10. Site 1132¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1132 is located on the Great Australian Bight upper slope in 218.3 m of water (Fig. F1). The primary objective at Site 1132 was to obtain a detailed record of Neogene shelf edge and upper slope cool-water carbonate deposition and, in particular, to intersect the thin proximal portion (to contrast with the distal section at Site 1130) of an inferred Paleocene-middle Eocene progradational siliciclastic wedge (Fig. F2) identified and mapped as seismic Sequence 7 (Feary and James, 1998, reprinted as Chap. 2). This east-west-oriented, elongate sediment body increases in thickness seaward to 230 m and then abruptly downlaps onto the underlying Cenomanian/lower Cenozoic unconformity. Sequence 7 extends at least 300 km along the Eucla Basin, seaward of a prominent basement high (see "Seismic Stratigraphy").

Site 1132 was projected to intersect seismic Sequences 2, 3, 4, 6A, 6B, and 7 to recover a record of Neogene cool-water carbonate deposition punctuated by hiatuses representing both sequence boundaries and intrasequence disconformities. The Sequence 2 interval at Site 1132 offered the opportunity to examine along-slope variations in sedimentation rates and depositional facies by comparison of this site with the much thicker Sequence 2 succession intersected at Sites 1127, 1129, and 1131 to the east. Similarly, comparison with the Sequence 2 interval intersected at Site 1130 on the uppermost slope provided an opportunity to describe downslope facies variability. Site 1132 was expected to intersect the most complete section through Sequence 3 of any of the Leg 182 drill sites to characterize the facies deposited in the interpreted aggradational shelf succession (Feary and James, 1998, reprinted as Chap. 2). Seismic data (Fig. F2) show that a thin unit containing mounds immediately overlies the Sequence 7 progradational wedge at Site 1132. This interval was mapped as the distal edge of Sequence 6B, interpreted as an Eocene-middle Miocene interval predominantly deposited on the shelf, but with a thinning wedge containing mounds extending into deeper water (Feary and James, 1998, reprinted as Chap. 2). Site 1132 was located to intersect one of these small Sequence 6B mounds.

The scientific objectives for Site 1132 were to

1. Recover a detailed record of shelf edge siliciclastic deposition at a proximal site, evaluate the sedimentary response to Paleogene sea-level fluctuations, and evaluate the complex interaction between sea-level variation, accommodation space, and subsi-dence;
2. Determine the characteristics of cool-water carbonate facies within the Neogene/Quaternary succession;
3. Determine paleoceanographic parameters within a shelf edge setting in Sequences 2 to 4 to complement other components of the shelf-to-basin transect;
4. Evaluate sea-level control on Neogene facies within an upper slope/shelf edge setting; and
5. Evaluate the diagenetic history and processes within Neogene facies in an upper slope/shelf edge setting.