Preliminary results and documentation of sediment cores CRP2 and CRP-2A from the Ross Sea off Cape Roberts, Antarctica

The site for CRP-2, 14 km east of Cape Roberts (77.006°S; 163.719°E), was selected to overlap the early Miocene strata cored in nearby CRP-1, and to sample deeper into the east-dipping strata near the western margin ofe he Victoria Land Basin to investigate Palaeogene climatic and tectonic history. CRP-2 was cored from 5 to 57 mbsf (metres below the sea floor) (core recovery 91 %), with a deviation resulting in CRP-2A being cored at the same site. CRP-2A reached down to 624mbsf (recovery 95%), and to strata with an age of c. 33-35 Ma. Drilling took place from 16 October to 25 November 1998, on 2.0-2.2 m of sea ice and through 178 m of water. Core fractures and other physical properties, such as sonic velocity, density and magnetic susceptibility, were measured throughout the core. Down-hole logs for these and other properties were run from 63 to 167 mbsf and subsequently from 200 to 623 mbsf, although density and velocity data could be obtained only to 440 mbsf because of hole collapse. Sonic velocity averages c. 2.0 km S-1 for the upper part of the hole, but there is an sharp increase to c. 3.0 km s-1 and also a slight angular unconformity, at 306 mbsf, corresponding most likely to the early/late Oligocene boundary (c. 28-30 Ma). Velocity then increases irregularly to around 3.6 km s-1 at the bottom of the hole, which is estimated to lie 120 m above the V4/V5 boundary. The higher velocities below 306 mbsf probably reflect more extensive carbonate and common pyrite cementation, in patches, nodules, bedding-parallel masses and as vein infills. Dip of the strata also increases down-hole from 3° in the upper 300 in to over 10° at the bottom. Temperature gradient is 21° k-1. Over 2 000 fractures were logged through the hole. Borehole televiewer imagery was obtained for the interval from 200 to 440 mbsf to orient the fractures for stress field analysis. Lithostratigraphical descriptions on a scale of 1:20 are presented for the full length of the core, along with core box images, as a 200 page supplement to this issue. The hole initially passed through a layer of muddy gravel to 5.5 mbsf (Lithological Sub-Unit or LSU 1.1), and then into a Quaternary diatom-bearing clast-rich diamicton to 21 mbsf (LSU 2. l), with an interval of alternating compact diamicton and loose sand, and containing a rich Pliocene foraminiferal fauna, to 27 mbsf (LSU 2.2). The unit beneath this (LSU 3.1) has similar physical properties (sonic velocity, porosity, magnetic susceptibility) and includes diamictites of similar character to those of LSU 2.1 and 2.2, but an early Miocene (c. 19 Ma) diatom assemblage at 28 mbsf (top of LSU 3.1) shows that this sub-unit is part of the older section. The strata beneath 27 mbsf, primary target for the project, extend from early Miocene to perhaps latest Eocene age, and are largely cyclic glacimarine nearshore to offshore sediments. They are described as 41 lithological sub-units and interpreted in terms of 12 recurrent lithofacies. These are 1) mudstone, 2) inter-stratified mudstone and sandstone, 3) muddy very fine to coarse sandstone, 4) well-sorted stratified fine sandstone, 5) moderately to well-sorted, medium-grained sandstone, 6) stratified diamictite, 7) massive diamictite, 8) rhythmically inter-stratified sandstone and mudstone, 9) clast-supported conglomerate, 10) matrix-supported conglomerate, 11) mudstone breccia and 12) volcaniclastic sediment. Sequence stratigraphical analysis has identified 22 unconformity-bounded depositional sequences in pre- Pliocene strata. They typically comprise a four-part architecture involving, in ascending order, 1) a sharp-based coarse-grained unit (Facies 6,7,9 or 10), 2) a fining-upward succession of sandstones (Facies 3 and 4), 3) a mudstone interval (Facies l), in some cases coarsening upward to muddy sandstones (Facies 3), and 4) a sharp-based sandstone dominated succession (mainly Facies 4). The cyclicity recorded by the strata is interpreted in terms of a glacier ice margin retreating and advancing from land to the west, and of rises and falls in sea level. Analysis of sequence periodicity awaits afirmer chronology. However, apreliminary spectral analysis of magnetic susceptibility for a deepwater mudstone within one of the sequences (from 339 to 347 mbsf) reveals ratios between hierarchical levels that are similar to those of the three Milankovitch orbital forcing periodicities. The strata contain a wide range of fossils, the most abundant being marine diatoms. These commonly form up to 5% of the sediment, though in places the core is barren (notably between 300 and 412 mbsf). Fifty samples out of 250 reviewed were studied in detail. The assemblages define ten biostratigraphical zones, some of them based on local or as yet undescribed forms. The assemblages are neritic, and largely planktonic, suggesting that the sea floor was mostly below the photic zone throughout deposition of the corcd sequence. Calcareous nannofossils, representing incursions of ocean surface waters, are much less common (72 out of 183 samples examined) and restricted to mudstone intervals a few tens of metres thick, but are important for dating. Foraminifera are also sparse (73 out of 135 samples) and represented only by calcareous benthic species. Changing assemblages indicate a shift from inshore environments in the early Oligocenc to outer shelf in the late Oligocenc, returning to inshore in the early Miocene. Marine palynomorplis yielded large numbers