Site 1261 is located at a water depth of 1899 mbsl on the gently dipping (~1°), northwest-facing slope of Demerara Rise, ~350 km north of Suriname. Sites 1260 and 1258 are to the northwest. and Sites 1257 and 1259 are to the north. Site 1261 is the shallowest site, forming the paleoceanographic depth transect across Demerara Rise. The major objectives were the following:
The seismic stratigraphy established for Demerara Rise, including the Horizons A, B, B', and C, was correlated to Site 1261 with line GeoB213. Lines GeoB204 and GeoB208 are orthogonal to GeoB213 and pass within 5 km of the site to the northwest and southeast, respectively. Industry line C2206a passes orthogonal to GeoB213 as well, 8 km to the northwest of the drill site.
The seismic stratigraphy for Demerara Rise shows increasing sediment thickness to the south (in board). At Site 1261, the uppermost sediment section has not been defined previously. It consists of a thin (30 ms; 24 m) package of parallel, coherent reflections that offlap from the upslope direction (south) and truncate against the seafloor ~10 km downslope from the site. This sequence is likely Quaternary in age, and the base of it is termed Reflector "O."
What has previously been defined as seismic Unit 1 (Miocene–Pliocene) underlies Reflector O. It is largely missing at the other sites. In the immediate vicinity of Site 1261, Unit 1 comprises a well-defined set of coherent seismic reflections of varying amplitudes. The topmost reflections truncate against the Reflector O, sometimes in an angular fashion. The base of the unit is Reflector A, just above which is a 50-ms-thick (~40 m) zone of incoherent reflections capped by a bright reflector. This interval appears to be a debris flow. Unit 1 is 415 ms thick (~367 m) at the drill site.
Unit 2 is below Reflector A, which is the presumed lower Miocene erosional unconformity. The base of seismic Unit 2 is correlated to Reflector B at 586-ms subbottom (550 mbsf). It is represented by a sequence of high-amplitude, parallel, coherent reflections that are relatively flat lying, dipping 0.5° to the north. Seismic Unit 3, between Reflectors B and C, is estimated to be 150 ms thick (175 m). Much of the acoustic energy in the high-resolution site survey profile (line GeoB213) is lost within the highly reflective Unit 2. Little detail is resolved in Unit 3 as a result. It appears as an acoustically transparent package with occasional semicoherent reflectors at the top and at the very base, just above Horizon C. It is difficult to correlate the Horizon B' and thus to distinguish Subunits 3a and 3b resulting from this low reflectivity. Horizon B' has been correlated to the base of the slightly coherent section near the top of Unit 3, but the tie is uncertain.
Reflector C, at the base of the section of interest, is an unconformity. No coherent subsurface data are recognizable in the survey data, but the nearby industry line C2206a shows the underlying section. At Site 1261, the unconformity appears as a disconformity, and it is difficult to actually pick up Reflector C as a single event. Further below, reflections form a broad anticline, fault bounded to the southwest and folded into a syncline to the northeast.
A listric normal fault is shown on line GeoB213 5 km southeast of the drill site. This fault has offset the entire sediment column. Apparent displacement across the fault is ~30–60 ms in the deeper portion of the section (at Reflector C), whereas in the upper portion, it is on the order of 200 ms. This discrepancy in offset can be accounted only by invoking either significant rotation or slumping in the upper sediment column. The fault splays at 300-ms subbottom to the seafloor, showing significant tilting of reflections in the interval between offsets and providing further evidence of slumping.
At Site 1261, two RCB holes were cored. Hole 1261A was spot cored to a depth of 236.9 mbsf, with cores taken between 0 and 22.5, 69.7 and 79.3, 131.4 and 141.1, and 189.1 and 198.7 mbsf. Continuous coring proceeded from 236.9–659.8 mbsf. The recovery in Hole 1261A was 73.4% for the cored intervals (Table T1). Hole 1261B was washed to 525.3 mbsf and cored to 674.1 mbsf. Recovery was 62.2% within the cored intervals.
