Intervals of complete stratigraphy were recovered by multiple coring, particularly through the middle–upper Eocene and Cenomanian–Campanian (Fig. F16). Rotary coring and moderate recovery prevented sampling complete stratigraphy everywhere, however. Where overlap existed in the Paleocene–lower Eocene (Fig. F17) and Campanian and Maastrichtian (Fig. F18) sequences, magnetic susceptibility was generally the best correlation tool, reflecting varying percentages of carbonate and clay. Within the Cenomanian–Santonian black shales, GRA bulk density allowed good correlation between holes where organic-rich laminated claystones alternate with more calcareous well-indurated sediments (Fig. F19).
In the lower and middle Eocene, splices were created for Sites 1258, 1259, and 1260 (Fig. F16). At these sites, the splice sections span the P/E boundary. Splices also were constructed that span the K/T boundary at Sites 1259 and 1260, where the boundary is conformable and contains the ejecta layer (Figs. F17, F18). A splice also was generated for Site 1257, where the base of the Paleocene is an unconformity. It was possible to create a splice section that covered the entire Campanian–Maastrichtian succession at Sites 1259 and 1260. For Sites 1259, 1260, and 1261, sampling splices were created that cover all or nearly all of the Cretaceous black shales.
Cenomanian–Santonian sequences, extraordinarily rich in organic carbon, were recovered at all five sites cored during Leg 207. The finely laminated, dark-colored organic-rich claystones typically contain between 2 and 15 wt% organic carbon, and they range in thickness from 30 m at Site 1258 to 95 m at Sites 1260 and 1261. These thick and extensive black shales are part of the global burial of huge amounts of organic carbon on the seafloor during OAEs 2 and 3.
Organic geochemical properties of the black shales that were measured during Leg 207 reveal aspects of the exceptional conditions of organic matter production and preservation involved in their formation. The results of Rock-Eval pyrolysis show that the bulk of the organic matter originates from algal and microbial primary production (Fig. F20). Extractable biomarker hydrocarbon compositions suggest that relative proportions of algal and microbial contributions of organic matter vary in different parts of the black shale sequences. Hydrocarbon distributions from some parts are dominated by the acyclic tetraterpenoid lycopane, which is indicative of microbial productivity (Brassell et al., 1981). In other parts, the algal C15–C19 n-alkanes dominate. The presence of both algal and microbial biomarkers suggests that organic matter production was enhanced by a consortium of primary producers. Brassell et al. (1981) postulate expansion of an intensified oxygen minimum zone into the photic zone led to black shale deposition at DSDP Site 144, which was recored as Site 1257 during Leg 207. This paleoenvironment probably permitted the coexistence of algae and the photosynthetic microbes that function best under dysaerobic and anaerobic conditions.
Organic matter is thermally immature, as shown by both the high Rock-Eval hydrogen index and low Tmax values (Fig. F20) and also by the dominance of nonrearranged steranes. In addition, improved organic matter preservation is implied by Corganic/Ntotal ratios that increase to ~40 as organic carbon concentrations increase (Fig. F21). Values above ~20 are usually considered typical of land-plant organic matter, but such elevated ratios are also common to Cretaceous black shales (Meyers, 1997). Consequently, most of the elevated C/N values that mimic those of land-derived organic matter are likely to be the result of retarded, selective alteration of algal and microbial organic matter. A likely scenario is that nitrogen-rich components were more readily degraded than other organic matter components during sinking of organic matter through a strongly developed oxygen minimum zone, thereby elevating the C/N ratio of the surviving organic matter (Twichell et al., 2002).
Contributions of land-derived organic matter are also evident in the black shales. Some of the n-alkane distributions of the 10 samples that comprise the biomarker survey are dominated by the C29 and C31 components diagnostic of land-plant waxes. These hydrocarbon compositions do not indicate a dominance of land-plant organic matter in the black shales, inasmuch as land plants contain larger proportions of hydrocarbons than do algae and microbes (e.g., Meyers, 1997), but they do confirm the presence of continental plant debris. However, the proportion of land-plant material appears to become important in some of the lower Cenomanian black shales from Site 1260 in which C/N ratios are between 40 and 60 (Fig. F21).
