contains many individual archaeo-cyathan bioherms; no basinal facies were re-ported (Rees et al., 1989). Deposits of the ooliticshoal facies are laterally extensive and common-ly include small algal-archaeocyathan bioherms(Rees et al., 1989). The bioherms were locallyecologically zoned as a function of depositionalenergy and in cases coalesced to form com-posite bioherm complexes up to 50 m thick.Burrow-mottled mudstone and skeletal or pe-loidal packstone were interpreted as shallow-water, low-energy deposits formed on eitherside of the oolite shoal complex (Rees et al.,1989). However, stratigraphic position, litho-logic characteristics, and facies relationshipssuggest that these deposits are entirely deepersubtidal in origin and formed basinward (east-ward) of the oolite shoal complex.

Douglas Conglomerate

Clastic rocks of the upper Byrd Group havebeen assigned to several formations includingthe Douglas Conglomerate, a succession ofcoarse clastic deposits exposed throughout theChurchill Mountains. Clasts derived from theShackleton Limestone were recognized earlyon within the Douglas Conglomerate (Skinner,1964, 1965); these clasts indicate uplift of theShackleton and older units prior to depositionof these conglomeratic strata. Laird (1981)made the case for a sharp but conformablecontact between the Shackleton and DouglasFormations. However, abundant faults in thisregion make this claim suspect, and locally atsections along the Starshot Glacier, the Doug-las Conglomerate rests unconformably onfolded Shackleton Limestone (Fig. 1; Rees etal., 1988; Rowell et al., 1988b). Its age hastraditionally been bracketed as being youngerthan the Lower Cambrian Shackleton Lime-stone and older than the Devonian Beacon Su-

pergroup. The Douglas Conglomerate at itstype locality is purportedly in depositionalcontact with fine-grained siliciclastic depositsof the Dick Formation (Skinner, 1964; Rees etal., 1988; see subsequent discussion).

Rees and Rowell (1991) provided the onlydetailed sedimentological study of the Doug-las Conglomerate, but noted that no completesection of the formation exists and that fault-ing has made stratigraphic correlations diffi-cult. Their facies analysis indicated depositionmainly in proximal to distal alluvial-fan en-vironments, but also in marine and lacustrinesettings. Clast compositions in the conglom-eratic alluvial-fan deposits are dominated byquartzite and limestone, the latter eroded fromthe Shackleton Limestone. However, the fol-lowing clast types were also noted: argillite,sandstone, metaquartzite, diorite, two-micagranite, gneissic granite, amygdaloidal spilite,rhyolite, basalt, chert, and dolomite (Skinner,1964, 1965; Rees et al., 1987; Rees et al.,1988). Clasts of Shackleton Limestone in theDouglas Conglomerate are locally folded andcontain some calcite veins, which indicate de-formation prior to Douglas deposition (Rowellet al., 1986). The Douglas itself is folded andcontains cleavage, indicating that Ross defor-mation outlasted deposition of the unit.

Starshot Formation

The Starshot Formation was defined byLaird (1963) to include shale, sandstone, andminor conglomerate exposed along the eastside of the lower Starshot Glacier (Fig. 1); itstype section is at Mount Ubique. Clasts in theconglomeratic units have compositions similarto those of the Douglas Conglomerate andalso contain archaeocyathan fossils, thus in-dicating that the Starshot Formation was de-posited contemporaneously with or after theShackleton Limestone. No depositional con-tacts have ever been described between theStarshot Formation and any other formation inthe region. Likewise, no fossils have been re-covered from the Starshot Formation, otherthan those inherited in clasts, so determina-tions of its minimum depositional age are gen-erally unconstrained.

The depositional environments of the Star-shot Formation are poorly understood, and in-terpretations range from deep-water turbiditicto shallow-water shelf (Laird, 1963; Laird etal., 1971). Laird et al. (1971) recognized prob-able shoaling of the deposits toward the northwhere conglomeratic facies occur. Descrip-tions of fine-grained parts of the Douglas Con-glomerate (Rees and Rowell, 1991) indicatethat in lithology and sedimentological char-

acter, they are similar to the bulk of the Star-shot Formation. Recent detailed analysis in-dicates that the Starshot Formation wasdeposited in environments that ranged fromshoreline and possible fluvial facies to deepershelf (Myrow et al., 2002). Trace fossils occurin several sections, but they are not common.The vast majority of the formation was de-posited in inner-shelf to shoreline environ-ments, in part as wave-modified turbidites(Myrow et al., 2002). Abundant paleocurrentdata indicate transport toward the east andnortheast. The presence of wave-modified tur-bidites and conglomeratic beds, the inferredfacies relationships with the Douglas Con-glomerate, and the paleocurrent patterns allsuggest development of a high-relief proximalhinterland adjacent to a storm-influenced sea(Myrow et al., 2002).

Dick Formation

The Dick Formation consists primarily offine siliciclastic deposits (shale through fine-grained sandstone) exposed in the northernChurchill Mountains (Fig. 1). Its type localityis ;15 km east of Mount Dick, south of theoutlet of Byrd Glacier at Stations P and N(Skinner, 1964). At Station P, 150 m of thisunit is exposed below the Douglas Conglom-erate. It is presumably conformably overlainby Douglas Conglomerate at this locality(Skinner, 1964; Rees et al., 1988), but little isknown about its age or relationship to otherstratigraphic units. Burgess and Lammerink(1979) claimed that it conformably overliesthe Shackleton Limestone (no details provid-ed), but this contact was considered a fault byRees et al. (1988) and we agree. The forma-tion contains thin polymictic conglomeratebeds with carbonate clasts whose character issimilar to that of clasts of the Douglas andStarshot Formations (Laird, 1963).

Rees et al. (1988) considered the Dick For-mation to have been deposited after the initialepisode of early Paleozoic deformation, andthey viewed it as genetically related to theDouglas Conglomerate. They interpreted theunit to represent fluvial to marginal-marine de-posits that were essentially the distal equivalentof the Douglas alluvial-fan facies. Rare tracefossils suggest that the deposits are at least part-ly marine. We did not study the Dick Forma-tion in detail, but our examination of the unitat section N of Skinner (1965) indicates that itcontains most of the sedimentological aspectsof the Starshot Formation. These include abun-dant fine-grained and very fine grained sand-stone beds, graded bedding, thick climbing-



ripple divisions, and minor carbonate-clastconglomerate beds as thick as 1 m.