of well-preserved forms from most of the 116 samples examined. The new in situ assemblagc found last year in CRP-1 is extended down into the late Oligocene and a further new assemblage is found in the early Oligoccnc. Many taxa are new, and cannot us yet contribute to an improved understanding of chronology or ecology. Marine invertebrate macrofossils, mostly molluscs and serpulid tubes, are scattered throughout the core. Preservation is good in mudstones but poor in other lithologies. Climate on land is reflected in the content of terrestrial palynomorphs, which are extremely scarce down to c. 300 mbsf. Some forms are reworked, and others represent a low growing sparse tundra with at least one species of Nothofagus. Beneath this level, a significantly greater diversity and abundance suggests a milder climate and a low diversity woody vegetation in the early Oligocene, but still far short of the richness found in known Eocene strata of the region. Sedimentary facies in the oldest strata also suggest a milder climate in the oldest strata cored, with indications of substantial glacial melt-water discharges, but are typical of a coldcr climate in late Oligocene and early Miocene times. Clast analyses from diamictites reveal weak to random fabrics, suggesting either lack of ice-contact deposition or post-depositional modification, but periods when ice grounded at the drill site are inferred from thin zones of in-situ brecciated rock and soft-sediment folding. These are more common above c. 300 mbsf, perhaps reflecting more extensive glacial advances during deposition of those strata. Erosion of the adjacent Transantarctic Mountains through Jurassic basalt and dolerite-intruded Beacon strata into basement rocks beneath is recorded by petrographical studies of clast and sand grain assemblages. Core below 310 mbsf contains a dominance of fine-grained Jurassic dolerite and basalt fragments along with Beacon-derived coal debris and rounded quartz grains, whereas the strata above this level have a much higher proportion of basement derived granitoids, implying that the large areas of the adjacent mountains had been eroded to basement by the end of the early Oligocene. There is little indication of rift-related volcanism below 310 mbsf. Above this, however, basaltic and trachytic tephras are common, especially from 280 to 200 mbsf, from 150 to 46 mbsf, and in Pliocene LSU 2.2 from 21 to 27 mbsf. The largest volcanic eruptions generated layers of coarse (up to 1 cm) trachytic pumice lapilli between 97 and 114 mbsf. The thickest of these (1.2 m at 112 mbsf) may have produced an eruptive column extending tens of km into the stratosphere. A source within a few tens of km of the drill site is considered most likely. Present age estimates for the pre-Pliocene sequence are based mainly on biostratigraphy (using mainly marine diatoms and to a lesser extent calcareous nannofossils), with the age of the tephra from 112 to 114 mbsf (21.44k0.05 Ma from 84 crystals by Ar-Ar) as a key reference point. Although there are varied and well-preserved microfossil assemblages through most of the sequence (notably of diatoms and marine palynomorphs), they comprise largely taxa either known only locally or as yet undescribed. In addition, sequence stratigraphical analysis and features in the core itself indicate numerous disconformities. The present estimate from diatom assemblages is that the interval from 27 to 130 mbsf is early Miocene in age (c. 19 to 23.5 Ma), consistent with the Ar-Ar age from 112 to 114 mbsf. Diatom assemblages also indicate that the late Oligocene epoch extends from c. 130 to 307 mbsf, which is supported by late Oligocene nannofossils from 130 to 185 mbsf. Strata from 307 to 412 mbsf have no age-diagnostic assemblages, but below this early Oligocene diatoms and nannofossils have been recovered. A nannoflora at the bottom of the hole is consistent with an earliest Oligocene or latest Eocene age. Magnetostratigraphical studies based on about 1000 samples, 700 of which have so far undergone demagnetisation treatment, have provided a polarity stratigraphy of 12 pre-Pliocene magnetozones. Samples above 270 mbsf are of consistently high quality. Below this, magnetic behaviour is more variable. A preliminary age-depth plot using the Magnetic Polarity Time Scale (MPTS) and constrained by biostratigraphical data suggests that episodes of relatively rapid sedimentation took place at CRP-2 during Oligocene times (c. 100 m/My), but that more than half of the record was lost in a few major and many minor disconformities. Age estimates from Sr isotopes in shell debris and further tephra dating are expected to lead to a better comparison with the MPTS. CRP-2/2A has recorded a history of subsidence of the Victoria Land Basin margin that is similar to that found in CIROS-170 km to the south, reflecting stability in both basin and the adjacent mountains in late Cenozoic times, but with slow net accumulation in the middle Cenozoic. The climatic indicators from both drill holes show a similar correspondence, indicating polar conditions for the Quaternary but with sub-polar conditions in the early Miocene-late Oligocene and indications of warmer conditions still in the early Oligocene. Correlation between the CRP-2A core and seismic records shows that seismic units V3 and V4, both widespread in the Victoria Land Basin, represent a period of fluctuating ice margins and glacimarine sedimentation. The next drill hole, CRP-3, is expected to core deep into V5 and extend this record of climate and tectonics still further back in time.

