Lower Miocene Diatom Biostratigniphy of the CRP-1 Drillcore, McMurdo Sound, Antarctica

Department of Gcoscieuces. University of' Nebraskii. Lincoln, NI< 08588-0340 - USA wppsala University, Institute of H;irlh Sciences. Dppsala - Sweden

Abstract - Lower Miocene diatoms and other siliceous microfossils in the Cape Robcrts Pro,ject drillcore (CRP-l), Robcrts Ridge, western McMurdo Sound, arc doeuinenled. Diatom biostratigraphy. along with "SrPWSr ratios from mollusk sliclls ainl '"Ar/^Ar data from volcanic materials. constrain the interpretation of the magnetic polarity stratigraphy and ;illows construction of an age model forCRP- 1 . Together, these records indicate an age spun from to -21 .S Ma at 147.69 mbsf(the bottom of the hole) lo -17.5 Ma at 43.15 mbsf (top ol'thelowerMiocene interval). Important diatom datums identified in tliis drillcore are the I-'AD and LAD of T I ~ a l a . s , s i o . ~ i ~ ~ a p m e f r ~ and the LAD of Cuviiciilis wiii .s . Variation in diatom abundance and assemblage composition through the lower Miocene reflects changcs in depositional environment. There are intervals of repeated ice cover (floating or grounded) in the lower Miocene section of CRP- 1, where diatom production was limited. and intervals where diatoms were growing in open, although at times shallow ( 4 5 m). marine seas. Relative numbers of planktic to benthic diatoms often parallel similar indices ofwater depth from sedimentologic and sequencestratigraphic interpretations. Approximately 70diatom taxa, in addition to 2 silicoflagellate and 2 ebridian taxa, are reported. No radiolarians were encountered. Three tentative diatom biostratigraphic zones are introduced. though not formally named in this report. A Ross Sea diatom zonation for tlie lower Miocene to upper Eocene is under development, for future publication. The fortl~coming zonation will incorporate diatom biostratigraphic information from CRP-l and CRP-2, and earlier stratigraphic drilling in McMurdo Sound, as well as inagnetostratigraphically calibrated claturns from the Southern Ocean.

INTRODUCTION

The Cape Roberts Project (CRP) is an international programme aimed at the recovery of Palaeogene sediments from McMurdo Sound, Ross Sea, Antarctica, through stratigraphic drilling. Two seasons of drilling were planned in order to core several holes through seaward dipping strata on Roberts Ridge, 10 to 15 km east of Cape Roberts. Cape Roberts Drillhole 1 (CRP-1) was drilled 16 km northeast of Cape Roberts at a water depth of 153.5 m. The site was chosen in order to drill the youngest seismic reflectors on Roberts Ridge (Cape Roberts Science Team, 1998). The hole was drilled to a depth of 147.69 metres below the sea floor (mbsf). and 113.32 m of core was recovered. Drilling was terminated prematurely due to a severe storm that removed fast ice to within 1 km of the drill rig and resulted in unstable drilling conditions (Cape Roberts Science Team, 1998). Quaternary and Miocene strata are present in this section, and the favoured age model for the lower -100 m of the drillcore is from -17.5 Ma to -21.5 Ma (Roberts et al., this volume).

ANTARCTIC DIATOM BIOSTRATIGRAPHY

Antarctic diatom biostratigraphy has advanced steadily over the last two decades. Initial description of lower Miocene diatom assemblages from Antarctica and the

Southern Ocean was presented by McCollum (1 975); with further discussions by Schrader (1976) and Weaver & Gombos (1981). Neogene and Oligocene diatom assemblages of the Southern Ocean have only recently been calibrated to the magnetic polarity time scale (Gersonde & Burckle. 1990; Baldauf & Ban-on, 1991; Harwood & Maruyama, 1992; Harwood, 1994).

Prior drilling in the western Ross Sea resulted in the development of a local biostratigraphic framework for the upper Eocene to lower Miocene (Harwood, 1986, 1989; Harwood et al., 1989a). Diatomaceous sediments were recovered during DSDP Leg 28 at sites 270 and 272 (McCollum, 1975; Savage & Ciesielski, 1983; Steinhauff et al., 1987), although diatom diagenesis and transformation to opal CIT (disordered cristobaliteltridymite) rendered the siliceous sediments in these sections largely unusable for diatom studies. Lower to middle Miocene diatom assemblages were also described fromreworked diatomite clasts in upper Miocene sediment in the central Ross Embayn~ent (Harwood et al., 1989b; Scherer, 1992). The CRP- 1 drillcore adds an important interval to this composite record that was previously not available.

The diatom stratigraphy in deep-sea sediments of the Southern Ocean is well-documented and calibrated to the magnetostratigraphic time scale, but available stratigraphic sections from the Antarctic continental shelf are largely discontinuous and not calibrated to absolute time scales. Some diatom datunls used in the Southern Ocean zonal

framework (Baldaiif & B ~ I T O I I , 199 1 ; l Iarwooul & Mariiyama, 1992) can be applied directly to the Antarctic continental shelf. Othc~~opcii-occan laxa. however, are not present or abundant due to biogeographic barriers and. tlms, have limited 1~iostr;itigr;ipliic value in Antarctic shelf sections.

Diatom floras recovered from the co~itincntal shelf include a mixture of neritic planktic and benthic littoral taxa, in addition to rarer occurrences of open-ocean floras characteristic of the Southern Ocean. Many fossil neritic and coastal taxa are poorly known, or undescribed. While these taxa may be of biostratigraphic value in future studies, no calibrated reference sections have been available to enable their application in age determination and correlation at this time. The focus of the present study is to establish the ranges of diatoms that have utility as biostratigraphic markers and to provide data on diatom occurrence and abundance that may aid in palaeo- environmental interpretations. A diatom biostratigraphic framework for the Antarctic continental shelf is being developed, and will be formally presented in a later work.