In summary, the age relationships of thevarious formations just described are poorlyknown because of discontinuous regional ex-posure, faulted formation contacts, lack ofbiostratigraphic control, and a near absence ofgeochronologic data. Many stratigraphicschemes have been suggested, as summarizedin Figure 2. We present data from several crit-ical sections that reveal important stratigraphicand age relationships among these formations.In combination with other stratigraphic andsedimentological information, as well as new-ly acquired U-Pb detrital-zircon dates, a co-herent stratigraphic scheme is presented for allsub-Devonian sedimentary units in the centralTransantarctic Mountains.

LOWER SHACKLETON LIMESTONE

The sandstone-rich lower member of theShackleton Limestone is exposed at CottonPlateau beneath Panorama Point, where it con-sists of up to 133 m of interbedded white- tocream-weathering, vitreous, quartz sandstoneand brown-weathering, white, fine-grained do-lomitic grainstone (Figs. 4, 5A). These beds,in both the upright and overturned limbs of alarge syncline, are in fault contact with adja-cent Goldie Formation (see previous descrip-tion of the contact; Goodge et al., 1999). Inthe less deformed Shackleton section abovethe lower member, the percentage of carbonateincreases up section, and the sandstone com-ponent eventually disappears as the sectionshifts into a thick, massive boundstone unit.A detailed section of the lower ShackletonLimestone at Cotton Plateau was measured onthe lower limb of the large overturned syn-cline (Fig. 4). This section contains meter-scale, mixed siliciclastic-carbonate cycles(Fig. 5B) that begin with 7–181-cm-thick,fine- to medium-grained sandstone beds (av-erage 5 60 cm) having sharp, commonly ero-sional basal contacts with underlying carbon-ate beds (Fig. 5C). These sandstone bedslocally show evidence for shallow-water de-position including paired clay drapes diagnos-tic of tidal deposition (Fig. 5D) and small-scale two-dimensional wave ripples. Thesandstone beds have gradational upper con-tacts that include zones of carbonate-richsandstone and sandy carbonate. Carbonatecaps of these cycles consist of grainstone withabundant hummocky cross-stratification (Fig.5E). These cycles appear (atypically) to recordupward-deepening patterns. The upward de-crease in sandstone from cycle to cycle andeventual stratigraphic transition from these cy-

clic deposits into a carbonate platform repre-sents a long-term transgression and establish-ment of a long-standing carbonate ramp.

As already mentioned, we consider the con-tact between the Shackleton Limestone andGoldie Formation to be a fault at all locationswe examined, including at Cambrian Bluffand Cotton Plateau, where Laird et al. (1971)and Stump (1995) described it as an uncon-formity. The faulted nature of the contact be-tween the lower Shackleton Limestone and‘‘inboard’’ Goldie Formation raises two pos-sible scenarios. First, faults may have devel-oped at or near a possible unconformity be-tween these units, owing to mechanicalweakness coupled with a sharp rheologicalcontrast, leaving the basal Shackleton relative-ly intact. Second, some amount of the lowerShackleton may be missing owing to faulttruncation. Therefore, we maintain that a de-positional relationship between Shackletonand inboard Goldie has yet to be demonstratedand that an unknown amount of the lowermostShackleton section may be missing regardlessof its original relationship with the underlyingGoldie rocks. Given the overall upward tran-sition from sandstone to carbonate observedat Cotton Plateau, one would predict that anyolder parts of the Shackleton would be dom-inated by quartz sandstone. A possible expo-sure of such a section occurs at the outlet ofthe nearby Princess Anne Glacier. This out-crop contains a thick section of white quartzsandstone and minor shale that Laird et al.(1971) attributed to the Shackleton Limestone.The strata contain evidence for shelf and shore-line deposition (e.g., mud cracks, paired mud-stone drapes, hummocky cross-stratification).An interval of coarse, fluvial cross-bedded con-glomerate, pebbly sandstone, and sandstoneoccurs near the top of the unit. The entire sec-tion, several hundred meters thick, is unrep-resented in the Cotton Plateau section just 17km away. Unfortunately, neither outcrop isfossiliferous, although given the facies char-acteristics, it is unlikely that much time is rep-resented by these strata. An archaeocyathanfossil was collected from the Cotton Plateausection at 127.95 m (Fig. 4), but this fossil ispoorly preserved and only indicates that thispart of the formation is Lower Cambrian.

To better determine the depositional age ofthe lower Shackleton Limestone, a series ofsamples for chemostratigraphic analysis wastaken approximately every 50–100 cm fromthe top through the basal limb of the synclinebelow Cotton Plateau (Fig. 4). Carbon isotoperatios were determined by using a FinneganMAT Delta plus mass spectrometer housed atthe University of Tennessee, Knoxville. These

carbon isotope ratios were measured againstestablished standards (PDB—Peedee belem-nite) and plotted against stratigraphic heightto produce a chemostratigraphic curve (Fig.6). This curve shows a prominent, long-term,positive isotopic excursion that begins at d13Cratios of 12‰ and rises to nearly 15‰; thecurve then gradually decreases to about 12‰.The prominent positive excursion occurs overnearly 75 m of shallow-marine carbonate-richsection and therefore appears to represent ageologically significant excursion of probableglobal significance. A number of systematicisotopic excursions are documented in LowerCambrian rocks, particularly for the Siberianstages, and 10 (I–X) positive excursions areknown (Kirschvink et al., 1991; Brasier et al.,1994; Brasier and Sukhov 1998). Six assem-blages of trilobites were noted in the Shack-leton Limestone by Palmer and Rowell(1995). The oldest assemblage contains forms,including redlichiids, that overlap in age withfallotaspidid trilobites, which occur at the baseof the Atdabanian Stage (Fig. 3) and are as-sociated with carbon isotope excursion IV(Brasier et al., 1994). The positive excursionat the base of the Shackleton Limestone isconsidered to be excursion IV on the basis ofthe following. First, the redlichiid trilobiteswere likely recovered from very low in theformation, presumably not much above themixed siliciclastic-carbonate deposits of thelower Shackleton. Second, the upper half ofthe underlying Tommotian is globally char-acterized by a long-standing negative excur-sion. Finally, no faunas of clearly Tommotianage have been recovered from the ShackletonLimestone. Thus, identification of the basalShackleton Limestone excursion as excursionIV dates the timing of marine transgressiononto the Antarctic craton in this region as be-ing early Atdabanian.

SHACKLETON–UPPER BYRD GROUPTRANSITION

Exposures of the Shackleton and DouglasFormations along the east side of the Holy-oake Range were examined by Rees and Row-ell (1991). They mapped several faults in theregion and concluded that no contacts betweenthese formations were depositional. Our study,aided by helicopter support, revealed thatthere is large-scale repetition of section inthrust sheets carried by faults that trend sub-parallel to the axis of the range. A deposition-al transition between the Shackleton Lime-stone and overlying siliciclastic units of theupper Byrd Group was discovered andmapped in successive thrust sheets on numer-


MYROW et al.