Clay mineralogy of Cape Roberts sediment cores

The clay mineral assemblages of the ca. 1600 m thick Cenozoic sedimentary succession recovered at the CRP-1, CRP-2/2A and CRP-3 drill sites off Cape Roberts on the McMurdo Sound shelf, Antarctica, were analysed in order to reconstruct the palaeoclimate and the glacial history of this part of Antarctica. The sequence can be subdivided into seven clay mineral units that reflect the transition from humid to subpolar and polar conditions. Unit I (35-33.6 Ma) is characterised by an almost monomineralic assemblage consisting of well crystalline, authigenic smectite, and therefore does not allow a palaeoclimatic reconstruction. Unit II (33.6-33.1 Ma) has also a monomineralic clay mineral composition. However, the assemblage consists of variably crystallized smectite that, at least in part, is of detrital origin and indicates chemical weathering under a humid climate. The main source area for the clays was in the Transantarctic Mountains. Minor amounts of illite and chlorite appear for the first time in Unit III (33.1-31 Ma) and suggest subordinate physical weathering. The sediments of Unit IV (31-30.5 Ma) have strongly variable smectite and illite concentrations indicating an alternation of chemical weathering periods and physical weathering periods. Unit V (30.5-24.2 Ma) shows a further shift towards physical weathering. Unit VI (24.2-18.5 Ma) indicates strong physical weathering under a cold climate with persistent and intense illite formation. Unit VII (18.5 Ma to present) documents an additional input of smectite derived from the McMurdo Volcanic Group in the south.

Chronostratigraphy on sediment core CRP-2 from the Ross Sea, Antarctica

Strontium isotope stratigraphy was used to date 16 discrete horizons within the CRP-2/2A drillhole. Reworked Quaternary (<1.7 Ma) and possible Pliocene (<2.4 Ma) sediments overlie a major sequence boundary at 25.92 meters below sea floor (mbsf). This hiatus is estimated to account for c. 16 Myr of missing section. Early Miocene to ?earliest Oligocene (c. 18.6 to >31 Ma) deposits below this boundary were cut by multiple erosion surfaces of uncertain duration. Strontium isotope ages are combined with 40Ar/39Ar dates, diatom and calcareous nannofossil datum and a palaeomagnetic polarity zonation, to produce an age model for the core.

