The 13 Leg 175 drill sites (Fig. 1) encompass a variety of oceanic settings and passive-margin depositional environments (see Pufahl et al., Chap. 18 and Wefer et al., Chap. 16, both this volume). The amounts and types of gases found in the sediments from the various locations are influenced by the differences among the sites. The 13 drill sites may be grouped into four sedimentary regimes, as follows.
Drilling at three drill sites on the lower continental margin explored the Quaternary history of fan-margin sedimentation north of the Congo River Canyon. This depositional regime is located under a neritic environment dominated by the freshwater contributions of the Congo River, seasonal coastal upwelling, and incursions of open-ocean water from the South Equatorial Countercurrent. The three drill sites are close to areas of commercial petroleum exploration, and drilling at these sites was limited to 200 meters below sea-floor (mbsf) because of safety concerns. Predrilling seismic surveys detected evidence of gas chimneys, which indicate free gas, and abundant bottom-simulating reflectors (BSRs) in the regions of the three Lower Congo Basin drill sites.
The three sites constitute a depth transect that provides a composite picture of hemipelagic sedimentation across this margin (Fig. 1). Site 1075 (4°47′S, 10°05′E) is located in 2995 m water depth. Sediments at this site consist of Pliocene–Pleistocene greenish gray diatomaceous clay and nannofossil-bearing diatomaceous clay having an average sedimentation rate of 100 m/m.y. Sediments from Site 1077 (5°11′S, 10°26′E; water depth 2392 m) are composed of intercalations of greenish gray diatom-rich, diatom-bearing, nannofossil-bearing, and nannofossil-rich clay. Site 1076 (5°04′S, 11°06′E) is situated in 1404 m water. Its sediments consist of Pleistocene diatom-bearing olive-gray clay and greenish gray clay that accumulated at rates that varied between 50 and 200 m/m.y. Sediments from all three sites are bioturbated and contain an average of 2.5 wt% organic carbon.
Two drill sites on the upper continental margin of Angola provide a history of upper Quaternary sedimentation in a setting that is free of strong fluvial or upwelling influences. The moderately productive coastal environment of this depositional regime contrasts with the more productive Lower Congo Basin sites and to the upwelling-dominated sites south of the Angola-Benguela Front. The Angola margin drill sites are close to areas of commercial petroleum production, and drilling at these sites was limited to 200 and 120 mbsf because of safety concerns. Predrilling seismic surveys detected evidence of gas chimneys and of abundant BSRs in the regions of the two drill sites.
The two sites describe a transect across the upper continental slope (Fig. 1). Site 1078 (11°55′S, 13°24′E) is located in 438 m water depth. Site 1079 (11°56′S, 13°19′E) is downslope in 749 m of water. Flaring of gas from a production platform that was ~25 mi distant could be observed from Site 1078, imparting special importance to the safety aspect of the shipboard gas analyses. Sediments at both sites consist of olive-gray silty clay with varying amounts of nannofossils and foraminifers. Sedimentation rates are high because of inputs of terrigenous materials from the arid and elevated coastal regions of Angola. The rates average between 600 and 400 m/m.y. over the upper two-thirds (260 k.y.) of Sites 1078 and 1079, respectively, before dropping to ~50 m/m.y. in deeper sediments. The shallow water depths and elevated sedimentation rates combine to improve organic matter preservation. The elevated preservation is reflected in organic carbon concentrations of ~2.5 wt%, which is greater than that anticipated from the moderate marine productivity in this sedimen-tary regime.
Sedimentary sequences at the drill sites in this depositional regime provide a composite latest Miocene–present history of the major upwelling system that extends from the mid-Namibia margin to north of the Walvis Ridge. The drill sites (Fig. 1) are seaward of the upwelling centers, but they contain second-order signals of enhanced marine productivity associated with the upwelling signals that are transported to their respective locations by the Benguela Current and its meanders and eddies.
Site 1080 (16°36′S, 10°49′E) is located south of the Angola-Benguela Front in water 2778 m deep. Sediments consist of dark greenish gray to olive-gray diatom-bearing to diatom-rich silty clays with varying amounts of calcareous microfossils. The sedimentary sequence is incomplete and disturbed, possibly involving turbidite deposition. Site 1081 (19°37′S, 11°19′E) is situated on the Walvis Ridge in water 794 m deep. Sites 1082 (21°06′S, 11°49′E; water depth 1280 m) and 1083 (20°57′S, 11°13′E; water depth 2180 m) are in the Walvis Basin on the southern flank of the ridge. Site 1084 (25°31′S, 13°02′E) is in 1992 m of water south of the Walvis Ridge on the northern landward edge of the Cape Basin. The sediments at the four sites contain similar alternations of bioturbated olive-gray to black clays rich in nannofossils and diatoms that overlie an olive-gray clayey nannofossil ooze. Sedimentation rates vary from 40 to 200 m/m.y. in the various lithostratigraphic units and subunits of these sites. Most of the sediments of this depositional regime are notably rich in marine organic matter; organic carbon concentrations exceed 10 wt% in many parts of the sequences.
A record of sediment accumulation from the middle Miocene to the latest Pleistocene is provided by the three sites in this depositional regime. These sites (Fig. 1) are south of the main high-productivity zone of the Benguela upwelling system, yet they all lie under the cold waters of the Benguela Current.
Site 1085 (29°22′S, 13°59′E) is at a water depth of 1725 m on the Namibia slope. Sediments at this site are greenish gray, foraminifer-bearing nannofossil oozes. Site 1086 (31°33′S, 15°40′E) lies in 782 m of water on the northern part of the Orange River paleofan. Sediments consist of bioturbated olive nannofossil oozes with varying amounts of foraminifers, silt, and clay. Site 1087 (31°28′S, 15°19′E) is in 1383 m of water on the South Africa slope. Sediments are greenish gray nannofossil oozes with varying amounts of foraminifers and clay. Sedimentation rates in this depositional regime are low, ranging between 20 and 70 m/m.y. at these three drill sites. Organic carbon concentrations in these sites are fairly low (averaging 1 wt%), which reflects the low-to-moderate productivity of this region.