Figure 4. Bed-by-bedmeasured section ofmixed siliciclastic-carbonate deposits of thelowermost ShackletonLimestone, Cotton Pla-teau. Abbreviations:Sh—shale; Sls—siltstone;VFS—very fine grainedsandstone; FS—fine-grained sandstone. Car-bonate: FG—fine-grainedgrainstone, CG—coarse-grained grainstone.



Figure 5. Shackleton syncline section, Cotton Plateau (Fig. 4). (A) Lower Goldie Formation(G) is overlain by interbedded quartz sandstone and dolostone of lower Shackleton Lime-stone (S). Arrow points to contact. Upper part of section consists of massive archaeocyathid-bearing dolostone cliffs. (B) Interbedded quartz sandstone and dolostone beds in 60–63 mzone (distances refer to measured section; Fig. 4) of lower Shackleton Limestone. Hammerfor scale (arrow). (C) Deep-channel fill of quartz sandstone in cross section at 98.81 m.Hammer for scale. (D) Paired clay drapes (arrows) in quartz sandstone bed at 74.15 m.Pencil (at left) is 14 cm long. (E) Hummocky cross-stratification in carbonate grainstoneat 27 m. Pencil is 14 cm long.

Figure 6. Chemostratigraphic curve of d13Cvs. stratigraphic height for lower Shackle-ton Limestone at Shackleton syncline sec-tion (Fig. 4).

ous spurs along an ;30 km north-south sec-tion of the Holyoake Range.

In the Holyoake Range, the upper Shack-leton Limestone consists of black to dark gray,burrow-mottled and skeletal wackestone thatgrades upward into large, massive, isolated ar-chaeocyathan bioherms (Fig. 7). Both thebioherms and the interbioherm strata on whichthe bioherms rest are capped by an extensive,multigeneration, black, phosphatic surface,which is overlain by a unit of shale and nod-ular lime mudstone in a brown, calcareousshale matrix. This facies grades upward intofossiliferous siltstone and then into a complex

assemblage of carbonate-clast conglomerateand fine-grained sandstone. The transition-zone strata were carefully measured in severallocalities, and two measured sections are pre-sented in Figures 8 and 9. A description andinterpretation of these facies follows in thestratigraphic order in which they occur.

Nodular Carbonate (ShackletonLimestone)

The nodular carbonate facies makes up atabular unit that is overlain by isolated ar-chaeocyathan bioherms (described subse-

quently). It consists of gray to dark gray, nod-ular beds of skeletal-pelletal wackestone andlime mudstone. This facies contains rare, thin,laterally discontinuous skeletal packstone andgrainstone beds with bioclasts of trilobites, ar-chaeocyathans, and eocrinoids. A few thin in-tervals (commonly ,2 m thick) of dark, thin-bedded lime mudstone occur within thisnodular carbonate.

The dark color and nodular bedding suggestthat this facies developed in a deep subtidalsetting colonized by burrowing organisms.The lack of interbedded archaeocyathanbioherms, ooids, and light-gray lime mud-stone (cf. Rees et al., 1989) suggests that itformed basinward of the ooid shoal facies.The thin interbeds of skeletal grainstone/packstone are interpreted to be storm depos-its that infrequently affected this deeper sub-tidal environment.

Archaeocyathan Bioherms (ShackletonLimestone)

Isolated bioherms at the top of the Shack-leton Limestone consist of massive-bedded,light gray to white algal boundstone with ar-chaeocyathans. The bioherms range in thick-ness from 0 to .38 m (Figs. 7, 10A). Mostof the bioherms are simple structures withonly one episode of growth. However, at a fewlocations the bioherms are composite struc-tures with at least two separate growth phases,each capped by an extensive phosphate-encrusted surface (Fig. 11). The biohermsform isolated bodies, and between them thephosphate-encrusted surface occurs directlyon top of the nodular carbonate unit describedearlier.

The upper surface of the lower growthphase of the composite bioherm at section 1


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Figure 7. Large light-colored archaeocyathan reefs (white arrows), tens of meters thick,just south of section 1 of the Holyoake Range. Bedding is vertical, and stratigraphic upis to right; dashed line represents bedding orientation. Reefs are surrounded by shale,nodular carbonate and shale, and fine-grained sandstone of Starshot and Holyoake (newname) Formations.

Figure 8. Stratigraphic section 1 from eastside of Holyoake Range. Standard grainsizes from Sh (shale) to Bo (boulder). M—micrite, B—boundstone, T—trilobites. Seelegend.

M

has local small, irregular depressions with upto 1.5 m of relief (Fig. 12A). These were filledby wackestone, shale, and carbonate-clast con-glomerate with boundstone fragments. A sec-ond growth phase of microbial boundstone iscapped with another thick composite phos-phatic surface (Fig. 12B). This composite sur-face is composed of multiple irregular layersof phosphatic micrite cement incorporating athin (10–15 cm thick) skeletal packstone/grainstone with a fine-grained fauna of hy-olithid, eocrinoid, and trilobite bioclasts aswell as quartz silt and phosphatic fragments(Fig. 12C). Many of the phosphatic surfacesare bored with 1–5-mm-diameter, nearly cir-cular borings.

Development of $40 m of relief and a lack

of interbedded oolite shoals suggest that thesealgal bioherms developed in a deep subtidalsetting basinward of the oolitic shoal complex.The phosphatic surfaces formed during earlycementation of the archaeocyathan biohermsand represent a complex submarine hard-ground. Most of the bioherms grew during asingle phase of growth, although some ofthem show at least two phases of growth sep-arated by hardground development and onlapof fine carbonate and shale. The carbonateconglomerate on the side of the biohermformed by submarine erosion of the growingbioherm, possibly by storms. The irregular up-per surfaces of the bioherms were likely pro-duced by nonuniform growth of the biohermor possibly by submarine erosion. Both the

cessation of bioherm growth and the succeed-ing development of the hardground were re-sponses to rapid drowning of the carbonateplatform. The hardground records sedimentstarvation during flooding and replacement ofcarbonate by phosphate that was likely intro-duced by upwelling. The borings indicate thehardgrounds were hard, cemented surfacesand that they acted as surfaces of sedimentbypass (Bathurst, 1975; Mullins et al., 1980).The multiple-stage hardground on top of thearchaeocyathan bioherms indicates terminaldrowning of the downslope bioherms, whichwe interpret as likely caused by tectonic sub-sidence related to initiation of orogenesis inthis region.

