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Geologic and fission-track studies in the Heritage Range,Ellsworth Mountains, and nunataks of West Antarctica

P.G. FITZGERALD* and T.F. REDFIELD, Department of Geology, Arizona State University, Tempe, Arizona 85287P.M. GOLDSTRAND, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830

C. HOBBS, Mount Cook, South Island, New Zealand

*present address: Department of Geosciences, University ofArizona, Tucson, Arizona 85721.

Nertical sampling profile on the Vinson Massif in theouthern Sentinel Range of the Ellsworth Mountains (fig-

ure 1) indicates that the mountains were denuded at least 4kilometers (km) in the early Cretaceous following the initialseparation of East and West Gondwana and the opening ofthe Weddell Sea (Fitzgerald and Stump 1991). The data didnot reveal the timing of initiation of this denudation but

apatite fission-track age analysis on a sample of CrashsiteGroup quartzite from Springer Peak (kindly given to us byG.F. Webers of Macalester College) suggested that the initia-tion of this denudation event may be found in the HeritageRange (Fitzgerald and Stump 1992). To test this hypothesis,we visited the Heritage Range in November and December1992. Samples of Jurassic granite were also collected from the

Whitmore Mountains and Pirrit Hills inlate December 1992 and early January

I 1993. Fission-track data from the50W 50 kmEllsworth Mountains and other ranges of

the Ellsworth-Whitmore Mountains78°S-crustal block will help to further constrain

not only the uplift and formation of these)Vinson Massifmountains but also the tectonic evolution

of West Antarctica during, and subsequent

Binghamto, the break-up of Gondwana. Compar-Peaksion of these data with fission-track data

I-from the Transantarctic Mountains (forexample, Stump and Fitzgerald 1992;

IFitzgerald 1992) will help resolve the)lt807S -nature of the East Antarctica-West Antarc-s tica boundary and any shared tectonic his-

I tory the two halves of Antarctica may havehad.

The Ellsworth Mountains are part ofthe Ellsworth-Whitmore Mountains block,

h Poleone of five ailochthonous crustal blocks0 that constitute West Antarctica (for exam-ilS pie, Storey et al. 1988). Rotated to their

pre-Gondwana breakup position (Schopf1969; Clarkson and Brook 1977; Watts andBramaii 1981; Grunow, Kent, and Daiziel1991), the Ellsworth Mountains werelocated adjacent to present-day CoatsLand, north of the Pensacola Mountains.Early geological mapping (for example,Craddock, Anderson, and Webers 1964)and more recent work (for example,Webers, Craddock, and Splettstoesser1992) suggested the stratigraphy of theEllsworth Mountains is a conformablesequence spanning the entire Paleozoic.This is in marked contrast to theTransantarctic Mountains where Protero-zoic and Early-Middle Cambrian sedi-

Figure 1. Map of West Antarctica and part of East Antarctica showing boundaries of crustalblocks [Antarctic Peninsula, Thurston Island, Marie Byrd Land, Ellsworth-WhitmoreMountains (EWM), and Haag Nunataks (HN)], modified from Storey et al. (1988).

ANTARCTIC JOURNAL - REVIEW 199343

mentary rocks (Ross Supergroup) were deformed, metamor-phosed, and intruded during the Cambro -Ordovician RossOrogeny, and subsequently eroded prior to the uncon-formable deposition of the Devonian-Triassic Beacon Super-group (Elliot 1975; Stump 1992). That there is no late Cambri-an unconformity in the Ellsworth Mountains, given their for-mer position within an extension of the Transantarctic Moun-tains, has long been regarded as enigmatic (Schopf 1969).Therefore, while in the Heritage Range, we examined thenature of the contact between the Crashsite Group and Her-itage Group.

Despite a delayed put-in to the Heritage Range on 21November, most of the sampling objectives for fission-trackthermochronology were accomplished. Vertical profiles werecollected from Bingham Peak [550 meters (m)], Mount Virginia(450 m), and Mount Twiss (650 m) in a transect along the southside of the Splettstoesser Glacier (figure 2). Samples were takenfrom quartzites within the Crashsite Group and from thinquartzite beds within the upper Heritage Group. A proffle wasalso collected at the Soholt Peaks (79°40.14'S 84°21.30'W). Thismassif is dominantly a Devonian dacite stock (Vennum et al.1992), although it was noted on close inspection that the crag-gy nature of some of these peaks (for example, spot height2,341 m) is a result of quartzite beds upturned and hydrother-mally altered as a result of dacite intrusion. A composite sec-tion of greater than 1,000 in sampled from the dacite onthe northwestern side and from quartzite beds within theSpringer Peak Formation to the south. On the south side of the

- Minnesota GlacierWelcome

-r7Mt. Virginia

La Peak

Mt. Tw

13 inghamPeak

Soholt

A Vertical sampling profile• Sampling locality

0102030 km

850S 840S 830

Union Glacier, samples were collected at Rhodes Bluff in atransect to the east as far as Mount Rossman (79048.30'S83°10.52'W). At the northern end of the Heritage Range, verti-cal profiles were collected from quartzites at Landmark Peak(500 m) and at Pardue Peak (500 m).

