Site 1258 is located in the greatest water depth (3192 mbsl) of the Leg 207 Demerara Rise depth transect. From the three holes at Site 1258, we recovered 79% of the ~1120 m cored (Fig. F4).
Five lithostratigraphic units were recognized at Site 1258 (Table T2). The oldest unit recovered (Unit V) is dominated by phosphoritic calcareous claystone. The superjacent unit (Unit IV) is composed dominantly of laminated calcareous organic-rich black shale and laminated chalk/limestone. The youngest three units (Units I–III) recognized are pelagic and composed dominantly of calcareous microfossils, siliceous microfossils, and clay. These pelagic sediments are pervasively bioturbated; pyrite and other iron sulfides are found throughout the pelagic interval but are most abundant in Unit III. Clay and zeolite are also relatively abundant in Unit III, whereas Subunit IIA contains a relatively high abundance of either siliceous microfossils or zeolite. The carbonate content for the pelagic Units I–III varies from 30 to 80 wt%. Sediments and sedimentary rocks recovered at Site 1258 range in age from Albian to Miocene (see "Biostratigraphy"), but the record is interrupted by at least two major and several minor hiatuses and/or gravity flow deposits.
Lithostratigraphic units recognized at Site 1258 generally parallel those reported for the other Leg 207 sites and Site 144 (Hayes, Pimm, et al., 1972). Differences among the lithologic divisions represent differences in the level at which unit boundaries were placed and can be attributed to uncertainty related to coring gaps at Site 144 and to differences in the definition of the Unit II/III boundary between Sites 1257 and 1258.
Unit I is predominantly a nannofossil ooze that exhibits a gradual color change downhole from pale yellow to light greenish gray. The unit was cored only in Holes 1258A and 1258B. It contains abundant black mottles, but other indications of bioturbation are rarely visible throughout. There is a sharp color change observed only in Hole 1258A from pale yellow/pale brown to greenish gray at 3.47 mbsf. However, neither sediment composition nor sediment fabric changes noticeably at this boundary, and it is interpreted as a diagenetic redox boundary that propagated down from the seafloor. Below this color change, the sediments are light greenish gray, with only subtle variations. The base of Unit I is placed at a distinct lithologic change from nannofossil ooze to nannofossil chalk and corresponds to a major stratigraphic hiatus from the early Oligocene to middle Eocene.
Unit II consists of light greenish gray to greenish gray nannofossil chalk containing 30–80 wt% carbonate content in dominant lithologies (Fig. F4). Zoophycos and Chondrites trace fossils are found throughout. The unit is divided into three subunits. Subunit IIA contains 10%–25% foraminifers and 10%–25% clay and zeolites. Subunit IIB is defined by a marked increase in abundance of diagenetic calcite and carbonate debris (values of up to 70% of material identified in smear slides). Subunit IIC is dominated by nannofossils and foraminifers, but clay content is as high as 45%. The upper contact of Unit II is placed at a distinct lithologic change from nannofossil ooze to nannofossil chalk, whereas the bottom contact corresponds to an equally prominent downhole increase in clay content.
Subunit IIA (Fig. F5A) is composed of carbonate-rich (nannofossils and foraminifers) pelagic sediment with minor amounts of clay and either zeolite or radiolarians and diatoms. The latter two were observed only in the upper and middle parts (base of lower Eocene) of the subunit. The top of this subunit is placed at a distinct lithologic change from nannofossil ooze to nannofossil chalk at 8.52 mbsf (Hole 1258A). The lithologic change coincides with the beginning of a steady decrease in carbonate content from 70–80 wt% to values stabilizing at ~30–50 wt%. There are also color changes associated with the top of Subunit IIA. In Hole 1258A, this ooze to chalk transition is associated with a change from greenish gray to light greenish gray at 8.52 mbsf, contrasting with the top of Subunit IIA in Hole 1258B, which is characterized by a change from pale yellow to darker pale yellow. This contrast in color between the two holes at the top of Subunit IIA indicates that this depth horizon in Hole 1258B is still above the speculated diagenetic redox boundary found in Unit I in Hole 1258A (3.47 mbsf); this boundary does not occur in Hole 1258B until slightly deeper in Subunit IIA at 7.02 mbsf. Below this redox color change, the sediments are characterized by lighter–darker color cycles, which persist throughout Subunit IIA at varying intensities. Black mottles and streaks are found throughout and are often concentrated in burrows. Sediments range from homogeneous to burrow mottled, and the number of discrete traces increases downhole. The base of Subunit IIA is placed at a marked increase in abundance of diagenetic calcite and carbonate debris (values of up to 70% of material identified in smear slides), which also coincides with a color change to a predominance of reddish brown.