Concentrations of interstitial methane in the sediment sequences at all five Leg 207 sites increase abruptly in the black shales, where they reach their peak values. The possible relation between sediment organic matter contents and gas concentrations was tested by measuring the organic carbon concentrations of the headspace sediment samples at three of the sites. A rough correspondence exists between higher TOC and greater gas concentrations. Marked excursions from a simple linear relation suggest that the type of organic matter, and not simply the amount, affects gas generation from the black shales. Because of the low thermal maturity of organic matter at all five sites (Fig. F20) and the predominance of methane in the interstitial gases, the origin of all the gases is almost certainly from in situ microbial activity. Dramatic decreases in methane concentration at the tops of the black shale boundaries suggest that methane oxidation, which consumes interstitial sulfate, proceeds in overlying units (see "Interstitial Water Geochemistry"). Moreover, active generation of gas must exist to replace the gas that migrates out of this lithologic unit and to maintain the elevated gas concentrations within the black shales.
IWs from 152 samples were collected at all five sites cored on Demerara Rise, covering a depth range from the sediment/seawater interface to 648 mbsf. This comparably dense IW sampling program was conducted to understand the impact of relatively deep-seated organic matter–rich black shale sequences on diagenetic processes along the paleoceanographic transect. At all sites (except Site 1261), sulfate and ammonium gradients are essentially linear from the sediment/seawater interface to the top of the black shale sequence (Fig. F22A, F22B). These results indicate the following:
Despite the reducing character of the sedimentary column at all Leg 207 sites, only very low IW concentrations of manganese and iron are attained within the black shales (Fig. F22D, F22E). Our favored working hypothesis for this observation is that these redox-sensitive metals were completely remobilized during or shortly after the host organic matter–rich units were deposited, implying conditions of severe synsedimentary oxygen depletion. In contrast, high dissolved barium levels (>300 black shales (Fig. F22C), indicating ongoing mobilization of barium where sulfate concentrations are lowest. Uphole diffusion of Ba and downhole diffusion of sulfate from the sediment/water interface give rise to authigenic barite formation in the overlying Campanian to Paleogene sediments (see "Lithostratigraphy").
The other prominent features seen in the Leg 207 IW data set are chloride anomalies (Fig. F22F). At Sites 1257, 1259, and 1261, we see increases in chloride concentration downhole to >60% relative to standard seawater. Based on the depth profiles obtained at Site 1257 (where data are available from significantly below the black shales), we infer that the shales acts as an aquifer for brines at these sites. At Sites 1258 and 1260, we observe relatively low salinity and chloride concentration anomalies between about 300 and 500 mbsf (up to 17% freshening relative to seawater). Low-chlorinity anomalies such as those seen at Site 1258 are not easy to interpret with confidence on the basis of shipboard data alone. The presence of significant concentrations of methane in headspace gas analyses (>50,000 ppmv) (see "Organic Geochemistry") are consistent with the anomalies having been caused by dissociation of gas hydrates. Alternative explanations for the chloride anomalies are clay dehydration reactions and dilution by meteoric water. The former possibility seems unlikely given the lithologies encountered, but the latter possibility cannot be excluded even though the nearest landmass is located ~350–400 km away.
The main objectives of Leg 207 were the recovery of continuous black shale sequences and critical boundaries such as the K/T, P/E, and E/O and the acquisition of a high-resolution stratigraphy vital to investigating these events. Four of the five sites were logged during Leg 207 (Holes 1257A, 1258C, 1260B, and 1261B). Downhole logging runs included the triple combo tool string with the Multi-Sensor Spectral Gamma Ray Tool (MGT) and the FMS-sonic and WST tool strings, acquiring borehole diameter and wall imagery, formation acoustic velocity, density, porosity, electrical resistivity, natural gamma radiation, and direct acoustic traveltimes. Logging data from Leg 207 show a high degree of correlation with the physical property data, which includes index properties, P-wave velocities, GRA densities, and NGR emissions measured on all recovered core samples.
