The Demerara Rise is a north-facing protruberance of the continental margin off Suriname, South America. It lies in water depths ranging from 1200 to 4400 m (Fig. F1). Leg 207 focused on the northern extent of this plateau (water depth = 1800–3400 m). Water depths increase gradually in this region until about the 2600-m isobath. On the north-facing portion, the edge of the Demerara Rise is steep, dipping from 2600 to 4500 m in <10 km distance (10°). On the northwest-facing portion, the seafloor dips gradually at ~1.5°. On the top of the rise, the seafloor appears relatively smooth and featureless on a large scale, with no obvious channels or topographic features of note. On the steep flanks of the rise, the seafloor is dissected by gullies (Fig. F1).
Although industry multichannel seismic data show structure down to 4 or 5 s below seafloor, discussion of the seismic stratigraphy in the context of ODP Leg 207 will be largely restricted to the upper sediment column, within ~600 ms of the seafloor. Five key seismic reflectors, or reflection horizons, separate seismic units and were correlated throughout the seismic data to provide the seismic stratigraphic framework. Seismic profile C2211 demonstrates some of the features characteristic of the industry profiles (Fig. F3). Seismic profiles GeoB219 and 220, transecting the study area from southeast to northwest through Sites 1259 and 1257, provide a good overview of the different seismic stratigraphic units within the target sediments (Fig. F4). These units are presented from oldest to youngest.
The lowest seismic unit, just below the section of interest, is represented by contorted and faulted reflections that truncate against a prominent flat-lying reflector termed Reflector "C." This high-amplitude and regionally correlatable horizon reflects the angular unconformity between Cenomanian black shales and underlying synrift sediments, which are mainly clays, siltstones, and sandstones of pre-Cenomanian age.
Above the angular unconformity is a ~70- to 100-ms-thick sequence of medium-amplitude, parallel, coherent reflections that are largely flat lying. The character of these reflections changes laterally from high amplitude to nearly transparent. The top of the unit is distinguished by a regionally correlatable high-amplitude reflection event termed Reflector "B." In general, Reflector B dips uniformly to the northwest. A single variable-amplitude but coherent reflector within this unit is correlatable across the rise. It is termed Reflector "B´." Unit 3 is broken into two subunits; between Reflectors C and B´ is Subunit 3b and between the Reflectors B´ and B is Subunit 3a. This unit terminates on the steep-walled flanks of the rise.
Unit 2 lies between Reflector B at its base and Reflector A at its top. Its thickness is highly variable since Reflector A is an erosional surface, possibly channelized in places. The unit tends to be composed of a distinctive sequence of parallel, coherent, flat-lying reflections. On the flank of the Demerara Rise, however, these reflections are incoherent, possibly due to mass failure (slumping and rotation). Unit 2 generally thins toward the north and toward the flanks of the rise.
The overlying seismic unit is characterized by a sequence of moderate-amplitude near-parallel reflections, although in some cases the unit or sections of the unit appear transparent. In these cases, the sediments have likely failed, disrupting any reflector coherency. This sequence of reflectors shows offlap, onlap, and, in some cases, slight folding. These complexities in form are related to faulting, as correlated with reflector offsets deeper in the succession or mass failure within the section. The sequence is variable in thickness from absent to >500 ms, generally thickening to the south and west (in board). In most cases, the top of this unit is the seafloor, although a thin veneer (<30 ms) with a finely bedded to homogeneous seismic character may overlie it. In these cases, Unit 1 appears unconformable with the overlying sequence and the top contact is called Reflector "O" (Figs. F3, F4). This thin veneer is termed Unit Q and is rarely resolved on seismic profiles.
Faults within seismic Unit 4 are common. They usually display characteristics of normal extensional faults, forming horst and graben structures with rotated and tilted reflectors between faults. Folding is also common. These characteristics are typical of a synrift margin. In spite of the intensity of faulting in seismic Unit 4, there is only occasional evidence for faulting in the overlying units, although there is abundant evidence of slope mass failures, especially on the steep flanks of Demerara Rise and within Unit 1.