Site 1258 is located at a depth of 3192.2 mbsl on the gently dipping western slope ~2° of Demerara Rise, ~380 km north of Suriname. The site is located on a ridge of Paleogene sediments outcropping on the seafloor. Site 1258 is the distal and deepest end-member of the paleoceanographic depth transect across Demerara Rise. The major objectives are similar to those of Site 1257:

1. Core and log a Paleogene–Albian section to evaluate paleoceanographic and paleoclimatic changes, with emphasis on major and abrupt events during this interval such as the E/O boundary, the P/E boundary, and the Cretaceous OAE.
2. Reconstruct the history of the opening of the equatorial Atlantic gateway by obtaining benthic proxy data. These data will help understand changes in bottom water circulation over Demerara Rise during the gradual opening of the seaway.
3. Recover continuous and expanded sediment records of the Paleogene and Cretaceous to reconstruct short- and long-term changes in greenhouse forcing.

The seismic stratigraphy established for Demerara Rise, including Horizons A, B, B', and C, have been correlated to Site 1258 strata. Reflector A, representing the top of a presumably early Miocene erosional unconformity, crops out at the seafloor at the site (within the resolution of the survey data). Between Reflectors A and B, seismic Unit 2 is 300 ms thick (265 mbsf), using downhole logging velocity information. Seismic Unit 3, between Reflector B at 300-ms subbottom and Reflector C at 480-ms subbottom, is a 94-m-thick flat-lying sequence that dips 1.5° to the north-northwest.

Seismic Unit 2 shows an echo character of reasonably coherent but slightly contorted reflections with offsets, which probably describes a sediment sequence that has undergone mass failure and rotational displacement. Seismic Unit 3 is divided into two subunits: the basal subunit (Subunit 3b) lies between Horizons B' and C (450- to 515-ms subbottom; 417–480 mbsf). It is defined on the basis of a series of strong, parallel, coherent reflections that are laterally contiguous and have been shown to correlate to the black shale interval.

At Site 1258, three RCB holes were cored. Hole 1258A was cored to 447.5 mbsf, with 83.9% recovery, and Hole 1258B to 460.9 mbsf, with 76.3% recovery (Table T1). Hole 1258C was washed from 0 to 120.0 mbsf. To obtain a splice for a lower–lower middle Eocene succession, missing because of faulting in Hole 1258A, Hole 1258C was spot cored between 120.1 and 206.1 mbsf. Spot coring the interval between 245.3 and 274.2 mbsf obtained a third copy of the K/T boundary interval. Subsequent washing until 384.8 mbsf was followed by coring of the hole until a total depth of 485.0 mbsf. The average recovery in Hole 1258C was 74.9%. Hard beds between 320 and 380 mbsf and between 415 and 420 mbsf reduced core recovery in these intervals.

Stratigraphy of Site 1258

Lithologic description of the cores and biostratigraphic age assignments revealed a rather continuous sedimentary succession with only a few hiatuses. Sediments at Site 1258 range in age from Miocene to middle Albian. An ~8-m-thick package of Miocene nannofossil ooze unconformably overlies a drape of calcareous ooze with radiolarians and diatoms of early Oligocene age. This sequence, in turn, unconformably overlies an expanded 143-m-thick succession of lower middle Eocene (planktonic foraminiferal Zone P10; calcareous nannoplankton Zone NP15) to lower Eocene nannofossil chalk. With a sedimentation rate of ~20 m/m.y., the middle–lower Eocene succession is a remarkably expanded and complete (to zonal level) section. Excellent RCB recovery provided continuous core overlap from the middle Eocene to ~10 m below the P/E boundary. The periodic variability present in the Eocene magnetic susceptibility data at Site 1258 will provide a good basis for postcruise cyclostratigraphic studies. Age control is excellent, with well-defined paleomagnetic datums present in the section (e.g., the top of Subchron C21n and the base of Subchron C22r). Preliminary investigation suggests that the dominant frequencies of the magnetic susceptibility data are likely Milankovitch Periods, with significant power at ~20, 40, and 100 k.y.

At all three holes, we recovered an apparently expanded section across the P/E boundary. As at Site 1257, the upper Paleocene clayey nannofossil chalk sequence is relatively thick. Zeolite or locally abundant opal-CT lepispheres replace siliceous microfossils in this interval. In all three holes, we recovered the K/T boundary, and the KT-ejecta layer is present in Core 207-1258B-27R. The subjacent upper Maastrichtian greenish gray nannofossil chalk with foraminifers and clay and lower Maastrichtian–lower Campanian zeolitic nannofossil claystone display cyclic color banding between light greenish gray and greenish gray on a decimeter scale. The succession is considerably expanded with planktonic foraminifers missing in most of the samples investigated. Radiolarians in the Campanian are well preserved. The lithology becomes increasingly clay rich downhole, and carbonate contents decrease to 35%. 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 12 m/m.y.

