Calcareous Nannofossils from Cape Roberts Project Drillhole CRP-3, Victoria Land Basin, Antarctica
D.K. WATKINS'
,S.W. WISI,. J R . ~
&G . VILLA^
'Department of Geosciences. University of Nebraska. Lincoln. Nebraska 68588-0340 - USA Department of Geology. Florida State University. Tallahassec. Florida 32306 - USA
'^Dipartimento di Scienze della Terra. Universiti di Parina. l'arco Area tlelle Seicnze. 157, 43100 Parma - Italy Received 17 November 2000: accepted i n revised form 26 October 200 1
Abstract - Fossil specimens of the exclusively oceanic calcareous nannoplankton were found only in the upper 200 m of the Cape Roberts Project 3 (CRP-3) cores.
. -
I hcse assemblages are depauperate and sporadic in occurrence. The majority of the assemblages contain fewer than 4 species, indicating a severe palaeoecological restriction. The clear domination of these assemblages by rcticuloienestrid species, coupled with the almost total exclusion of Coccolithns p e l q i c i i s , indicates cold surface water conditions in the Victoria Land Basin during the lives of these poor algae. These cold surface water conditions, perhaps coupled with abnorn~al surface water chemistry, excluded almost all of the age-diagnostic species from the area.The occurrence of Tra11sversoponti.s pulcheroides, however, suggests a biostratigraphic assignment of lower Oligocene for this part of the core. The CRP-3 assemblages occur within fine-grained clastic sedimentary rocks, but only those that are in close stratigraphical proximity with coarser-grained lithologies.
INTRODUCTION
Scientific drilling at Cape Roberts, Antarctica ( F i g . 1) h a s yielded three overlapping records spanning the Oligocene through Quaternary history of sedimentation in the sifted Victoria Land Basin. By taking advantage of dipping strata and only minimally buried subcrops of older material, the Cape Roberts Project (CRP) has been able to core over 1600 metres o f Cenozoic s e d i m e n t . C R P - 1 , drilled in 1997, recovered a thin sequence of Neogene and Quaternary glaciogenic sediment. Drilling in the 1998 season yielded CRP-2 and CRP-2A, containing Miocene to Quaternary and O l i g o c e n e t o M i o c e n e sections, respectively. Results from both CRP-1 and CRP-2 indicate that the Pliocene and most of the Miocene are absent or relatively thin in the study area. During 1 9 9 9 , the last field season of the project, CRP-3 (located at 77.0 106OS, 163.6404OE) penetrated through a thick section of glaciogenic clastics of Oligocene a g e , unconformably overlying intruded B e a c o n S u p e r g r o u p sedimentary rocks of probable L a t e Palaeozoic age.
Calcareous nannofossils occur in parts of the s e c t i o n s recovered f r o m all t h r e e years of C a p e Roberts drilling. Results from CRP-l (Villa & Wise, 1998) suggest that calcareous nannoplankton were a b s e n t from the Victoria L a n d Basin d u r i n g the Quaternary. Villa & Wise (1998) observed only rare s p e c i m e n s of T/zoracosp/7c/era (a calcareous
dinoflagellate) in the CRP-1 Quaternary. Results from both CRP-1 and C R P - 2 (Watkins & Villa, 2000) suggest that calcareous nannoplankton did not inhabit this part of the Ross Sea during the early Miocene, as nannofossils do not occur in rocks of this age. T h e stratigraphically-highest calcareous nannofossils include Dictyococcites bisectus, indicating a minimum a g e of latest Oligocene. Late Oligocene nannoplankton i n the Ross Sea of the Cape Roberts region consisted of sparse, depauperate assemblages dominated by small Reticiilofenestra a n d Dictyococcites species that only occasionally invaded the study a r e a . T h e early Oligocene record of nannoplankton in t h e area is complicated b y reworking (Watkins & Villa, 2000), making it difficult to determine if and when nannoplankton actually inhabited the study area.
METHODS
The entire section from CRP-3 above the contact with the Beacon S u p e r g r o u p was sampled and examined for calcareous nannofossils. Samples were chosen preferentially from fine-grained lithologies 01-
at least from sedimentary rocks with fine-grained matrix material. All samples were examined initially by using smear slides of raw sediment. In most cases, no additional sample preparation was done. A few coarser-grained s e d i m e n t s were processed by a
*Corresponding author (dwatkins@unlserve.unl.edu)
MO D.K. Watkins ct al.