METHODS

A total of 155 samples from the Miocene interval of the CRP-1 drillcore were collected for diatom analysis. Sample spacing for study of lower Miocene diatoms is variable. but it is less than 1 m for most intervals (Tab. 1). All samples were checked initially by examination of a strewn slide of raw sediment. This was prepared by separation and disaggregation in 50 1111 of water and settling for 30 seconds to remove coarse material. A strewn slide was made from the suspended material for a quick check of diatom presence and abundance. If warranted, additional concentration was performed by sieving through a 25 pm sieve, and in some samples, density separation was performed using a Sodium Polytungstate solution of 2.2 specific gravity. Slides were routinely examined at 500x with increase to 1250x magnification for positive identification.

DIATOM OCCURRENCE IN CRP-1

Marine diatom occurrence is discontinuous throughout the lower Miocene section of CRP-l, with intervals of rich diatom assemblages separated by non-productive intervals of reworked, fragmented diatoms (Fig. 1 , Tab. 1 ). Intervals of abundant in situ diatoms occur within the following core depths: 58.75 to 77.06 mbsf; 80.12 inbsf; 85.20 to 102.25 mbsf; 116.48 to 118.57 n~bsf ; 127.79 to 134.3 1 mbsf: 141.80 to 142.61 mbsf; and 146.51 to 147.69 mbsf. All other intervals are interpreted to reflect intervals when diatoms were not growing over the CRP-1 site, likely due to cover by floating or grounded ice. Alternatively, extremely rapid sedimentation and water column turbidity wouldlimit diatom production anddilute diatomabundance in the sediment. Poor diatom preservation and the occurrence of <30 diatom fragments in 5 fields of view at 250x are interpreted to reflect intervals of glacial recycling

of marine scili~iiciits (I-'ig. l . -Tab. 1 ). These diatom pour asse~iihla~:es were eroded locally from lateral coi.n-lii~ivrs of iinderlying lithostratigraphic units or were ~-vworkrd from older striit;~. Iii~ci~viils where more than 50 f'r;ii:mr~iis were observed in 5 fields-of-view (Fig. 1 . Tub. 1 ) ;ire interpre[ed to represent open-water productivity ;nul in ,si/ii cliiitoiii sediinciittition. There is n o cvidcn(,i.- lor reworkinpoflowcr Oligocene or older Paleogene di;itoms into the Mioccne sequence of CRP- 1. T h e known stratigraphic ranges of diatoms in CRP-l are prcsenled i n figure 2 and tiihlc 2.

The Miocenescction ofCRP- 1 includes several ccologiciil associations ofmarinediatoms. These reflect envii-onnicniiil conditions of variable water depth. A preliminary assess~iicut of diatom assemblage ecology from CRP-I is shown in table 1 .Thefollowingcharacteristic assemblages arc noted in order of decreasing water depth: (1) assemblages dominated by pclagic and neritic marine planktic Cosciiwclisciis and Stephcinopyxis spp.. desigiiiited ' l " (planktic) on table 1 : (2) assemblages with high ~ ~ l ~ i i ~ i d i i ~ i ~ ~ of Paraliu, a filamentous tychoplanktic ('T') diatom from the benthic environment, which often occurs in liigli numbers in the plankton; (3) assemblages with high numbers of large benthic diatoms ('B'), such as !,vlini(i, Aracluwidisciis. and a large unknown diatom (referred to here as genus and species uncertain A, as well as Co(~con(>;,s spp., Rhabdone~no spp. and G1~o11~117citophora sop. The association of 'B+P' is generally a reflection of benthic diatom transport into deep water and mixing with open- marine planktic diatoms.

The distribution of diatoms is discussed, and presented in table 1, in relation to stratigraphic units from the base of the core upwards to the unconformity with overlying Pleistocene sediments at -43 nibsf. Diatom study of the Pleistocene section of CRP-1 is presented in a companion paper (Bohaty et al., this volume).

Lithostratigraphic Unit 7.1 (147.69 to141.60 11112.sf)

mudstone - A rich assemblage of mixed benthic and planktic diatoms is present near the top and base of this lithostratigraphic unit. The middle of this lithostratigraphic unit from 145.76 to 143.73 mbsf contains rare diatoms of indeterminate ecology.

Lithostratigraphic Unit 6.3 (141.60 to 119.28 mbsf), diamictite - The diatom assemblages within this lithostratigraphic unit are variable in the different samples, ranging between benthic, planktic and barren of diatoms. The upper part of the unit contains a higher concentration of benthic diatoms than the lower section (Tab. 1).

Lithostratigraphic Unit 6.2 (119.28 fo108.76 I I I ~ ~ ) , sandstone - This lithostratigraphic unit contains a poor assemblage of diatoms and fragments of diatoms, except for the sample at 11 8.56-1 18.57 mbsf, where planktic and tychoplanktic diatoms occur in higher abundance.

Lithostrc~tigraphic Unit 6.1 (108.76 to 103.41 mbsf), diamictite - Three samples from this lithostratigraphic unit contain only rare fragments of marine diatoms.