Index property trends, corroborated by the porosity, density, and velocity logs, generally indicate normal consolidation from the surface to the depth of the K/T boundary. Significant perturbations in these profiles are found coincident with the P/E and K/T boundaries as well as various hiatuses. At Sites 1257–1260, acoustic velocity increases linearly with depth until the P/E boundary (Fig. F23). In the interval between the P/E and K/T boundaries, velocity remains about a fixed baseline, varying with cyclical changes in sediment composition and degree of lithification. These variations are well represented in the high-resolution FMS images, MGT gamma ray and porosity logs, and MST GRA densities, all providing an excellent opportunity for cyclostratigraphic analysis. At Site 1261, a 310-m-thick Neogene sequence of normally consolidated clay-rich material overlies a 60-m-thick debris flow and a highly indurated middle Eocene sequence that is unique in character to this particular site (Fig. F23).
Across the K/T and through most of the Cretaceous, all sites show a high degree of similarity in the physical property and downhole logs. Highly lithified Maastrichtian chalks gradationally change to low-velocity, high-porosity calcareous claystones with glauconite-rich horizons at their base. This change is readily tracked in both the downhole MGT natural gamma ray logs and the NGR records from the MST.
Through the black shale intervals, the wireline data have provided a continuous record of the stratigraphy that is readily interpreted in terms of organic-rich, clay-rich intervals interbedded with carbonate-cemented layers. High-resolution MST data through the shales shows excellent agreement with the downhole logs. Coupled with high recovery through these sequences, the generation of a core-log composite stratigraphy at all logged sites should be possible. Figure F24 shows the black shale sequence in Hole 1260B, with the carbonate-cemented layers readily picked out as low gamma ray and porosity and high resistivity (white bands in the FMS images), density, velocity, and photoelectric effect values.
Gamma ray levels through the shales are characteristically high and correlate exceptionally well between the log and MST profiles. All the sites show a distinct two level division within the sequence. Spectral information from the Hostile-Environment Natural Gamma Ray Sonde (HNGS) and MGT suggest that this is mainly a function of uranium content, which may indicate leaching or postdepositional concentration (Fig. F24).
An overall increase in the porosity of the organic-rich claystones tends to be more significant at Sites 1261, 1259, and 1257, where a slight increase in the porosity profile may reflect the existence of excess pore pressures or subtle lithologic variations (Fig. F23).
The seismic stratigraphy for the Demerara Rise study area is characterized by key reflectors (Reflectors O, A, B, B', and C) were introduced that defined four main seismostratigraphic units (Units 1–4) and one minor unit (Unit Q). A seismic track map with seafloor bathymetry is presented in Figure F2. Within each site chapter, physical property data (in particular, velocity and density) derived from laboratory measurements on core and from downhole logging were used to generate synthetic seismograms. These seismograms were matched to seismic reflection profiles across each site to provide quantitative ties from core and logging results, which are in the depth domain to seismic data that are in the time domain. The results of these ties are summarized in Figure F25. Although these data are presented in each site chapter, there was no attempt to correlate seismic reflection profile data to lithology. Using the information derived from the synthetic seismograms, seismic profiles across each drill site have been depth migrated and matched against the summary lithologic column (Figs. F26, F27, F28, F29, F30). These results permit the seismic profiles to be interpreted with certainty and allow lithologic units to be correlated site to site and regionally across Demerara Rise with confidence.