Fig. I - Location of Cape Roberts Project drill sites and environs.
gravitational settling t e c h n i q u e in w h i c h approximately 0.1 cm^ of sediment was disaggregated and suspended in sufficient purified water t o constitute a column of approximately 2 cm height in a small closed vial. This suspension was allowed to settle undisturbed for 60 seconds. at which point an aliquot of the supernatant was withdrawn with a pipette and mounted on a cover glass. This procedure removed grains larger than approximately 2 0 micrometres from the s u p e r n a t a n t , and t h u s concentrated the finer, nannofossil-bearing s i z e fraction. Many of these slides w e r e prepared originally to concentrate siliceous microphytoplankton fossils, and use of these existing preparations greatly expedited our work.
Nannofossils occur in only some samples in the CRP-3 section. Even in these samples, nannofossils are an extremely rare sedimentary component (much less than 1%). As such, the traditional estimators of calcareous nannofossil abundance as a sedimentary component yield little information, as they all would be either B (Barren) or V (Very rare). Conventions for reporting the semi-quantitative relative abundance of different calcareous nannofossil species a r e also difficult t o apply t o t h e s e e x t r e m e l y s p a r s e assemblages. As a result, we have reported the raw number of each different s p e c i e s counted in t w o longitudinal transects of the cover slip at 1250x.
RESULTS
A total of 2 4 8 samples were examined f o r nannofossil occurrence and biostratigraphy i n the CRP-3 sequence above 782 metres below sea floor (mbsf). Fast-track samples and prepared ( n o n - acidified) diatom settled slides were used to locate promising horizons for further examination. Additional smear slides were then prepared and examined t o further d e f i n e t h e nannofossil-bearing intervals.
Calcareous nannofossils were observed only from the upper 1 9 4 m of C R P - 3 core. T h e nannofossil assemblages are not ubiquitous in this upper 194 m, but occur restricted to certain stratigraphic intervals that are separated by rock that is ban-en of calcareous nannofossils ( F i g . 2). T h e characteristics of the nannofossils in these intervals are discussed below.
Calcareous nannofossils occur in the upper two samples examined from CRP-3. Two specimens (one each of Dictyococcites daviesii and Dictyococcites productus) were observed in a sample from 2.85 mbsf. The sample at 5.01 mbsf contains a few more specimens of these taxa and also several specimens of Reticulofenestra 1~0111pdenensis. This assemblage is similar to the in situ assemblages from the upper Oligocene documented by Watkins & Villa (2000), so they may be assumed to be in place. However, the CRP-3 assemblages occur in sediments that are very
. .
0 50 100350 400 R F C A
Nannofossil abundance Diatom abundance
/-'?g. 2 - N u m e r i c a l a b u n d a n c e o f c a l c a r e o u s n a n n o f o s s i l a s s e m b l a g e s f r o m CRP-3 ( c e n t e r c o l u m n ) c o m p a r e d to d i a t o m abundance (right column). Nannofossil abundance is the number of s p e c i m e n s e n c o u n t e r e d in t w o random traverses of t h e slide a t 2 5 0 x . Diatom abundance is exprcsscd a s follows: R = rare: F = few: C = common: and A = abundant. Note that the distribution of nannofossil-bearing s a m p l e s is lion-random ( s e e text). also that diatom abundance and calcareous nannofossil abundance trends are not coincident.
c l o s e to a s e a f l o o r that has been actively eroded during tlie Neogene and Quaternary (as demonstrated by the lack of o v e r l y i n g s e d i m e n t a r y cover). I n addition. this upper part of tlie cored section is highly drilling-disturbed. Thus, it is impossible to ascertain w h e t h e r t h e s e a r e in situ lower O l i g o c e n e assemblages, or have been transported in either time or space.
Calcareous nannofossil assemblages occur in the fine-grained matrix of several sandy rocks from 17.82 through 46.42 mbsf. These assemblages consist almost exclusively of Dictyococcites daviesii, with only a few fragn~entary specimens of Thoracospliaera lieimii and Thoracosphaera saxea in sample 44.98 nibsf. These calcareous dinoflagellate cysts are often produced at t i m e s of abnormal s u r f a c e water conditions ( e . g . , eutrophication events, high fluvial discharge), and are reminiscent of the Quaternary assemblage in CRP- 1 documented by Villa & Wise (1998).