Lithostratigraphic Unit 5.8 (103.41 to 92.19 mbsf), mudstone - This lithostratigraphic unit contains a rich assemblage ofplanktic diatoms, although the lowest sample at 103.39-103.40 mbsf is nearly barren. An assemblage of well-preserved diatoms, with relatively abundant

5.3 62 11-62 12 <50 - strewn, sieve

62.76-62.78 <30 - strewn

63 14-63.16 <l0 - strewn

63 37-63.38 X - strewn

65 .80658 1 <l00 P+T strewn

66.16-66.17 >l00 P strewn; sieve

66.64-66.66 <l00 P+T strewn, sieve

5.4 67.42-67.44 <l00 P+T strewn 67 56-67.57 >l00 P strewn

169.73-69.74 l !: l ; 1;trewn: sieve

69.77-69.79 P+T strewn

70.23-70.25 4 0 0 P+T strewn, sieve 72.58-72.59 strewn

74.85-74 87 <l00 P+T strewn

74.91-74.92 >l00 strewn; sieve

82 38-82 40 1 <30 1 P lstrewn

83 24-83 26 1 <30 1 P lstrewn

81.39-81.41

81 41-81.42

81.97-81 99

87.06-87.08 strewn

strewn, sieve

strewn, sieve

922692.28 1 <l00 I P /strewn; sieve

927092.72 1 5 0 1 P lstrewn; sieve

<30

c30 c50

9 5 04-95.06 I <50 1 P lstrewn, sieve

P

P

P

5.8

strewn

114 76-114.77 < l0 strewn

115.95-115.96 <50 strewn

11648-11649 <l0 strewn: sieve

strewn

strewn

strewn; sieve

93 14-93 15 1 c50 1 P lstrewn

93 24-93.33 1 <50 1 P lstrewn

93.869390 1 1 0 0 1 P lstrewn; sieve

127 79-127 80 6 0 - strewn

127 79-12781 c30 - strewn

128 39-128.41 c50 P+B strewn, sieve 129 27-129 29 <50 P+B strewn: sieve

129.59-129.60 1 <50 [ B Istrewn 130.40-131 42 1 <50 1 P+B lstrewn: sieve

6.3 1131.41-131.43 l l p!B 131 67.1 31.68 strewn, sieve 13259-132 61 strewn

134 18-134.20 P+B strewn, sieve

134.30-134.31 <loo strewn, sieve

136.85-136.86 <30 - strewn

38.75-138.76 1 c50 1 - [strewn

39 77-139.78 1 <l0 1 - /strewn

141 52-141 54 1 <l0 1 - lstrewn 141.80-141.92 1 <l00 [ P+B FT. strewn, sieve; float

142.52-142.55 1 <50 1 P+B strewn, sieve 142.60-142.61 1 <50 1 - /strewn 142.85-142 91 1 <50 1 P+B lstrewn, sieve

1144.38-144.40 l d; l p;B ;;:m; 144 80-144.82 145.18-145.21 strewn

145.46-145.50 1 <l0 strewn

145.50-14551 <30 strewn

145.68-145.70 4 0 strewn; sieve

143.61-143.3 143.73-143.77

144.01-144.03

146.34-146 36 strewn

146 51-146.52 strewn; sieve, float

6 0

<30

<30

Note: abundance represents the number of diatom fragments observed in five fields-of-view at 250x. Diatom assemblages are classified into general ecological groups. where 'P' = planktic: ' T = tychoplanktic; and 'B' = benthic. Intervals lacking sufficient data for ecologic interpretation are represented by '-..

147 19-147.20 147 48-147.52

147 68-147.69

B - -

<50 1 P+B [strewn, sieve

<50 1 P+B lstrewn, sieve

<loo 1 P lstrewn; sieve

strewn; sieve

strewn

strewn

Fig. 1 - Summary log and diatom abundance for Miocene section of CRP-l. Relative diatom abundance is plotted in relation to lithologic descriptions. lithostratigraphic unit designations. and depositional interpretations. Black bars represent diatom occurrence with increasing abundance to the right. Five fields-of-view were observed (at 250xj in a raw strewn slide of each sample. Data were collected in six categories: barren (X). <10. <30. <50. <100. or >l00 diatom fragnients per five fields-of-view. Only samples equal to or exceeding <50 fra- "merits are depicted: lower abundance categories most likely represent intervals of diatom reworking. Intervals dominated by benthic diatoms are noted with a lower case "b."

Chaetoceros frustulcs, occurs at 9 1.22-9 1.23 rnhsl'. l , i t l io .~tr~i t i~~t~dphic Unit 5.7 (92. I9 to 81. I 0 tnl),y/),

siltstone and inndxione - This lithostratigraphir u n i t contains an assemblage of'planktic diatoms that t l c c ~ ~ i s e in abundance upwards.

Litl~ostrc~ti~y~~cipl~ic Unit 5.6 (81.16 to 78.<S.7 t i ~ / ) , y / ) ,

sandstoi~e and silt,stone - Three samples were cx;imiru'd from this lithostratigraphic unit. Planktic marine (lia~oms are present in relatively high abundance, but asc liiglily fragmented.

Lif l ios trat ig~l i ic Unit 5.5 (78.85 to 70.28 hf), sandstone - This lithostratigraphic unit contains ;I ric'li floraof planktic diatoms, which continues into the overlying lithostratigraphic unit. The base of the unit i l l 78.0 I - 78.62 mbsf was deposited under very shallow waler. us indicated by the presence of henthic diatoms and rare planktic diatoms. The assemblages are highly fragmented.

Lifliostratigraphic Unit 5.4 (70.28 to 63.20 m h f ) , sandstone - This lithostratigraphic unit contains a rich flora of planktic and tychoplanktic diatoms. The uppermost sample at 63.37-63.38 mbsf is barren of diatoms, except for rare diatom fragments and rare clasts of diatomaccous sediment. Authigenic minerals of dolomite, calcite or siderite are present along with fine grains of mudstonc.

Lifliostratigraphic Unit 5.3 (63.20 to 61.51 i f i f ) ,

diamictite - Four samples were examined from ihis lithostratigraphic unit. The diatoms are highly fragmcntcd and poorly preserved. Paralia, Stepl~anopyxis and the ebridian Pse~ic/a~~i~izodocl~iiim lingii are present. Pieces of diatomaceous sediment occur as small clasts in the sieved fractions, suggesting a source from existing sediment o f a contemporaneous age.

