Late Mesozoic-Cenozoic history ofthe Ross Sector

PETER-NOEL WEBB

Department of Geology and Mineralogyand

Institute of Polar StudiesThe Ohio State University

Columbus, Ohio 43210

The Ross Sector contains a significant and varied record oflate Cretaceous and Cenozoic geologic history. The separationof the New Zealand subcontinent from Antarctica in the lateCretaceous, the separation of Australia in the Paleogene, theuplift of the Transantarctic Mountains from the Cretaceous,and the submergence of the Ross Sea continental shelf fromthe Cretaceous controlled or at least strongly influenced themajor preglacial and glacial events within and even beyondthis part of Antarctica. Lateral and vertical tectonic eventsinfluenced the fragmentation of late Mesozoic basins, thedevelopment of more localized Cenozoic basins, the formationof seaways within and across Antarctica, the evolution ofoceanic circulation patterns and biotic migration routes withinand around Antarctica, the growth of fault-related volcaniccenters, the provision of ice nucleation centers, and the phys-iographic restriction and channeling of ice sheets and shelves(figures 1, 2).

The geologic history of the Ross Sector is separated into twomajor phases: a preglacial or pre-late Oligocene (>25 millionyears) phase, and a glacial late Oligocene to Recent phase. Thisseparation is presently based on a single control point in thecentral Ross Sea, at Deep Sea Drilling Project (DSDP) site 270.It is premature, then, to suggest that marine glaciation origi-nated at or about 25 million years ago around the entire RossSea periphery. Terrestrial glaciation presumably commencedearlier than the latest Oligocene, but again it is premature toidentify with any certainty specific times or locations for theorigins of alpine or ice sheet glacial history. It is assumed thatthe Transantarctic Mountains acted as one of the major icenucleation centers.

Cenozoic. About 28.5 percent (18 million years) of Tertiarytime is documented (Webb 1980,1981a, 1981b). The Paleogene(65 to 24 million years ago) record is extremely poor. TheNeogene (24 to 2 million years ago) is much better known,with 73 percent (16 million years) currently documented. Dat-ing techniques as they apply to the Cenozoic are summarizedin figure 3.

Late Cretaceous-Cenozoic basins. The Transantarctic Moun-tains and mountain ranges of part of West Antarctica providethe major boundaries of the Ross Sea depression or basin, asedimentary depocenter since the Cretaceous. Webb (1979,1980, 1981a, 1981b) applied the name "Transantarctic Strait"to this north-south trending depression and suggested that itprovided a moderately deep marine link between the proto-Pacific and Atlantic oceans, at least until being obstructed bya grounded west antarctic ice sheet (figure 2). It has also beensuggested that distinct basins formed within this "Transant-arctic Strait," with centers of maximum sediment accumula-tion shifting in concert with regional tectonic events (Webb

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Figure 1. Structural features of Ross Sector. Major proven andpostulated features within and adjacent to the TransantarcticMountains are taken from Gairet al. (1969), Warren (1969), Grindleyand Laird (1969), and McGregor and Wade (1969). V = significantoccurrences of Miocene-present day volcanic centers. 1 = HallettVolcanic Province, 2 = Melbourne Volcanic Province, 3 = ErebusVolcanic Province. Ross Sea continental shelf information fromHayes and Davey (1975). + = western Ross Sea positive gravityanomaly.

1979). Discrete basins are poorly delineated at this time, andit is unlikely that regional basins were completely isolatedfrom one another. Western and eastern Ross Sea basins existedin the Paleogene, separated by a north-south oriented sub-marine high of Paleozoic crystalline basement rocks (figure 1).In the southwest, the relatively small and elongate McMurdoBasin (Wilson et al., Antarctic Journal, this issue; figure 3)developed following the eruption of Mount Bird and otherRoss Island volcanic centers from the Pliocene onward. ShouldRoss Island have erupted above sea level earlier, the history ofthe McMurdo Basin might be extended into the Miocene. Theabsence of volcanic material in Miocene sediments of severaldrill holes in the area suggests that the McMurdo Basin is arelatively young feature (less than 5 million years old). TheMcMurdo Basin maintained marine connections to the westernRoss Sea basin throughout its history.