Grain size analysis of sediment coring site CRP-2 from the Ross Sea, Antarctica

The purpose of this note is to present results of grain size analyses from 118 samples of the CRP-2/2A core using sieve and Sedigraph techniques. The samples were selected to represent the range of facies encountered, and tend to become more widely spaced with depth. Fifteen came from the upper 27 m of Quaternary and Pliocene sediments, 62 from the early Miocene-late Oligocene strata (27 to 307 mbsf), and 41 from the early Oligocene strata beneath (307 to 624 mbsf). The results are intended to provide reference data for lithological descriptions in the core logs (Cape Roberts Science Team, 1999), and to help with facies interpretation. The analytical technique used for determining size frequency of the sand fraction in our samples (sieving) is simple, physical and widely practised for over a century. Thus it provides a useful reference point for analyses produced by other faster and more sophisticated techniques, such as the Malvern laser particle size analysis system (Woolfe et al., 2000), and estimates derived from measurements taken with down-hole logging tools (Bücker, pers. com., 1999).

Geochemistry on smectites in sediment cores CRP-1 and CRP-2 from the Ross Sea, Antarctica

TEM (transmission electron microscopy) observations and microanalyses on smectite microparticles in the sediments of the CRP-2A core were carried out to determine their origin (authigenic or detrital) and the source rocks. Smectites are dioctahedral and are Fe-rich members of the nontronite-beidellite series. They generally display both flaky and hairy shapes, but no large compositional difference between the two forms was observed. Flaky smectites are detrital while hairy smectites probably formed in situ through the reorganisation of previous flaky particles. The source rocks for smectites are probably represented by the McMurdo Volcanic Group to the south, but also by the Ferrar Dolerites and Kirkpatrick Basalts in the Transantarctic Mountains. CRP-2A smectites are Fe and Mg richer than those of the coeval or not coeval levels of the CIROS-I, DSDP 270 and 274 cores. The average compositions of smectite in CRP-1 and CRP-2A cores show a downcore trend toward more alluminiferous terms, which might reflect the increase of the chemical weathering processes on the continent.

Environmental analysis on sediment core CRP-2 from the Ross Sea, Antarctica

Cape Roberts Project drill core 2/2A was obtained from Roberts Ridge, a sea-floor high located at 77° S, 16 km offshore from Cape Roberts in western McMurdo Sound, Antarctica. The recovered core is about 624 m long and includes strata dated as being Quaternary, Pliocene, Miocene and Oligocene in age. The core includes twelve facies commonly occurring in associations that are repeated in particular sequences throughout the core and which are interpreted as representing different depositional environments through time. Depositional systems inferred to be represented in the succession include: outer shelf with minor iceberg influence, outer shelf-inner shelf-nearshore to shoreface under iceberg influence, deltaic and/or grounding-line fan, and ice proximal-ice marginal-subglacial (mass flow/rainout diamictite/subglacial till) singly or in combination. Changes in palaeoenvironmental interpretations up the core are used to estimate relative glacial proximity to the site through time. These inferred glacial fluctuations are then compared with the global eustatic sea level and d18O curves to evaluate the potential of glacial fluctuations on Antarctica influencing these records of global change. Although the comparisons are tentative at present, the records do have similarities, but there are also some differences especially in possible number (and perhaps magnitude) of glacial fluctuations that require further evaluation.

Geochemical analysis of minerals from sediment core CRP-2 in the Ross Sea, Antarctica

Distribution patterns, petrography, whole-rock and mineral chemistry, and shape and fabric data are described for the most representative basement lithologies occurring as clasts (granule to bolder grain-size class) from the 625 m deep CRP-2/2A drillcore. A major change in the distribution pattern of the clast types occurs at c. 310 mbsf., with granitoid-dominated clasts above and mainly dolerite clasts below; moreover, compositional and modal data suggest a further division into seven main detrital assemblages or petrofacies. In spite of this variability, most granitoid pebbles consist of either pink or grey biotite±hornblende monzogranites. Other less common and ubiquitous lithologies include biotite syenogranite, biotite-hornblende granodiorite, tonalite, monzogranitic porphyries (very common below 310 mbsf), microgranite, and subordinately, monzogabbro, Ca-silicate rocks, biotite-clinozoisite schist and biotite orthogneiss (restricted to the pre-Pliocene strata). The ubiquitous occurrence of biotite±hornblende monzogranite pebbles in both the Quaternary-Pliocene and Miocene-Oligocene sections, apparently reflects the dominance of these lithologies in the onshore basement, and particularly in the Cambro-Ordovician Granite Harbour Igneous Complex which forms the most extensive outcrop in southern Victoria Land. The petrographical features of the other CRP-2/2A pebble lithologies are consistent with a supply dominantly from areas of the Transantarctic Mountains facing the CRP-2/2A site, and they thus provide further evidence of a local provenance for the supply of basement clasts to the CRP-2/2A sedimentary strata.