The Paleocene/Eocene Thermal Maximum (PETM) is present in this subunit. In Section 207-1258A-19R-5, the sediment changes gradually from greenish gray chalk to brownish clayey chalk followed by dark greenish, faintly laminated, carbonate-free clay. The contact with the underlying chalk (Section 207-1258A-19R-5, 116 cm) is sharp. The onset of the PETM is marked by the last occurrence of bioturbation 2 cm into the chalk.
Subunit IIB consists of calcareous chalk with foraminifers (upper part of Fig. F5B), with a distinct yellowish red and yellowish brown color in parts. The color is more intense downcore. Typically, light greenish gray to olive-gray burrows and mottles are observed, which seem to represent reducing environments around the burrowed sediment. The subunit is mottled throughout and is pervasively bioturbated except for short intervals marked by basal layers of opaque minerals. Discrete trace fossils include Zoophycos, Chondrites, and Planolites. Traces of pyrite are present.
This subunit is marked by a significant increase in abundance of diagenetic calcite and carbonate debris. The sediment also contains abundant foraminifers (20%–40%), and clay content increases downhole. In contrast to the overlying and underlying lithologic Subunits IIA and IIC, nannofossils are rare and poorly preserved in this particular interval. Carbonate content varies between 40 and 65 wt%.
The Cretaceous/Tertiary (K/T) boundary is located in this subunit. Interval 207-1258B-27R-1, 49–51 cm, contains a well-preserved 1.5-cm-thick impact ejecta layer of green microspherules (Fig. F5B). The spherules appear to be arranged in layers and grade from 2–3 mm diameter at the bottom of the layer to 1.5 mm at the top; some laminations are lighter than others. Pyrite is also present at the base of this ejecta layer. Beneath the ejecta layer is a very thin interval of white sediment (3–5 mm thick), and below this is another single layer of green spherules. The lower contact is sharp and represents an abrupt lithologic and color change from the dark green ejecta layer above to the mottled light green-gray calcareous chalk below.
Subunit IIC consists of a greenish gray nannofossil chalk with foraminifers and clay, which is moderately burrowed with black and white mottles (Fig. F5C). Distinct Zoophycos burrows (black) are common to abundant throughout the entire subunit. Chondrites burrows appear downcore and are abundant. Barite and pyrite concretions up to several centimeters in diameter are common in the lower part of Subunit IIC. A pronounced cyclic color banding (decimeter scale) in various shades of light greenish gray and greenish gray is observed throughout this subunit (Fig. F5C). Smear slide analysis showed abundant diagenetic (blocky) calcite, foraminifer fragments, and carbonate debris. Carbonate content varies between 50 and 80 wt%.
Unit III consists of calcareous nannofossil clay displaying cyclic color banding between light greenish gray and greenish gray on a decimeter scale (Fig. F6A). Barite and pyrite crystals are abundant throughout this unit. The boundary between Subunit IIC and Unit III is defined by a sharp change in the composition, texture, and bioturbation of the sediment, although the color (greenish gray) remains the same. The primary constituents of Unit III are clay and calcite. Nannofossil, foraminifer, and coccolith contents increase downhole. Trace amounts of pyrite, quartz, and zeolites are present in smear slides. The contact is also characterized by a drop in carbonate content from ~65 wt% in Subunit IIC to ~35 wt% in Unit III. Bioturbation increases from moderate in Subunit IIC to pervasive throughout Unit III. Bioturbation slightly obscures the color cycles. Planolites, Chondrites, and Zoophycos burrows are abundant and occasionally have black halos of monosulfide. The lower contact of Unit III is placed at the transition to laminated lithologies.