Geophysical results suggest that the Cretaceous-Cenozoicsedimentary succession of the Ross Sea basins are 3 to 4 kilo-meters thick.

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22 ANTARCFIC JOURNAL

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Figure 2. Sub-ice topography beneath present day west and eastantarctic ice sheets and Ross Ice Shelf. Dashed line suggestspossible early Cenozoic marine passages within and across Ant-arctica, prior to the development of ice sheets grounded belowsea level. Modified from Bentley (1972).

PALEOGENE NEOGENE QUATERNARYPAtIO. EOCENE LOLIGO. MIOCENE I PLIOCENE PLEISTO HOLOCENE

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FORAMINIFERA

•• •i. I ______________• • •• • • • _________• . ••••• _____________

•- -K/AR - (IL AUCONITE• •• NANNOPLAN/( TON• ••• • • •• ••• • • • • ••• • • • • 1000 ••• 0/ATOMS

•• • ••• K/AR VOLCANICS• • • • •

•• CARBON "•• URANIUM/ THORIUM

•VERTEBRATE FOSSILS

Figure 3. Stratlgraphic distribution of major fossil groups throughthe Ross Sector Cenozoic. Isotope techniques used In dating arealso plotted.

Cretaceous. Cretaceous rocks are not known to crop out any-where in the Ross Sector. Cretaceous foraminifera and radi-olaria have been redeposited into Miocene-Quaternary sedi-ments. Cretaceous sediments presumably occur ow in succes-sions south of the latitude of McMurdo Souna. Cretaceoussediments are also likely near the base of the succession at thenorthern extremity of the continental shelf in the northernRoss Sea. It is probable that these sediments closely resemblethe organic-rich marine and nonmarine successions of theOtway Basin and Campbell Plateau.

Paleocene. No Paleocene fauna, flora, or sediments areknown from the Ross Sector, but they are likely to be presentlow in the continental shelf successions. Deep water separated

the early Cenozoic Ross Sea from the Campbell Plateau,although the northwestern area of the Ross Sea and the south-east Australia-Tasmania area were still in close proximity.Similar littoral-shallow shelf sediments and faunas probablyoccur in the latter two areas, and the existence of a "Transant-arctic Strait" facilitated biotic movement between the south-western Pacific and Atlantic oceans.

Eocene. No Eocene in situ outcrops are known. Transportedblocks of Eocene sediment, in a wide variety of facies, occurin Quaternary deposits of southern McMurdo Sound. Sedi-ments include breccia, conglomerate, greensands, shell beds,oyster beds, carbonaceous sandstones, and calcareous siltymudstone. Fossils include mollusca, foraminifera, palyno-morphs, microplankton, and wood. Eocene silicoflagellatesand microflora are present in Miocene and younger sediments.No Eocene volcanics are known. The varied sediment typessuggest that the bathymetry within the "Transantarctic Strait"ranged from littoral depths to several hundred meters. Theabundance of recycled Beacon Supergroup and older Paleozoicdetritus in these sediments indicated that the TransantarcticMountains constituted a significant physiographic featureduring the Eocene.

Oligocene. Only the latest Oligocene (26 to 24 million yearsago) is documented at present. At DSDP site 270, in the centralRoss Sea, approximately 29 meters of Oligocene breccia, green-sand, and carbonaceous sandstone is preserved above a Paleo-zoic crystalline basement. This succession was deposited dur-ing a late Oligocene marine transgression. Basin deepeningcontinued late into the Oligocene, with deposition of 33 metersof glaciomarine pebbly mudstones. Benthic and planktonicforaminifera, microplankton, and palynomorphs are commonin this Oligocene sequence. No Oligocene volcanic rocks areknown.

