A complex Neogene depositional history on Demerara Rise is indicated by the prevalence of nonsynchronous slump deposits and hiatuses at the five sites drilled during Leg 207 (Fig. F2). The limited number of samples in this study precludes a very detailed analysis of the relationships among these events but does allow inferences to be made concerning their likely timing and magnitude.
At Site 1257, the shallowest Sample 207-1257C-1R-2, 140 cm (84.9 mcd), has an OCR of 4.08, with 1510 kPa of overburden stress removed (Table T1; Fig. F3). The high OCR of the sample argues for a significant erosional event at this site. The timing of the erosional event must be associated with either the absence of Pliocene–Pleistocene material or the early Oligocene–late Miocene hiatus identified in the recovered sediments (Fig. F2). Generally, sedimentation rates on Demerara Rise were between 15 and 20 m/m.y. for the early and late Eocene, slowing to 8–12 m/m.y. during the early Oligocene, and then increasing substantially during the Miocene (up to 65 m/m.y. at Site 1261) (Erbacher, Mosher, Malone, et al., 2004). The absence of significant Pliocene–Pleistocene deposits from all sites visited during Leg 207 suggests that significant accumulation of these recent sediments on the outer margin flanks is untenable. Assuming a sedimentation rate of 5 m/m.y. for the Eocene–Oligocene sediments at Site 1257 (Shipboard Scientific Party, 2004b), and taking an average bulk density of 1.59 g/cm3, the missing material represents ~270 m (Equation 5) of Eocene–Oligocene chalks that would account for nearly 54 m.y. of deposition. If the estimated sedimentation rate is increased to 10 m/m.y., the duration of deposition is only 22 m.y. and approximates the length of the early Oligocene–late Miocene hiatus at Site 1257. If the eroded material is assumed to be solely Miocene equivalent deposits of nannofossil clay, with an average bulk density of 1.72 g/cm3 (recovered at Site 1261), the missing overburden represents ~220 m of sediment (Equation 5), and with a sedimentation rate of 65 m/m.y. (from Site 1261), the missing sediment accounts for only 3.5 m.y. of deposition. The amount of overburden thus requires either a prolonged period of accumulation throughout the Oligocene and early Miocene followed by a single large erosional event or a prolonged period of deposition and erosion that ended in the middle Miocene, allowing the accumulation of significant Miocene nannofossil clays during a period of 3–4 m.y. The accumulation of this material over 3.4 m.y. is consistent with the stratigraphy from Site 1261, where 270 m of nannofossil clay were deposited between 6 and 3 Ma (Shipboard Scientific Party, 2004b). Together, this evidence suggests a late Miocene age for the lost overburden at Site 1257 and implies a stable period of deposition in the late Miocene that followed a prolonged period of continued deposition and erosion lasting from the early Oligocene to middle Miocene. The prevalence of nonsynchronous Oligocene–Miocene slump deposits at Sites 1259, 1260, and 1261 fits with the interpretation of a dynamic period of deposition and erosion extending from the early Oligocene–middle Miocene (see Fig. F2).
OCRs from the single sample from Site 1259 and the shallow samples from Site 1261 indicate other less substantial erosional events (Fig. F3). At Site 1259, an OCR of 3.21 in Sample 207-1259A-3R-5, 127 cm, represents 310 kPa of lost overburden, equivalent to ~50 m of eroded nannofossil clay. An ~30-m-thick slide deposit of reworked lower Oligocene calcareous ooze rests on top of the lower Miocene calcareous ooze and chalk where Sample 207-1259A-3R-5, 127 cm, was taken (Shipboard Scientific Party, 2004a), implying that the maximum amount of eroded material was ~80 m and occurred sometime after the early Miocene.
There is no evidence for a large erosional event in the late Miocene or Pliocene–Pleistocene at Site 1261, where the only substantial Neogene deposits were recovered during Leg 207. A single underconsolidated sample was recovered from 72 mcd at Site 1261, near the top of lithostratigraphic Subunit IB, with an age range of late Miocene–middle Pliocene (Fig. F3). Because this interval was spot cored, a detailed age model was not developed, but shipboard results indicate that sedimentation rates between 70–140 m/m.y. are possible for the upper 70 mcd of Site 1261 (Shipboard Scientific Party, 2004c). These high rates, combined with the low permeability of the Unit II sediments (10–9 cm/s), may have resulted in disequilibrium compaction, a state where sediment loading exceeds the rate at which pore water can be expelled from the consolidating strata. Downhole, the OCRs of samples from Subunit IB indicate very slight overconsolidation.