N o calcareous nannofossils were observed from s a m p l e s i n [lie interviil 4 4 . 0 to 77.1 1 nibsf. T h i s ihsence is nottihlc. as consi(li.~rahle efforts were made to locate calcareous nannolossils within the fine- gr;iinccl scclimentiiry rocks 01' this diatom-rich interval.
S p a r s e . d e p a u p c r a l c calcareous n a n n o f o s s i l assemblages occur i n some of the samples in t h e interval from 77.46 tliroi~gh 95 iiibsf. Only one t o three specimens per double trtinsect were observed,
~ ~ e f ' l e c t i n g low abinuhinccs of n a n n o f o s s i l s . l)ict\ococcites (Icn'icxii was preseiit i n all of these assemblages, and often was the only species observed.
O t h e r species observed i n this interval i n c l u d e d Dictjococcitc.',produetns, Rcticiilo,fenestra l ~ u i ~ i p d c ~ ~ e i i s i s , and a singlc s p e c i m e n o f T i - ( i 1 1 , ~ 1 ) e r s o / ; o i 7 f i , ~ . T h e singlc T r a n s v e r s o p o ~ i t i . ~ specimen is partially obscured in tlie s m e a r s l i d e (Sample 94.72 mbsf). preventing definitive species a t t r i b u t i o n . It has features c o m m o n to both T.
o11liquipo11.s a n d T. pii1clieroicle.s. but t h e p o o r preservation and presentation on the slide make i t i m p o s s i b l e to see the surface ornamentation that would differentiate the exact species. It has been d e n o t e d on t h e r a n g e chart (Tab. 1) as
T.
piilc/ieroii/es, but may actually be T. obliquipoiis.
T h e interval from 100 to 13 l mbsf contains the most abundant nannofossil assemblages in the entire s e d i m e n t a r y s e c t i o n recovered by C a p e R o b e r t s d r i l l i n g ( F i g . 2 ) . T h e relatively high n u m b e r of:
specimens in this interval allows some meaningful q u a n t i t a t i v e a n a l y s i s of t h e s e a s s e m b l a g e s . Dict~ococcires dmiesii occurs in all of these samples and comprises the majority of specimens in these a s s e m b l a g e s . T h e r e is a s i z a b l e variation i n tlie percentage of D. daviesii. This is exemplified by considering the two most populous samples ( 1 14.3 1 and 114.89 mbsf), which contain 60% and 88%- D.
daviesii (respective1 y j. Reticulofenestrii l i ~ ~ ~ ~ p d e n e n s i s and D. proihictus, together comprise 10-20% of these a s s e n ~ b l a g e s , although the contribution of each of these two species varies widely amongst the samples in t h i s i n t e r v a l . Dictyococcites b i s e c t u s a n d Dictyococcites scrippsae a r e c o m m o n a c c e s s o r y species. A single specimen of C l ~ i i i s m o l i t l ~ ~ i s altus o c c u r s i n t h e r i c h e s t s a m p l e in t h e s e q u e n c e ( 1 1 4 . 8 9 mbsf; Fig. 2). I n addition, three samples ( 1 1 4 . 3 1 , 1 1 4 . 8 9 , a n d 1 2 1 . 3 6 n ~ b s f ) c o n t a i n Trcmsversopontis p~ilclieroiiles.
T h e nannofossil-rich interval from 1 0 0 to 131 mbsf is underlain by about 10 m of largely barren sedimentary rock comprised of coarse-grained clastic s e d i m e n t s . S a m p l e s at 1 4 1 . 3 2 a n d 1 4 2 . 9 3 mbsf c o n t a i n isolated specimens of D. c/cii.;iesii and R.
hampdenensis, respectively. The interval from 157.7 to 161.9 mbsf records another episode of relatively high nannofossil abundance accompanied by good preservation. These assemblages reflect the same basic population structure as those from 100 to 131 mbsf, with D. claviesii being tlie dominant species and other
Tab. 1 - Range chart of calcareous iiannofossils identified in samples from CRP-3. Tlie "Ahi~ndam'e" column denotes the a h t ~ i n l ~ i i n ~ r of calcareous nannofossils as a sedimentary component, where B = barren. V = very rare ( I 2 specimens per longitudinal liaveisv o f t h e 4 0 - m m s m e a r ~ l i c l e ) . R = r a r e ( 3 - 1 0 s p e c i m e n s p e r t r a v e r s e ) . a n d I-' = few ( 1 0 1 0 0 speeimi.'iis per traversi.'). T i n ,
"Preservation" c o l u m n d e n o t e s the average preservation of t h e fossils. with P = poor ( s i - \ i ~ r c l y dissolved s o that s l n ' r n * s identification is frequently impaired). M = moderate (species identification only occasionally impaireil hy diagcnesis), and G ;>(çii (mild or no diagenesis. species identification not impaired).
