7. Site 1129¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1129 is located on the uppermost slope/shelf edge adjacent to the Eucla Shelf in 202.5 m of water (Fig. F1). It was the shallowest, most proximal site of a depth transect of three sites (including intermediate-depth Site 1131 and deepest Site 1127). Site 1129 was designed to sample an impressive set of prograding sigmoidal clinoforms constituting seismic Sequence 2 as defined by Feary and James (1998, reprinted as Chap. 2) and projected to be Pliocene-Pleistocene in age (Fig. F2). Sequence 2 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 (see "Seismic Stratigraphy" in the "Site 1127" chapter). The sequence spans the entire width of the Eucla Basin (~350 km) and 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 1129 was located to intersect an expanded record of the oldest part of this clinoform sequence, as compared to Sites 1127 (younger part) and 1131 (middle part).

In addition, seismic data show 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 1129 was designed to intersect a thick interval of possible mounds at the top of the clinoform sequence, to determine the extent of biogenic contribution and to determine the factors controlling mound development. Seismic data show that the zone containing possible mounds extends deeper in the section beneath the outer shelf (see "Seismic Stratigraphy" in the "Site 1127" chapter), indicating that such mounds have been a characteristic feature of the shelf edge and uppermost slope throughout the interval represented by Sequence 2.

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, 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 history within an uppermost slope/shelf edge 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 that may have been near, at, or above 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 1129, we also sought to recover an incomplete Sequence 3 record before drilling to the target depth within Sequence 4.