ALBERT

Beschreibung: Twenty four core samples from CRP-1, seven from Quaternary strata (20-43.55 meters below sea floor or mbsf) and seventeen from early Miocene strata (43.55 to 147.69 mbsf), have been analysed for their grain-size distribution using standard sieve and Sedigraph techniques. The results are in good agreement with estimates of texture made as part of the visual core description for the 1 :20 core logs for CRP-1 (Cape Roberts Science Team, 1998). Interpretation of the analyses presented here takes into account the likely setting of the site in Quaternary times as it is today, with CRP-1 high on the landward flank of a well-defined submarine ridge rising several hundred metres above basins on either side. In contrast, seismic geometries for strata deposited in early Miocene times indicate a generally planar sea floor dipping gently seaward. Fossils from these strata indicate shallow water depths (〈 100 m), indicating the possibility that waves and tidal currents may have influenced sea floor sediments. The sediments analysed here are considered in terms of 3 textural facies: diamict, mud (silt and clay) and sand. Most of the Quaternary section but only 30% of the early Miocene section is diamict, a poorly sorted mixture of sand and mud with scattered clasts, indicating little wave or current influence on its texture. Although not definitive, diamict textures and other features suggest that the sediment originated as basal glacial debris but has been subsequently modified by minor winnowing, consistent with the field interpretation of this facies as ice-proximal and distal glaciomarine sediment. Sediments deposited directly from glacier ice appear to be lacking. Mud facies sediments, which comprise only 10% of the Quaternary section but a third of the early Miocene section, were deposited below wave base and largely from suspension, and show features (described elsewhere in this volume) indicative of the influence of both glacial and sediment gravity flow processes. Sand facies sediments have a considerable proportion of mud, normally more than 20%, but a well-sorted fine-very fine sand fraction. In the context of the early Miocene coastal setting we interpret these sediments as shoreface sands close to wave base.

Beschreibung: The relationship between whole-core compressional wave velocities and gamma-ray attenuation porosities of sediments cored at CRP-1 is examined and compared with results from core-plug samples and global models. Both core-plug and whole-core velocities show a strong dependence on porosity: this relationship appears to be independent of lithology. In the range from 0.1 to 0.4 of fractional porosity (Miocene strata), plug velocities are generally 0.2 - 0.5 km s-1 higher than whole-core velocities. Possible reasons include decreased rigidity in the whole core and diagenetic changes in the plugs. Possibly both velocity measurements are correct but neither is fully representative for in situ conditions. It appears that the core-plug results are more compatible with data from other regions than the whole-core data. After removing first-order compaction control from the whole-core porosity record, a second-order control by clay content can be quantified as a simple positive linear regression (R=0.6). In contrast, after correction for first-order control, porosity and velocity are not significantly influenced by lonestone abundance except for rare, very large lonestones.

Beschreibung: Heavy minerals from the lower Miocene and Quaternary sediments of the CRP-1 drillcore have been investigated in order to characterise the mineralogical composition of the sediments and to reconstruct their source areas. The sediments are dominated by very high concentrations of pyroxenes. Hornblende, stable heavy minerals (zircon, garnet, epidote. titanite and apatite), opaque and altered minerals are quantitatively minor. The downcore distribution of the pyroxenes indicates 5 cycles. Each cycle starts with diamictites and proximal glacimarine sediments containing maximum concentrations of pyroxenes of up to 80% in the heavy mineral fraction, and grades into distal glacimarine sediments, with a lower pyroxene content. The heavy minerals indicate a mixed source for all CRP-1 sediments. The proximal sediments consist mainly of minerals derived from a crystalline basement and clastic sedimentary rocks, such as are widespread in the Transantarctic Mountains. However, they also contain a minor component derived from volcanic rocks. The distal sediments, in contrast, are dominated by a volcanic source, which was probably situated to the south of the drillsite, in the region of the present-day Ross Ice Shelf.

Beschreibung: Clay mineral assemblages in lower Miocene and Quaternary sediments recovered at the CRP-1 drillsite, McMurdo Sound shelf, Antarctica, were analysed in order to reconstruct source areas of the sediments and late Cenozoic Antarctic icedynamics. The assemblages are dominated by smectite, illite and chlorite. Kaolinite occurs in trace amounts only. The lower part of the drillcore, below c. 70 mbsf, is characterised by high illite and chlorite, but low smectite concentrations. This assemblage is typical of a source consisting mainly of granitic and metamorphic rocks, such as are widespread in the Transantarctic Mountains and on the East Antarctic craton. In contrast, the upper c. 70 mbsf of the core is characterised by three intervals with high smectite, but low illite and chlorite concentrations at around 59, 45 and 33 mbsf. The smectite-rich assemblage is typical of a hinterland composed of volcanic rocks, such as are found in the region of the present-day Ross Ice Shelf. The clay minerals are mainly detrital and therefore document the changing influence of different source areas through time. Correlation of the clay mineral record of CRP-1 with that of the CIROS-1 and MSSTS-1 cores implies that the oldest sediments at CRP-1 are younger than 21.8 Ma, and that there is no overlap between CRP-l and the other cores.

Beschreibung: The mineralogy of the lower Miocene and Quaternary sediments of the drillcore CRP-1 (McMurdo Sound, Ross Sea, Antarctica) has been analysed using the X-ray diffraction method. Quartz, plagioclase feldspars, K-feldspars are the most important non-clay minerals. Amphiboles occur throughout the core in minor amounts. The composition of the sediments points to an origin in the Transantarctic Mountains for the majority of the detrital components. There, the plutonic and metamorphic rocks of the basement, the sediments of the Beacon Supergroup and the volcanic rocks of the Ferrar Dolerite could serve as possible source lithologies. The quartz abundance is strongly linked to the gain size of the sediments with maxima correlating with coarse gain sizes. The downcore distribution of the other detrital minerals is relatively invariable, and does not indicate a major change in the source area during the time represented by the CRP-1 sediments. Some diagenetic alteration of the sediments is indicated by the occurrence of minor amounts of opal-CT and by some clinoptilolite below110 m depth.

Beschreibung: The CIROS-1 drillhole, which in 1986 reached a depth of 700 m below the seafloor, is still the only deep hole that can provide information on the velocity structure of the upper crust in McMurdo Sound and the Ross Sea, Antarctica. A careful review and quality control of the downhole logging data of CIROS-1 resulted in a new porosity depth function that is consistent with porosity data from the MSSTS-1 and CRP-1 drillholes. Using existing porosity-velocity equations, it was possible for the first time to obtain reliable velocity information for the upper 700 m of strata off the Victoria Land coast. The calculated synthetic seismograms, based on downhole velocity and density data, fit very well with the existing seismic lines IT90A-71, PD90-12, and NBP9601-89. The quality of the correlation confirms that the average velocity of the top 700 m of strata is about 2 000-2 300 m/s, and not 2 800-3 000 m/s, as was previously assumed. In consequence, these distinctly lower velocities result in shallower depths for the seismic unconformities V3/V4 andV4/V5 and thus may have important implications for further drilling off Cape Roberts.