Calcareous Nannol'ossils from Cape Rohcrls Pro]cci Drillhole CRP-3 Tab. I - Continued.
reticulofenestrids comprising the secondary taxa.
T m s v e r s o p o n f i s p~ilcl~eroicles is also present in at least one of the samples in this interval, indicating the persistent nature of this taxon below its occurrences near 1 14 mbsf.
An interval barren of calcareous nannofossils extends from 162 to 185 mbsf, interrupted only by the occurrence of D. daviesii in sample 178.01 mbsf.
The lowest interval of nannofossil presence occurs between 185 and 193.2 mbsf. Dictyococcites daviesii was observed in all nannofossil-bearing samples in this interval. A f e w samples a l s o c o n t a i n e d R.
11ci111~1clenei1sis and D. pi-ocl~~ctus. A single specimen of Chimolit17us o a ~ i z a r i ~ e ~ ~ s i s was observed in a sample from 185.25 mbsf. This relatively large nannofossil is characteristic of upper Eocene and lowest Oligocene sediments throughout the oceanic system, but is more c o m m o n in h i g h e r latitudes. T h e youngest documented occurrence of this species is within the Blackites spinosus Zone of earliest Oligocene age at Site 51 1 on the Falkland Plateau (Wise, 1983).
A single specimen of D. daviesii was found in a sample from 266.27 mbsf. Additional slides from this sample do not contain any nannofossils, suggesting
t l i i ~ t this single occurrence might be a laboratnry c o n t a m i n a n t . N o oilier ~ittii~iofossils have been observed i n samples from 195 to 781.4 mbsf, This is
1101 i~ncxpected. given the coarse-grtiined ~ititure of scdiiiieiit t l i i ~ t domim!tes this interviil, the sus~~ectcci dcpositiotiiil enviro~imcnt. and liigh scclimetitiitio~i rate for this material. I n addition. some of tlie samples between 600 iind 780 iiibsf yield clay-sizecl xi-~tliigenic st~iectite precipitiited intcrstitiiilly within the s ; i ~ ~ l s t o n c liost rock (Wise et al., this volume). These inciic;ite fluid ~ i i o v e ~ i i e ~ i t s i i i i c l cliitge~ielic processes t l i i i t may liave iiffectecl niinnofossil preser~iitioti in the lower part of the C e n o ~ o i c section.
in gcncnil. the sparse assemblages i n the i-ippcr 200 111 of CRP-3 arc not age iliagnostic, Tlie absence of many importdt~t groups ol' Palaeogcnc calcareous tian~iopla~ikto~i, inducling the discoasters. splienolitlis.
Iiclicospliaeritls. 'arid even cnccoliilii~ls. was due undoubtedly to ccologici.il exclusion rather than biostr;itigr;ipliicaI cotistriii~its, In this light.
biostratigrapliical iirgu~iients that arc based on [he absence of taxt~ tiiiist be viewer! wit11 cinition. Given this ciiveat, there is evidence thiit suggests a lower to mid-Oligocene placement for the upper 200 111 of (:RI'-3, bitsecl on the hick of oilier taxa known 10
occur i n the sci-Iitiienti~ry sequence of the Victoria Land Basin (i.e. CIROS-1 and CRP-?A), The absence
of several s p e c i e s of the genus R e ^ c i i / i j j ' ( ~ ~ ~ ( ~ . v / ~ ' ~ ~ .
