The record of paleoenvironmental change at the ANDRILL McMurdo Ice Shelf site, Antarctica, from downhole measurements
|Location||International Geological Congress,oslo 2008|
|Author||Williams, Trevor۱; Morin, Roger۲; Jackolski, Chris۳; Jarrard, Richard۴; Henrys, Stuart۵; Niessen, Frank۶; Magens, Diana۶; Powell, Ross۳|
|Holding Date||08 October 2008|
The ANDRILL McMurdo Ice Shelf (MIS) project drilled 1285 metres of sediment representing the last 13 million years of glacial history. Downhole geophysical logs are complementary to data acquired from the drill core itself, and we describe here the application of the logs to understanding paleoenvironmental change at MIS. Prior to interpretation, a 1.6% depth correction was applied to the depth values of the downhole logs to bring them in line with the core depths, and natural gamma radiation logs were stacked and corrected for signal attenuation through the drill pipe. Natural gamma and temperature logs cover the whole interval from the sea floor to 1018 metres; magnetic susceptibility, electrical resistivity, borehole images, and other logs covered the open-hole intervals between 692-1018 and 237-342 metres.
The downhole logs show a clear correspondence to lithology. The natural gamma radiation logs have high values in the sub-glacial diamict facies and drop to low values in the open-water diatomite facies. Similarly, low resistivity values indicate the relatively high porosity of diatomite, and high magnetic susceptibility indicates the high terrigenous input in the diamict, particularly from magnetite-rich formations such as the Ferrar dolerite. Although the core recovery at MIS was excellent (~98% recovery), nevertheless some small intervals were not recovered and the downhole logs provide the only means to fill in the missing lithology. In addition to showing broad-scale sedimentary patterns, the logs (like the core physical properties) can also be used to give information on subtle lithological trends and features that are difficult to quantify from visual inspection of the core itself. Examples include varying terrigenous content of diatomites and varying degrees of cementation. Cluster analysis and factor analysis of the downhole logs add a further dimension, bringing out features that are less apparent in any one log taken singly. Overall, downhole logs provide a valuable contribution to understanding the paleoenvironmental record at the McMurdo Ice Shelf.