Miocene. Miocene marine, terrestrial, and volcanic rocks arewidely distributed through the western part of the Ross Sector.The early and middle Miocene glaciomarine sediments of thecontinental shelf consist of deepwater pebbly mudstones con-taining up to 40 percent diatoms. In rare instances, the diatomcontent rises to more than 90 percent. The more than 900meters of early-middle Miocene provides a blanket coverabove the coarse-grained preglacial clastic successions. Appre-ciable methane and ethane shows are known from two drillhole successions in these Miocene sediments. Late Miocenesediments are not known from the Ross Sea continental shelf.This may be due to lack of exploration. Latest Miocene gla-ciomarine sediments occur within the valley basins or paleo-fjords of the eastern Transantarctic Mountains. Basal tills andwater-lain deposits occur at many high-elevation locationsalong the Transantarctic Mountains. These are poorly dated.They are certainly Miocene in age, and some deposits couldbe as old as Paleogene.

Early Miocene volcanic centers are known in the southernTransantarctic Mountains. Middle and late Miocene volcanicsare concentrated north of the McMurdo Sound area. Duringthe Miocene, marine sediments of Ross Sea were derived fromalpine, trunk, and piedmont glaciers and very restricted iceshelves. Currents within the "Transantarctic Strait" moveddebris-laden icebergs from the Ross Sea periphery to open seamelt zones in the central and northern Ross Sea. Prolific pho-toic zone productivity contributed a significant biogenic com-ponent to the clastic material derived from icebergs.

Pliocene. Pliocene rocks are known from the TransantarcticMountains, the offshore volcanic islands, and the Ross Sea

1981 REVIEW 23

continental shelf. The Scallop Hill Formation and Pecten grav-els are thin units rich in fossils and widely distributed aroundthe periphery of McMurdo Sound. These were quite shallowwater deposits. Pliocene sediments of the McMurdo Soundarea are dominantly volcaniclastic in composition, while thosefrom the paleofjords of the Transantarctic Mountains containmostly igneous and metamorphic material. Fossil materialincludes mollusca, polyzoa, barnacles, corals, serpulids,sponges, echinoids, fish bones, foraminifera, radiolaria, andostracods. The dating of volcanics associated with thesemarine sediments suggests that only part of the Pliocene ispresent. A thick (265 meters) open shelf hemipelagic glacio-marine succession occurs at DSDP site 271. These sedimentscontain abundant diatoms, radiolaria, and silicoflagellates,testifying to the continued presence of open water marineconditions and high photic zone productivity. Again, onlypart of the Pliocene is thought to be present here. Pliocenevolcanism was widespread throughout the TransantarcticMountains north of McMurdo Sound.

Quaternary. A blanket of Quaternary sediment 40 meters orless thick is distributed widely across the Ross Sea floor. Adistinctive disconformity separates these sediments from theunderlying Cenozoic sediments. The age of sediments justabove the disconformity is not determined with certainty inmany areas; late Pliocene ages are possible. The mode of originof these Quaternary successions is also the subject of debate.They have been explained as the basal till of a grounded marineice sheet and as glaciomarine sediments deposited from ice-berg sedimentation in an otherwise ice-free water column.Recent ages have been established for littoral deposits aroundthe periphery of McMurdo Sound, but underlying marginalmarine and freshwater sediments have been dated as latePleistocene or Pliocene.

Tectonic considerations. The Victoria Orogeny is the mostsignificant event in the history of the western Ross Sector. Asmost recently defined (Webb 1979), it spans the late Mesozoicand Cenozoic and includes subsidence (with marinetransgression) to form the Ross Sea, and emergence (withmarine regression) to form the Transantarctic Mountains. Asingle fault, or more likely a plexus of faults, separate thesubsiding and emerging crustal blocks. The geology of thejunction or fault zone is located near and seaward of the presentwestern Ross Sea coastline. This junction zone is critical to afuller understanding of the vertical displacements and dura-tion of Victoria Orogeny tectonism. Most existing data comefrom widely separated and strongly contrasted environments,within the Transantarctic Mountains on one side of the faultzone and the central Ross Sea on the other. If major faultlineations are just offshore, we have an opportunity to examineemerging and subsiding blocks in a totally marine environ-ment. The gathering of such sedimentological and paleonto-logic evidence will require offshore drilling in up to 400 metersof water.