including l?. I I Z ; U I I K . I , l?. I ~ i ~ q i i , and I?. ininn/ii/o. in ClZP-3 is probably I>iostrt~tigrtipliical in ~ i i ~ t u r e (i.e., not (lie result of e t i v i r o ~ i n i e ~ ~ t t ~ l exclusion). These stilii~! di;imcter (<5 ~ l i i c r o ~ ~ i c t r e s ) rciic~tlol'e~iestr~cIs first occur at r. 340 mbsf i n CRP-?A. at a level that is no older than early late Oligocenc (Wilson et al., 2000). Tlms, their absence i n CRP-3 suggests that tliis sectioii is stratigrapliict~lly below tlie lower upper O l i g o c e n e . In a s i m i l a r way. the ;il)se~ice of l ? i ~ / i ~ : ~ ~ i o f i ' ~ ~ e s i r i ~ o ~ i m a n i e i ~ . ? i . ? indicalcs tliai this a s s e ~ i i b l a g e is not latesl Eocenc it1 a g e . l?en'c~~/i~f'ene.s/~'a o i ~ ~ ~ ~ a r n e n s i s II~LS a well-cdlibratecl upper Eocene ninge i n tlie Soutliern Ocean. More importantly. i t is known 10 occur i n C I R O S - l (Edwards & Wtighorn. 1989: Wei. 1992: Monechi &
Reale. 19971 am1 (as a reworked cotiipo~ie~ilj in CRP- 2 (Walkins & Villa. 2000). Therefore, its absence from CRP-3 is most probably 1)iostnitigrapliical.
T w o aclditiotiiil lines of evidence suggest a n e'tiriiest Oligoccne a g e f o r the upper 2 0 0 mbsf of C R P - 3 . T h e last appeiirance clat~ini ( L A D ) of T r a n s v e r s e p o n / ~ , s ~~111chyroicie.s. in s a m p l e 114.13 ~iibsf. is a useful biostratigrapliical indicator.
The liiglicst stri~tigrapl~ical occurrence for this species has been d o c u ~ i ~ e ~ ~ ~ e c l from neiir the midpoint o f the lowcst Oligoccne Bl(icki!("i ,v/?/'~ios~i.s Zone on the l-'alklatid Plateau (Wise. 1983). This last occurrence datum is best expressed in tlie relatively shallow Deep
Sea Drilli~ig Project (DSDP) Site 51 1 . although i t is evident even i n the deeper water DSDP Site 5 1 3 . Although T /111/~~11~'roii/i~s hits not been observed i n all ( ~ i ; i ~ i ~ i o l ' o s s i l - b c i i r i ~ ~ g j siimplcs from C R F - 3 . its
~ c c u r r e n c e in tlie three richest samples of the inlcrviil from 100 to 13 1 mbsf imply thiit this intcrv;il o f CR1'-3 is below the lowest Oligocctic LAD of this species. Secondly, Clfi~i,smoli~/tus oimtm-neiisis is a
rcliilivcly large ~ii~tinofossil thiit is chiiriicleristic of upper Eocene aiul lowest Oligocene se(.litiients iliroiigho~it the occiinic system. AltIi<nl~i cosmopoiilan i n clistrib~~tion, i t tends to be much more abundant in sediments deposited at higher l;ititu(lcs. The, youtigest d o c u ~ i ~ e m e d occurrence of this species is within the Riaclki/e.s ,s/~;~~ij,sii,s Zone of earliest Oligoccne age at Site 51 1 on tlie Falkland Plateau (Wise. 19831. The significance of the occurrence of the single specimen of C. otinuii'uen.si.s ( s a m p l e 1 8 5 . 2 5 mbst') i s
~jucstion~~ble, The fiict that this spccies occurs only as an isolaiei-l individual specimen is possibly i\ rcs~tlt of reworking.
T h e available l>iostrt~tigra]?liical evidence from
,
Â¥i 1 , . L C I I L ~ L I ~ ., iii~nnofossils consistently s u g g e s t t i nossigtiti~ent of an eariicst Oligocene i\gc for the upper 200 mbsf of CRP-3, None of the lines of cvidence is partieuli~rly strong, given the depauperate and sparse iii~t~ire of tlie ciilcnrcous ~iantiolbssii iisscmblages. This a g e dei.crti1inatioii i s con"oborai.cd by d i i ~ t o m biostr;itign~pIiy (Ht~rwood & 13oIiaty, this volume). and is tin ititernally-consistent pan of the geocliro~iological i\s.e model f o r this locality ( I h ~ n n a l i et ill., this volume).