The Ellsworth Mountains consist mainly of the Lower(?)-Upper Cambrian Heritage Group, overlain by the UpperCambrian-Devonian Crashsite Group (Webers et al. 1992).The Heritage Group contains tuffaceous diamictite,greywacke, shale, conglomerate, marble, and quartzite,whereas the Crashsite Group is dominated by quartzites. Inthe Bingham Peak—Soholt Peaks area the lowermost unit ofthe Crashsite Group (Howard Nunataks Formation) is pre-dominantly quartzite with interbedded argillite. Prior to ourfieldwork, the only known transition was from limestone (theMinaret Formation, upper Heritage Group) to fine-grainedsiiciclastic rocks (Howard Nunataks Formation, lower Crash-site Group) (Sporli 1992). We examined exposures of the con-tact between the Heritage Group and the Crashsite Group inthe Webers Peaks and a section measured at Bingham Peak(figure 3). We also visited a previously undescribed locality onthe west flank of the Soholt Peaks (79°40.10'S 84 027'W), wherea minimum of 20 in light-gray marble of upper MinaretFormation is overlain by a minimum of 62 in cobble-to-pebble conglomerate and sandstone. The conglomerate andsandstone unit (described in caption to figure 3) represents abraided river depositional environment, with possibleinterfingering of tidal sand flat depositional environments.

The conglomerate indicates that the upliftand erosion or a rainy iocai source usn-ered in the period of deposition of theCrashsite Group. At Bingham Peak, amarked facies change [as noted by otherworkers at this and other localities (seeSporli 1992)], occurs across the contact.The cross-contact disparity between car-bonate and siliciclastic sedimentation isconsistent with the interpretation of theconglomerate as indicative of late Cambri-an tectonism. Discovery of a conglomerateat the base of the Crashsite Group solvesthe enigma of a previously unrecognizedUpper Cambrian tectonic signal in theEllsworth Mountains (Goldstrand et al. inpress).

The field party of four was extractedfrom the Heritage Range on 15 December1992 without incident. Two members ofthe party were deployed to CASERTZ (cor-ridor aerogeophysics of the southeast Rosstransect zone) fieldcamp in late Decem-ber. Utilizing the Twin Otter, we collectedvertical profiles from Mount Goodwin(81 006.68'S 85042.38'W) in the Pirrit Hills(800 m) and over 400 in a ridge at thenorthwestern end of the Mount Seeligmassif in the Whitmore Mountains

790 00'WParduePeak '..

79° 15W

79° 30'W

79° 45'W

I Sporli

Rhodes Bluff 82S\

Mt. Rossman

81

Figure 2. Map of central and northern Heritage Range showing sampling localities asreferred to in the text.

ANTARCTIC JOURNAL - REVIEW 199344

a

S

Shales, sandstones

Marine and terrestrial Iillils- Dark brown - black qseetzurcs

White quartzites

Quartzites-----------Light and dark marble?Dark green quartzites andbrown argitlice and flaws,conglomerateMarble - and quartzite - clanconglomerate, quartile at bsncBlack nhale and maablr

Grsyseacke,aegitlrtrandconglomeNte

Atitlins' luffs/IdiansMarble-clant conglomerate

QUARTZITE - nearshore and intertidal

QUARTZITE - shallow to nearshoze,marine, tidal dominated

INTERBEDDEI) AEGILLITE ANDSANDSTONE - inter shelf

ARGILLITE - outer shelf, stormdominated

MARBLE - shallow marine carbonate,piatfonrn.slocta dominated

ARGILLACEOUS MARBLE- inner sheff,storm dominated

INTERBEDDED AROILLITE ANDSANDSTONE' outer shelf, storm dominated

COVERED

AROILLITE muter shelf, stortli dominaled

BINGHAM PEAKNorthSouth

Permian(km)

ftrana-Caobonileiraas-,,-

SOHOLT PEAKS Is " Devonian

to/sANDs'rordE and mterte&Jedconglomerate,\pper9sandy-gravelly braided river. Cambrian

CONGLOMERATE with interbedded sandstone,\ - Upper - 8gnanelly-sandy braided river ,- Cambrian ---SANDSTONE - nearshore, tidal dominated MiddleCONGLOMERATE - gravelly braided river Camiwian

