8. Site 1261¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1261 is located at a water depth of 1899 meters below sea level (mbsl) on the gently dipping (~1°) northwest-facing slope of Demerara Rise, which is ~350 km north of Suriname (see Fig. F1, in Shipboard Scientific Party, this volume ["Site Survey and Underway Geophysics"]). Site 1261 is the shallowest site of those forming the paleoceanographic depth transect across Demerara Rise. The major objectives were the following:

1. Core and log a Paleogene–Albian section to evaluate paleoceano-graphic and paleoclimatic changes, with emphasis on major and abrupt events during this interval that include the Eocene/Oligocene (E/O) and Paleocene/Eocene (P/E) boundaries 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 to understand changes in bottom water circulation over Demerara Rise during the gradual opening of the seaway.
3. Recover continuous and expanded sediment records of the Paleogene and Cretaceous in order to reconstruct short- and long-term changes in greenhouse forcing.

The seismic stratigraphy established for Demerara Rise, including Horizons "A," "B," "B´," and "C," has been correlated to Site 1261 with line GeoB213 (Fig. F1). Lines GeoB204 and GeoB208 are orthogonal to line GeoB213 and pass within 5 km of the site to the northwest and southeast, respectively. Industry line C2206a passes orthogonal to line GeoB213 8 km to the northwest of the drill site (Fig. F2).

The seismic stratigraphy for Demerara Rise shows increasing sediment thickness to the south (in board). At Site 1261, the uppermost sediment section has not been defined previously. It consists of a thin (30 ms thick; 24 m) package of parallel coherent reflectors that offlap from the upslope direction (south) and truncate against the seafloor ~10 km downslope. This sequence is Quaternary in age, and the base of it is Reflector "O" (Fig. F1).

What was previously defined as seismic Unit 1 (Miocene–Pliocene) (see Shipboard Scientific Party ["Site Survey and Underway Geophysics"], this volume) underlies Reflector O. It is largely missing at the other sites. In the immediate vicinity of Site 1261, Unit 1 comprises a well-defined set of coherent seismic reflections of varying amplitudes. The topmost reflections truncate against Reflector O, sometimes in an angular fashion. At the base of the Unit 1 is a 50-ms-thick (~40 m) zone of incoherent reflections capped by a bright reflector that appears to be a debris flow. The bottom of the unit is Reflector A. Unit 1 is 415 ms thick (367 m thick) at the drill site.

Unit 2 is below Reflector A, the presumed early Miocene erosional unconformity. The base of seismic Unit 2 is correlated to Reflector B at 586 ms (550 m). It is represented by a sequence of high-amplitude parallel coherent reflections that are relatively flat lying, dipping 0.5° to the north. Seismic Unit 3, between Reflectors B and C, is estimated to be 150 ms thick (175 m). Much of the acoustic energy in the high-resolution site survey profile (line GeoB213) is lost within the highly reflective Unit 2. Little detail is resolved in Unit 3 as a result. It appears as an acoustically transparent package with occasional semicoherent reflectors at the top and at the very base just above Horizon C. It is difficult to correlate Horizon B and thus to distinguish Subunits 3a and 3b due to this low reflectivity. It has been correlated to the base of the slightly coherent section near the top of Unit 3, but the tie is uncertain.

Reflector C, at the base of the section of interest, is an unconformity. No coherent subsurface data are recognizable on the survey data, but the nearby industry line C2206a shows the underlying section. At Site 1261, the unconformity appears as a disconformity and it is difficult to pick Reflector C as a result. Further below, reflections form a broad anticline, which is fault-bounded to the southwest and folded into a syncline to the northeast.

A listric-normal fault is shown on line GeoB213 (Fig. F1) 5 km southeast of the drill site. This fault apparently has affected the entire sediment column. Offset across the fault is ~30–60 ms in the deeper portion of the section at Reflector C, whereas in the upper portion it is on the order of 200 ms. This discrepancy in offset can be accounted for only by invoking either significant rotation or massive slumping in the upper sediment column. The fault splays at 300 ms subbottom, showing significant tilting of reflections in the interval between offsets, which is further evidence of slumping.

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.

Ms 207IR-108