A condensed glauconite-rich horizon in Core 207-1258B-44R separates the Campanian clayey chalk from the ~60-m-thick black shale sequence below. This interval contains the lower part of OAE 3, a complete OAE 2, and an expanded succession of laminated shales of Cenomanian–middle Albian age. The preservation and abundance of calcareous microfossils is poor to good, with glassy foraminifers present in the Cenomanian part of the black shales. A middle Albian disconformity separates the laminated black shales and limestones from the underlying TOC-rich claystones with phosphatic concretions. Although rich in predominantly marine organic matter (up to 5%), the sediments lack obvious laminations. Clay-rich beds in the latter yield some extremely well preserved microfossils. In addition, ammonites as small as a centimeter in diameter are abundant in some laminae. Rare and thin bioclastic limestone intervals are intercalated; they may represent occasional small-scale storm deposits. The base of Site 1258 is dated as late early Albian age (Tenuipteria primula planktonic foraminiferal zone and Subzone NC8a–b).

Recovery of Critical Intervals

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 OAEs.

In the three holes cored at Site 1258, sediments spanning the P/E boundary were recovered. The boundary interval comprises the last occurrence of benthic foraminifer Gavelinella beccariiformis, followed by a sharp contact between light green chalk and dark green clay. The sharp contact reflects the sudden decrease of carbonate content from values of ~60 wt% CaCO₃ in the upper Paleocene to ~13 wt% in the lowermost Eocene associated with the P/E boundary. Green clay-rich sediments prevail from the P/E boundary to ~1.90 m above the boundary. Light green carbonate-rich sediments are the dominant lithology of the lower Eocene. Site 1258 appears to provide the first highly expanded P/E boundary succession known from tropical oceans.

The K/T boundary was recovered in all the three holes cored at Site 1258, and the K/T-ejecta layer is present in Core 207-1258B-27R. The base of the K/T boundary is marked by a 1-mm-thick layer of clayey spherules overlying upper Maastrichtian chalks and is covered by a thin (2–3 mm) drape of whitish nannofossil chalk of potentially reworked upper Maastrichtian sediments. Alternatively, this lower lamina of spherules may represent particles that settled through a soupy portion of fine carbonate (nannofossils) that had been either fluidized or suspended by dewatering of the upper Maastrichtian sediment column induced by the K/T impact. The 2-mm-thick white layer is overlain by a 2-cm-thick graded bed of medium to fine sand-sized green spherules, which are, in turn, overlain by gray clay. The Parvulorugoglobigerina eugubina planktonic foraminiferal zone (P) is remarkably expanded (~3 m thick) and is overlain by planktonic foraminifers belonging to Danian foraminiferal Subzones P1a–P1c, suggesting that the recovered succession is expanded and complete (at the zone level). The recovery of the ejecta layer is, to our knowledge, the first such in South America. Demerara Rise lies ~5000 km southeast of the impact crater in Yucatan. The distribution of documented ejecta deposits west, north, and northeast of Yucatan is one of the supporting lines of evidence to suggest the K/T bolide approached from a southeasterly direction.

A ~60-m-thick Coniacian–Albian succession of laminated black shales and laminated limestones, including OAE 2, was recovered in all three holes at Site 1258. Average recovery of this interval was 77%, and correlation between the holes and the high quality FMS logs will allow for a continuous reconstruction of the interval. The transition between Campanian chalks and the underlying black shales is represented by a hiatus covering the entire Santonian. Coniacian organic-rich sediments are slumped or missing. OAE 2 is represented by an interval of distinctly laminated black shales with TOC values up to 28 wt%. The main lithology consists of dark olive-gray to black finely laminated calcareous claystone with organic matter (black shale) and clayey chalk and limestone with organic matter. The unit shows well-developed submillimeter-scale laminations and has a strong petroliferous odor. Rhythmic color variations between dark olive gray to black are visible on a decimeter scale. Carbonate contents vary between 5 and 95 wt%, and the constituents include nannofossils (concentrated in fecal pellets), foraminifers, and shell fragments. TOC values range from ~5 to 28 wt% in the black shales. 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. The black shales facies continue until the upper middle Albian, including the middle Cenomanian Event, and the lowermost black shales are dated as late middle Albian (Biticinella breggiensis planktonic foraminiferal zone). Storm deposits with oyster and other pelecypod fragments are present, especially toward the base of the black shale interval. Occasional layers of diagenetic calcite with a distinct bluish tint are noted. Microscope analysis reveals that the morphology of the calcite growth resembles that of authigenic methane carbonate cements and may derive from the high methane content in the black shales. To date, no upper Albian biostratigraphic markers have been identified that would enable a stratigraphic definition of the uppermost Albian OAE 1d.

