Site 741

Abstract

Principal results: Ocean Drilling Program (ODP) Site 741 is situated in the inner part of Prydz Bay, on the shelf of East Antarctica (68°23.16'S, 76°23.02'E, 551.4 m water depth). Below a 24-m-thick cover of probable glacial deposits, approximately 100 m of sediments was sampled. The sediments form part of an approximately 2-km-thick, slightly tilted sequence resting on the basement at Prydz Bay. The sampled interval is stratigraphically placed at about 1500 m above the stratigraphic level of the top of the preglacial sediments in Hole 740A, which is about 600 m above the surface of basement. The main objective of coring was to examine the upper part of the sequence, especially any marine intervals, in order to (1) explain the development of the Lambert Graben, a structure that has been linked to the rifting between the Indian subcontinental block and the Antarctic continent and (2) determine the hitherto unknown preglacial conditions of the area. Drilling began on 28 January 1988. Based on the experience at Site 740, rotary drilling was used. Drilling terminated at a final depth of 128.1 m below seafloor (mbsf) because no preglacial marine sediment was found. The core recovery was generally poor (average 26.3%), but 5 of 14 cores had less than 6% recovery. The sediments encountered at Site 741 can be divided into four lithologic units. Unit I (0-0.66 mbsf) consists of olive-gray diatom ooze with minor quartz silt, presumably of Holocene age. Unit I shows a sharp color contrast to Unit II (0.66-4.1 mbsf), which is a silty clay containing up to 1% gravel and 20% diatoms; the proportion of diatoms decreases with depth. Unit III (4.1-24.05 mbsf) consists of fragments of gneiss up to 12 cm in length. Core recovery within this interval is poor; thus, the thickness and overall composition of Unit HI is unknown. Unit IV (24.05-128.1 mbsf) consists