Physical properties of sediment core CRP-3

Drillhole CRP-3 in northern McMurdo Sound (Ross Sea, Antarctica) targeted the western margin of the Victoria Land basin to investigate Neogene to Palaeogene climate and tectonic history by obtaining continuous core and downhole logs. Well logging of CRP-3 has provided a complete and comprehensive data set of in situ geophysical measurements down to nearly the bottom of the hole (920 m below sea floor (mbsf)).
This paper describes the evaluation and interpretation of the downhole logging data using the multivariate statistical methods of factor and cluster analysis. The factor logs mirror the basic geological controls (i.e., grain size, porosity, clay mineralogy) behind the measured geophysical properties, thereby making them easier to interpret geologically. Cluster analysis of the logs delineates individual logging or sedimentological units with similar downhole geophysical properties. These objectively and independently defined units help differentiate lithological and sedimentological characteristics (e.g. grain size, sediment provenance, glacial influence).
For CRP-3, the three factor logs derived from the downhole measurements reflect sediment grain size (proxy for lithology), the occurrence of diamict and/or conglomerate (glacial influence), and sediment provenance. It is possible to divide the borehole into three main sections on the basis of cluster analysis of the three factor logs. The top section down to about 200 mbsf is dominated by mudstone with clearly different physical properties from the mudstones occurring below this depth. Beneath 200 mbsf sandstones dominate the lithology. Two types of sandstones were characterised in the early Oligocene/late Eocene sequence, with a division between the two types occurring at about 630 tnbsf. These two types of sandstones, which are differentiated mainly on the basis of their magnetic and radiogenic properties, can be correlated by detrital mode provenance analysis. Comparison of the results of the factor and cluster analyses with the reflection seismic profiles shows that the major change in sediment source from the Victoria Group to the Taylor Group at 630 mbsf (earliest Oligocene) is not seen by seismic sequence analysis. This observation may have important consequences for the entire Ross Sea seismic stratigraphy.