Densities, porosities, and seismic velocities of some rocks from Victoria Land, Antarctica

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Abstract
Density, porosity, and compressional wave velocity have been measured in the laboratory for 75 samples representing the major rock units of Victoria Land and the adjacent sea floor. Granitic and metamorphic rocks from the Late Precambrian—Early Paleozoic basement show significant differences in saturated density (2⋅66 and 2⋅77 Mg m−3) and velocity (4⋅7 and 5⋅3 km s−1). Low rank metasedimentary and sedimentary rocks from north Victoria Land give intermediate values. Field-measured velocities in granite are O: 9 km s−1 higher than laboratory velocities for surface granitic rocks, but are in the same range as velocities from granite cored below 156 m at DVDP Site 6. The basement rocks are overlain by the Beacon Supergroup, a flat-lying sandstone sequence up to 2 km thick and of Devonian to Triassic age. The physical properties of Beacon samples show a wide range (density 2·22-2·61 Mg m−3; porosity 6–21%; velocity 2·2–4·4 km s−1); there appears to be no consistent relationship between these properties and lithology or stratigraphic position. The Beacon Supergroup is intruded by extensive sills of Jurassic dolerite 30 to 300 m thick, and these are much more consistent in density and velocity (2·90Mg m·−3 and 5·8 km s−1). Field measurement, however, indicates a velocity 1·1 km s−1 higher at depth. Because no other major known rock type in the region has a higher velocity, Beacon and older rocks beneath the uppermost major dolerite sill will be inaccessible by seismic refraction techniques. Early Cenozoic sediments are known mainly from erratics around McMurdo Sound, but have recently been found at DSDP Site 270, 400 km to the east. They have a shallow marine character and are typically calcareous. The range in density (2·40-2·71 Mg m−3) and velocity (3·0–4·2 km s−1) overlaps considerably with the Beacon samples. The widespread Late Cenozoic sediments beneath the Ross Sea, which are largely pebbly mudstone of glacial origin, are significantly different (1·68–2·12 Mg m−3 and 1·6-2·6 km s−1) from the older pre-glacial strata. The McMurdo Volcanic Group includes Late Cenozoic basaltic and intermediate rocks, which at DVDP Site 3 have densities and velocities of 2·77 and 2·21 Mg m−3, and 4·6 and 2·6 km s−1 for a flow-dominated and a pyroclastic sequence respectively. These data are consistent with Robinson's suggestion of a 2 km-thick layer of volcanic ash overlying basalt beneath the sea floor south and west of Hut Point. However, farther out in McMurdo Sound the velocity gradient beneath the sea floor is steep, and the upper kilometre of strata there is believed to include both Early and Late Cenozoic sedimentary rocks.