Lifhosfratigraphic Unit 5.2 (61.51 to 53.70 inlxf), interbeddedsiltstone, diamicfite andbreccia - T h e diatoms shift from a benthic assemblage at the base to one reflecting a deeper, open-water environment at 57.51-57.52 mbsi' (Tab. 1, Fig. 1). Water depth for the lower interval is estimated at 50 m or less in this environment, the approximate depth where the euphotic zone would illuminate the seafloor. Diatoms are sparse in the upper four samples of this lithostratigraphic unit.

Litl~ostratigraphic Unit 5.1 (53.70 to 43.55 M), sandstone and miidstone - This diatoms of this unit are predominantly fragmented and are thought to be recycled due to low abundance and poor preservation. One sample at 44.08-44.09 mbsf contains a sparse lower Miocene planktic diatom flora that is similar to assemblages present in lower levels of CRP- 1.

Tab. 2 - Diatom biostratigraphic datunis used in age assignment of the Miocene section of CRP-1.

Depth Range (mbsf) Datum Pub1 ishecl A se (Ma) Source

Absent Absent

59.58 - S8 75 88.81 - 91 22

103.39 - 102.24 146.79-146.83

Absent Absent

FAD Actinocyclus ingelis FAD Dentic~~lopsis ~naccollumii

LAD Thalassiosira praefraga LAD Asteromphalus syn~metricus

FCAD 'I'halassiosira praefraga LAD Cavitatus rectus

LAD Kisseleviella carina LAD Lisitzinia ornata

16.3 (,C5Cn.ln) 16.6 (C5Cn.3111

18.4-17.8 (C5En to CSDrj 18.5 (C5En)

20.3 or 20.6 (C6Nj -20.5

-25 (at 145 m in CIROS-l) 24.5 (C6Cr)

Note: ages are revised, by interpolation. to the Berggren et al. (1995) time scale. Datums for each taxon arc abbreviated as 'FAD' = First Appearance Datum. 'LAD' = Last Appearance Datum. and 'FCAD' = First Common Appearance Datum. Sources for age information are abbreviated as follows: Hc = Harwood (1989). H&M = Harwood & Maruyama (1992). H+ = Harwood et al. (1992), A+ = Akiba et al. (1993). and Y&A = Yanagisatva & Akiba (1998).

Lower Miocene Diatom Bioslr;iligr;ipliy ol' CKIJ I . MoVlui'tIo Sound 503

Present Absent

CRP- 1 ,X,. ,.,X-..

BIOGEOGRAPHIC CONSIDERATIONS

As with diatom assemblages and distributions from the CIROS-1 (Cenozoic Investigatons in the Ross Sea) and MSSTS-1 (McMurdo Sound Sediment and Tectonic Studies) drillcores, there is considerable difficulty in correlating Antarctic shelf sequences to established Southern Ocean diatom biostratigraphic schemes (Harwood & Maruyama, 1992). This is a likely result of local environmental effects such as temperature, salinity, turbidity, proximity to ice and ice cover, as well as regional palaeobiogeographic controls on diatom bio- provinces in the southern high latitudes. Such bio- geographic barriers appear to have limited the southward migration of Southern Ocean zonal taxa such as Lisitzinia ornofa, Rocella gelida, Azpeitia gombosi, Coscinodiscus rhombiciis, Rossiella s~miiietrica, Bogorovia spp., Tl~c~lassiosira sp~~~nellaroides, Hemiaulus tciz/rus and Ni~zschia maleinterpretaria (Harwood, 1991).

DIATOM BIOSTRATIGRAPHY

Important Miocene diatom datums recognised in CRP-l are listed in table 2 and highlighted in table 3.

Diatom occurrence and range in relation to a tentative zonal scheme are summarised in figure 2. These data bracket the interval recovered by CRP-1 and provide a means of con-elating CRP-l to other shelf sections in the Ross Embayment. Interpreted stratigraphic relationships between these sections, based on diatom biostratigraphy, are shown on figure 2. In some instances, the basis for determining the stratigraphic level of first and last diatom occurrences is not well-established at this time, nor is the sequence of diatom events known from a single continuous section. The sequence of datums will be established through future drilling. Ages listed below and marked by asterisk are revised to conform to the Berggren et al. (1995) time scale, as adjusted by Barron (pers. comm.). This recalibration accounts for the differences in cited ages between this paper and those of Baldauf & Barron (1991) and Harwood & Maruyama (1992).

The absence ofActinocycl~is ingens in CRP- 1 suggests an age older than 16.2* Ma within magnetic s~ibchron C5Cn. In (Ban-on & Baldauf, 1995) at 43.15 mbsf. The top of the lower Miocene section (at 43.15 mbsf) is older than 16.7* Ma based on the absence of Denficulopsis ~~~accol lumi i . This datum occurs within magnetic subchron C5Cn.3n in ODP Site 748 (Harwood et al., 1992), at an age of 16.7* Ma according to Berggren et al. (1995). Both of

D.M. l liirwood ct :d.

Tub. 3 - Miocene siliceous microfossil occurrence in CRP-1 - selcclnl l;ix;i.

these datums approximate the boundary between the early and middle Miocene. Furthermore, both of these taxa are well-documented in Ross Sea continental shelf sediments.

Similarly, the absence of Kisseleviella cariiza at the bottom of CRP-1 suggests an age younger than -25 Ma (the LAD of K. cariiza). Kisseleviella carina has a highest appearance in CIROS-1 at 145.21 mbsf. This datum suggests the lower Miocene CRP-1 section may be correlative with all or part of the upper -145 m of the CIROS- 1 drillcore.

