L TH

SEYMOUR

SO^

ISLAND

BOOMAN POINT

Upper Cretaceous microfossilbiostratigraphy of Seymour Island,

Antarctic Peninsula

BRIAN T. HUBER, DAVID M. HARWOOD, and PETER N. WEBB

Department of Geology and Mineralogyand

Institute of Polar StudiesThe Ohio State UniversityColumbus, Ohio 43210

During the 1982 Ohio State University expedition to SeymourIsland, we sampled and measured detailed sections in the Up-per Cretaceous Lopez de Bertodano Formation. Approximately200 samples were collected from three sections, the thickestbeing 1,460 meters (figure 1). The purpose of this study is todocument the microfossil assemblages from this locality and touse these assemblages to develop a biostratigraphic zonationand establish Southern Hemisphere correlations. Foraminiferaare the responsibility of B. T. Huber and calcareous nan-noplankton and diatoms of D. M. Harwood.

The Cretaceous stratigraphy of Seymour Island (figure 1) hasbeen described by Andersson (1906), Bibby (1966), and Mac-

ellari and Huber (1982), and formations were named by Rinaldi(1982). The Lopez de Bertodano Formation was considered (onthe basis of ammonite evidence) to be Campanian to possiblyMaastrichtian age by Spath (1953), whereas Howarth (1966) andOlivero (1981) suggested a middle to late Campanian age. Al-though the conformably overlying Sobral Formation lacks am-monites, Rinaldi (1982) considered it to be uppermost Cre-taceous to lower Tertiary.

Prior to the 1982 field season, documentation of marine mi-crofossils from Upper Cretaceous strata of the James Ross Basinwas very limited; Holland (1910) described two agglutinatedforaminiferal taxa and Macfadyen (1966) described 17 otherpoorly preserved taxa. The foraminifera were considered Seno-nian based on their co-occurrence with ammonites of this age.

Calcareous nanno plankton. Well preserved calcareous nan-noplankton assemblages are dominated by Braarudosphaerabigellowi and include the following Late Cretaceous species:Microrhabdulus decoratus, Cribrosphaerella ehrenbergii, Eiffellithusturriseiffeli, Prediscosphaera cretacea, Kamptnerius magnificus, andArkhangelskiella cymbiformis. Three Maastrichtian species occur:Braarudosphaera turbinae (late Maastrichtian), Nephrolithus fre-quens (late Maastrichtian), and N. corystus Wind (latest Campa-nian—middle Maastrichtian) (figure 2). A more thorough treat-ment of the nannoplankton is in progress (Harwood inpreparation).

Nephrolithus frequens has a bipolar distribution and is mostabundant in higher latitudes, often comprising 20 percent of

AlANTARCTICA

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Figure 1. Geologic map of Seymour Island showing sections measured by Rinaldi et al. (1978) and Macellari and Huber (1982). Sample localitiesreferred to in the text are also shown.

72 ANTARCTIC JOURNAL

floral assemblages south of 50°S latitude (Thierstein 1981). Lati-tude controls the stratigraphic range of N. frequens, because itsfirst occurrence is older toward the poles. Therefore, the max-imum stratigraphic range is assumed on Seymour Island. Al-though commonly regarded as late Maastrichtian, the oldestreported occurrence of this taxon is latest middle Maastrichtianat Deep Sea Drilling Project site 249 (Wind 1979). Worsley mostlikely included N. car ystus in his N. frequens 'et al" category,because he reports N. frequens "et al" from the lower Maastrich-tian in polar shelf sections (Worsley 1974, figure 10).

The assemblage of Late Cretaceous taxa from Seymour Islandbelongs to the Nephrolithus frequens province of Worsley andMartini (1970) and the Falkland Island nannofossil province ofWind (1979). This study extends the known range of each ofthese provinces southward by more than 13° latitude.

Foraminifera. Over 157 foraminiferal species have been re-covered from the Lopez de Bertodano Formation on SeymourIsland. Low diversity agglutinated assemblages alternate withassemblages of mixed calcareous benthic, planktonic, and ag-glutinated forams. The absence of calcareous taxa in manylevels of the formation may be due to subaerial leaching. Webelieve that the variations noted in foraminiferal faunas reflect awide variety of paleoenvironmental conditions.

Dominant taxa in the agglutinated assemblages include spe-cies of Rhabdamm ma, Bath ysiphon, Hyperammina, Psamrnosphaera,Ammodiscus, Reophax, Haploph ragmo ides, Cyclammina, Trocham-mina, Cribrostornoides, Spiroplectamrnina, and Ammobaculites.Large agglutinated taxa, some attaining diameters of 3 milli-meters, occur in the lower third of the sections.

