Core- and log-based geophysical investigations of neogene deposition on continental margins of the Southern Ocean.

摘要

Geophysical studies were undertaken during two Ocean Drilling Program cruises in and around the Southern Ocean. Log data were collected during Legs 181 and 188, and rock-magnetic data were collected from samples taken during Leg 181.; A combination of downhole logging and continuous-core geophysical measurements was used to create synthetic seismograms to ground-truth seismic profiles recorded in Prydz Bay, Antarctica, and to derive high-resolution mass accumulation rate (MAR) curves of Neogene deposition around the margins of the Southern Ocean.; The synthetic seismograms identify the causes of several major regional reflectors in the Prydz Bay region by tying the reflectors to observed petrophysical changes in holes drilled on the continental shelf and rise. The MAR curves document changes in the extent of Antarctic glacial ice and its relationship to deep current activity in the surrounding Southern Ocean. Also recorded in the MAR curves are movements of oceanographic fronts, the northward drift of Australia and Tasmania from Antarctica, changes in the climate of New Zealand and the desertification of Australia.; Rock-magnetic models were created with synthetic-magnetite hysteresis data. They suggest that for broad lognormal grain size distributions, centered around the pseudo-single domain size range, changes in hysteresis parameters are not a reliable indicator of changes in mean magnetic grain size. Extending this concept to the use of relative magnetic grain size as a proxy for relative terrigenous grain size, natural sediments tend to have lognormal grain size distributions sufficiently broad that this is an issue. Therefore, hysteresis parameters are a problematical proxy for relative changes in mean terrigenous grain size. Parameters sensitive to the abundance of magnetic material, however, are good proxies for terrigenous concentration. These predictions were explored (1) by Leg 181 studies, (2) by combined rock-magnetic measurements, statistical analysis of wireline logs, and conventional grain size analyses, and (3) by evaluation of all three as indicators of terrigenous grain size and abundance.; The results were used to document changes in the intensity of the Deep Western Boundary Current east of New Zealand, expansion of the East Antarctic Ice Sheet and acceleration of the tectonic uplift of the Southern Alps.