OCRs at Site 1257, and to some extent in the two samples from Site 1258, tend to decrease downhole. Below the heavily overconsolidated sample from Section 207-1257C-1R-2, the OCR for Section 207-1257C-6R-3, a Paleocene-age nannofossil chalk, also indicates an overconsolidated state as would be expected when a significant overburden has been removed from a sedimentary sequence (Fig. F3). However, the magnitude of lost overburden represented by the difference between Pc´ and 
´ is only 1000 kPa (Table T1). Conversely, Sample 207-1257C-16R-5, 135 cm, representing the less porous Albian claystones underlying the black shales at Site 1257, had an OCR of 0.35 (underconsolidated) and a difference between Pc´ and ´ of –1750 kPa. Using the average eroded overburden (1250 kPa) from Samples 207-1257C-1R-2, 140 cm, and 6R-3, 140 cm, a modified OCR of 0.19 is determined using
´ + ´eroded material). (19)
The largely underconsolidated nature of this sample implies that normal consolidation processes have been inhibited and that excess pore pressure is present below the black shales at this site. Although the black shale sample from Site 1257 had an OCR of 1.39, the modified OCR (Equation 19) for this sample is actually 0.72 and implies that the black shale sequence at this site was moderately underconsolidated at the time of maximum loading, which, given the arguments for the timing of the erosional event, would have been in the late Miocene. It is difficult to reconcile the highly underconsolidated sample from beneath the black shales at Site 1257 with the almost normally consolidated black shale sample. Part of the difference may be accounted for by sample disturbance, which may have reduced the Pc´ estimate for Sample 207-1257C-16R-5, 135 cm. Alternatively, the black shale sample may be more normally consolidated because of the higher permeability, which allows flow channeling toward the outcrops on the margin flanks, while the lower permeability underlying Albian claystones remain overpressured. In spite of the mechanism for the drop in the OCR beneath the black shales at Site 1257, a downhole decrease in the OCR at Site 1257 is recognized, with highly overconsolidated near-surface sediments giving way to underconsolidated sediments at the bottom of the hole (Fig. F3).
Sample 207-1258B-45R-4, 45 cm, the second Cretaceous black shale sample tested, was underconsolidated with an OCR of 0.39 (Fig. F3). Unlike Site 1257, where evidence for significant erosion was found, the Maastrichtian–early Paleocene chalk from above the black shales at Site 1258 appears to be normally consolidated (OCR = 1.07). The low OCR (0.39) for the black shale from Site 1258 indicates that fluid pressures in excess of hydrostatic are present. The difference in the current in situ effective stress and the Pc´ from this sample gives a rough estimate of the present excess pore pressure of 1990 kPa (Table T1).
Permeability profiles for Sites 1257, 1259, and 1261 were constructed using the best estimates of effective stress profiles to predict the in situ void ratio profiles and applying the laboratory derived e-log(k) relationships to these profiles (Figs. F7, F8, F9). Differences in the predicted and measured void ratio profiles translate into uncertainties in the permeability profiles. These differences likely arise from variability in downhole and intersite sediment composition or the degree of diagenesis but may also include variability in stress history that cannot be identified without a more detailed sampling program. At Site 1261, this uncertainty would include the thickness of material removed during the debris flow of Subunit IC, which represents a gap of ~30 m.y. in the sediment record (Fig. F3).
The general observation that underconsolidation increases with depth at all the studied sites suggests that fluid pressures increase above hydrostatic toward the black shales. On the flank of Demerara Rise, the black shales at Site 1257 are currently normally consolidated to overconsolidated, but significant overpressuring exists in the underlying Albian claystones. At the time of maximum loading, the black shales at Site 1257 would have been slightly underconsolidated, as inferred from the OCRm for Sample 207-1257-13R-1, 130 cm (Table T1). The only other sample from the black shales (taken from Site 1258) was highly underconsolidated. Although these very different stress histories may simply highlight the natural variability between and within sites on the Demarara Rise, they may also suggest that as one moves away from the steeper eastern flank, where the black shales outcrop on the outer margin, fluid pressures increase. In this scenario, the hydrostatic pressures in the black shales along the eastern flank of the rise are being maintained by lateral fluid expulsion into the water column.
Composite permeability profiles from Sites 1257, 1259, and 1261 indicate that the Cretaceous shale deposits are between 3 and 5 orders of magnitude more permeable than the Maastrichtian–Paleocene chalks and clays that overly them (Figs. F7, F8, F9). The existence of a hydraulic seal, associated with the deposition or diagenetic alteration of the low-permeability Paleocene chalks and clays, is a possible mechanism for generating the underconsolidation of the black shales and older underlying sediments. With a hydraulic seal in place, continued microbial generation of methane (Meyers et al., 2004), compaction-related dewatering, or connections to deeper-sourced fluid reservoirs and the presence of an active flow regime could all prevent normal consolidation of the black shales and underlying sediments.
Similar to the permeability structure of other passive margins, such as the New Jersey margin (Dugan and Flemings, 2000), the architecture of the Demerara Rise appears to support a large horizontal component to fluid flow. The black shale deposits serve as a high-permeability, laterally continuous conduit. Increasing sediment thickness toward the center of the rise provides a gradient in overburden to drive flow. The presence of an active flow regime within the black shales may partially explain geochemical evidence for hypersaline pore fluid in the Cretaceous shale deposits at three of the five sites (Sites 1257, 1259, and 1261) (Erbacher, Mosher, Malone, et al., 2004). A reduction in chlorinity concentrations beneath the shale sequence at Site 1257 indicates that the profiles are not the result of a diffusion gradient driven by a chloride source in the underlying synrifted sediments (Shipboard Scientific Party, 2004a).
The complex stress history of sediments on Demerara Rise makes it difficult to tie regional evidence for large-scale slope instability to elevated fluid pressures in the sedimentary column. If the Maastrichtian–Paleocene chalks and clays are acting as a hydraulic seal, then depending on the efficiency of this seal, fluid pressures would rise within the underlying black shales. Overpressure would build and either exceed the overburden pressures making the formation unstable or drive a large horizontal flow regime. These fluids would likely be focused toward the sea surface along existing faults or through the high-permeability black shales toward outcrops located on the flanks of Demerara Rise. Ongoing microbial degeneration of organic matter within the black shales (Meyers et al., 2004) would contribute to the development of overpressures and generate methane-enriched pore fluids that would migrate laterally along aquifers or vent on the margin flanks.