Lithologic descriptions of the cores and biostratigraphic age assignments reveal a sedimentary succession with several hiatuses. Sediments at Site 1261 range in age from Pleistocene to Cenomanian. Approximately 13 m of nannofossil ooze of Pleistocene age at the top of the section unconformably overlie an apparently continuous ~300-m-thick succession of lower middle Pliocene–upper Miocene nannofossil clay. Sedimentation rates in the Neogene are extraordinarily high with values of 65 m/m.y. A ~60-m-thick upper Miocene matrix-supported conglomerate is the base of the Neogene at Site 1261. A disconformity separates this debris flow from a concordant succession of middle (planktonic foraminiferal Zone P14; calcareous nannoplankton Zone NP17) to lower Eocene calcareous chalk, porcellanite, and limestone. A 4-m.y. hiatus covering the lower Eocene (planktonic foraminiferal Zone P6) lies above the P/E boundary, which was recovered in Hole 1261A.
As at the other Leg 207 sites, the upper Paleocene clayey nannofossil chalk contains zeolites or locally abundant opal-CT lepispheres that replaced siliceous microfossils in this interval. Foraminifers in the middle part of this succession are very well preserved. The K/T boundary layer is absent from this site because of a hiatus covering most of the lower Danian (planktonic foraminiferal Zone P1). The subjacent upper Maastrichtian greenish gray to upper Campanian claystone with nannofossils is fairly condensed and displays cyclic color banding between light greenish gray and greenish gray on a decimeter scale. Foraminifers and nannofossils are rare in this interval, and a significant increase in abundance of diagenetic calcite and carbonate debris is observed. Planolites, Chondrites, and Zoophycos burrows are abundant, as are barite and pyrite crystals. Average sedimentation rates in the Maastrichtian–Campanian interval were 3.3 m/m.y. Magnetostratigraphic age control in this interval is excellent, with well-defined paleomagnetic datums C29r to C32r present in both holes.
There are numerous very dark-colored intervals, cyclic in appearance, in the lower part of the Campanian. A condensed glauconite-rich interval separates the claystone from a ~8-m-thick laminated black shale sequence. The contact between upper Campanian and the mid-Cretaceous black shales is very sharp and may represent a hardground. Site 1261 represents the most expanded black shale sequence of the Leg 207 paleoceanographic depth transect. Approximately 10 m of Santonian sediments overlies ~10 m of lower Coniacian black shales. The Turonian epoch is virtually completely represented and underlain by 40 m of upper Cenomanian black shales. Preservation and abundance of calcareous microfossils is rated poor to moderate. The maximum TOC content in these sediments is 16 wt%. Rock-Eval analyses indicate Type II kerogen, which is consistent with a marine origin of the organic matter. Fish scales, bone fragments, and amorphous to cryptocrystalline phosphatic nodules are common.
A disconformity separates the Cenomanian-age laminated black shales and limestones from the underlying quartz sandstone and silty claystone that is upper Albian–Cenomanian in age, according to a single nannofossil datum. The oldest sediment cored at Site 1261 is a limestone with abundant oysters of the genus Aucillina.
The main objective of Leg 207 was to recover sediments containing microfossils through major and abrupt paleoclimate events of the Paleogene and Cretaceous Periods, such as the E/O boundary, P/E boundary, and the Cretaceous OAEs.
Sediments spanning the P/E boundary were recovered in Hole 1261A. The boundary interval comprises a sharp contact between light green chalk over dark green clay followed by a reddish clay-rich interval similar to Sites 1258 and 1259. The sharp contact reflects the sudden decrease of carbonate content between the upper Paleocene and the lowermost Eocene associated with the P/E boundary. The K/T boundary was not represented at this site.
A ~87-m-thick Santonian to upper Cenomanian succession of laminated black shales and laminated foraminiferal limestones, including OAEs 3 and 2, was recovered in both holes at Site 1261. This interval is the most expanded record of the Santonian to Coniacian OAE 3 of Leg 207 (20 m). The variability of the claystone and chalks/limestone comprising the black shales resulted in strong signal-to-noise ratios in both the GRA bulk density and natural gamma ray data sets. These data sets, combined with good RCB recovery over a significant portion of the black shale interval, allowed for the construction of a continuous composite section with only three small gaps. If these alternations prove to be periodic, then there is a good opportunity for high-resolution age control with orbital tuning.
Site 1261 is the third of the five sites dominated by the presence of an IW brine. Chloride increases in a linear fashion to the base of the black shales, where the highest chlorinity encountered on Demerara Rise is observed (907 mM; 62% greater than standard seawater). Unfortunately, it was not possible to extract pore water from the very thin interval of quartz sandstone that was recovered immediately below the shales. It could not be verified, therefore, whether the brine was sourced laterally through the black shale sequence, as was suggested by the Site 1257 chloride profile.
