Rhynchonellid brachiopods from the Oligocene of King George Island, West Antarctica

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1 POLISH POLAR RESEARCH Maria Aleksandra BITNER 1 and Michael R. A. THOMSON 2 Institute of Paleobiology Polish Academy of Sciences Twarda 51/ Warszawa, POLAND bitner@twarda.pan.pl 2 British Antarctic Survey Geological Sciences Division High Cross, Madingley Road Cambridge CB3 OET, UNITED KINGDOM mrath@pcmail.nerc-bas.ac.uk Rhynchonellid brachiopods from the Oligocene of King George Island, West Antarctica ABSTRACT: Rhynchonellid brachiopods are reported for the first time from the Oberek Cliff Member of the Oligocene Polonez Cove Formation of King George Island, South Shetland Islands, West Antarctica. Rhynchonellids are very rare in the Polonez Cove Formation, and absent in the younger Tertiary strata of King George Island, the Miocene Destruction Bay Formation. Key words: Antarctica, Brachiopoda, Oligocene. Introduction The clastic deposits of the Polonez Cove Formation crop out on the southern part of King George Island, South Shetland Islands, West Antarctica (Fig. 1). The formation was divided into four lithostratigraphical members by Birkenmajer (1980, 1982). The lowest unit, the Krakowiak Glacier Member, is composed of non-marine tillites, whereas the upper units, the Low Head, Siklawa, and Oberek Cliff members, consist of glacio-marine deposits represented by conglomerate, sandstone and siltstone (Birkenmajer 1987, 1994; Porębski and Gradziński 1987). Although these rocks are richly fossiliferous at certain levels (e.g. so-called Pecten Conglomerate), brachiopods are very rare and poorly preserved. So far, they have

2 84 Maria Aleksandra Bitner and Michael R.A. Thomson 59" W 58* W Fig. 1. Location map of King George Island (arrowed) in Antarctica, and the outcrop of the brachiopod-bearing strata (asterisk) of the Oberek Cliff Member (Polonez Cove Formation, Oligocene). been described from the Low Head Member (Bitner and Pisera 1984, Bitner 1997) which is the most richly fossiuferous unit (Gazdzicki 1984). The fossils, although poorly preserved, are diverse; apart from brachiopods, there are coccoliths (Gaździcka and Gazdzicki 1985), foraminifers, ostracods (Blaszyk 1987), worms, bryozoans, bivalves, gastropods (Gazdzicki and Pugaczewska 1984), and echinoderms (Jesionek-Szymańska 1984). The age of the Polonez Cove Formation was previously suggested to be Pliocene (Barton 1965; Birkenmajer 1980,1982), but calcareous nannoplankton studies (Gaździcka and Gazdzicki 1985, Birkenmajer, Dudziak and Tokarski 1988) and isotopic data (Birkenmajer and Gazdzicki 1986, Birkenmajer et al. 1991, Dingle, McArthur and Vroon 1997) indicate that it is Oligocene. In the last 20 years, regular visitors to the South Shetland Islands have noticed substantial reductions in the amount of ice cover. One of the most dramatic incidences is that of the almost complete disappearance of the Krakowiak Glacier on the eastern side of Polonez Cove (compare left hand half of fig. 40 in Smellie et al. 1984, taken in February 1975 with Pl.l in this paper, a photograph taken from exactly the same viewpoint in January 1997). In 1975, what is now Chlamys Ledge

