4. Site 12571

Shipboard Scientific Party2

BACKGROUND AND OBJECTIVES

Site 1257 is located at 2951 meters below sea level (mbsl) on a terrace above the steep northern slope (>10) of northwest Demerara Rise, which is ~400 km north of Suriname. As the second deepest location of the cruise, Site 1257 serves as an intermediate member of the paleoceanographic depth transect across Demerara Rise. It was promoted to recore Deep Sea Drilling Project (DSDP) Site 144, which was spot cored during Leg 144 in 1970. The major objectives were the following:

  1. Core and log an Albian–Oligocene section to evaluate paleoceanographic and paleoclimatic changes during the Paleogene and Cretaceous, with an emphasis on major and abrupt events during this interval (e.g., the Eocene/Oligocene [E/O] boundary, the Paleocene/Eocene [P/E] boundary, and the Cretaceous oceanic anoxic events [OAEs]).
  2. Reconstruct the history of the opening of the Equatorial Atlantic Gateway by obtaining benthic foraminifer proxy data. These data will help us to understand changes in bottom water circulation over Demerara Rise during the gradual opening of the seaway.
  3. Define the depth of key seismic reflectors in the seismic stratigraphy compilation in order to create a detailed drilling and sampling plan for the remaining sites along the Demerara Rise depth transect.

The steep northwestern slope indicated in Figure F1 of Demerara Rise, where Site 1257 is located, is part of the southern transform fault that separated South America and West Africa during the rift phase of the southern central Atlantic in the Late Jurassic–Early Cretaceous. Although spot cored, results from Site 144 demonstrated the potential for the presence of all the target sediments.

Seismic Stratigraphy

The seismic stratigraphy of Site 1257 is demonstrated in Figure F1. Within 2.5 km northwest and 1.5 km north-northeast of the drill site, the seafloor begins to fall off at a 10 angle, from the site elevation of 2951 mbsl to 4400 mbsl on the abyssal plain. The site itself is on a slight mound that appears to be an erosional remnant. Reflector "A," representing the top of this erosional unconformity, crops out at the seafloor at the site (within the resolution of the survey data). Between 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 173 ms thick (144 m two-way traveltime) at this location using checkshot and downhole logging velocity information (see "Downhole Logging"). Seismic Unit 2 shows an incoherent reflection character here, describing a disturbed sediment package or the effect of side echoes from local complex topography. A high-amplitude reflection event of short lateral duration occurs at 110 ms subbottom (90 meters below seafloor [mbsf]) at the site.

Reflector B at 173 ms subbottom (144 mbsf) marks the top of seismic Unit 3. It is a flat-lying sequence at this site that dips gently to the northeast at an angle of 1.7. Reflector B is hummocky on a local scale, probably cut by channels, and is underlain by several high-amplitude reflections and then a short transparent zone to the top of Reflector "B" at 217 ms subbottom (181 mbsf). This section is described as Subunit 3a in "Seismic Unit 3" in Shipboard Scientific Party ["Site Survey and Underway Geophysics"], this volume. Subunit 3b underlies Subunit 3a and is the sequence between Reflector B and the Reflector "C," which occurs at 272 ms subbottom (232 mbsf).

Most acoustic energy is lost below Reflector C in the survey data, and the section is difficult to describe. A few hyperbolic reflections in this interval are visible. Industry line C2206 crosses in a northeast–southwest direction just 1 km northeast of the drill site (Fig. F1 in Shipboard Scientific Party ["Site Survey and Underway Geophysics"], this volume). In this profile, the sequence of reflectors below Horizon C appears folded into a possible small anticline below the drill site and contacts Reflector C as an angular unconformity.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

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