The diatoms Tl~alassiosirafraga and T. praefraga are useful biostratigraphic markers for the lower Miocene. The reported occurrences of T. frasa in Antarctic diatom studies should be attributed to T. praefraga, as the Antarctic forms do not exhibit the distinctive marginal processes of T.fraga. It is possible that the presence of these processes

is either environmentally controlled, or that one form is the resting spore morphology. Alternatively, T.fraga may not range into the southern high latitudes and the occurrences there are exclusively T. pmefraga. Further study is warranted on Southern Ocean and Antarctic shelf deposits to document the biostratigraphic ranges and morphologic variation of these taxa. Until resolution of this situation, these diatoms are reported here as the T.fraga/T. praefraga complex. Gersonde (1 990) illustrated examples from this group in the Southern Ocean that have two marginal rings of strutted processes. which are denser in arrangement than that indicated in the emended description of T. fraga by Akiba & Yanagisawa (1986).

The range of the Tlzalassiosira fraga/T. praefraga complex (synonyms: Coscinodiscus sp. 1 of McCollum, 1975; Tlzalas.~iosirafraga of Gersonde & Burckle, 1990,

of Baldanl' A Barron, 199 1, of l-larwood et al., 1 O<S9b. ancl of Haswoo0 & Maruyama. 1992) provides two key hiostratigr:ipl~ic datums for CRP- 1 . An ;igc of 1 8.3* M;I is suggested lor depth 59.68 to 58.75 mhsf based on the I ,as[ Appearance Datum (LAD) of 77;(1/(1.~.sio,sira / ) /Y~( : / ; . ( I~ ( I . The age for this datum is derived from Yanagistiwa & Akiba(1008) fromtheNorth Pacific, yd they also indicate a discontinuous range for this diatom up to 17.3 Ma. Harwootl & Maruyama (1992) record this datum in the Southern Ocean at ODP Hole 75 1 A within {he lower part of magnetic subchron C5Dr (Harwood et al., 1992) with an age of 17.8 Ma according to Bcrggren et al. (1995). Other reports place this datum slightly older within the upper part ofC5En (Gersonde & Burckle, 1990; Barron & Baldauf. 1995). This datum defines the boundary between T. f r ~ s~ibzones "b" and "c" of Harwood & Maruyama (1 992).

The First Common Appearance Datum (FCAD) of T. praefraga (102.24 to 103.39 mbsf) is reported at 20.3 Ma (Yanagisawa & Akiba, 1998) from the North Pacific region. This agrees with afirst occurrence in the lower part of magnetic subchron C6n or the upper part of C6r from Antarctic drillholes (Gersonde & Burckle. 1991; Baldauf & Barron, 199 1 ; Harwood & Maruyania, 1992; Harwood et al., 1992). We reinterpret the magnetostratigraphic interpretations of Harwood et al. (1992) for ODP Sites 747 and 748 guided by the above ages for T. pmefragt; (T. fraga). We use the published age of 20.3 Ma for the first common appearance datum of T. praeficlgc~. In CRP- 1, it first appears as a common element of the diatom assemblage of lithostratigraphic Unit 5.8, at 102.49 mbsf, above lithostratigraphic Unit 6.1, a diamictite, which is barren of diatoms. The presence of the Unit 6.1 diamictite probably causes a truncation of the lower range of T. praefraea, as well as other diatoms characteristic of this interval, such as Fmgilariopsis sp. A. This implies that the age for the FCAD of T. praefraga is younger in CRP-1 than 20.3 Ma.

The last occurrence of Cavitatus rectus is a useful datum for the lower Miocene of Japan (Akiba et al., 1993), where a zone is proposed based on this datum at -20.5 Ma, but at a stratigaphic level beneath the highest occurrence of Kisseleviella carina. Yanigasawa & Akiba (1998) reduce this datum to a secondary event at 20.7 Ma, but still below the highest occurrence of Kisseleviella carina. The stratigraphic relationship of these two taxa appears to be reversed both in the CRP-1 and CIROS-1 drillcores, where the last occurrence of Kisseleviella carina is below the last occurrence of Cavitat~is rectus.

A tentative diatom zone for the Ross Sea is suggested for the interval below 141.80 mbsf in CRP-1 (Fig. 2). Three zones are bounded by diatom events and zones proposedfor the Antarctic continental shelf from drillcores (Harwood, 1986; Harwood et al., 1989a) and reworked sediment clasts (Harwood et al., 1989b; Scherer, 1992). Many, but not all of the selected diatomevents are supported by data from the Southern Ocean. Two of the bio- stratigraphic zones proposed here are interval zones, and this is not optimal. A zonation based on first occurrences is more desirable and will develop as more is known of the ranges of lower Miocene taxa. A working zonation is

CA VITA W S RECHJS PARTIAL RANGE ZONE

/J(I.s~J. Not defined :it this time. biit i t lies at or above the K i s s e l ~ ~ ~ ~ i e l l ( ~ cciriiui Zone of Harwood ( 1986).

Top. l iighest occurrence of C(~vitatus rectus. Age raiige in W - 1 . This zone ranges u p to

146.79 mbsl'which is assigned an age of -2 1 .S Ma (Roberts et al., this volume).

STEPHAN0IÈYX/, SPINOLs"IL$IMA INTERVAL ZONE

Base in CRP-1. Highest occurrence of Cavitatus recfus. Top in CRP-1. Lowest occurrence of the Tli~~lassiosim

fraga/praefraga complex. Age range in CRP-1. This zone ranges from -21 .S Ma

at 146.79 mbsf up to - 20 Ma at 102.25 mbsf (Roberts et al., this volume). This age range is based on theoccurrence of diatom index taxa in sediments above and below the interval zone, which itself contains few calibrated age markers. TheS. spinosissi~~~a Interval Zone is characterized by heavily silicified neritic diatoms including S. .~pi~iosissiim. It is unlikely that this zone can be used for correlation well beyond CRP-1.