3 Rhynchonellid brachiopods from the Oligocene 85 (Birkenmajer 1998) was covered by a small remnant glacier that made access southward along the cliff extremely hazardous. In the intervening years, ice retreat has exposed a ledge of rock 150 m wide and several hundred metres long, extending from the foot of Chopin Ridge to the top of the cliff bordering Polonez Cove. As its proposed name implies, Chlamys Ledge is the source of rocks which contain relatively common occurrences of bivalves. The rocks belong to the Oberek Cliff Member of the Polonez Cove Formation and consist largely of pale coloured, medium- to coarse-grained sandstones. Although first noticed from their occurrence as hematite-stained moulds in loose blocks scattered all over the ledge, the fossils were traced back by A. Gaździcki to the source outcrop at the back of the ledge. The total fauna of the sandstones is limited. The most common forms are bivalves (Chlamys is the most common, but there are also ovate bivalves with fine concentric ribbing, and a single specimen of Pinna was also found), but A. Gaździcki (personal commun.) also recorded rare stromatolites, and there are the brachiopods which form the focus of this note. The brachiopods were collected by one of us (MRAT) in austral summer of and by Andrzej Gaździcki during the Polish Antarctic Expedition to the Arctowski Station in The studied material is housed in the Institute of Paleobiology of the Polish Academy of Sciences, Warszawa under the collection number ZPAL Bp.XLII and in the Geoscience Division of the British Antarctic Survey, Cambridge under the numbers P Systematic part Order Rhynchonellida Kuhn, 1949 Gen. et sp. indet. (PI. 2, Figs 1-3) Material. The brachiopods come from the upper part of the Oberek Cliff Member of the Oligocene Polonez Cove Formation of King George Island, South Shetland Islands (Bitner 1997). They were collected in front of receding Krakowiak Glacier on a gently sloping and recently exposed subglacial surface of the Chlamys Ledge (PI. 1, Figs 1-2; compare also fig. 40 in Smellie et al. 1984). The fossil brachiopods are represented by 16 moulds and several fragments of moulds; all specimens, with one exception, have damaged beaks; many are deformed or crushed. Remarks. The poorly preserved material does not permit detailed taxonomic study. The specimens are relatively large (dimensions of the only specimen P with the preserved beak: length 41.1 mm, width 42.6 mm, thickness 18.5 mm),

4 86 Maria Aleksandra Bitner and Michael R.A. Thomson subpentagonal in outline, wider than long, with the greatest width usually at the middle. The brachial valve is more convex than the pedicle one which may be nearly flat in some specimens. The beak seems to be high and straight. The anterior commissure is uniplicate with a broad, low brachial fold. The shell is ornamented with between 12 and 17 distinct, coarse, rounded ribs. On the brachial valve of one specimen there is a trace of a median ridge which does not extend all the way to the apex. Although the brachiopods are preserved only as moulds, the type of convexity, outline, and the uniplicate anterior commissure clearly indicate that they belong to the order Rhynchonellida. In the Southern Hemisphere, both in the Tertiary strata as well as in the Recent waters, the rhynchonellid genus Notosaria Cooper is the only genus with similar ornamentation, i.e. coarse, rounded ribs (Foster 1974, Lee 1978, Lee and Wilson 1979, Owen 1980, Wiedman et al. 1988, Bitner 1996). However, all the Notosaria species are much smaller than the investigated specimens. Notosaria also differs in having a greater number of ribs, namely Discussion Brachiopods are rare and poorly preserved in the Oligocene Polonez Cove Formation. They have been described previously from the lower unit of the formation, the Low Head Member (Bitner and Pisera 1984). However, because of the scanty material only three specimens could be determined to the generic level (Bitner and Pisera 1984, Bitner 1997): the terebratulids Liothyrella Thomson and Neothyris Douville, and the smooth rhynchonellid Cryptopora Jeffreys. The present specimens represent the first brachiopods from the upper part of the formation, the Oberek Cliff Member. Moreover, they belong to the Rhynchonellida, a group which is very rare in the Tertiary strata of King George Island: the better preserved and more diversified brachiopod assemblage from the Lower Miocene Destruction Bay Formation contains no rhynchonellids at all (Biernat, Birkenmajer and Popiel- -Barczyk 1985). It is worth mentioning, however, that in the brachiopod assemblage of the Eocene La Meseta Formation from Seymour and Cockburn islands rhynchonellids are common and represented by six genera (Owen 1980, Wiedman et al. 1988, Bitner 1996), however in Recent waters of the Antarctic region they are also rare with only two genera present (Foster 1974). Acknowledgements. The authors thank Andrzej Gaździcki for donation of the material and Krzysztof Małkowski (both from the Institute of Paleobiology, Warszawa) for critical review of an earlier version of this paper. The photos were taken by Ms. Grażyna Dziewińska (Institute of Paleobiology, Warszawa) to whom the authors are very grateful.

