6. Site 12591

Shipboard Scientific Party2

BACKGROUND AND OBJECTIVES

Site 1259 is located in a water depth of 2354 meters below sea level (mbsl) on the gently dipping (~1) north-facing slope 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 subcropping near the seafloor and is the second shallowest of all sites forming the intended 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 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.

Seismic Stratigraphy

The seismic stratigraphy established for Demerara Rise, including Horizons "A," "B," "B," and "C," has been correlated proximally to Site 1259 with line GeoB219 (Fig. F1); the closest of the three holes drilled at this site is 170 m from this line. Industry line C2206a crosses the site orthogonal to line GeoB219 (Fig. F2). Lines GeoB219 and GeoB220 form a single northeast-trending strike line that ties Sites 1257 and 1259 (Fig. F1).

Reflector A, representing the top of a presumably early Miocene erosional unconformity, outcrops at the seafloor on the nearest seismic line. The Miocene section recovered in the core is probably not present at the extrapolated site position on the seismic line. Adjacent to the site position, however, seismic Unit 1, which lies above Reflector A, appears transparent and incoherent in seismic profile. It is variable in thickness but in general thickens in the upslope (southerly) direction.

At Site 1259 and between Reflectors A and B, seismic Unit 2 is 465 ms thick (~405 m using laboratory measured velocity information). The topmost sequence (180 ms [~130 m]) in this seismic unit consists of incoherent reflections that are not transparent as in Unit 1. The equivalent stratigraphic succession, 10- to 15-km upslope, consists of parallel high-amplitude reflections that terminate abruptly. Sediments composing this seismic facies at the drill site location likely have undergone some degree of mass flow displacement that disrupts reflector coherency.

Below 130 m, Unit 2 is represented by a 120-m-thick sequence of high-amplitude parallel coherent reflections followed by low-amplitude coherent reflections down to Reflector B at 463 ms subbottom (442 meters below seafloor [mbsf]). Seismic Unit 2 dips (~0.6) to the north, slightly less than the seafloor dip. The parallel coherent reflections are occasionally separated by thin (15–20 m thick) horizons of low amplitude and poor coherency. They tend to pinch out up- and downslope. This echo character likely represents pelagic deposition with rare interbedded mass flow deposits. The high-amplitude sequence probably represents a different lithology than the underlying low-amplitude facies. This entire sequence of parallel reflections does not continue downslope to Site 1257 but appears eroded and composed of slope-failure sediment.

Reflector B is a high-amplitude laterally coherent double-wavelet reflection that correlates with the approximate position of the Cretaceous/Tertiary (K/T) boundary at previous sites. It laterally correlates to Site 1257 from Site 1259. Seismic Unit 3, between Reflectors B (462 ms subbottom [~400 mbsf]) and C (571 ms subbottom [~505 mbsf]) is a ~105-m-thick flat-lying sequence that dips <1 to the north. The upper part of this unit between Reflectors B and B is called Subunit 3a. It is acoustically incoherent and relatively transparent, extending from ~400 to 445 mbsf (462–503 ms subbottom). The basal part of Unit 3 is Subunit 3b, which lies between Horizons B and C (503–571 ms subbottom [~445–505 mbsf]). It is defined on the basis of a series of strong parallel coherent reflections that are laterally contiguous for several kilometers. At the base of Subunit 3b, just above Reflector C, is an incoherent low-amplitude reflection event that may represent another interbedded mass flow deposit.

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). At Site 1259, Reflector C is an angular unconformity, with underlying reflections cropping against it at relatively shallow angles (~2) as they appear on the strike line GeoB219. On the industry seismic line C2206a, which intersects the site in a dip profile (northeast), these low-angle reflections are not resolved and the underlying sequence appears locally conformable (i.e., a disconformity).

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-106

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