Reflector C is clearly defined as the base of the black shales, and the underlying sediments of Unit 4 are Albian-age claystone, clayey siltstone, and sandstone. Site 1260 sampled the deepest below Horizon C (40 m). It is uncertain as to whether any of these sediments were truly synrift or whether they represent undeformed shallow water clastic sediments following rifting. Horizon C is presently ~2650 mbsl at Site 1261 (the shallowest site) and dips 0.7° to the north (Fig. F31). At Site 1258 (the deepest site), Horizon C is 3700 mbsl.
The complete sediment sequence from the seafloor to Horizon C is in excess of 1000 m to the south of Site 1261. It gradually thins to the north and rapidly thins near the flanks of the rise. It is ~200 m thick on the upper slope of the flank and then rapidly drops to zero, as the sediment is not sustained on the very steep slopes.
Seismic Unit 3 lies between Reflectors C and B. Horizon B is within seismic Unit 3, and it correlates with the top of the black shale sequence (lithologic Unit IV) at each site, except Site 1259. At this location, it correlates slightly above Unit IV, probably due to an expanded gradational sequence of organic-rich glauconitic claystone above the black shales. The top of seismic Unit 3 is Reflector B, a regional high-amplitude reflector that correlates with the K/T boundary at every site. This surface dips rather uniformly at ~1° to the northwest. The thickness of Unit 3 is locally variable but, in general, increases from ~120 m in the south (Site 1261) to >300 m thick in the northwest between Sites 1258 and 1260.
Seismic Unit 2 lies between Reflectors B and A. Horizon A correlates everywhere to the top of lithologic Unit II, a variable nannofossil chalk sequence that is mostly Eocene in age but possibly as young as early Miocene. Reflector A is believed to result from an episode of extensive erosion during the early Miocene. The surface of Reflector A dips toward the northwest and is ~2100 mbsl at Site 1261 and 3200 mbsl at Site 1258. This surface appears to be channelized, but seismic line density is insufficient to discern details. Unit 2 is highly variable in thickness as a result of differential amounts of erosion but, in general, thins toward the northwest. At Site 1260, for example, the seismic unit is 180 m thick, whereas at Site 1259 it is 400 m thick.
Reflector A forms the base of seismic Unit 1. Its top is typically the seafloor because Reflector O and seismic Unit Q are rarely present. Unit 1 correlates with unlithified or semilithified sediment, such as the 300-m-thick succession of Miocene–Pliocene nannofossil clay at Site 1261. The unit is absent at other sites. Seismic lines show that its thickness is highly variable because its lower bounding surface is Reflector A. Its maximum thickness is >350 m, and it thins toward the west and northwest.
Seismic Unit Q rarely is resolved on seismic reflection profiles, but at Site 1261, it is nearly 30 m thick (Fig. F30). At this location, Reflector O correlates with the base of lithologic Subunit IA, a Pleistocene nannofossil ooze. Reflector O represents an unconformity, separating Pleistocene and younger sediment from the middle Miocene. Reflections within Unit 1 sometimes truncate against Reflector O.
In summary, strong physical property contrasts in lithologic units, as described from Leg 207 drilling results on Demerara Rise, translate to excellent correlation with seismic units and even with some single-reflection events. Depth-migrated seismic reflection profiles consistently match summary lithologic columns as a result. Seismic reflection data can be used to confidently correlate from site to site and across the Demerara Rise study area.
The stratigraphy of sites cored along the depth transect during Leg 207 were integrated to interpret the mid-Cretaceous (Albian) to Neogene history of Demerara Rise. This history reflects multiple depositional episodes during the late rifting and drifting stage of a continental margin that saw different sedimentological regimes separated by several hiatuses of varying duration. Transform movements related to the opening of the equatorial Atlantic gateway are believed to have influenced the sedimentary history during the Cenomanian–Campanian stages.