5 Rhynchonellid brachiopods from the Oligocene 87 References BARTON CM The geology of South Shetland Islands. HI. The stratigraphy of King George Island. Br. Antarct. Surv. Scient. Rep., 44: BIERNAT G., BiRKENMAJER K. and POPIEL-BARCZYKE Tertiary brachiopods from the Moby Dick Group of King George Island (South Shetland Islands, Antarctica). Studia Geol. Polon., 81: BiRKENMAJER K Discovery of Pliocene glaciation on King George Island, South Shetland Islands (West Antarctica). Bull. Acad. Polon. Sci., Terre, 27: BiRKENMAJER K Pliocene tillite-bearing succession of King George Island, South Shetland Islands (West Antarctica). Studia Geol. Polon., 74: BiRKENMAJER K Oligocene-Miocene glacio-marine sequences of King George Island (South Shetland Islands), Antarctica. In: A. Gaździcki (ed.), Palaeontological Results of the Polish Antarctic Expeditions. Parti. Palaeont. Polon., 49: BiRKENMAJER K Geology of Tertiary glacigenic deposits and volcanics (Polonia Glacier Group and Chopin Ridge Group) at Lions Rump (SSSI No. 34), King George Island, West Antarctica. Bull. Polish Acad. Sci., Earth Sciences, 42: BIRKENMAJER K New place names introduced in South Shetland Islands and Antarctic Peninsula by the Polish Geodynamic Expeditions, Polish Polar Res., 19: BIRKENMAJER K. and GAŹDZICKI A Oligocene age of the Pecten Conglomerate on King George Island, West Antarctica. Bull. Polish Acad. Sci., Earth Sciences, 34: BIRKENMAJER K., DUDZIAK J. and TOKARSKI A.K Palaeogene calcareous nannoplankton from a neptunian dyke in the Low Head Member: its bearing on the age of the Polonez Glaciation in West Antarctica. Studia Geol. Polon., 95: BIRKENMAJER K., GAZDZICKI A., GRADZINSKI R., KREUZER H., PORĘBSKI S.J. and TOKARSKI A.K Origin and age of pectinid-bearing conglomerate (Tertiary) on King George Island, West Antarctica. In: M.R.A. Thomson, J.A. Crame and J.W. Thomson (eds), Geological Evolution of Antarctica. Cambridge University Press. Cambridge; BITNER M.A Brachiopods from the Eocene La Meseta Formation of Seymour Island, Antarctic Peninsula. In: A. Gaździcki (ed.), Palaeontological Results of the Polish Antarctic Expeditions. Partii. Palaeont. Polon., 55: BiTNER M.A Cenozoic brachiopod fauna of Antarctica. In: P. Głowacki (ed.), Polish Polar Studies. 24th Polar Symposium, Warszawa, Institute of Geophysics of the Polish Academy of Sciences, Warszawa: BITNER M.A. and PlSERA A Brachiopods from "Pecten Conglomerate" (Polonez Cove Formation, Pliocene) of King George Island (South Shetland Islands, Antarctica). Studia Geol. Polon., 79: BŁASZYK J Ostracods from the Oligocene Polonez Cove Formation of King George Island, West Antarctica. In: A. Gaździcki (ed.), Palaeontological Results of the Polish Antarctic Expeditions. Parti. Palaeont. Polon., 49: DINGLE R.V., MCARTHUR J.M. and VROON P Oligocene and Pliocene interglacial events in the Antarctic Peninsula dated using strontium isotope stratigraphy. Jour. Geol. Soc, London, 154: FOSTER M.W Recent Antarctic and Subantarctic brachiopods. Antarct. Res. Series, 21: GAŹDZICKA E. and GAŹDZICKI A Oligocene coccoliths of the Pecten Conglomerate, West Antarctica. N. Jb. Geol. Palaont., Mh., 12: GAZDZICKI A The Chlamys coquinas in glacio-marine sediments (Pliocene) of King George Island, West Antarctica. Fades, 10:

