Four to five seismic reflectors can be distinguished on the outer Demerara Rise; these are labeled as reflectors O, A, B, B', and C. The reflectors have been dated by correlation to industry well Demerara A2-1, DSDP Site 144, and gravity cores taken during cruise M49-4 (Fig. F5).
Where present, Reflector O marks the base of a thin veneer of Quaternary clay-rich carbonate oozes (no more than a few tens of meters thick; see line GeoB01-208). Reflector O is underlain by a package of mostly thin layers indicated by a group of distinctive reflectors of different strength. Toward the distal end of Demerara Rise, reflectors are anastoming and partly missing, presumably caused by erosion events (e.g., lines GeoB01-217, GeoB01-211, and GeoB01-212). Correlation of industry well Demerara A2-1 with line GeoB01-204 suggests a Miocene age for this package.
Reflector A is produced by an erosional surface of latest Oligocene to early Miocene age as dated by industry well Demerara A2-1 and a Meteor 49-4 gravity core taken from an outcrop of Reflector A on line GeoB01-215. The reflector is more or less uniformly dipping to the northwest in the eastern part of Demerara Rise (see lines GeoB01-211 and GeoB01-212). The roughness of Reflector A increases toward the northwest and northeast of Demerara Rise. There, deeply incised channels indicate extensive erosion into the Paleogene sedimentary sequence (see lines GeoB01-215, GeoB01-216, GeoB01-204, and GeoB01-219). Channels dip to the northwest, north, and northeast toward the slopes of Demerara Rise.
Reflector A pinches out on the northern slopes of Demerara Rise where sediments older than this reflector crop out. It is difficult to trace Reflector A along the shallow dipping western slope of Demerara Rise to proposed Site DR-3C. Line GeoB01-221 shows a package of branching reflectors that probably correlates with Reflector A and indicates the presence of several Oligocene outcrops.
Reflector B is the first prominent reflector below the erosional surface marking Reflector A and marks the top of a Santonian to Cenomanian black shale sequence. In general, Reflector B dips uniformly to the northwest. This reflector is present in all lines except one and downdips along Line GeoB01-217, where its absence appears to reflect local extreme downcutting associated with the erosive event denoted by Reflector A.
Reflector B' is a relatively weak reflector and is thought to represent the top of a package of Albian to Barremian marls and claystones underlying the mid-Cretaceous black shales.
Reflector C is strong and reflects the angular unconformity between Barremian/Aptian marine claystones and underlying synrift sediments. This prominent reflector is the deepest in the set of reflectors and a distinct feature in all seismic lines.
Extensional faults are present throughout the investigated area. Simple horst and graben structures (e.g., line GeoB01-208) with tilted blocks and extensional faults (e.g., lines GeoB01-213 and GeoB01-204) can be observed. The faults are related to Atlantic drifting and gravitational extension in the studied area. The general strike of the faults is northeast-southwest. The major fault in line GeoB01-213 can be traced to other lines such as GeoB01-204, GeoB01-219, and BeoB01-217.
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