Two different boundary contacts were captured for the lower contact of Unit III. In Hole 1258B, a gradual boundary between Units III and IV is observed in Section 207-1258B-44R-2, covering ~40 cm, and is expressed in both lithologic and color changes (Fig. F6A, F6B, F6C). The transition is gradual from the major lithology of Unit III, greenish gray calcareous nannofossil clay, to a greenish gray quartzose nannofossil chalk with abundant glauconite and fish remains, to a black calcareous claystone with organic matter in Unit IV. The minor lithology in Unit III, quartzose nannofossil chalk, is composed mainly of glauconite, quartz, some feldspar, and nannofossils (Fig. F7A). The base of Unit III in Hole 1258C displays a sharp lithologic and color contact between Units III and IV in Section 207-1258C-15R-3, 96 cm (Fig. F6D). Above the contact, pyrite nodules with glauconite were observed. The color changes from greenish gray to black. The top 15 cm of Unit IV is massive, suggesting that this interval is slumped. The Unit III/IV contact was not recovered in Hole 1258A but the core catcher of Core 207-1258A-41R contains abundant glauconite followed by black shale from Unit IV at the top of Core 42R.
Lithostratigraphic Unit IV primarily consists of dark olive-gray to black finely laminated calcareous claystone with organic matter (black shale) and clayey chalk and limestone containing organic matter (Figs. F7B, F8A). The unit shows well-developed submillimeter-scale laminations and has a strong petroliferous odor. Rhythmic color variations between dark olive gray and black are visible on a decimeter scale throughout (Fig. F8A). The lighter colors reflect higher carbonate content in general and strong diagenetic calcite growth in particular. Thus, carbonate contents are highly variable and range from as high as ~95 wt% in individual carbonate-rich layers to as low as ~5 wt% in black claystones (Fig. F4). Carbonate constituents include nannofossils (concentrated in fecal pellets), foraminifers, and shell fragments (e.g., inoceramids). Total organic carbon (TOC) values range from ~5 to 28 wt% in the black shale. Rock-Eval pyrolysis analyses indicate Type II kerogen, which is consistent with a marine origin of the organic matter. The organic matter is clearly visible in thin sections (Fig. F7B) (Sample 207-1258B-45R-4, 114–117 cm; TOC = 28 wt%). Fish scales and bone fragments (Francolite) (Fig. F7C) and amorphous to cryptocrystalline phosphatic nodules (Collophane, up to 2 cm diameter with soft and hard consistency) are common throughout. They are present either parallel to bedding or concentrated in thin gravity deposits and appear as white to light brown blebs and streaks on core surfaces (Figs. F8B, F9B).
The black shales in the uppermost part of the unit show clear signs of mass movement; the sediment is distorted, contacts dip steeply, and laminae are not visible. Shell material, clasts of laminated claystone, and phosphoritic clasts are scattered throughout the sediment and show a distinct matrix-supported fabric (Fig. F6C). In Section 207-1258C-15R-3, 96 cm, the topmost part of the black shales is distorted (Fig. F6D). This deformation is seen in thin sections as distinct kinks in the laminated fabric.
In the lower part of the unit, the rhythmic chalk lithology is replaced by well-cemented limestones with sharp basal contacts, grading, and gradual laminated transition into black shale (Fig. F8C). Oyster and other pelecypod fragments are present, especially toward the base, and can form discrete lags (Fig. F9C). The limestones are rich in planktonic foraminifers, which are concentrated in layers. Diagenetic calcite formation is common. Occasional layers of diagenetic calcite with a distinct bluish tint are present (Fig. F9A). The calcite may grow perpendicular to the bedding, forming several-millimeter-high pinnacles. The graded units are interpreted as tempestites and/or turbidites.