Shale and Nodular Carbonate/Shale (NewHolyoake Formation)

Directly overlying the hardground surface,both between the bioherms and on their flanks,is dark gray to brown, calcareous fissile shale.This passes upward into nodular carbonate ina similar-colored calcareous shale that onlapsthe uppermost parts of the taller bioherms(Fig. 10B). The nodular carbonate consists oflime mudstone nodules 2–10 cm thick and upto 40 cm wide in a brown shaly matrix. Thenodules include rare trilobite and very rare hy-olithid bioclasts and up to 1% disseminatedpyrite. Near the top of some bioherms, inshale just below the first occurrence of thenodular carbonate facies, are rare, small (1–2m wide, ,1 m high) archaeocyathan biohermsthat are capped by a thick phosphatic surface.The archaeocyathans in these bioherms arelarge and locally replaced by phosphate (Fig.12D).

This shale records deep-water depositionabove the phosphate-encrusted bioherms in re-sponse to drowning of the carbonate platform.The nodular carbonate and shale also consti-tute a deep-water facies, and although the in-crease in carbonate content may indicate achange in climate (Weedon, 1986), it morelikely represents a response to shoaling and astratigraphic shift into overlying calcareoussiltstone (described in the next section). Shaleand nodular shale within pockets in the upperpart of some bioherms (between lower and up-per growth stages described earlier) and the


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

small bioherm bodies within overlying shaleand nodular carbonate facies reflect an intervalof repeated drowning and attempted biohermgrowth. Because of its distinction as a map-pable unit, we define this facies of the tran-sition zone as the ‘‘Holyoake Formation,’’ asproposed later in this paper.

Calcareous Siltstone/Very Fine Sandstone(Starshot Formation)

This facies consists of brown-weathering,calcareous, siltstone to very fine 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 faciesincludes a few very widely spaced (severalmeters) decimeter-scale shaly intervals. Thesandstone is blocky weathering and generallyfeatureless in outcrop. This facies occursabove shale with nodular carbonate, as alreadydescribed, and for less than 1 m of this faciestransition, the siltstone contains thin (,4 cm)nodules of gray micritic limestone (Fig. 13B).The lowermost 5–10 m of section above thisnodular-facies transition contains abundantfossils of trilobites and hyolithids. This faciescontains locally developed black phosphaticmaterial that occurs both as disseminatedgrains in siltstone beds up to 1.1 m thick andas nodular beds up to 8 cm thick. The lattercontain abundant ,2-mm-diameter borings(Fig. 13C).

Beds of pebbly fine- to medium-grainedsandstone from 30 to 70 cm thick occur withinthis facies. These are nongraded and matrix



Figure 10. (A) Onlap of shale (o) against flank of large archaeocyathan reef (m). Smallarrow indicates position where onlapping bed in foreground contacts the reef. The reef is;40 m thick. (B) Transition from uppermost Shackleton Limestone archaeocyathidmound (m) into Starshot Formation at section 1 of the Holyoake Range. Nodular limestoneand shale (n) directly overlie mound and are succeeded by fossiliferous calcareous siltstoneand very fine sandstone (s). Pencil (circled) is 14 cm long.

supported with widely dispersed, well-rounded,pebbles that make up ,20% of the lithology.The pebbles consist mostly of quartzite, whitelimestone, and intraclasts of fine sandstonegenerally ,7 cm in diameter, although onebed contains a sandstone intraclast that is 453 5 cm in cross section. At section 1, thisfacies occurs in the 10 m of strata directly

below the first stratigraphic occurrence of con-glomerate beds.

Few diagnostic sedimentary structures oc-cur in this facies, but the presence of abundanttrilobite and hyolithid fossils as well as boredphosphate nodules indicates a marine deposi-tional setting. The relative paucity of shalybeds and a stratigraphic transition into con-

glomerate and sandstone facies (describedsubsequently) indicates that this siltstone/veryfine sandstone was deposited in nearshore toshoreline environments. The pebbly sandstonebeds have many of the characteristics of debrisflow deposits such as matrix support and lackof grading. These beds likely formed as shallow-marine debris flows, possibly representingstorm-generated deposits.

Trilobite FaunaThe fauna from the calcareous siltstone/

very fine sandstone units is of Early Cambrianage. Intercontinental correlation of EarlyCambrian trilobites is very difficult, and con-siderable uncertainties exist (Palmer and Row-ell, 1995; Palmer, 1998). However, the faunareported here has closest affinities to faunasfrom South Australia and south China. Al-though the trilobites are slightly deformed andindifferently preserved, there appear to bespecimens of Redlichia, Megapaleolenus, andHsuaspis cf. bilobata Pocock. The latter isprobably conspecific with a trilobite describedby 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 camefrom a locality very close to ours, it is pos-sible that the very same interval was sampled.Hsuaspis occurs in China in rocks of lateChiungchussuan to medial Tsanglangpuan age(Zhang et al., 1995) and in Australia in bedsconsidered to be Botomian equivalents (Jell inBengtson 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 aneed to reevaluate the age of assemblage 5 ofPalmer and Rowell (1995) from the underly-ing Shackleton Limestone, determinations ofwhich have been poorly constrained butthought to be a slightly younger Early Cam-brian age.

Mixed Conglomerate, Sandstone, and SiltyShale (Douglas Conglomerate)

This particularly complex facies consists ofinterbedded conglomerate; sandstone bedswith a wide range of grain sizes, bed thick-nesses, and sedimentary structures; and blacksilty shale. Conglomerate beds range fromsandy pebble conglomerate to poorly sortedcobble/boulder conglomerate. Individual bedsare generally ,2 m thick, but amalgamatedunits of conglomerate range up to 17 m inthickness. Beds range from massive, unorga-nized and poorly sorted to well graded (Fig.14A) with well-organized transitions, e.g.,


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Figure 11. Drawing of upper part of composite reef from section 1, Holyoake Range. Viewis to the west. Two distinctive phosphate-encrusted hardground layers near the top of thereef (heavy black lines) indicate that drowning occurred in at least two phases. The initialdrowning event was followed by shale deposition and renewed growth of the archaeocy-athan reef. The discontinuous carbonate conglomerate above the shale likely indicateserosion of the reef, possibly by submarine currents (e.g., Schlager, 1998, 1999). Lateralexpansion of the reef over the carbonate conglomerate was followed by the final drowningevent and formation of the laterally persistent phosphatic hardground at the top of thereef. Nodular carbonate and shale then buried the reef following drowning. It is unclearwhether the small archaeocyathan bioherm in the center of the drawing is a third phaseof reef development or is a three-dimensional extension of the second phase of reef growth.Small faults offsetting the hardground at the top of the reef cannot be traced into theoverlying shale or siltstone.