THALASSZOSZRA PRAEFRAGA RANGE ZONE

Base. Lowest occurrence of Tlzalassiosira praefraga. Top. Highest occurrence of Thalassiosira praefraga. Age range in CRP-1. This zone ranges from - 20 Ma

at 102.25 mbsfto between 18.0 and 18.8 Ma at 58.75 mbsf (Roberts et al., this volume); the highest occurrence of the T. fraga/praefiaga complex occurs in an interval of reversed polarity (58-65 mbsf) in magnetozone R l .

Discussion. Harwood & Maruyama (1992) report the highest occurrence of T. praefraga in the lower part of subchron C5Dr but other reports place this datum slightly older in the upper part of C5En (Gersonde & B~~rckle , 1990; Barron & Baldauf, 1995).

The interval between 58.75 and 43.15 mbsf is not zoned at present.

CORRELATION TO OTHER DIATOM- BEARING ANTARCTIC SHELF SEQUENCES

ROSS ICE SHELF PROJECT, SITE J/9 (RISP Jl9)

The diatom assemblages from CRP- 1 resemble diatom floras recovered from diatomaceous sediment clasts within the RISP Site J-9 (82O22'S; 68O38'W) cores beneath the Ross Ice Shelf (Harwood et al., 1989b; Scherer, 1992). Lower Miocene sediment clasts contain between 72 to

506 D.M. I fill-wood c l i l l .

92% diatoms. with little terrigenoiis debris, rcl'leetiq the widespread distribution of diatoms duringtthc early Miocene in the Ross Sea. Diatoms were clearly abundant at this time around the Antarctic margin. yet little is known about these asseniblages because lower Miocene reference sections are limited.

DliEP SEA DRILLING PROJECT (DSDP) LHG 28

McCollum (1 975) described many of the diatom taxa encountered in the present study from strata recovered during DSDP Leg 28. Steinhauff et al. (1987) correlated the drillcores from Site 270 and 272, noting differences in the diatom assemblages, specifically the presence of Kisseleviella cari17a in DSDP Site 270 and its absence in DSDP Site 272.

A gap of 3 my is inferred between these sites within the early Miocene (Steinhauff et al.. 1987). Only one sample examined from DSDP Site 270. Core Interval 13-3, 1 10- 1 12 cm, contained diatoms (Steinliauffet al., 1987) due to diagenetic alteration of diatomaccous scdimcnts to opal C-T and opal Q (chest) in other intervals. The intervals in DSDP Site 272 that are correlative to the lower Miocene of CRP-1 are similarly altered to opal C-T in Unit 2B and to opal Q in Unit 2C (Hayes et al., 1975). The distribution of Kisseleviella cari77a in Core Section 13-3 of DSDP Site 270 and its absence in both DSDP Site 272 and CRP-l indicate the gap between the DSDP drillholes may be represented, in part, within CRP-l . The range of Tlialassiosira praefraga enables a good correlation between DSDP Hole 272 (Cores 20 to 29) (Savage & Ciesielski. 1983) and CRP- 1 (- 102 to 59 mbsf).

The lower Miocene interval in CRP-1 is probably above the interval recovered by the MSSTS-1 drillcore. as indicated by the absence of diatoms Pterotlzecc~ reticulata, Kisseleviella cc1ri17a and Lisiizi17ia ornata (Fig. 2).

CIROS- 1

The diatom record from the upper -145 m of the CIROS- 1 drillcore (Harwood. 1989) is similar to the lower Miocene interval of CRP-l. Both of these intervals are stratigraphically above the highest occurrence of Kisseleviella carina and Pterotheca reficulafa. Re- examination of the diatom record from the upper Oli- 0-ocene and lower Miocene interval of CIROS-1 is needed, preferably in parallel with the detailed documentation of the diatom floras from CRP- 1. A younger age of the upper 145 m of CIROS-1 than inferred by Reick (1989) and Harwood et al. (1989a) will resolve the disagreement in age of diatoms datums noted between the MSSTS-1 and CIROS-l drillcores. An age of - 18.6- 19.4 Ma based on strontium isotope ratios from biogenic carbonate from -45 mbsf in CIROS-1 (M. Lavelle, pers coni~n.) initially suggested the younger age and potential for overlap between CRP- 1 and upper CIROS-1 drillcore.

CONCLUSIONS

The clociiment;ition of diatom occiirrcnc'c and abundance is presented here to aid c ~ i v i r o n ~ ~ ~ c n ~ a l reconstruction tliroi.igIi (he lower Miocene interval of CRP- 1 . The diatoms reflect variation in environiiicnt through ice cover to open sea. Variation in iliatoin cibi~ndanceand asse~i~blagccomposition through ~Iic lo\vi:r Miocene reflects changes in depositional envimiimi~iit. Several intervals of repeated ice cover (floiiliiii; or grounded) are interpreted for the lower Miocene section of CRP- 1, wherediatom production was limited, ancl intci'v;ils where diatoms were growing in open. although ;I[ tinu's shallow ( 4 0 m), marine seas.