Cenozoic terrestrial palynomorphs from the late Oligocene to early Miocene section of sediment core CRP-2/2A (Table 1)

Sparse terrestrial palynomorphs (spores and pollen) were recovered from glacigene Lower Miocene and Oligocene core samples from the Cape Roberts Project (CRP) drillhole CRP-2/2A, Victoria Land Basin, Antarctica. Rarity of palynomorphs probably results from the spares periglacial vegetation in the surrounding landscape at the time of deposition, as well as dilution from rapid sediment accumulation. The Miocene and Late Oligocene vegetation is interpreted as including herb-moss tundra with low-growing woody plants (including Nothofagus and podocarp conifers) in more protected areas, similar to that encountered in the Miocene of CRP-1. Species richness and numbers of specimens increase downhole, a trend that begins very gradually below ~307 mbsf, and increases below ~443 mbsf through the Early Oligocene. These lower assemblages reflect low diversity woody vegetation dominated by several species of Nofhofagus and podocarps, growing in somewhat milder conditions, though still cold temperate to periglacial in the Early Oligocene. The CRP-2/2A core provides new biostratigraphical information, such as the First Appearance Datums (FADS) of Tricolpites sp. a near the Oligocene/Miocene boundary, and Marchantiaceae in the Early/Late Oligocene transition: these are taxa that along with N. lachlaniae, Coptospora spp. and Podocarpidites sp.b characterize assemblages recovered from outcrops of the Pliocene Sirius Group in the Transantarctic Mountains. Some elements of the extremely hardy periglacial tundra vegetation that survived in Antarctica into the Pliocene had their origin in the Oligocene during a time of deteriorating (colder, drier) climatic conditions. The CRP results highlight the long persistence of this tundra vegetation, through approximately 30 million years of dynamically changing climatic conditions. Rare Jurassic and more common Permian-Triassic spores and pollen occur sporadically throughout the core. These are derived from Jurassic Ferrar Group sediments, and from the Permian-Triassic Victoria Group, upper Beacon Supergroup. Higher frequencies of reworked Beacon palynomorphs and coaly organic matter below ~307 mbsf indicate greater erosion of the Beacon Supergroup for this lower part of the core. A color range from black, severely metamorphosed specimens, to light-colored, yellow (indicating low thermal alteration), reworked Permian palynomorphs, indicates local provenance in the dolerite-intruded Beacon strata of the Transantarctic Mountains, as well as areas (now sub-ice) of Beacon strata with little or no associated dolerite well inland (cratonwards) of the present Transantarctic Mountains.

Investigation of microfossils on sediment core CRP-2 from the Ross Sea, Antarctica

Marine diatoms are the primary biostratigraphical and paleoenvironmental tool for interpreting the upper Palaeogene and lower Neogene strata recovered during the second drilling season of the Cape Roberts Project at site CRP-2 in the western Ross Sea, Antarctica. Silicoflagellates, ebridians, and a chrysophyte cyst provide supporting biostratigraphical information. More than 100 dominantly planktic diatom taxa are recognised. Of these, more than 30 are treated informally, pending SEM examination and formal description. Many other taxa are noted only to generic level. Lower Oligocene (c. 31 Ma) through lower Miocene (c. 18.5 Ma) diatoms occur from 28 mbsf down to 565 mbsf. Below this level, to the bottom of the hole at 624.15 mbsf, diatom assemblages are poorly-preserved and many samples are barren. A biostratigraphic zonal framework, consisting of ten diatom zones, is proposed for the Antarctic continental shelf. Ages inferred from the diatom biostratigraphy correspond well with geochronological data from argon dating of volcanic materials and strontium dating of calcareous macrofossils, as well as nannofossil biochronological datums. The biochronostratigraphical record from CRP-2/2A provides an important record of diatom events and mid-Cenozoic environmental changes in the Antarctic neritic zone.