Site 1260 is located at a water depth of 2549 meters below sea level (mbsl) on the gently dipping (~1°) northwest-facing slope of Demerara Rise, which is ~380 km north of Suriname (see Fig. F1 in Shipboard Scientific Party, this volume ("Site Survey and Underway Geophysics"). The site is located on a ridge of Paleogene sediments subcropping near the seafloor. Site 1260 is at an intermediate depth of the intended paleoceanographic depth transect across Demerara Rise. The major objectives were the following:
The seismic stratigraphy for Site 1260 is described by line GeoB215, which transects northeast–southwest, and line 207-L1S, which crosses orthogonal to line GeoB215 (Figs. F1, F2). A nearby industry multichannel seismic line (C2211) passes nearly parallel to line 207-L1S ~1.7 km to the southwest (Fig. F3).
Seismic Unit 1 lies between the seafloor and Reflection "A." It is very thin or absent at the drill site, as Reflector A forms the top of a ridge of seismic Unit 2. This ridge is presumably a remnant from early Miocene erosion. To the southwest and northeast of this ridge, seismic Unit 1 comprises a series of low-amplitude coherent reflections that onlap this ridge and are unconformable against the seafloor.
Between Reflectors A and "B" lies seismic Unit 2. It is 365 ms thick (two-way traveltime) at this location, calculated to be ~315 meters below seafloor (mbsf) using velocity data from downhole logs, a checkshot survey, and laboratory-measured compressional (P)-wave velocities on core samples. The topmost sequence in this seismic unit consists of contorted reflectors that pinch out against the seafloor within 1.5 km downslope, which may represent a slumped interval. Below this interval, Unit 2 is characterized by a sequence of crenulated but coherent reflection horizons separated by transparent or incoherent intervals. In the downslope direction, these reflections are reasonably flatlying, dipping slightly less than the angle of the seafloor.
Reflector B is a double-wavelet reflection on line GeoB215 (Fig. F1) of high amplitude relative to events above and below. It is laterally coherent both downslope and alongslope but slightly crenulated, like the reflections above. Seismic Unit 3, between Reflector B (365 ms subbottom) and Reflector "C" (522 ms subbottom), is a ~170-m-thick flatlying sequence that dips 1.5° to the north-northwest. This unit is divided into two subunits. Subunit 3a is an acoustically transparent zone between Reflectors B and "B´." Subunit 3a is ~100 ms thick (87 m). The basal subunit (Subunit 3b) lies between Horizons B´ and "C" (464–522 ms subbottom [~392–485 mbsf]). It is defined on the basis of a series of strong parallel coherent reflections that are laterally contiguous and correlate to the black shale interval at the previous sites. Horizon C forms a regional unconformity that separates the Upper Cretaceous black shales and younger sediments from Albian and older synrift sediments (seismic Unit 4).
On industry profile C2211 (Fig. F3), seismic Unit 4 is clearly defined. Immediately beneath the drill site, reflectors of Unit 4 form a small syncline or basin fill sequence resulting from horst and graben faulting. At this site, Horizon C appears as a disconformity. South of the drill site, this basin is clearly truncated by a fault, with adjacent tilted beds truncating against the Horizon C in an angular unconformity. Further in board, another normal fault separates these tilted beds from a flatlying sequence. This fault is probably a growth fault that apparently propagates to the modern seafloor.
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