167 Prospectus


The sites proposed for drilling form three transects cutting across the California Current system at right angles to the coast and one long transect from north to south along the coast. The three east-west transects cut across the California Current at about 40°N (the Gorda Transect), at 35°N (the Conception Transect), and at about 30°N (the Baja Transect). These transects will reconstruct past offshore gradients of upwelling intensity in northern (summer upwelling), central (year-round upwelling, cool), and southern (year-round upwelling, warm) regimes. The depth range of the Gorda Transect, from ~1000 to ~4200 m, is ideal for the reconstruction of past deep-water properties. Similarly, a group of sites in central and southern California form another depth transect covering much the same depth range. The north-south "Coastal" Transect will connect the three offshore transects to assess latitudinal translation of upwelling cells. Very high sedimentation rates along the margin will allow study of oceanographic variations at century-millenial scales, as well as at the lower resolution orbital and tectonic scales. The coastal transect contains sites through the California Borderlands and will address high resolution climate changes in the Southern California countercurrent, evolution of intermediate water properties on a productive continental margin, and sedimentation in anoxic to suboxic basins.

All sites will be triple-piston cored; two APCs will go to refusal, whereas the third will be taken to at least 50 m (further, if time is available). Extended core barrel (XCB) drilling will extend the drill holes to their total planned depth. Deep-water sites on each of the transects and two sites on the continental slope will be drilled to basement, which ranges between 200 and 450 m below seafloor (bsf). Basement will be sampled at all sites that reach it using the motor-driven core barrel (MDCB), if appropriate. This tool should collect a core about 4.5 m long in hard rock. Many sites in the coastal transect will be shallow, only to APC refusal or slightly deeper, to obtain the geographic coverage that is necessary to better understand the Pleistocene and late Pliocene climatic interval. We have also chosen four of the coastal sites to penetrate to at least 400 m, or to basement, to have shallow-water-depth drill sites for the important late and middle Miocene time intervals.

Gorda Transect

The Gorda Transect (Fig. 1) consists of the following Sites: CA-1, CA-2, CA-4, and CA-5; and the following alternates, in order of priority: Sites CA-6, CA-3, and CA-7. All sites will be important for the upper Neogene transect, because Site CA-1 should penetrate sediments with an age of ~1(2.5 Ma, Site CA-4 will sample sediments to ~5.1 Ma, Site CA-6 will penetrate to ~7.7 Ma, Site CA-3 to ~2.8 Ma, and Site CA-7 should sample sediments with a maximum age of 1(2.5 Ma. Site CA-2 (1951 m, Delgada slope) and Site CA-5 (4242 m, Delgada fan) will be important for understanding the complete Neogene, provided that enough time is available for the deeper drilling objectives at each site; both sites should have a mostly continuous record to about 30 Ma. To fit within the time constraints of Leg 167, drilling lower middle Miocene through Oligocene sediments at both Sites CA-2 and CA-5 has been assigned a second priority status, and is not in the current drilling plan. Despite its shallow depth, Site CA-2 most probably resides upon the Pacific plate, so Sites CA-2 and CA-5 have also moved in tandem, northward along the California margin.

The sampling of gas hydrates is a primary objective at Site CA-1. The gas-hydrate interval should be located above a bottom-simulating reflector at about 160 mbsf. In addition to standard APC operations, the pressure-core sampler may be deployed if conditions are appropriate.

Conception Transect

The Conception Transect (Fig. 1) consists of the deep-water Site CA-11 (3873 m, 22.5 Ma crust) and two shallow Sites CA-9 (Santa Lucia slope; 974 m, ~3(4 Ma at total depth) and CA-15 (Tanner Basin; 1182 m, 4(8 Ma at total depth). The two shallow sites are in a deformed region of the North American continental margin, and Site CA-11 has moved northward with respect to them since the middle Miocene. We hope to recover a high-resolution record in the Santa Lucia slope region (Site CA-9), near the important upwelling center at Point Arguello. We believe that we will get an older sedimentary record from Tanner Basin (Site CA-15) to compare with Site CA-11.

Baja Transect

The deep-water Site CA-14 (3495 m) will be compared to a margin site in Animal Basin (Site CAM-2; 2047 m; Fig. 1). Both sites should contain middle Miocene sediments, however, Site CAM-2 probably contains a significant hiatus. Nevertheless, we should be able to compare a coastal and deep-water record for much of the interval from the beginning of the late Miocene to the Holocene. Both sites will be drilled to basement.

Coastal Transect

The shallow sites from other transects (Sites CA-1, CA-2, CA-9, CA-15, and CAM-2) make up a major part of the coastal transect. In addition, an important site on the central California margin (Site CA-8; 2508 m) and a series of shallow drill sites in the California Borderlands basins of southern California (Fig. 1) will be drilled to complete the regional coverage. Site CA-8 will sample a high-resolution sedimentary record that should span into the upper Miocene and that will tie the northern and southern drilling sites together. The proposed California Borderland sites (Sites BA-1, East Cortez Basin; BA-2, San Nicolas Basin; and BA-4, Santa Monica Basin) and alternate (Site CAM-3; San Clemente Basin) are all shallow, designed to sample only Pleistocene sediments. They will provide important information on the evolution of intermediate waters and the development of upwelling centers in this region. They will also be important for understanding organic carbon diagenesis and tectonic evolution of the California margin. Provided that important safety concerns can be met, a return to the location of ODP Site 893 in the Santa Barbara Basin is an objective of Leg 167. If we can redrill Site 893, we will gather important ancillary data on pore-water chemistry and sediment physical properties that were lost in the first drilling effort as well as provide new sediments for analysis.

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