THE SIGNIFICANCE OF NANNOFOSSII OCCURRENCE
Calct~reous ~ia~'niofossils arc extremely nirc i n the C R P - 3 s d m p l e s . None o f the s a ~ i i p l e s ex a m i n e d contain calcareous n ~ i n n o f o s s i l s i n rock-forming quantities. Conce~~lriitions o f ~iannof'ossils in m a n y samples iire so low that the possibility of their being (icrived from reworking is significant. Ini-Iced. at least some. similiirly sparse assemblages from CRP-2 were s h o w n 10 be reworked (Wntkins & Villa. 2 0 0 0 ) . A1~houa.h extretiie spursiiy is suggestive of
rework in^.
i t is by no means ;I clefiiiitive criterion. Reworking can only be clctiiotistrated where thcre is a c l e a r l~iostri~tigrdpliical incongruity. For excitiiple. very s p a r s e bin well-preserved s p e c i m e n s o f l?i~/iriilo/mes!ni oiii~iiiniensis i n CR1'-2 (449.35 to 474.63 ~ i i b s f ) indicate ;I Iatesl Eocene iigc. whereas diiitom l~icisln~tign~pliy. stronlium isotope stntti~rapliy.
and p a l a e o ~ n a g t i c t i c a l g e o c h r o n o i o g y indicate a inkiclle early Oiigocene age for this interval (Schercr cl al.. 2000: Lavelle, 2000: Wilson cl al.. 20001. I n the case of tlie CRP-2 material, more than one line of evidence indicdtcs the biostrdtigapliical i~icongruily that bctrays the rcu~orkccl nature ol' l?. oanu~rnen.si.s.
There is no compelling evidence for reworking of calcii~-eons tia~itiofossils in the CRP-3 section. As
stiited above, the little biostratigrapliiciil evidence derived Irom the calciireotis ~itin~iofossil assembliiges is c o n s i s t e n t with the s i l i c e o u s microfossil biosinitigriiphy and the geoclironologici~l age model.
Thus. i f there was any reworking of ~nannofossils. it icciin'cd so briefly alter i ~ i i t i i i l deposition iis to be biosin~tigriipliically undeiectiible. Given ibis (i~iterniil]
consistency. the iisseii>I>lages from CRP-3 are assumed to be autoclitho~ious represenuitions of tlie calcareous iii11111o~)1ii1ikto11 tliitt sporadically inlin1~itccl [lie surfitcc
\viitcrs of the Victciriii 1.ant.l Basin d u r i i i ~ [he e;ir!icst Oligocene.
'The d i s t r i b i ~ t i o ~ i of e i ~ l c : i r e o i ~ s ~ii~iiiiofossil assemblages reliitive to Iithology is not random in the upper 2 0 0 1111~sf of (:RI'-3 ( F i g . 2 ) . C a l c a r e o u s
~iiinnot'ossils teiici to be present i n the he-grftiliecl inittrix of sundsto~ies. N t i ~ ~ ~ i o f o s s i l s iire ahscnl from conglomemtes iincl the coarsest-grained sandstones.
T h i s m a y i n d i e m e tlic a b s e n c e of c a l c a r e o u s
~iaii~ioplii~ikto~i i n the surfiice waters near the gliiciiil ice margin. Altcrruitively. [lie al>sence of nannol'ossils iiiiiy he a inere artii'Lld of removiil by current winnowing ( i n some cases1 or cliagcnesis. The l i ~ t e r iillemaiivc is siiggcstcd by tlie ciilcitic cement that is c o m m o n in tlicsc c o a r s e - g n i i n e d intervals. More perplexing is the absence o f calcareous nannofossils from tlie finest-gminecl inter~iils (Fig. 2). Normally, i t is expected that ~ i i ~ n n o f o s s i l s should be the most i l l ~ u i ~ i a n l i n sedimenLs that are the fi~iest-gi-aincd. 21s these sediments represent depositional environments thiit are iiiost distal to the gliiciiil ice ~ i i i i r g i ~ i . I n a d d i t i o n . s u c h f i n e - g n i i n c d s e d i m e n t s a r c o f t e n associated with sea level high s t a n d . when the l o o c l i n g of oceanic siirfiice waters into ilic biisin shoiilcl be at its maximum. However. ibis is not she case with the CRP-3 assemblages. A IIIOSI ~ i o t a b l c e x ~ i m p l e of this discrepancy is illiistriitccl b y the p r o m i n c m siltstone interval f r o m 5 2 so 6 5 ~ i i b s f ( F i g . 2 ) . which is c h a r a c t e r i z e d b y iibundii~is to common diatoi'ns iincl other siliceous ~iiicrofossils liut contitins n o ~iannofossils, This interval, i~iterpreted as a high stii~id systems n-iict ( C a p e Roberts Science Team. 2000), should have the greatest conee~itn~tion o f ~niinnofossils based o n its proposed del~ositional environ11ic1it and sequence striitigri-aphic ii'iterpret~ition.