Geologic and geophysical evidence suggests vertical dis-placements at the eastern side of the Transantarctic Mountainsof about 4 to 5 kilometers. If we assume subsidence of thecentral Ross Sea from near sea level in the late Oligocene, ratesof subsidence and basin filling are about equivalent (about 40to 45 meters per 1 million years). Subsidence may have com-menced earlier in other parts of the Ross Sea. Paleontologicand fission-track dating data suggest an emergence rate for the

Transantarctic Mountains of at least 60 meters per 1 millionyears during the Cenozoic. The rate of uplift during the Pli-ocene may have been in excess of 100 meters per 1 millionyears. This may account for more widespread volcanism dur-ing the Pliocene.

A detailed understanding of the tectonic history of the west-ern Ross Sea is particularly important to discussions of therelationship between the west and east antarctic ice sheetsduring the late Cenozoic. To what extent did the emergingTransantarctic Mountains dam the east antarctic ice sheet andprevent massive streaming into the Ross Sea? Deep-sea isotopedata have been interpreted as suggesting a major buildup ofthe east antarctic ice sheet in the middle Miocene (approxi-mately 15 million years ago). Was part of the east antarctic icesheet able to flow through or around the Transantarctic Moun-tains to form antarctic bottom water (.sw) in the Ross Sea; ordid ABW enter the southern ocean by other drainage routes? Ifthe Transantarctic Mountains prevented a major flow of icefrom East Antarctica, could the west antarctic ice sheet havecontributed the ABW that apparently reached northern Pacificareas in the middle Miocene?

This work was supported by National Science Foundationgrant DPP 79-07043.

References

Bentley, C. R. 1972. Subglacial topography. Morphology of the Earthin the Antarctic and sub-Antarctic (Folio 16, Plate 7). In B. C. Hee-zen, M. Tharp, and C. R. Bentley (Eds.), Geologic maps of Antarctica,Antarctic maps folio series. New York: American Geographical Soci-ety.

Gair, H., Sturm, A., Carryer, S. J . , and Grindley, C. W. 1969. Thegeology of northern Victoria Land (Folio 12, Plate 12). In V. C.Bushnell (Ed.), Geologic maps of Antarctica 1:100,000, Antarctic mapsfolio series. New York: American Geographical Society.

Grindley, G. W., and Laird, M. C. 1969. Geology of the ShackletonCoast (Folio 12, Plate 14). In V. C. Bushnell (Ed.), Geologic maps ofAntarctica 1:100,000, Antarctic maps folio series. New York: AmericanGeographical Society.

Hayes, D. E., and Davey, F. J . 1975. A geophysical study of the RossSea. Initial Reports of the Deep Sea Drilling Project, 28, 887-908.

McGregor, V. R., and Wade, F. A. 1969. Geology of the western QueenMaud Mountains (Folio 12, Plate 15). In V. C. Bushnell (Ed.), Geo-logic maps of Antarctica 1:100,000, Antarctic maps folio series. NewYork: American Geographical Society.

Warren, C. 1969. Geology of the Terra Nova Bay-McMurdo Soundarea, Victoria Land (Folio 12, Plate 13). In V. C. Bushnell (Ed.),Geologic maps of Antarctica 1:100,000, Antarctic maps folio series. NewYork: American Geographical Society.

Webb, P. -N. 1979. Paleogeographic evolution of the Ross Sector dur-ing the Cenozoic. In T. Nagata (Ed.), Memoirs of the National Instituteof Polar Research, Special Issue 13, 206-212.

Webb, P. -N. 1980. Late Cretaceous-Cenozoic geological history of theRoss Sector (Review). Paper presented at the Sixteenth SCAR, RossSea Symposium, Queenstown, New Zealand, October 1980.

Webb, P. -N. 1981. Cenozoic geology of the Ross Sector, Antarctica.Abstracts with Programs. The Geological Society of America 94thannual meeting, Cincinnati, 577. (a)

Webb, P. -N. 1981. Late Mesozoic-Cenozoic geology of the Ross Sec-tor, Antarctica (Extended Abstract). Journal of the Royal Society ofNew Zealand, 11(4). (b)

Wilson, D. D., McGinnis, L. D., Burdelik, W. J . , and Fasnacht, T. L.1981. McMurdo Sound upper crustal geophysical study. AntarcticJournal of the U.S., 16(5).

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