Unlike previous Leg 207 sites, organic matter degradation reactions occur outside the organic-rich black shales. Sulfate decreases to zero in the upper 200 mbsf within the rapidly deposited (65 m/m.y.) Pliocene nannofossil clay. Correspondingly, ammonium, a common respiration product of organic matter diagenesis, increases sharply to ~1.5 mM within the upper 140 mbsf, and then slowly increases with depth to a maximum of 1.8 mM in the black shales. Methane increases modestly to ~2,000 ppmv in the upper 500 mbsf, then increases sharply near the top of the black shales to a maximum of ~110,000 ppmv. In contrast, alkalinity does not increase with depth as expected within the sulfate reduction zone, but decreases from the seafloor to a minimum at ~480 mbsf. The decreasing alkalinity corresponds with a decrease in calcium between ~250 and 375 mbsf and the interval of well-cemented Eocene chalks and limestones, indicating carbonate precipitation.
Index properties, P-wave velocities, and GRA densities were measured on core samples from Site 1261. Downhole logging runs included the triple combo and the FMS-sonic tool strings, acquiring borehole caliper, acoustic velocity, formation density, porosity, electrical resistivity, and natural gamma ray emission data. The WST was run for check shot velocities, but the tool could not be lowered past a bridge at 210 mbsf. Five WST stations were acquired in this upper interval. Logging data show the seafloor to be at 1887.8 mbsl, 12.1 m shallower than the drillers depth.
Logging data in the upper ~380 mbsf are highly suspect as a result of a highly variable hole diameter through this interval. Consequently, correlation between laboratory-measured densities and logging densities are poor. Log velocity data are not affected as severely by the hole diameter and correlate well with discrete core measurements in the laboratory and the check shot velocities measured with the WST tool. Physical property data show this interval is normally consolidated, with linear increases in density and velocity and a decrease in porosity. A significant perturbation in the porosity and velocity curves correlates with a major debris flow interval between ~320 and 380 mbsf.
Logging density data become stable below ~380 mbsf. They show a significant increase between 380 and 500 mbsf, concomitant with higher velocities. This interval corresponds with the Eocene sequence that has high clay and siliceous fractions relative to overlying sediments. A gradual decline in velocity and density corresponds with the P/E boundary. A subsequent sudden increase in these properties correlates with the K/T boundary and the underlying Maastrichtian chalk sequence. Absolute values show an overall drop with a high degree of scatter through the Cenomanian and Turonian black shale sequence. Velocity and density maximums in this interval correspond with limestone or coarse-grained beds, and low values in density and velocity correspond with organic-rich intervals.
Shallow marine quartz sandstones with ammonite casts and limestones with oysters represent the oldest sequence recovered at Site 1261. They are overlain by a black shale sequence that dates from upper Cenomanian to Santonian, but most of it is Cenomanian to Turonian with thinner Coniacian and Santonian intervals. Occasional clayey bentonite layers indicate the proximity of volcanoes. The laminated, coarse-grained foraminiferal limestones in the succession may be related to winnowing, grain flows, or reflect changes in carbonate productivity. Continuous deepening characterizes the remaining Upper Cretaceous succession. During the OAE 2, high TOC values and very distinct laminations indicate bottom water anoxia. The top of the black shale interval is a sharp contact between the Santonian and overlying glauconite-rich upper Campanian claystones. At present, it is unclear whether the observed glauconite-rich intervals in the black sediment deposits of the Turonian and Coniacian reflect periods of oxygenation, condensation, or both.
Oxic conditions were established by the late Campanian, when sedimentation on Demerara Rise changed from hemipelagic to pelagic. The cyclic pattern of trace fossil abundance, however, suggests that a reduction in bottom water oxygenation recurs.
Maastrichtian- to Pleistocene-age sediments at Site 1261 consist of pelagic deep marine clayey chalk, claystone, limestone, and clays. Sedimentation rates varied from 3.3 m/m.y. in the Maastrichtian, 7 m/m.y. in the Paleocene and early Eocene, to 8.9 m/m.y. in the middle Eocene. These are the lowest values observed during Leg 207 for these intervals. Average sedimentation rates for the upper Miocene–lower Pliocene reach the extremely high values of 65 m/m.y. The pelagic succession is interrupted by a few hiatuses—an lower Paleocene hiatus covering ~4 m.y., a ~2.5-m.y. hiatus in the lower Eocene, and a hiatus representing ~31 m.y. covering the upper Eocene, Oligocene, and lower Miocene. These hiatuses may reflect periods of slow deposition and/or erosion.