Instei~cl. it is barren of calcareous ru~iinofassils. This remains a puzy.ling feature of the CR1'-3 nannofossil reconl.
A s s e m b l a g e s a r e s p e c i e s p o o r in the upper 2 0 0 ~ i i b s f of C R P - 3 ( T a b . I ) . M a n y o f the iissemblages contain only one or two species. wliile the richest assemblage ( 1 14.89 nibs!') cuntiii~is only seven d i f f e r e n t taxa. M o s t o f t h e s e s p e c i e s ;ire r c t i c ~ i ~ o f e ~ i e s l r i c l ~ l l a c o l i t h s . with / l i c ( ~ o c o c c i l e . s iiiivic.sii coi'nprising the bulk of the speciniens. None of the other (non-reticiilol'cnestricl) tiixii make up a numerically significant fraction o f the assembliigc.
. .
1 hese o t h e r tiixii i n c l u d e Tran'^vc~i'.so~on/i.s i i i u iChici.smo/i111ii.s, two taxa k n o w n to have i~il-iiil~ited the
Southern Occnn i . l u r i n ~ ihe O l i g o c e n e . Only o n e siiiglc specimen of Cocco/i~/~ii.s /~o/ci,qicsis, ii species
lliiit is considered cosmopoliliin in the Oligocenc. \\,as observecl (in su~inple 100.75 mbsfl. This specimen is poorly preserved. with corroded niiirgins, differing markedly from the prescrviition of otlier specimens i n this stimple. This cliffcrentiiil preservation, couplct.1 with the singularity of its occurrence, suggests that i t may be ii reworkei.1 specimen. Significantly iil>se~it are representatives of "typical" lower O l i g o c e n e
~ i s s e i i i l ~ l i ~ g c s i~iclucliiig !)i.scoii^'lei; .Sp/iciiolii/in.s, Eric'sonia. I/e/icosphne~~ci. and I.s//~~i~o/i~/~ii.s. Altliougli the hick of 1 l i . s c o d ~ e r and Splu'no/i//in'i is ~probiibly a reflection of the cool iempcrtitures of the surface water in the Victoria Land Basin. tlie other taxa arc well known from the Southern Ocean during the early O l i g o c c n e . T i n s s u g g e s t s that the sin-face water concliiions in tlic Victoriii Land 13iisi11 were subnormal during tlic eiirly Oligocene.
CONCLUSIONS
The extre~iicly rare ~iannol'ossils ihiu occur i n the C R P - 3 s e c t i o n lire chiirdcteri~.ed by low s p e c i e s r i c h n e s s . low diversity. t i i u l clo~iiiniition of the assemblages by retici~lofc~icstrid pliicolitlis. These (.lepiiuperate ~ i i ~ n n o f o s s i l iisscml~liiges i~idiciitc that marginal c o ~ i d i t i o ~ i s for calcareous nannoplatikton existed ciuring the Oligocene in McMnrcIo Sound. The lack of splienolilhs and discoiislers inclicdtes very cool si~rface ~ i ~ t o - S tliat were 1ie;ir the lower thermal limits for n a ~ ~ n o p l r t n k i o n growth. I n addition to thermal c o ~ i s t r a i n t s . [lie high doininiincc/low diversity iisse~iibliiges suggest that subnormal surface ~ i ~ t c r coiulitio~is m a y have existcd i n the Victoria I.and Basin d u r i n g t h e early O l i g o c e n e . T h e avuikible bioslri~tigrapliicol evidence from nannofossils suggests
it11 earliest Oligocene age t'or the upper 200 mbsf of
CR1'-3. However. she largely ~iionotaxic natnre of t h e s e a s s e m b l a g e s indicates that conclitions f o r eaicarcous nannopliinkton were very ~iiiirgiiial. so tlidt
a n a s s i g ~ i m c n t b a s e d o n the a b s e n c e of taxi1 is tenuous at best,
ACKNOW1.EI~GEMI~NTS - The ni~tliors wisli to thank the organizers of the Cape Roberts I'roject for inviting tlicm 10
piirticipalc in this interesting endeavour on the fronlici's, I t hiis been great fim, Funcling for our eiTons were provided by the Nutioiiiil Science l-'oiiiidatio~i 01'1'-9419770 (to DKW
& S\A'\V),
H a n n a h M . I . , l-'lorimlo I:., H a n v o i ~ l D . M . . l~ii'lcling C.]<. fl C R P Srcm, Cl<l??I2,\, V i c l w i z t l.;ta?d 13:tsin , i ~ ~ ~ i t v c l i c : s . T<,rro S c i c n c c 'TC~~III, 2OOl. ~ l ~ r ~ ~ , ~ ~ ~ ~ l r ~ ~ l i g r ~ ~ ~ ~ l ~ y 111s C I l F . 2 A/iliir!Kv, 7 . 443-452.
