Fluids play a pivotal role in the dynamics of continental margin evolution. Subseafloor fluids link many physical and chemical processes through the transport of energy and solutes over a wide range of scales. Defining these fluids, their flow systems, controlling mechanisms and rates on both active and passive margins are important scientific questions that have become target themes for the Integrated Ocean Drilling Program (Ge et al., 2002).
In normally consolidated sediments, hydrostatic pressure is maintained in the pore spaces by the expulsion of pore waters as burial compaction progresses. If sediment permeability is sufficiently low or if sedimentation and the accumulation of overburden pressure are sufficiently high (as in many high-productivity shelf environments), pore fluids cannot escape quickly enough and overpressures are generated. In these instances, the retained fluid carries part of the overlying sediment load. As time progresses, pore pressures can reach lithostatic levels and cause fracturing, affect local and regional slope stability, and induce flow channeling (Hart et al., 1995).
On continental margins, downhole variability in formation permeability and horizontal differences in overburden stresses result in the focusing of fluids laterally along preferential flow channels (Dugan and Flemings, 2000). Where these flow channels outcrop on margin flanks, slope instabilities may occur near the seafloor. Modeling on the New Jersey margin has shown that lateral flow along highly permeable Miocene fluid conduits initiates slope failure and feeds cold seeps on the seafloor (Dugan and Flemings, 2000), and similar evidence of mass wasting exists in the Eastern Nankai Trough on the Tokai margin, where active seepage is concentrated along outcrops of coarse turbidites (Henry et al., 2002). In both situations, the margins are characterized by large downhole variations in permeability and lateral heterogeneities in sediment thicknesses.
A similar situation exists on Demerara Rise, an extension of the continental margin off the coasts of French Guyana and Suriname. The rise is covered with 2–3 km of sediment and generally lies <700 meters below sea level (mbsl). Basement rocks on the rise are Precambrian and early Mesozoic and are overlain by pre-Albian synrifted sediments (Erbacher, Mosher, Malone, et al., 2004). The rise's northwestern edge is characterized by a gentle sloping surface that deepens >4000 mbsl while maintaining a generally constant thickness of sediment cover, whereas on the eastern flank, sediments thin in the seaward direction, with basement outcrops occurring below 3500 mbsl.
Five sites drilled on Demerara Rise during Ocean Drilling Program (ODP) Leg 207 constitute a depth transect extending from 1900 to 3200 mbsl. Recovered sediments are primarily Cretaceous and Paleogene deposits defined by five major lithsotratigraphic units (Figs. F1, F2). Three regional unconformities are identified in the recovered sequences and occur
The changing depositional environment on Demerara Rise produced a diverse suite of sediments whose petrophysical properties are complicated by postdepositional diagenetic alterations and periods of erosion or nondeposition. This study presents laboratory test results that describe the permeability, compressibility, and stress history of 12 samples from Sites 1257, 1258, 1259, and 1261 (Fig. F3). The results of laboratory testing allow for the construction of composite effective stress and permeability profiles of the recovered sediments and are used to investigate the timing and magnitude of documented unconformities and the evolution of pore pressure and possible fluid flow regimes on Demerara Rise.