clast-supported cobble conglomerate to pebblysandstone to cross-bedded sandstone. Beds arecommonly lenticular with concave-up deeplyscoured bases. Conglomerate beds have highlyvariable clast compositions from siliciclasticdominated to carbonate dominated. Large,outsized intraclast blocks of sandstone rangeup to 1 3 0.4 m in cross section and are par-ticularly abundant in areas of intimate inter-bedding of conglomerate and sandstone (Fig.14B). Clasts clearly derived from the under-lying Shackleton Limestone are abundant andlocally outsized, particularly in section 2(Figs. 9, 15, A and B), in which conglomeratebeds rest directly on the upper surface of abioherm. Large white biohermal limestoneclasts range up to 2 3 0.8 m across and in-dicate local erosion and transport of biohermblocks. Large outsized blocks also locally oc-cur stratigraphically well above the bioherms.One outsized block of fenestral mudstone, 1.53 1.6 m in cross section, occurs in section 1,147 m above the high point of the underlyingbioherm (Fig. 14C).

Green light-red-weathering sandstone beds(Fig. 14B) range from fine grained to coarsegrained with floating small pebbles. On aver-age, these beds are medium to coarse grainedand moderately to poorly sorted. Bed thick-nesses range from 4 cm to .1 m and average30–40 cm. These beds make up amalgamatedunits up to 12 m thick. Fine-grained sandstonebeds show parallel lamination, trough cross-

bedding (Fig. 14D), and locally developedcurrent ripple lamination.

Black shaly siltstone and silty shale unitsare widely dispersed in this facies. These aregenerally a few tens of centimeters thick, butrange up to 14 m thick. In some cases, thefine-grained units contain 1–3-cm-thick veryfine grained to fine-grained sandstone bedsthat locally reach up to 15 cm in thickness.

This diverse facies was almost certainly de-posited in a variety of subenvironments. It oc-curs in a stratigraphic position above the pre-viously described facies such that it recordscontinued shoaling and increased proximity ofsource regions. The bed thicknesses andcoarse grain size of the conglomerate indicateproximal deposition, particularly given thelarge outsized meter-scale blocks. Theseblocks, locally derived from the ShackletonLimestone, indicate local tectonic uplift anderosion, particularly as strata from lower inthe formation are also represented as clasts.The conglomerate beds first appear in section1 (Fig. 8) in close association with phosphate-rich siltstone and farther up section with or-ganic-rich silty shale. Thus, at least some ofthese beds were either deposited in, or inproximity to, a subaqueous setting. Althoughthe phosphatic beds are likely marine in ori-gin, the black silty shale beds could be eithermarine or marginal marine. Thicker units ofmixed conglomerate and medium- to coarse-grained sandstone most likely represent fluvial

to marginal-marine deposits. Meter-scale grad-ed beds (Fig. 14A) are likely channel depositsthat were either interbedded with fluvial sheet-flood or shoreline sandstone beds. Similar fa-cies were described as part of a wider rangeof coarse clastic facies of the Douglas Con-glomerate by Rees and Rowell (1991). Theyinterpreted the wider suite of facies as rangingfrom proximal to distal alluvial-fan deposits.

The strata described from the Shackleton–up-per Byrd Group transition zone likely representa shift from clearly shallow-marine (trilobite-bearing) siltstone through marginal-marine andthen alluvial-fan deposits. Much of the shallow-marine deposits were deposited in fan-deltas.The stratigraphic framework and implications ofthese deposits are discussed next.

STRATIGRAPHIC REVISION

The depositional ages and stratigraphic re-lationships of the Neoproterozoic to lower Pa-leozoic formations in the central Transantarc-tic Mountains have been difficult to discernbecause of structural complexity, abundant icecover, and a general lack of fossils. Detailedstratigraphic and sedimentological data pre-sented in this paper, in addition to ages of de-trital zircons from these units (Goodge et al.,2002), allow for a significant stratigraphic re-vision (Fig. 16). The depositional transitiondescribed herein between the uppermostShackleton Limestone and the siliciclastic de-posits of the upper Byrd Group is critical inthis regard.

A multitude of stratigraphic schemes andinferred depositional ages of the various for-mations have been proposed (Fig. 2). In manycases, previous workers had limited access toparticular formations and did not examine oth-ers, which hindered stratigraphic synthesis.For instance, Rees, Rowell, and various col-leagues (Fig. 2, columns E through I) exam-ined the Shackleton and Douglas Formations,but did not study the Starshot Formation. Con-versely, all previous workers uniformly placedthe Cobham and Goldie Formations in theBeardmore Group and generally consideredthem to be Neoproterozoic in age because oftheir apparent great thickness, lack of fossils,and inferred position beneath the Lower Cam-brian Shackleton Limestone. However, thebulk of rocks previously mapped as GoldieFormation are now known to be late EarlyCambrian in age or younger. Thus, we suggestthat the Beardmore Group should be redefinedto include only those rocks that are known, orreasonably inferred to be older than the Shack-leton Limestone, namely, the Cobham For-mation and ‘‘inboard’’ Goldie assemblage



of Goodge et al. (2002). The ‘‘inboard’’ Gol-die is best defined as that part of the Beard-more Group lacking detrital minerals havingages younger than 600 Ma. This criterion re-stricts the mapped extent of the group to theeastern Cobham Range and the vicinity ofCotton Plateau (Fig. 1). The available detrital-zircon ages only indicate that this restrictedBeardmore Group is younger than ca. 1000Ma, although a new zircon U-Pb age of 668Ma from gabbro associated with pillow basaltsat Cotton Plateau (Goodge et al., 2002) indi-cates that deposition is at least in part lateNeoproterozoic. Although the same geochro-nological constraints allow the ‘‘inboard’’Goldie to be as young as the lowermostShackleton Limestone, it is unlikely to beCambrian. Sandstone samples with a youngdetrital signature (younger than 520 Ma) fromthe Lowery and Beardmore Glacier areas, aswell as the northern Churchill Mountains, sug-gest that the outer ranges contain younger‘‘outboard’’ Goldie deposits with a differentprovenance (Goodge et al., 2002). Theseshould hereafter be mapped as part of the Star-shot Formation (Figs. 1, 16).

