Site 1258 is located in a water depth of 3192.2 meters below sea level (mbsl) on the gently dipping western slope (~2°) of Demerara Rise, which is ~380 km north of Suriname (see Fig. F1 in Shipboard Scientific Party ["Site Survey and Underway Geophysics"], this volume). The site is located on a ridge of Paleogene sediments outcropping on the seafloor. Site 1258 is the deepest member of the paleoceanographic depth transect across Demerara Rise. The major objectives were the following:
Site 1258 is on the northwest-facing slope of a small promontory of Demerara Rise, with a local seafloor gradient of 1.5° toward the northwest. The seafloor falls away more steeply ~1 km from the site. To the southeast (upslope), the sedimentary sequence thickens above Reflector "C" (post-Albian). The seismic stratigraphy for the site is described by line GeoB221, which transects northeast–southwest, and line 207-L1S, which crosses orthogonally to line GeoB221 (Figs. F1, F2). A nearby industry multichannel seismic line (line C2211) passes nearly parallel with line 207-L1S, which is ~5.5 km to the southwest (Fig. F3).
Reflector "A," representing the top of a presumably early Miocene erosional unconformity, crops out at the seafloor at the site (within the resolution of the survey data). Between the Reflector A (the seafloor in this case) and Reflector "B" is seismic Unit 2 (seismic Unit 1 is missing from this site). Unit 2 is 300 ms thick (two-way traveltime) at this location, calculated to be 265 meters below seafloor (mbsf) using downhole logging velocity data from the site. The unit shows an echo character of reasonably coherent but slightly folded reflections with offsets, which probably describes a sediment sequence that has undergone mass failure and rotational displacement. A relatively coherent package of higher-amplitude reflections of short lateral duration is present at 220 ms subbottom (189 mbsf).
Reflector B at 300 ms subbottom (265 mbsf) marks the top of seismic Unit 3, and Reflector C at 480 ms subbottom (~459 mbsf) is its base. It is a flat-lying sequence at this site that dips gently to the north-northwest at an angle of 1.5°. The sequence thins and pinches out in the downslope direction from the drill site. Reflector B is not well defined at Site 1258 because the reflections are incoherent. Coherency improves laterally, especially in the upslope (southwest) direction. Seismic Unit 3a, defined at the interval between Reflectors B and "B´" (300–450 ms subbottom [265–417 mbsf]), is relatively transparent and incoherent in profile, with low-amplitude coherent parallel reflections that become apparent only toward its base. Subunit 3b underlies Subunit 3a and is the sequence between Reflectors B´ and C (450–515 ms subbottom [417–480 mbsf]). It is defined on the basis of a series of strong parallel coherent reflections that are laterally contiguous.
On industry profile line C2211, the entire section from Reflector A to B´ appears relatively incoherent with contorted reflections (Fig. F3). More than 16 km upslope (southwest), the same interval comprises a coherent parallel set of reflections that terminates in a listric fashion against this incoherent unit. The character and upslope truncation suggests most of the interval of seismic Unit 2 and Subunit 3a at the drill site has slumped as a rotational mass failure. The package of reflectors composing seismic Subunit 3b, however, appears contiguous and may be undeformed. Reflectors below Horizon C appear folded into a possible small anticline below the drill site and contact Reflector C as an angular unconformity. There are numerous listric-normal faults and folds in this section (Fig. F3).
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Ms 207IR-105