r MARISLE.shallowmwincciinghmicratc '--:-:B

Middle2Cambrianand Lower2Cambrian

—o

Figure 3. Central column. Stratigraphic column of Paleozoic rocks exposed in the Ellsworth Mountains. Total stratigraphic thickness in theEllsworth Mountains exceeds 13 km, being divided into approximately 7.5 km of clastic and carbonate rocks of the Heritage Group andapproximately 3 km the Crashsite Group quartzites, overlain by approximately 2.5 km of sandstones, shales, and glaciomarine deposits(modified from Webers et al. 1992). Right-hand column. Detailed stratigraphic column of strata exposed at Bingham Peak. Springer PeakFormation—thinly laminated black argillite with thin interbeds of sandstone; sandstone interbeds with Bouma sequences Tb-e, flute casts, softsediment folds, and hummocky cross stratification. More than 122 m thick. Minaret Formation conformably overlies Springer Peak Formation.Basal Minaret consists of black, thinly laminated to thinly bedded calcareous argillite, sparite, and biosparite. The majority of the section is lightto gray marble. Bioclastic beds are common and argillaceous beds rare. Oncolitic and flat-pebble conglomerates are also present.Approximately 35 m thick. Howard Nunataks Formation—white marble directly overlain by approximately 55.5 m of interbedded black to dark-green argillite and thinly bedded brown sandstone. Dominantly horizontally laminated argillite and sandstone with abundant pseudonodules andload casts, and rare horizontal burrows. Thin graded and hummocky cross-stratified sandstone beds pass up into tan, thickly-bedded quartzite.Tabular, herringbone, and trough cross-stratification dominate higher in the formation, although argillaceous rip-up clasts, symmetric ripplesand mudcracks are also common. Left-hand column. Detailed stratigraphic column of strata exposed on the west flank of the Soholt Peaks.The conglomerate is dark gray-brown, structureless to thickly bedded, with su brou nded -to-su bang u lar, poorly sorted, and poorly imbricatedclasts. Quartzite clasts make up 100 percent of the basal conglomeratic clasts, although approximately 8 m above the contact, clastpercentages are approximately 70 percent quartzite, 15 percent dacite, 10 percent basalt, and 5 percent granitic. Percentages of volcanic andplutonic clasts then decrease upward with an accompanying increase in percentage of quartzite clasts. Minor dacite clasts are present at thetop of the section. The sandstone is a black, micaeous sublitharenite with abundant horizontal laminations, symmetric ripples, and tabularcross-stratification in the lower section. Sandstone units are lenticular, and thin-to-thickly bedded. Higher in the section, horizontally laminatedcross-stratified quartzarenites predominate.

(82025.77'S 104 006.45'W) from 30 December 1992 to 1 January1993. Enroute to the Thiel Mountains to collect more verticalprofiles, we stopped at Hart Hills (83 042.36'S 89 029.07'W) tocollect a sample. A hard landing at this locality temporarilycrippled the Twin Otter and brought this part of the field sea-son to a close.

Returned samples will be processed for age determina-tion and track-length measurement at the fission-track labo-ratories at Arizona State University and the University of Ari-zona. The pilots and crew of Kenn Borek Air are thanked, as isVXE-6 for logistic support. Sue Selkirk is thanked for drafting.This research is supported by National Science Foundationgrant OPP 91-17441.

References

Clarkson, P.D., and M. Brook. 1977. Age and position of the EllsworthMountains crustal fragment. Nature, 265, 615-616.

Craddock, C., J.J. Anderson, and G.F. Webers. 1964. Geological outlineof the Ellsworth Mountains. In R.J. Adie (Ed.), Antarctic geology.Amsterdam: North-Holland.

Elliot, D.H. 1975. Tectonics of Antarctica: A review. American Journalof Science, 275A, 45-106.

Fitzgerald, P.G. 1992. The Transantarctic Mountains in southern Vic-toria Land: The application of apatite fission track analysis to arift-shoulder uplift. Tectonics, 11, 634-662.

Fitzgerald, P.G., and E. Stump. 1991. Early Cretaceous uplift in theEllsworth Mountains of West Antarctica. Science, 254, 92-94.

Fitzgerald, PG., and E. Stump. 1992. Early Cretaceous uplift in thesouthern Sentinel Range, Ellsworth Mountains, West Antarctica.In Y. Yoshida, K. Kaminuma, and K. Shiraishi (Eds.), Recentprogress in antarctic earth science. Tokyo: Terra Scientific.

Goldstrand, P.M., P.G. Fitzgerald, T.F. Redfield, E. Stump, and C.Hobbs. In press. Stratigraphic evidence for the Ross orogeny in theEllsworth Mountains, West Antarctica: Implications fo the evolu-tion of the Paleo-Pacific margin of Gondwana. Geology.

Grunow, A.M., D.V. Kent, and I.W.D. Dalziel. 1991. New paleomagnet-ic data from Thurston Island. Implications for the tectonics ofWest Antarctica and Weddell Sea opening. Journal of GeophysicalResearch, 96, 17935-17954.

Schopf, J.M. 1969. Ellsworth Mountains: Position in West Antarcticadue to sea-floor spreading. Science, 164, 63-66.

Sporli, K.B. 1992. Stratigraphy of the Crashsite Group, EllsworthMountains, West Antarctica. In G.F. Webers, C. Craddock, andJ.F. Splettstoesser (Eds.), Geology and paleontology of theEllsworth Mountains, West Antarctica (Geological Society ofAmerica Memoir 170). Boulder, Colorado: Geological Society ofAmerica.

Storey, B.C., I.W.D. Dalziel, S.W. Garrett, A.M. Grunow, R.J.

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