Early workers recognized that the presenceof Shackleton Limestone clasts in both theDouglas and Starshot Formations indicatedthat these units were equivalent to and/oryounger than that formation. Most subsequentworkers considered the Douglas to be youngerthan, and locally in unconformable contactwith, the underlying Shackleton Limestone(Fig. 2), and we concur. Several lines of evi-dence indicate age equivalence between theStarshot and Douglas Formations (Fig. 16).First, ‘‘Douglas’’-like conglomeratic units oc-cur in rocks mapped as Starshot Formation inthe Mount Ubique area. Second, these con-glomeratic parts of the Starshot occur spatiallybetween more inboard regions mapped asDouglas Conglomerate that are dominated bycoarse conglomerate and more outboard re-gions of the Starshot Formation that are al-most exclusively sandstone and minor shale.Third, paleocurrent patterns in these sandyunits are uniformly oriented from inboard ar-eas toward more outboard regions (i.e., flowfrom west to east and southwest to northeast)(Myrow et al., 2002). Finally, the age distri-butions of detrital zircons from samples ofthese formations are similar (J. Goodge, un-publ. data).

Rees et al. (1988) and Rowell et al. (1988b)clearly demonstrated an unconformity be-tween the Shackleton and Douglas Formationsat one locality, but noted fault contacts be-tween these units at other locations. The de-positional transition from the Shackleton

Limestone into the younger siliciclastic unitsof the upper Byrd Group described in this pa-per indicates that, in this area, the latter en-tirely postdate Shackleton carbonate deposi-tion (i.e., none is even partly coeval). Thesiltstone/sandstone facies within this upward-coarsening transition (Fig. 8, 23.8–63.5 m) issimilar in grain size, texture, color, and weath-ering to the bulk of the Starshot Formationand is thus included in that formation. Over-lying coarse-grained units are assigned to theDouglas Conglomerate. It is apparent fromspatial distributions of conglomeratic faciesand paleocurrents that the coarse-grained unitsof the Douglas Conglomerate make up a thicksedimentary wedge of limited lateral extent.This is a logical geometry because this unitrepresents the deposits of an alluvial-fan com-plex (Rees and Rowell, 1991). The distal fin-gers of this wedge make up the conglomeratebeds of the Starshot in the Mount Ubique area.The result of progradation of this alluvialwedge into a marine setting is that sandstonefacies both underlie and are laterally equiva-lent to this conglomeratic body (Fig. 16). Infact, this wedge is also overlain by a thicksection of sandstone (Fig. 16), as seen at HighRidge (Fig. 1). Here, an upward-coarseningsuccession identical to that in sections 1 and2 is followed by hundreds of meters of sand-stone and minor shale facies typical of theStarshot Formation.

The presence of sandstone below, adjacentto, and above a conglomeratic wedge helpsexplain a number of other stratigraphic com-plexities, including the relationship of theseunits with the Dick Formation. As describedearlier, our brief study of the Dick Formationat co-type section N of Skinner (1964, 1965)revealed a remarkably similar suite of beddingcharacteristics and internal sedimentary struc-tures to those of the Starshot Formation. Inaddition, detrital-zircon ages from the DickFormation at section N include populations at495, 510, 555, 1020, and 1130 Ma and a rangeof older Proterozoic and Archean ages; theseages are very similar to zircon-age distribu-tions from the Starshot Formation elsewhere(J. Goodge, unpubl. data). These data and oth-er geologic relationships lead us to considerthese units as equivalents. According to Skin-ner (1964), Burgess and Lammerink (1979),and Rowell and Rees (1989), the Dick For-mation is overlain by, and in conformablecontact with, the Douglas Conglomerate at co-type section P. This is therefore analogous tothe transition from the Starshot Formation(with Botomian fossils, ca. 515 Ma) to theDouglas Conglomerate in the Holyoake sec-tions described previously herein. The pres-

ence of young 500–490 Ma detrital zircons inthe Dick Formation at co-type section N in-dicates that the strata at this locality are po-tentially younger by ;20 m.y. than at its otherco-type section P, including part or all of theDouglas Conglomerate. If the Dick Formationis at different locations both older than, andyounger than, a (presumably) large part of theDouglas Conglomerate, then the latter likelyforms a wedge within the sandy Dick For-mation, as described for the Starshot Forma-tion. The base of the Dick Formation is notexposed, but we would predict that it overliesa shaly unit that itself directly overlies theShackleton Limestone, as described for theHolyoake Range in this paper.

We see no benefit in continuing to use thename ‘‘Dick Formation’’ because this unitappears to be identical to the Starshot For-mation in lithologic and sedimentologic char-acter, detrital-zircon geochronology, andstratigraphic relationship with the DouglasConglomerate. We thus suggest that the name‘‘Dick Formation’’ be abandoned and that ex-posures previously mapped as Dick Formationbe placed in the Starshot Formation (Fig. 16).The name ‘‘Starshot’’ is preferred given itsearlier introduction (Laird, 1963). In summa-ry, we suggest that the Starshot Formationshould include all rocks originally mappedwithin this formation plus rocks of the ‘‘out-board’’ Goldie Formation and all of thoserocks previously mapped as Dick Formation.We suggest continued use of the term‘‘Douglas Formation’’ for those rocks domi-nated by conglomerate.

Holyoake Formation

The depositional transition between theShackleton Limestone and overlying clasticdeposits of the upper Byrd Group marks a ma-jor lithologic break that reflects profoundchanges in depositional systems, depositionalhistory, and tectonics. This transition occursat the contact between the phosphatized uppersurface of the Shackleton Limestone and theoverlying fine-grained shale and mixed nod-ular carbonate and shale deposits. The shalydeposits form an extensive stratigraphic unitthat can be mapped throughout the HolyoakeRange and may be present within adjacentranges as well. Previous workers did not rec-ognize this unit as a separate mappable litho-some and therefore would have likely includ-ed these deposits as a facies within theDouglas Conglomerate or Starshot Formation.This lithologically distinct unit, which con-sists of fine-grained dominantly siliciclasticfacies sandwiched between carbonate-


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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 andonlap the hardground surface. Pen is 14 cmlong (circled). (B) Phosphatic hardgroundsurface (thin arrow on right edge) near topof composite archaeocyathan reef. Beddingis vertical. Pencil is 14 cm long. (C) Close-up of phosphatic grainstone fill (g) andmultiple hardground surfaces (white ar-row) on top of composite archaeocyathanmound (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 is14 cm long.