~ l s ~ l l l ~ ~ ~ l c , \k'Iosi:v l.;md 13;tsin A~~l;trclic:#. 'Tl~is ~ > I t ! ~ n c . \Vci L\'.. 1992. L!p~l~tlccl xn:lt,~?oro$sil slri!ligfiq~l>y o r l l l c C:lIlOS-l H a n w < ~ l D . M . & Bol,:ny S.M., 2001. l<irly Oligoccnc biliccot8s c c , lM c M u n I o Sound 1 1 2 ~ ~ ~ Sea). I'm,: 0 1 l l ' S r . Iirsu11.s.
tmicrol'owil l ~ i c ~ s l ~ ~ ~ l i g r ~ ~ ~ ~ l ~ y o f (:;!pc l l o l x w s l'v,>,iccl core 120. 11115-l 117.
<:1<1'-3, \Jicloria l.;$~>cl 13;tsin. ,\1112trclic:1. 'Tl~is ~>II$IIIC. \ i ' i w S,\!.. Jr.. 1 9 8 3 . h~lcbo,.oic an<I C c n o i ~ > i c c ~ ~ l c ; ~ r c ~ ~ u ~ l.:ncllc h4., 2011U. SIrm?li~n>~, isoI,>pc s ~ c ~ ~ l i g ~ ~ ~ ~ ~ l ~ y :and :age ~m<xIcl Shs ~n$~~~,,oS<~ssils r ~ c ~ ~ ~ ~ e r c ~ l 1,) l)ctl> Sea l j s i l l i ~ , g l'rojccl l..cg 71 i n
CI<l'-212A, Vicl,>ri>~ l,z!n~l 13asin. A n l a r c l i c ~ ~ . 7?,rr,( ,l~,ior,ic<~. 7 . II,C l - ~ ; ~ l k I i ~ $ ~ < l I ' I t ~ l c i ~ ~ ~ r e z i w ~ , S , > ~ ~ l l ~ n , e s l , \ l l ~ ~ n l i c O C C ~ ~ I ~ . It?:
1 , ~ ~ I w i g \ A T , K n ~ ~ l ~ c ~ ~ i ~ , ~ ~ i k ~ ~ v \l.,\. cl :,l., l!~i!i<!l /<<7,0rI.s ,t/'i/?t I)I,c/} Sea llnllinv, Projw!, 71, Pan 1. ,481-550,
\Vise S.\&'.. .Ir., S m e l l i e J.1 A g l l i b F.S.. I n n a n 1 R . D . & Kriswl;
L.A.. 2001. Aiiliiigc~liic smeciilc clay L-O;IIS i n CRP-3 diilleorc,
\ k t ~ > r i ; $ l.:m<l 13:55i$1. ~ \ I I I W C I ~ C ~ ~ . :i ~ p o ~ ~ i l ~ l c ~~,<I~c:IIw o r Clthicl rlo$$,: it l~r,>gccss r q x ~ ~ l . ' r l ~ i s \<>lw?k'
\\jilsc>~~ (;.S,, l ~ l m r i ~ l < ~ l:., S:tgnoIli I-., \Jcrm<tl> K.I.. & I C ~ ~ I ~ c r l s AI'..
2flOO. ~ ~ ; ~ ~ ~ ~ c I c ~ s I ~ : ~ I ~ ~ K ~ ~ I I ~ ~ o r O l i g o c ~ ~ ~ c - h ~ l i ~ ~ c c ~ ~ e g l ~ ~ c i w ~ ? z ~ r i w U S ~ I ? IIK A t ~ l w c l i c I<t$ioz?. Prr<, ,l~mtr!ic<t, 5 , 479-484. SIXII~ Srw,~ Cl<l'.2/2,\. Victoria I.ZIMI 13ui11. . A n ~ m c ~ i c ; ~ . 'fi'rro
\Valkins D . K . & Villa. C;.. 201111. Palneogenc calcareous ~iai,iml'ossils A~,li/rlirn. 7. 63 1-W6.