6 88 Maria Aleksandra Bitner and Michael R.A. Thomson GAŹDZICKI A. and PUGACZEWSKA H Biota of the "Pecten conglomerate" (Polonez Cove Formation, Pliocene) from King George Island (South Shetland Islands, Antactica). Stadia Geol. Polon., 79: JESIONEK-SZYMAŃSKA W Echinoid remains from "Pecten Conglomerate" (Polonez Cove Formation, Pliocene) of King George Island (South Shetland Islands, Antarctica). Stadia Geol. Polon., 79: LEE D.E Aspects of the ecology and paleoecology of the brachiopod Notosaria nigricans (Sowerby). Jour. Roy. Soc. New Zealand, 8: 395^117. LEE D.E. and WILSON J.B Cenozoic and Recent rhynchonellide brachiopods of New Zealand: systematics and variation in the genus Notosaria. Jour. Roy. Soc. New Zealand, 9: OWEN E.F Tertiary and Cretaceous brachiopods from Seymour, Cockburn and James Ross Islands, Antarctica. Bull. Br. Mus. Nat. Hist. (Geol.), 33: PORĘBSKI S.J. and GRADZIŃSKI R Depositional history of the Polonez Cove Formation (Oligocene), King George Island, West Antarctica: a record of continental glaciation, shallow-marine sedimentation and contemporaneous volcanism. Stadia Geol. Polon., 93: SMELLIE J.L., PANKHURST R.J., THOMSON M.R.A. and DAVTES R.E.S The geology of the South Shetland Islands: VI. Stratigraphy, geochemistry and evolution. Br. Antarct. Surv. Scient. Rep., 87: WIEDMAN L.A., FELDMANN R.M., LEE D.E. and ZINSMEISTER W.J Brachiopoda from the La Meseta Formation (Eocene), Seymour Island, Antarctica. In: R.M. Feldmann and M.O. Woodburne (eds), Geology and paleontology of Seymour Island, Antarctic Peninsula. Geol. Soc. Amer., Memoir 169: 449^157. Received December 18, 1998 Accepted February 11, 1999 Streszczenie W oligoceńskich utworach ogniwa Oberek Cliff, najwyższej jednostki formacji Polonez Cove odsłaniającej się na Wyspie King George (Szetlandy Południowe, Antarktyka Zachodnia), stwierdzono obecność ramienionogów należących do rzędu Rhynchonellida (fig. 1, pi. 1-2). Rhynchonellidy należą do bardzo rzadkich skamieniałości w osadach formacji Polonez Cove, natomiast w stosunkowo bogatym zespole ramienionogów z młodszych utworów trzeciorzędu Wyspy King George - dolnomioceńskiej formacji Destruction Bay, rhynchonellidy nie są znane. Interesującym jest, że w eoceńskim zespole formacji La Meseta wysp Seymour i Cockburn (Półwysep Antarktyczny) rhynchonellidy reprezentowane są aż przez 6 rodzajów.

7 POLISH POLAR RESEARCH, VOL. 20 M.A. BITNER and M.R.A. THOMSON, PL. 1 Chopin Ridge Krakowiak Glacier ^^HlMiiii liiiillimi Ch'/am/s, Ledge $;> 'f ^'' fe'.ia :Sfe.sje-v SŚĘl^S^^^I- 1. Panoramic view of the cliff and succession south of Lions Rump, King George Island. Arrow indicates collecting site found by M.R.A. Thomson in February Close up of the same area; surface exposed after receding of the Krakowiak Glacier. Arrow indicates collecting site. Photos taken by A. Gaidzicki, January 1997

8 POLISH POLAR RESEARCH, VOL. 20 M.A. BITNER and M.R.A. THOMSON, PL. 2 Cfc> 0. V ' ) # jfc ^^zx_j ^^1 ^r.,'. ^^B Y^ < ^\ W ' ^ł L> #&' J ^ V'jraHEJr -^ EU r ^^ ^.jl^ ^^H ^ 1 V ^H *^^K ' V Ff v V ^ ^1 [ 1* Rhynchonellid brachiopod, gen. et sp. indet., Polonez Cove Formation, Oberek Cliff Member, Chlamys Ledge, King George Island; a ventral view, b dorsal view, c lateral view, d anterior view. 1, P ; 2, ZPAL Bp.XLII/2; 3, P AU x 1.