Geochemistry

Active microbial organic matter diagenesis is focused within the organic-rich black shales of Unit IV, similar to Site 1257. Pore water sulfate decreases linearly throughout the overlying units to the top of the black shales at ~390 mbsf. Correspondingly, ammonium, which is produced by organic matter degradation, increases linearly through the same interval. In contrast, alkalinity, a byproduct of sulfate reduction, does not increase linearly with depth but varies with multiple maxima and minima. The correspondence between alkalinity and calcium profiles and intervals of increased lithification suggests alkalinity and calcium variability is controlled by carbonate diagenesis. As expected, where pore water sulfate approaches zero (approximately the top of the black shales), methane contents increase sharply to high values (~3000–60,000 ppmv). Interstitial gas volumes and methane/ethane ratios are higher at Site 1258 than at Site 1257, probably reflecting the higher TOC contents of Units IV and V at Site 1258. Unlike Site 1257, however, Site 1258 pore waters are not characterized by the presence of brine. Salinity and chlorinity decrease with depth beginning at ~300 mbsf, then sharply decrease within the clayey chalks of Unit V. The minimum chloride value at the base of the hole is 465 mM, a 17% decrease from seawater chlorinity. At present, the origin of the low-salinity fluids is undetermined.

Physical Properties and Logging

Core physical property data (MST and discrete measurements) and downhole logging physical property data show excellent agreement. The downhole profiles show marked variability linked to lithologic change. In general, they exhibit a normal consolidation profile down to ~267 mbsf, with a strong negative excursion in velocity and density encompassing the P/E boundary. Below 267 mbsf, the profiles remain constant or decrease slightly, correlating with lithologic Subunit IIC to Unit V. The data variability in this interval also shows patterns of cyclicity. The most pronounced change correlates with lithologic Unit IV, the black shales. In this case, lower density and highly variable velocity values, which reach in excess of 2600 m/s, characterize the unit. Other significant deviations from the normal consolidation trend correlate with events and hiatuses identified in the lithostratigraphy, such as the P/E boundary, the K/T boundary, and the top and bottom of the black shale sequence. These pronounced physical property contrasts yield strong reflection characteristics and allow for good correlation with the seismic stratigraphy.

Depositional History

Lower–middle Albian clayey and TOC-rich sediments represent the oldest sequence recovered at Site 1258 and reflect open marine conditions in a marginal or epicontinental setting. They are unconformably overlain by middle Albian–Coniacian black shales. At present, it remains unclear whether the contact between these two lithologies represents Reflector C, which separates synrift from drift sediments on Demerara Rise. If so, the change between synrift and drift sedimentation at Site 1258 occurred within one planktonic foraminiferal zone, which would give the reflector a late middle Albian age (B. breggiensis planktonic foraminiferal zone). Although no major facies change is observed between middle Albian and upper Cenomanian, the lack of late Albian index fossils point to a hiatus in this interval. The abundance of storm deposits in the lower part of the succession might indicate a paleowater depth above storm-wave base and continuous deepening characterizes the remaining Lower Cretaceous succession. During the OAE 2, highest TOC values and very distinct laminations indicate severe bottom water anoxia. Slumping and reworking of organic-rich sediments and black shales occurred during the Coniacian. The top of the black shales is represented by a hiatus covering the Santonian and lower Campanian. At present, it is unclear whether the observed mass flows in the Coniacian are part of the tectonic movements related to the opening of the equatorial Atlantic gateway. The contact between upper Coniacian slumped black shales and condensed lower Campanian glauconite-rich chalk is erosional.

Oxic conditions were established by the early 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 distribution of trace fossils, indicative of oxygen deficiency, suggests recurring decreases of bottom water oxygenation. The rather continuous Maastrichtian–Eocene pelagic record at Site 1258 records the results of the K/T impact with a graded bed of spherules. The spherules are up to 2.5 mm in diameter, an exceptional size considering the distance to the proposed impact crater.

Interpretation of the seismic records, in conjunction with the observation of mass-wasting deposits and "missing" intervals between holes, indicates that a significant portion of the cored stratigraphic column has failed in a slope instability event or events. The likely scenario is a rotational slump whereby material has been displaced but not transported far and the stratigraphy is left largely intact.