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) andshale of upper Holyoake Formation to veryfine calcareous sandstone (s) w/limestonenodules 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 onbedding plane. Pencil tip is 2 cm long.

platform deposits (Shackleton Formation) andcoarse, synorogenic molasse (Douglas andStarshot Formations), is herein assigned to aseparate formation. Because this unit occurswithin the Holyoake Range, we suggest thatit be designated the ‘‘Holyoake Formation.’’This formation is exposed at numerous sec-tions along the eastern Holyoake Range, in-

cluding sections 1 and 2 (Figs. 8 and 9), butis not present where uplift and erosion of theShackleton Limestone caused angular uncon-formity with overlying conglomerate units ofthe Douglas Conglomerate (Rees et al., 1988;Rowell et al., 1988b). The base of this for-mation is defined as the lowermost shale thatrests on the phosphatized surface at the top of

the Shackleton Limestone. The top of the for-mation is defined as the top of the extensivenodular carbonate and shale unit that underliescalcareous siltstone to very fine grained sand-stone of the basal Starshot Formation. The for-mation reaches a maximum thickness of ;45m and thins appreciably over the bioherms ofthe upper Shackleton Limestone. Locally, insection 2 and possibly elsewhere, conglom-eratic channels of the overlying DouglasConglomerate cut out both the Starshot For-mation and the thinned Holyoake Formationat the high point of a bioherm (Fig. 9). Thetype section of the Holyoake Formation isdesignated at the section 1 locality (Fig. 1;GPS [Global Positioning System] location82813.1859S, 160816.1229E).

The Holyoake Formation probably also ex-ists in more outboard regions, between theShackleton and Starshot Formations, but thisstratigraphic relationship has not been ob-served. Such a transition would be similar tothose at inboard localities except that the over-lying clastic deposits would be mostly sand-



Figure 14. Douglas Conglomerate at section 1 of Holyoake Range. (A) Graded conglom-erate bed 1.8 m thick at 70.2 m (Fig. 9). Yellow, size 9 men’s boot on left for scale (;30cm). Arrow points in direction of stratigraphic top. (B) Interbedded conglomerate andsandstone. Camera case for scale. (C) Large block of black fenestral mudstone with calciteveins (b) within conglomeratic bed at 166 m (Fig. 9). Block is 1.5 3 1.6 m in cross section.Interbedded sandstone and shale on right. Hammer for scale. (D) Trough cross-beddedsandstone and pebbly sandstone near the top of section. Pencil is 14 cm long.

Figure 15. Section 2 of the Holyoake Range.(A) Conglomerate (c) resting directly oneroded upper surface of large archaeocy-athan mound (m). Hammer (lower center)for scale. Large blocks of archaeocyathanbioherm (b) occur in basal conglomeratebeds in Douglas Conglomerate. (B) Top oflarge archaeocyathan mound at section 2 ofthe Holyoake Range. Both the mound (m)and the flanking shale/nodular limestone fill(sh) are truncated by boulder conglomerate(c). Large block (b) of archaeocyathanbioherm in conglomerate. Hammer forscale.

stone of the Starshot Formation with minor, ifany, conglomerate (i.e., no Douglas Conglom-erate). In fact, the High Ridge section (Fig. 1)records such a transition. Here, the HolyoakeFormation is overlain by deposits with onlyone 95-cm-thick conglomerate bed, and therest is almost entirely sandstone and minorshale of the Starshot Formation. The Holy-oake Formation is therefore considered an ex-tensive unit of the middle Byrd Group thateverywhere overlies Shackleton Limestone inthe study area and underlies sandy and con-glomeratic deposits of the Starshot and Doug-las Formations.

DISCUSSION

New data outlined in this paper on ByrdGroup strata provide multiple constraints onthe timing of deposition of various units andthe onset of Ross orogenesis. Previously pub-lished trilobite and archaeocyathan biostrati-graphic data from the Shackleton Limestonegenerally constrained determinations of theage of this unit, but the unfossiliferous basalunit of the formation was never dated, and thestratigraphic top of the formation was neverrecognized, let alone dated. Chemostratigraph-ic data presented in this paper strongly suggest

the lower sandstone-rich part of the formationto be early Atdabanian in age (isotopic excur-sion IV of Brasier et al., 1994; Brasier andSukhov, 1998). Although this section, whichwe interpret to be in fault contact with theGoldie Formation at Cotton Plateau, may notbe the absolute base of the formation, lowerparts may not be much older. This quartz-richsandstone of the basal Shackleton has ana-logues with relatively thin Cambrian basaltransgressive sandstone units across Laurentiaand elsewhere. Thus, Cambrian seas appar-


MYROW et al.

Figure 16. Inferred age rela-tionships of previously definedunits in the central Transant-arctic Mountains. ‘‘Inboard’’and ‘‘outboard’’ componentsof the Goldie Formation weredistinguished by Goodge et al.(2002). Revised stratigraphyincludes newly defined Holy-oake Formation and GoldieFormation that is restricted todeposits older than the Shack-leton Limestone (BeardmoreGroup).

ently transgressed this part of the Antarcticmargin during the early Atdabanian.

The ensuing carbonate platform existedthroughout the rest of the Atdabanian and sub-sequent Botomian stages. The first document-ed depositional contact at the top of theShackleton Limestone is described herein andrecords a major depositional shift in the geo-logic history of the Transantarctic Mountains.Archaeocyathan bioherms developed duringthe last stage of deposition of the formationare encrusted with multiple thick, phosphatichardgrounds. The preserved growth morphol-ogy of these bioherms and lack of evidencefor subaerial exposure indicates the biohermswere killed as a result of rapid drowning ofthe carbonate ramp. Multiple hardgroundsnear the apex of the bioherms suggest that de-mise of these structures was a multiphaseevent. The phosphatic crusts capping the bio-herms developed at, or near, the sediment-water interface in a deep-marine setting(Schlager, 1998), indicating that they formedover a considerable amount of time duringwhich there was very little or no other sedimen-tation. Rapid initial drowning led to depositionof the regionally extensive phosphatic crust andsubsequent deposition of the Holyoake Forma-tion, a unit of black shale followed by mixednodular carbonate and shale facies. The lattergrades upward into trilobite-bearing calcare-ous siltstone of the Starshot Formation andthen into mixed conglomerate and sandstonefacies of the Douglas Conglomerate. Onlap-ping relationships between the overlying deeper-water siliciclastic rocks and the phosphate-encrusted bioherm further indicate thecarbonates were drowned prior to depositionof the siliciclastic sediments. The siliciclasticfacies transitions record progressive shoaling

from initial deep-water deposition (shale andnodular carbonate encased in shale) throughshallow-marine to subaerial deposits (con-glomerate facies). This large-scale upward-coarsening succession records a regionallysignificant shift from passive carbonate-rampdeposition to a major influx of terrigenoussediment. This was triggered by a major tec-tonic event, the structurally active phase of theRoss orogeny, which resulted in faulting, up-lift, progressive unroofing, and widespread de-position of an enormous clastic wedge. Thetrilobite fauna from the lowermost siltstonedeposits of this transition zone (basal beds ofStarshot Formation) date the onset of thisevent as being within the Botomian Stage (ca.515–510 Ma). This age is consistent with ca.490 Ma and older ages of detrital zircons inthe overlying Starshot and Douglas Forma-tions (J. Goodge, unpublished data). It is mea-surably younger, however, than the onset ofbasement deformation in this area, dated to anEarly Cambrian interval between ca. 540 and520 Ma (Goodge et al., 1993).