Lithostratigraphic Unit V primarily consists of dark olive-gray to black phosphoritic calcareous claystone with organic matter (black shale). The cores show strong drilling disturbance in parts. Unit V sediments resemble the black shale lithology of Unit IV. Differences are the lack of obvious lamination, the paucity of "tempestites/turbidites," the distinct drop in carbonate content, and the regular presence of phosphoritic layers and ammonites/bivalves on bedding planes (Fig. F7D). Rare and thin bioclastic limestone intervals are intercalated; they may represent occasional small-scale tempestites or turbidites. The carbonate content drops to values between 5 and 25 wt%, whereas the organic carbon content is relatively high, with up to 5 wt% TOC of marine origin.
The oldest interval recovered at Site 1258 is a unit dominated by TOC-rich phosphoritic layers (Unit V) of early–late Albian age. These rocks lie below or in Reflector C (see Shipboard Scientific Party ["Site Survey and Underway Geophysics"], this volume), which represents the trace of an angular unconformity on Demerara Rise and thus may represent synrift deposition. The lithology is quite homogeneous overall but shows distinct bedding and faint lamination. Macrofossils are frequent (fish ammonite fragments), but trace fossils are absent. The lack of bioturbation and benthic organisms, the relatively high TOC content (up to 5 wt% marine organic matter), and the common occurrence of phosphoritic layers suggest a sheltered, probably shallow but fully marine (common ammonites) environment. Rare and thin bioclastic limestone intercalated with the TOC-rich claystones may be tempestites (storm deposits), limiting the water depths to storm wave base or less.
During the middle Albian–Coniacian, high productivity and low bottom water oxygen levels resulting in the preservation of large volumes of organic and phosphoritic matter prevailed at Site 1258. This phase is recorded by sediment dominated by dark laminated calcareous claystones and laminated chalks/limestones of Unit IV. TOC concentrations in Unit IV approach 30 wt%, and the organic matter is of marine origin. Zeolites (clinoptilolite) and rare radiolarians suggest siliceous microfossils were a common component of the sediment. Carbonate values are highly variable (5–95 wt%) and reflect diagenetic dissolution and precipitation of calcite.
Open marine conditions and oxic bottom waters were established in the early Campanian, as indicated by the bioturbated pelagic marls of Unit III. The sharp contact between the bioturbated marls of Unit III and the laminated shales and limestones of Unit IV in Hole 1258C reflects a hiatus, but an expanded glauconite-rich interval in Hole 1258B is consistent with the transition being recorded by a highly condensed interval. Furthermore, the trace fossils and abundance of pyrite may indicate that bottom waters were relatively dysoxic in the late Campanian and early Maastrichtian. Variations in sedimentary fabric and minor sedimentary components, however, suggest that conditions at the seafloor and/or overlying water column fluctuated on geologically short timescales.
The K/T boundary interval seems to be complete in Hole 1258B with the uppermost Maastrichtian of Subunit IIC below, followed by a discrete ejecta interval showing a graded spherule layer, and basal Paleocene calcareous claystone above. The spherules are up to 2.5 mm in diameter, an exceptional size considering the distance to the proposed impact crater.
The P/E boundary record also seems to be relatively complete at Site 1258. A 3-cm-thick clay layer in Hole 1258C falls within an interval that contains the P/E boundary (see "Biostratigraphy"). The lack of carbonate in this layer is consistent with sedimentary changes expected at the PETM.
Gravity flow deposits and temporal gaps are common in the pelagic record at Site 1258. The most prominent one occurs in Hole 1258A, where ~20 m of sediment appears to be missing relative to Holes 1258B and 1258C. This difference is possibly due to faulting or mass-failure displacement.