Several biostratigraphic events provide initial aye control for this drillcore, which is now dated by correlation to the magnetic polarity time scale. ^SrIs6Sr ratios from mollusk shells and alAr/'^Ar data from volcanic m;itcri;ils (Roberts et al., this volume). Together, these rccorJs indicate an age span from to -2 1 .S Ma at 147.69 mhsl'dhe bottom of the hole) to -17.5 Ma at 43.15 mbsf (top ol'the lower Miocene interval). Important diatom datunis identified in this drillcore are the FAD and L A D ol' the ThaIassiosirafrap/praefrc~gc~ complex and the LAD of Cavitatus rectus. Detailed documentation of the diatom assemblages in the future, guided by this age model, will enable the CRP-1 sequence to stand as a reference section for the lower Miocene of the Antarctic continental shelf

ACKNOWLEDGEMENTS

The CRP-1 drillhole was drilled as a cooperative research program of six nations (Australia. Germany, Italy, New Zealand. United States, United Kingdom) to address theclimatic, tectonic, palaeobiologic and geologic history of the Western Ross Sea, Victoria Land Basin and the Transantarctic Mountains. This report on the diatoms was prepared through funds from a U.S. National Science Foundation grant OPP-9420062 to Harwood and Scherer. The authors acknowledge the efforts of the CRP International Steering Committee in organising this programme. J. Ban-on, R. Gersonde and A. McMinn provided valuable comments to improve this paper.

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Plate 1 -Diatoms from CRP-l drillcore (scale bar equals 1 0 uin). 1 & 2) Asferoinphalns sp. aff. A. symmefriciis. 99.02 mbsf: 3 & 4) Thalassiosira fraga/praefraga complex. 99.02 mbsf; 5 & 6 ) Thalnssiosira iiameiiii. (5) 59.58 mbsf and (6) 99.02 mbsf: 7) H~alodiscus sp. A. 141.80 mbsf: 8) Melosira sp.. 141.80 mbsf: 9) Porosira sp. A. 141.80 mbsf: 10) Parcdia clai,igera. 53.50 mbsf: 11) Paralia so! var. iiiarfiiialis. 59.58 mbsf: 12) Coscinodiscus ociiloide~. 53.50 mbsf: 13) Coscinodisciis sp. A. 99.02 mbsf.

Plate 2 - Diatoms from CRP-1 drillcore (scale bar equals l 0 pm). 1-3) Genus and species uncertain A. 59.58 mbsf: 4) Cbaetoreros setae, 99.02 mbsf: 5) Xanthiopyxis sp. A. 99.02 mbsf: 6 & 7) Sfephanopvxis r111.1.i~. 53.50 inbsf: 8) Stephanopxxi.~ sp. resting spore. 99.02 mbsf: 9) Stephanopyxis sp. A. 53.50 mbsf: 10) Stepliciiuipy.\.is sp.. 59.58 mbsf: 1 1 ) S~eplimopv.\-;,S sp. B . 59.58 mbsf: 12 & 1 3 ) Stephanopyxis sp. C. 59.58 mbsf: 14) Pse~idainmodocl~i~iin 1i11,yii (ebridian). 99.02 mbsf.

Plate 3 -Diatoms from CRP-l drillcore (scale bar equals 10 ~ m ) . 1 & 2) Cocconeis sp. cf. C. cmtiquiis var. temiisfriata. 141.80 mbsf: 3) Cocconeis sp,. 53.50 mbsf: 4) Cocconeis sp.. 146.51 mbsf; 5 ) Cocconeis sp.. 146.51 mbsf: 6 ) Cocconeis sp.. 141.80 mbsf: 7 ) Diploneis sp.. 141.80 mbsf: 8) Graniiiicitophora marina. 14 1.80 mbsf: 9) Rhahdo~~eiiia japonicagro~~p. 99.02 n~bsf: 10) Rhahdonemn sp. B . l 4 l .SOmhsf:Il) Rhabdoiieniasp.. 99.02 mbsf: 12) Grammafophora charcofii. 99.02 mbsf: 13) Rhbdonema sp.. 99.02 mbsf: 14) Rhabdonemajaponica. 141.80 mbsf: 15) Slieslnikoria sp.. 99.02 mbsf: 16) R I ~ ~ ~ l ~ ~ l o n e m a sp. cf. R. ele,qcins. 59.58 mbsf: 17) Enfopvia sp. girdle band. 53.50 mbsf: 18) R h i ~ s o l e n i a sp. B.. 141.80 nlbsf:

h r Inllo\ving list i,s prcsc~i~led ;is ;I pitide 111 i l l ~ ~ s ~ i i i t i o ~ i s n l l l i r ~ l i i i l o ~ ~ i s l i~escnf in ( '1<11 I ;iiiil h i Iin"biions \vlicti.' l o i iw i l i s l s 111 s ~ i ~ o n ~ i i i s t ,111 I I~, IIIIIII~I. l " ' i i ; i f io i~s; i~c i~ i prc-l'er~'iirt' In! o r c ~ ~ r r e ~ ~ c c ~ I'IOIII \v i ! ln i~ 11n' Soi111n~i11 ( t m ~ i i ;ind 111c Koss I~ I I I~~; I~ I I~~I I I . I ' ~ ~ l i l ~ ~ ~ ; i ~ i o ~ i s o l ;"n~;iirs! IIW 1111 ilit. ii11 I I I I~ I I .iiii~ii

o f A ~ i l i i r r ~ i c lower Miocrnr ( l i i i~oms ;lrc MeCol l i i~ i i i D I S ) . Sr11iiicl1'1 ( Did), Sc11i:ulrr fv I ' r i~ i ic i I D / d ) . (ioiii l i i is 1 10 / I). Wri iv f i c<Â ( i t > i i i l i i i ~ 1 D K I 1.

( tomhos c<: ( ' i r l s i r l ~ k i (lciS.i). II;ir\vo~i(l ( IOSh). l l ; i ~wo ix l ( IOW). l l i i i w o i i ~ l c¥ ill. 1 ll.W)li). ( i r i s i i i ~ ~ l c ,Q H i m ~ I J ~ ~ i I1~YOl, I{ : ik l ; i i~l 'A I t . i i n n i I I 'D1 I. .snd 1 I;II\\'oo(! c<: ~ i i r l l y i ~ i l ~ i ~ (I1)02j. ~,OWIT M i o c t ~ ~ e ~i i ; i loin floiiis I'iotii l!u* N ~ ~ \ v r y i ; t i i Sr;i itic siiiii1;ii 111 Ilu' A~~ i i i i t ' l i c Iliii;is, !<t ' \ ' i \" f i i r i t i I '. l i ~ i i n Ill is ir?,ioii ;in- Srlir;~cler i 1 I'CIIIIC~~ ( 11J7d). I"ci i i~ci i D I S I . ; i i i ~ l D/i iui i ill/(. rl ill. (107S~.

