9. Site 1131¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1131 is located on the uppermost slope, adjacent to the Eucla Shelf, in 332.4 m of water (Fig. F1). Site 1131, together with the shallower Site 1129 and deeper Site 1127, form a depth transect designed to sample an impressive set of prograding sigmoidal clinoforms constituting Sequence 2 as defined by Feary and James (1998, reprinted as Chap. 2) and projected to be of Pliocene-Pleistocene age (Fig. F2). This seismic sequence forms a thin succession over the outer shelf (70-90 m), reaches peak thickness at the present shelf edge (350-550 m), and thins as a wedge farther seaward beneath the modern slope. The sequence occurs over the entire width of the Eucla Basin (~350 km) and reaches its thickest extent in the vicinity of Site 1131. This site, together with adjacent sites on this transect (Sites 1127 and 1129), offers the opportunity to develop an in-depth understanding of shelf edge and upper slope depositional and progradational processes. Complex reflection onlap and erosional truncation patterns within the clinoform package reflect hiatus or erosional episodes. Site 1131 was located to intersect an expanded record of the middle part of this clinoform sequence, as compared to Sites 1127 (younger part) and 1129 (older part).

In addition, seismic data indicated the presence of mounded seismic facies that have been interpreted as possible deep-water biogenic features (Feary and James, 1995, 1998 [reprinted as Chap. 2]). Site 1131 was designed to intersect a laterally restricted zone of mounds possessing distinct bathymetric and subsurface seismic expression to determine the extent of biogenic contribution and the factors controlling mound development.

The principal objective at these sites was to collect a transect of detailed high-resolution profiles through an upper Neogene shelf edge (high energy) to upper slope (low energy) succession deposited within a cool-water carbonate environment and to determine the response of such a depositional system to Pliocene-Pleistocene sea-level fluctuations.

Additional objectives were to

1. Obtain a high-resolution record of upper Neogene paleoceanographic variation within an upper slope setting as a component of the shelf-to-basin paleoceanographic transect;
2. Evaluate the diagenetic history of calcitic sediments that have accumulated in an environment at or near wave base; and
3. Characterize fluid circulation and the heat-flow regime within uppermost slope/shelf edge sediments.

In addition to providing a detailed understanding of seismic Sequence 2 depositional dynamics at Site 1131, we also sought to recover an incomplete Sequence 3 record before reaching the target depth in Sequence 4.