Fluctuations in abundance and diversity of praebuliminidsand members of the family Chilostomellinae can be correlatedacross sections I, II, and III (figure 1). The buliminids presentinclude species of Praebulirnina, Pyramidina, Neobulimina, andBulim inc/la. Other notable calcareous benthics includeEouvigerina hispida, Bolivina incrassata, B. decurrens, and Anoma-linoides piripaua. Numerous species of the families Cau-casiniidae, Nonionidae, Alabamindae, Osangulariidae, andAnomalinidae are also represented. A single well preservedspecimen of Frondicularia rakauroana (Finlay) was found in sam-ple 101. This taxon was reported previously only from theupper part of the Haumurian stage (upper Maastrichtian) ofNew Zealand (Webb 1971, 1972).

All three sections contain a variety of well preservedglobigerinids. Hedbergella holrndelensis and Globigermnelloides mu!-tispinatus are quite common whereas Hedhergella rnonmouthensis,Globotruncanella havanensis, Rugoglobigerina rugosa, R. rotundata,R. macrace phala, Heterohelix globuloso, H. glabrans, and Gum-be! itria cretacea occur in low abundance. The above taxa gener-ally range from the late Campanian through Maastrichtian(Pessagno 1967; Webb 1971; Sliter 1976).

The foraminiferal faunas resemble Maastrichtian as-semblages from southern South America (Charrier and Lahsen1968), New Zealand (Webb 1971), and particularly site 327 onthe Falkland Plateau (Sliter 1976).

Diatoms. Rare fragments of diatoms were encountered duringexamination of calcareous nannofossil preparations. Since fewUpper Cretaceous diatom localities are known in the world, anintensive study of diatoms is underway. Diatoms will be thor-oughly examined in the future (Harwood in preparation).

Sample 70 (figure 1) from the upper part of the Lopez deBertodano Formation contains a rich assemblage of Paleogenemarine diatoms including: Goniothecium odontella, Pterothecapokrovskajae, P. danica, Eunotogramma weissi, Chasea bicornis, C.

Figure 2. Photographs of microfossils from the Lopez de BertodanoFormation on Seymour Island. (a) Hedberge!la holmde!ensis fromsample 28; (b) Globotruncanella monmouthensis from sample 38;(c-d) Braarudosphaera turbinae from sample 148; (e)Glob igerinelloides multispinatus from sample 28b; (f-h)Nephrolithus corystus from sample 28b; (i-k) Nephrolithus fre-quens from sample 148.

oronata, Rh izosolen ia cretacea, Step1ianopixis spp. Acan tliodiscusschmidtii, Hemiaui!us kondai, H. glescri, and others. Sample 76(110 meters higher in the section) contains a slightly differentdiatom assemblage including: Pterothcca crucifera, P. coriluta,Pseudopyxillia aculeata, Pseudoru tila na mon ilL', Hcmiau/us schmid-tmi, H. echinulatus, Trincaria arietnium, Arachnoidiscus chrenbcrgii,Kent rodiscus armatus, Corbisema geometrica (silicoflagellate), andothers.

Many of these taxa are reported from the Cretaceous andPaleogene. However, the absence of the characteristic Late Cre-taceous genus Gladius and silicoflagellates Ltram u/a furcula andValacerta hortonii and V. tumidula (all of which are present lowerin the section) is significant.

Discussion. The calcareous nannoplankton provide the great-est degree of age resolution. In this preliminary report, weseparate the Lopez de Bertodano Formation into four broadsubdivisions as stated below. Examination of these subdivisionsis continuing and this will further refine the positions of bio-stratigraphic boundaries.

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1983 REVIEW 73

The presence of Nephrolithus corystus and the absence of N.frequens in sample 28b (figure 1) suggests an upper Campa-nian-lower middle Maastrichtian age (Wind 1979, 1983) forthis sample and the section below it. Sample 38 contains both N.corystus and N. frequens suggesting an uppermost middleMaastrichtian age as noted in Deep Sea Drilling Project site 249by Wind (1979). Sample 148 is upper Maastrichtian as indicatedby the occurrence of N. frequens and Braarudosphaera turbinae.

The above information enables placement of the Campanian/Maastrichtian boundary below sample 38 and the middle/upperMaastrichtian boundary between samples 38 and 148. The lastoccurrence of ammonites (Zinsmeister and Macellari AntarcticJournal, this issue) and foraminifera 20 meters below sample 70and the absence of characteristic Late Cretaceous diatoms andsilicoflagellates in sample 70 and above suggests that the Cre-taceous/Tertiary boundary may occur in the uppermost part ofthe Lopez de Bertodano Formation on Seymour Island.

The Lopez de Bertodano Formation has the potential to beone of the more continuous Cretaceous/Tertiary sequences. Anextremely rapid sedimentation rate (equal to approximately 200meters per million years) and the detrital nature of this se-quence is exceptional as most other boundary sequences arecarbonate-rich and contain an hiatus.

This study was supported by National Science Foundationgrants DPP 80-20096 to D. H. Elliot and W. J . Zinsmeister (field-work) and DPP 80-18749 AOl to P. N. Webb (laboratory work).

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