J. geol. Soc. London, Vol. 139, 1982, pp. 763-770, 1 fig, 1 table. Printed in Northern Ireland A comparison of the ammonite faunas of the Antarctic Peninsula and Magallanes Basin M. R. A. Thomson SUMMARY: Ammonite-bearingJurassic and Cretaceous sedimentary successions are well developed in the Antarctic Peninsula and the Magallanes Basin of Patagonia. Faunas of middle Jurassic-late Cretaceous age are present in Antarctica but those of Patagonia range no earlier than late Jurassic. Although the late Jurassic perisphinctid-dominated faunas of the Antarctic Peninsulashow wide-ranging Gondwana affinities, it is not yet possible to effect a close comparison with faunas of similar age in Patagonia because of the latter's poor preservation and our scant knowledge of them. In both regions the Neocomian is not well represented in the ammonite record, although uninterrupted sedimentary successions appear to be present. Lack of correspondence between the Aptian and Albian faunas of Alexander I. and Patagonia may be due to major differences in palaeogeographical setting. Cenomanian-Coniacian ammonite faunas are known only from Patagonia, although bivalve faunas indicate that rocks of this age are present in Antarctica. Kossmaticeratid faunas mark the late Cretaceous in both regions. In Antarcticathese have been classified as Campanian, whereas in Patagonia it is generally accepted, perhaps incorrectly, that these also range into the Maestrichtian. Fossiliferous Jurassic and Cretaceous marine rocks are rize first those of the Antarctic Peninsula and then to well developedin theAntarctic Peninsula, Scotia compare them with those of Patagonia. Comparisons Ridge andPatagonia (Fig. 1A).In Antarcticathese between Antarctic ammonite faunas and other Gond- rocks are distributed along the western and eastern wana areas wereoutlined by Thomson (1981a), and margins of theAntarctic Peninsula, formerly the the faunas of the marginal basin were discussed in magmatic arc from which the sediments were derived. Thomson et al. (1982). Upper Jurassic-Lower Cretaceous fore-arc rocks are represented by the Fossil Bluff Formation of Alexan- der I. (Taylor et al. 1979), whereas Middle and Upper Antarctic Peninsula region Jurassic back-arc deposits occur in south-eastern Pal- mer Land and Orville Coast (Latady Formation; Row- Middle Jurassic ley & Williams 1982), and the Cretaceous sequence of theJames Ross I. area hasbeen directly compared Middle Jurassic faunasare known only from the with that of the Magallanes Basin of Patagonia (Bibby southern Behrendt Mountains (Fig. 1A). Species pre- 1966). Local intra-arc marine sequences occur in the sent include Normannites cf. vulgaricostatus Wester- South Shetland Is (Smellie et al. 1980) and Adelaide I. mann, Skirroceras cf. bigoti (Munier-Chalmas), (Thomson 1972), and the Nordenskjold Formation of Teloceras cf. lotharingicum Maubeuge, Mega- NE Graham Land (Farquharson 1982) may represent sphaeroceras cf. rotundum Imlay, and Nothocephalites a sedimentary phase that pre-dated the main period of (?) sp. (Quilty 1970). Unfortunately these interesting arc building. Bajocian and Callovian species are not well preserved By contrast, in Patagonia (Fig. 1B) stratigraphically and they occur in isolatednunataks whose strati- equivalent sedimentary rocks are largely confined to graphical relationships with the rest of the sequence the Magallanes or Austral Basin (Natland et al. 1974) are obscure. that developed in a back-arc setting. Although much of this region is covered with Cenozoic rocks, Jurassic Late Jurassic and Cretaceous units are exposedalong the western margin of the basin and have been proved at depth to Late Jurassic ammonites are far more abundant and the E in wildcat and hydrocarbon-producing wells. In occur in the South Shetland Is, north-eastern Graham late Jurassic times a marginal basin opened within the Land, Adelaide I., Carse Point, Alexander I., Lassiter magmatic arc of theeastern Andes, but this was and Orville Coasts and Behrendt Mountains. Howev- short-lived and closed in the mid-Cretaceous. The er, with few exceptions they are poorly preserved and rocks of South Georgia (54"20'S, 36'40'W) represent confident identifications are normally possible only an isolated fragment of this basin (SuBrez & Pettigrew when several specimens are available. Everywhere the 1976). faunas aredominated by perisphinctids,a difficult Ammonites are varied and locally abundant in all group tounderstand even when dealing with good these areas, and it is the aim of this paper to summa- material. 