Inception of Ross orogenesis within the su-pracrustal successions of the central Transant-arctic Mountains is therefore recorded by thedrowning and final stratigraphic occurrence ofarchaeocyathan reefs. It is also coincident witha dramatic change in composition, prove-nance, and depositional setting of marginal-basin siliciclastic deposits. Detrital-zirconages from the Starshot and Douglas Forma-tions show that they were deposited well intothe Late Cambrian and probably into the EarlyOrdovician. Given that the relatively thinHolyoake Formation is Botomian in age, theoverlying clastic units of the Byrd Group spanat least the rest of the Cambrian (.20 m.y.).These relationships provide important new

timing constraints on clastic units of poorlyknown age. The stratigraphic relationshipsdiscussed here not only provide precise con-straints on the timing of supracrustal defor-mation, but they also help to clarify the re-gional tectonic patterns (discussed next).

Assigning the onset of supracrustal defor-mation to within the Botomian Stage is con-sistent with stratigraphic relationships in otherareas. Rowell et al. (1992) concluded that theearliest Ross deformation was likely to beMiddle Cambrian in age, on the basis of (1)the presence of folded rocks beneath the Mid-dle Cambrian Nelson Limestone in the Nep-tune Range of the Pensacola Mountains, and(2) the Douglas unconformity above the Low-er Cambrian Shackleton Limestone (defined atthat time to be Toyonian and older). The newfossil control on the basal Starshot Formationdescribed herein indicates that deformationcommenced somewhat earlier in the late EarlyCambrian. New evidence shows that the Pen-sacola Mountains underwent an initial majorphase of late Early to early Middle Cambriandeformation and subsequent episodes of lateMiddle Cambrian to possible Ordovician ac-tivity (Rowell et al., 2001). In the QueenMaud Mountains, deformation of MiddleCambrian limestone of the Taylor Formation(Rowell et al., 1997; Encarnacion et al., 1999)is inferred to have occurred in the MiddleCambrian to Ordovician, well after the initialphase of tectonism described here. Together,these results support the idea of protractedand/or temporally punctuated deformationduring the Ross orogenic cycle (Rowell et al.,1992, 2001; Goodge et al., 1993; Goodge,1997), which can be inferred on the basis ofradiometric age control to span the periodfrom 540 to 480 Ma.



TECTONIC IMPLICATIONS

Although the transition zone between theShackleton and upper Byrd Group clastic de-posits is apparently recorded only within a rel-atively small outcrop area, this interval pro-vides keys to understanding the nature andscale of crustal deformation. Our revisedstratigraphic framework suggests that low-angle faults within the Shackleton Limestoneand between the older Shackleton Limestoneand the upper Byrd Group clastic units are ofreverse geometry. Kinematic displacementmay vary from one area to another, but rec-ognition that ‘‘outboard’’ Goldie Formationrocks are in fact part of the Starshot Forma-tion, and therefore younger than the Shackle-ton Limestone, refutes earlier assumptions thatfor all Shackleton-‘‘Goldie’’ contacts, theShackleton rests either unconformably orstructurally upon older strata. Those exposureswith Shackleton above newly recognized Star-shot Formation rocks (‘‘outboard’’ Goldie) arenow viewed as an older-over-younger struc-tural juxtaposition. Intraformational faults,which are well exposed at Cambrian Bluffnorth of Nimrod Glacier (Laird et al., 1971),probably represent initial structural repetitionof the Shackleton carbonate ramp.

The consistent west to east and southwestto northeast (outboard) paleocurrent data fromthe Starshot Formation (Myrow et al., 2002)agrees with the spatial distribution of con-glomerate within the Byrd Group. Rapiddrowning of the outboard carbonate platformwas likely caused by flexural loading of crustunderlying the inner margin to the west (nearthe present-day axial zone of the orogen). Wespeculate that loading was initially caused bythe development of imbricate east-vergentthrust sheets farther to the west, but furthersubsidence may have resulted from the nega-tive buoyancy of subducting oceanic litho-sphere (Dickinson, 1995). Nonetheless, thetiming of the stratigraphic record relative toknown deformation events in the region indi-cates tectonic rather than global eustatic con-trol. As deformation progressed, the carbonateplatform was uplifted and eroded, so that insome places it was entirely removed and olderNeoproterozoic rocks were exposed and erod-ed to become clasts in the coarser molasse fa-cies. Locally, incomplete removal of the car-bonate led to an unconformity of foldedShackleton Limestone overlain by DouglasConglomerate. Differences in the compositionof the Douglas Conglomerate—relative abun-dance of carbonate versus siliciclastic clasts—resulted from spatial and temporal changes insource rocks during progressive deformation

and unroofing. Farther outboard, rapid subsi-dence occurred as a result of thrust loading ofthe inboard crust. The stratigraphic responseto thrusting was drowning of the bioherms anddevelopment of a phosphatic hardground, fol-lowed by deposition of shale and then progra-dation of a thick clastic wedge. Although thespecific geometry of thrusting and flexuralloading remains to be established, the strati-graphic transitions reported here offer a broadframework in which to understand the inter-play of tectonism and sedimentation. In ad-dition, the implied vertical and lateral strati-graphic changes reflect the shift frompreorogenic passive carbonate sedimentationto synorogenic deposition of molassic strataspanning proximal alluvial-fan to shelfenvironments.

ACKNOWLEDGMENTS

This work was supported by National ScienceFoundation (NSF) award OPP-9725426 to Goodge.We are grateful for the tremendous logistical andhelicopter support provided by the NSF Polar Op-erations section, Antarctic Support Associates, andPetroleum Helicopters, Inc. Shaun Norman provid-ed unsurpassed assistance in all aspects of the fieldwork. Discussions with Malcolm Laird, Bert Row-ell, and Peg Rees have been exceptionally helpfulin guiding our ideas. Reviews by Bert Rowell andEd Stump were extremely helpful.

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