A cli~i~plvcfiiis ~t'fiiiriiis (I~II'II~IcT~~) K l ~ i e ~ ~ l i i . ~ ~ ~ ~ . : l I ~ ~ I I ~ C V , IOCi4, p, 0.5, plalt- ?.i. I'igircs I ;nil1 2: K;inaya, 105'1, 11. OS. phile l. l i ; ~ i i i c ' I 17.

Ci~cciinei.~ costafa group Gregory 1857: Krcbs, 1083, 1). 285, plate l , figure 8; I-larwoocl. 1986, p. 85. plate 6. figures 5, 10. I I and I 6.

Cocconeis sp. A of Harwood. 1986, p. 85. plate 6, figure 6 Parallel (Kllerheckia) clavigera Grunow in Van l leurck: Woni;ir(l~. 1967, p. 15, figures 1 am1 2: l larwoocl. 1986, p. 86. plate 2. f i ~ i r r 1 l (Plate 1 . 10). Coccomi.s sp. D o f Hai-wood, 1986. p. 85, plate 6. figure 0

Coccoiieis spp. (Plate 3.3-6) I'arolia sol var. marginalis Pcragallo, 192 1 . p. 90. p l i ~ t c 5. f ip i rc .? ( P l i l t ~ 1.1 1).

Corelliron sp.. i ~ i e l ~ i d i n g spines

Coscinoclisciisociiloides Karsten, 1905, p. 81 ;Van I-leurck, 1909. p. 49. plate 12, figure 167 (Plate 1.12).

Coscinodisciisocz~lusiridiis (Bailey, 1856)Cleve. 1883. p. 147: Harwood. 1986, p. 85, plate 2, figures 1-3.

Cosci~wdisciis radiatzis Ehrenberg. 1839. p. 18., plate 3, figure 1 a-c. Porosira sp. A. (Plate 1.9)

Coscinodiscus sp. A o f Harwood. 1986, p. 85, plate 5. figures 1, 7 and 8 (Plate 1.13).

Proboscis praebarboi (Schracler) Jordan & Priddle, 199 1 . p. 57. Rhizosole~iiaprae/~~~rboi Schrader. 1973b. p. 709-7 10. plate 24. figures 1-3: Gombos & Ciesielski. 1983, p. 604. plate 24, figure 10: Akiba & Yanagisawa. 1986, p. 497, plate 42. figures 8 and 9: plate 43, figures 1-0. Cymatosira bilzarensis Pantocsek: Harwood, 1989, p. 78, plate 4.

figure 33, Rhabdonema japonicum group Tempere & Brun: Harwood. 1986. p. 86. plate 7, figure 3. (Plate 3.9 & 14). Ductyliosolen anfarcticiis Castracane: Haiwood. 1986. p. 85, plate 5.

f'hiila,s,vio.sir~ iniii,s('fni Sc.lirit~i, 111 S rh r i r i inul I ' \ t~q. IOOfi, p. SO, j~l:iti.' -1 , l i j ~ i i i r s I S. l ' l n~{<s \ \~ t~ \ i i ' i i iil'l. i i r ~ ~ , ; i i l i i / ~ i Srl i i i tdi ,~~. in l lii iwooil ri ill,, IOSOli. [il;ili'4, I l > ' l l l l ~ . I . (I'lilh' I.."!<' 11).

Stepliaiio11)wi.s sp. C. (Plate 2.12 & 13)

S/~p/iuiiofiyxi.s 1urri.s group (Grevillc & Arnott) Rails. in Pri~chaixl. I S6 l : Schr;i(ler XL Fciincr. 1976. p. 1000, plate 30. figures 1 - 10 :iiul 14: plate 37. figiires 17-1 0.

I'.seiidainii~o~locliiiiin liiigii Bohiity & I larwood. i n press. pliite 5. figure I : plate 8. figures 1 - 10. / ' . s~~~ i i i i i i i ~ i~ iodoc /~ i i im sp. CS, I-'. dic/yoidi's Hovasse, Ling. 1984. figure 2. # S . ##6 and #13: I-larwood. 1986. p. 87. 1)l;itc 2. figircs 16 and 17; Harwoocl c l al., 198911, p. 82. plate 4. Siguic 13. (Plate 2,14).

Slictodisciis I i ~ i r ~ ~ i i ~ ~ ~ i ~ i ~ ~ i z i ~ s Grcvillc: Ilarwood. 1986. p. 87, 1)liite l . figures 8 am1 0: I larwood, 1989. p. 8 1 , plate 1 . figure A: Desikachary & Sreclatha. 19SO. p. 236. plate 102. figures 6-7. plate 104. figures 2,4,6-7,

"Tigeria"group. Not Ibrinally named to date, but likely represents an u~~clescribctl gcnus. WC herein usean informal generic nameof"'I'i,qci-ici" o r forms previously referred to as S y e d r a / Frqi l~ ir ic i sp. A o f I larwood. 1989. p . 81 .. Synedra*? sp. 1 BI-ady i n Harwood et al.. 19891).

I'scii~l~~minodochium sphericurn Hovasse. 1932, p. 463. figure 16: Perch-Nielsen. 1975. p. 881. plate I. figures 17 and 18; Bohaty & Harwood. in press, plate 5. figure 6: plate 10. figures 7- 8.