0016-7649/8211100-0763$02.00 01982 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/6/763/4887516/gsjgs.139.6.0763.pdf by guest on 28 September 2021 764 M. R. A. Thomson 0 300 I 1 km 75' 70'W 65" FIG. 1. Sketchmaps of theAntarctic Peninsula (A)and southern South America (B) to show the localities mentioned in the text. AV = Ablation Valley, CP = Carse Point, C2 = Cape Zumberge, FB = Fossil Bluff, L1 = Low Island, SHI = Snow Hill Island. Thick lines in Fig. 1B indicate the western and eastern limits of the Magallanes Basin. Oxfordian ammonites arepoorly known but isolated Shetland Is. Although this is an Oxfordian species it specimens suggest that this stage may be represented occurs with a bivalve fragment having coarse concen- locally. A possible fragment of thelate Oxfordian- tric ornament similar tothat of the Kimmeridgian early Kimmeridgian Orthosphinctestransatlanticus Retroceramushaasti (Hochstetter) type. Poorly pre- (Steinmann) from James Ross I. was said to be loose servedperisphinctids from theBehrendt Mountains (Spath 1953), but Bibby (1966, p. 22) indicated that have beencompared to Oxfordian Discosphinctes perisphinctids occur in the rocks nearby. Other exam- (Quilty 1970). ples from Alexander I., compared to the same species Kimmeridgian ammonite faunas are also difficult to (Howarth 1958), are doubtfully conspecific. A juvenile identify, although bivalve studies (Crame 1982) indi- mayitid,resembling Epimayites trunsiens (Waagen), cate that sedimentary rocks of this age are widespread has been identified by the author from Low I., South in the region. Collections from the Orville Coast area Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/6/763/4887516/gsjgs.139.6.0763.pdf by guest on 28 September 2021 Ammonitefaunas of the Antarctic PeninsulaandMagallanes Basin 765 (Fig. 1A) include many fragmentary and sometimes Ablation Valley, Alexander I., wherea Haplophyl- tectonically distortedperisphinctids reminiscent of 1ocerasIBochianites fauna is present (Thomson 1979). mainly Kimmeridgiangenera such as Subdichoto- Although similar faunas inIndonesia have been in- rnoceras, Torquatisphinctes and possibly Pachysphinc- cluded in the Tithonian (Westermann et al. 1978), in tes (Thomson 1980). Although it is difficult to rule out Alexander I. they occur much higher in the succession similarities with some Tithonian species, local occur- than the late Tithonian Blanfordiceras fauna and in rences of Katroliceras and Subplanites (?) confirm the association with Spiticeras aff. spitiensis (Blanford), presence of Kimmeridgian strata.The presence of Rairnondiceras, earlyCretaceous belemnites (Thom- Aspidoceras at one locality could indicate an age be- son 1979) and Berriasian bivalves (Crame 1982). Thus, tween Kimmeridgian and Middle Tithonian. an early Berriasian age is considered most probable. A Kimmeridgian age for a possible Pachysphinctes Higher levels of the Berriasian are represented at from some of the oldest strata exposed at Ablation Callisto andTombaugh cliffs whereisolated occur- Point, Alexander I. (Thomson 1979) is supported by rences of Bochianitesgracilis Thomson, Substreblites its association with the bivalves Retrocerarnus haasti (?) sp., Hirnalayites (?) sp., Neocosmoceras aff. sayni and R. subhaasti (Wandel) (Crame 1982). Ammonites (Simionescu) and Sarasinella aff. hondana Haasare from the Nordenskjold Formation (Farquharson 1982) known (Thomson 1974). have been severely compressed but include haplocera- Although there is an apparent stratigraphical con- tids, Tararnelliceras and avariety of perisphinctids formity within the Fossil Bluff Formation up to beds resembling Torquatisphinctes and a finely ribbed with Aptian and Albian faunas (below), the interven- Lithacoceras, for which Kimmeridgiana or early ing strata are almost devoid of ammonites,and the Tithonian age seems most likely. bulk of the Neocomian is apparently unrepresented in TheTithonian faunas are less problematicalthan the ammonite record (Taylor et al. 1979). This absence those of the (?) Oxfordianand Kimmeridgian. has yet to be explained satisfactorily but the possibility Although they are no better preserved than the latter, of there being an undetectedstratigraphical break those ofAlexander I. in particular are distinctive must be kept in mind. It is also possible that some enough to allow comparison with the faunas of many aconeceratid faunas from the Fossil Bluff area might other regions (Thomson 1979). Perisphinctids domin- be better regarded as Barremian in age (below), rather ate with avariety of Virgatosphinctes and Aulaco- than early Aptian (Thomson 1974). No species which sphinctoides that can be matched with both central might indicate Valanginian and Hauterivian levels has South American and Himalayanhladagascan species. yet been identified in Alexander I., but Covacevich Phylloceratids,lytoceratids (including the distinctive (1976) documented the Valanginian in the South