The oldest sediments recovered during Leg 207 are early Albian in age. At the deepest site of the depth transect (Site 1258), they are represented by organic carbon–rich claystones with phosphatic pebbles. Ammonites and microfossils recovered from this interval indicate open marine conditions in a probable epicontinental basin. Upslope (at Site 1260), an increase of siliciclastic detritus indicates a sedimentary environment closer to the sediment source. Probable tidal flat deposits of unknown age (Albian?), which were deposited in a marginal marine setting, represent the oldest sequence recovered at the shallow end of the transect (Site 1259). These deposits suggest that Demerara Rise was a submarine high separated from South America by a shallow marine epicontinental basin as early Cretaceous shallow-water carbonates and Aptian–Albian open marine marls have been recovered in industry wells farther south.
Shallow-water environments are also indicated by marine quartz sandstone with occasional ammonite casts and shell debris recovered at the shallow sites (Sites 1261 and 1259) and at Site 1257, where they unconformably overly middle to upper Albian open marine clays. These sandstones are late Albian to upper Cenomanian in age.
Black shale deposition at Demerara Rise started in the middle Albian at the deep end of the transect. At this location, they are overlain by a lower–middle Cenomanian sequence of storm-induced layers intercalated within black shales, which might be laterally equivalent to the quartz sandstone upslope. No upper Albian sediments have been identified. Elsewhere, middle–late Cenomanian black shales overlie a prominent unconformity that separates them from the underlying Albian–lower Cenomanian clays and sandstones, which represent the synrift deposits on Demerara Rise.
Cyclic successions of middle–upper Cenomanian organic matter–rich black shale with abundant phosphatic pebbles and fish remains and laminated foraminiferal packstone are believed to represent the first shallow hemipelagic sediments on Demerara Rise. They reflect high surface water productivity and dysoxic bottom water conditions. The thickness of the Cenomanian sequence on Demerara Rise changes greatly (8–80 m). This may be the result of either varying subsidence histories of different small slope basins or the middle–early Turonian sea level rise and the consequent transgressive onlap. However, the onset of black shale deposition at the shallow sites dates late Cenomanian. There, very dark TOC-rich shales yield very reduced planktonic faunas and calcareous nannoplankton of the genus Ephrolitus, which indicate a very shallow marine to lagoonal setting.
High-productivity conditions, probably related to local upwelling, continued until the Coniacian with most extreme conditions during the OAE 2. Clayey bentonite layers indicate the proximity of volcanoes. Occasional glauconite-rich horizons are interpreted as either an oxygenation event or resulting from reduced sedimentation rates or both. Debris flow deposits, slumped intervals, and condensed horizons in the upper Turonian–lower Campanian indicate the position of the Leg 207 sites at a steeper slope and with turbulent sedimentary conditions. Black shale deposition on Demerara Rise ends in the Santonian–early Campanian.
No middle–late early Campanian-age sediments have been recovered, and the upper Campanian yields glauconite-rich horizons indicating reduced sedimentation rates. It is believed that the cessation of black shale sedimentation, very reduced deposition, rates, and hiatuses are related to oceanographic modifications and transform motions following the opening of the equatorial Atlantic gateway. Oxic conditions were established by the late Campanian, when sedimentation on Demerara Rise changed from hemipelagic to pelagic. The abundance of radiolarians in the Campanian, however, indicates increased surface water productivity. The cyclic pattern of trace fossil abundance suggests recurrence of a reduction in bottom water oxygenation.
Maastrichtian–Oligocene-age sediments at Demerara Rise consist of pelagic deep marine nannofossil and foraminiferal chalks and oozes deposited in an upper bathyal setting. The succession records the lithologic consequences of the K/T boundary impact and the abrupt global warming associated with the P/E boundary. The high abundance of radiolarians in middle Eocene sediments is related to upwelling conditions. Because of a major erosional event in the early Miocene, Oligocene sediments on Demerara Rise are often reworked or reduced in thickness.
The presence of Neogene sediments on the slopes of Demerara Rise is rather patchy. As a probable result of Amazon plumes, upper Miocene- to Pliocene-age sediments are clay rich and sedimentation rates can be very high (Site 1261). However, no upper Pliocene and only very thin veneers of Pleistocene and Holocene clay-rich sediments are present.