LITHOSTRATIGRAPHY

Site 1257 is a reoccupation of DSDP Site 144 and represents an intermediate-depth site (current water depth = 2951 mbsl) of the Leg 207 Demerara Rise depth transect. Among the three holes at Site 1257, we recovered >90% of the ~285 m drilled—a far more complete sample of the stratigraphic record at this site than recovered in earlier drilling (Fig. F2).

Five lithostratigraphic units were recognized at Site 1257 (Table T2). The oldest unit recovered (Unit V) is dominated by calcareous silty claystones to clayey calcareous siltstones. The superjacent unit (Unit IV) is composed dominantly of laminated, organic-rich calcareous claystones and laminated limestone. The youngest three units (Units I–III) recognized are pelagic and dominantly composed 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 Unit II contains a relatively high abundance of siliceous microfossils. Unit I is carbonate poor. Sediments and sedimentary rocks recovered at Site 1257 range in age from Albian to Miocene (see "Biostratigraphy"), but the record is interrupted by at least eight hiatuses and/or mass flow deposits.

Lithostratigraphic units recognized at Site 1257 generally parallel those reported for Site 144 (Hayes, Pimm, et al., 1972). There is an offset between unit numbers herein and those used at Site 144 because we recognized a thin upper unit that was not recovered during earlier drilling (Fig. F2). Most other differences between the lithostratigraphic divisions at Sites 144 and 1257 represent differences in the level at which unit boundaries were placed and can be attributed to uncertainty regarding the lithologies in coring gaps at Site 144. However, we recognized an increase in clay and zeolite content (the Unit II–III transition herein) higher in the section than the Unit I/II division at Site 144. Finally, we apparently did not penetrate the lowest unit drilled at Site 144.

Unit I

Interval: 207-1257A-1H-1, 0 cm, through 1H-2, 7 cm

Depth: 0.00–1.57 mbsf

Thickness: 1.57 m

Age: late Neogene

Lithology: nannofossil clay

Unit I is predominantly a nannofossil clay that exhibits a gradual change downhole from pale brown to pale olive (Fig. F3). The unit was only recovered in Hole 1257A. It is burrow mottled, and iron sulfide is present both in discrete burrows and as isolated blebs throughout the unit. The base of Unit I is placed at a sharp color change from pale olive to pale yellow.

Unit II

Intervals: 207-1257A-1H-1, 7 cm, through 10X-3, 55 cm; 207-1257B-1R-1, 0 cm, though 6R-1, 0 cm; and 207-1257C-1R-1, 0 cm, through 1R-3, 95 cm

Depths: Hole 1257A: 1.57–76.85 mbsf; Hole 1257B: <40.00–78.70 mbsf; and Hole 1257C: <82.00–85.95 mbsf

Thickness: 75.28 m

Age: early Oligocene–early Eocene

Lithology: nannofossil ooze with foraminifers, nannofossil chalk with foraminifers, and siliceous nannofossil chalk with foraminifers

This unit consists of light greenish gray to greenish gray chalk and ooze containing 40–80 wt% carbonate in dominant lithologies (Fig. F4). Oozes are restricted to the upper 20 m of Unit I. Zoophycos and Chondrites are found throughout. The unit is divided into two subunits based on biogenic silica content. Subunit IIA contains only traces of radiolarians, whereas siliceous microfossils make up 10%–40% of material in smear slides from Subunit IIB. The upper contact of Unit II is placed at sharp color change that corresponds to an increase in carbonate content, whereas the bottom contact is gradational and corresponds to a downhole decrease in the abundance of siliceous microfossils and an increase in clay and zeolite abundance. It is placed at the base of a gravity flow deposit (Fig. F3) that falls near the P/E boundary, but recovery problems and the gradational nature of the lithologic transition makes precise correlation uncertain.

Subunit IIA

Interval: 207-1257A-1H-1, 7 cm, through 4H-2, 75 cm

Depth: 1.57–23.85 mbsf

Thickness: 22.28 m

Age: early Oligocene

Lithology: nannofossil ooze with foraminifers and nannofossil chalk with foraminifers

Subunit IIA is composed of carbonate-rich (foraminifers and nannofossils) pelagic sediments near the chalk–ooze transition; these lithologies alternate on a 10- to 20-cm scale through most of Subunit IIA. The top of this subunit is placed at a sharp color change from pale olive to pale yellow at 1.57 mbsf (Hole 1257A). The color change coincides with an increase in carbonate content from 15 to 80 wt% and an increase in the degree of induration. There is also a sharp color change from pale yellow to greenish gray at 5.05 mbsf in Hole 1257A. However, neither sediment composition nor sediment fabric changes noticeably at this boundary and it is interpreted as a late diagenetic redox boundary that propagated down from the seafloor. Below this color change, the sediments are light greenish gray with only subtle variations. Dark bands are present (e.g., Sample 207-1257A-3H-4, 58–60 cm [17.13–17.15 mbsf]) and are often associated with burrows. These bands are interpreted as redox boundaries or other chemical gradients from early diagenesis. Black mottles, streaks, and blebs are found throughout the unit 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 the first consistent downhole occurrence of siliceous microfossils.

Subunit IIB

Intervals: 207-1257A-4H-2, 75 cm, through 10X-3, 55 cm; 207-1257B-1R-1, 0 cm, through 6R-1, 0 cm; and 207-1257C-1R-1, 0 cm, through 1R-3, 95 cm

Depths: Hole 1257A: 23.85–76.85 mbsf; Hole 1257B: <40.00–78.70 mbsf; and Hole 1257C: <82.00–85.95 mbsf

Thickness: 53.00 m

Age: early Oligocene–early Eocene

Lithology: siliceous nannofossil chalk with foraminifers to nannofossil chalk with siliceous microfossils and foraminifers

Subunit IIB is composed of a carbonate-rich (foraminifers and nannofossils) chalk containing common to abundant siliceous microfossils. Most of the subunit is pelagic, but there are at least four ~1-m-thick gravity flow deposits (Fig. F3C). Both the top and the bottom contacts of Subunit IIB are gradational. The top of the subunit is placed at the first consistent downhole occurrence of siliceous microfossils. Siliceous microfossils increase in abundance downhole from Sections 207-1257A-6X-1 through 9X-4 and 207-1257B-1R-1 through 4R-4 in both the dominant and minor lithologies; they compose 40% of material in some smear slides. Below these levels, the abundance of siliceous microfossils drops and that of zeolites and clay content increase. The boundary between Subunits IIB and III is placed at the base of a gravity flow deposit below which clay content increases (Fig. F4) and zeolites are more common than siliceous microfossils. The gravity flow also falls near the P/E boundary, but redeposited material and one subjacent sample (Hole 1257C) contain Eocene microfossils, demonstrating that the flow deposit is unrelated to the Paleocene/Eocene Thermal Maximum (PETM).

As in Subunit IIA, the color of Subunit IIB varies between light greenish gray and greenish gray. Subhorizontal millimeter-scale dark green bands are present and are interpreted as early diagenetic redox fronts. Black layers, blebs, and streaks are common and seem to be associated with burrows. Bioturbation is heavy to pervasive throughout Subunit IIB. Material filling discrete burrows varies only slightly from the background lithology. Tan burrow fill contains a relatively high abundance of larger microfossils, whereas light burrow fill contains a relatively high abundance of nannofossils. Finally, in those samples checked, the material in Zoophycos and Chondrites traces contains more siliceous microfossils than material outside the burrows.

Unit III

Intervals: 207-1257A-10X-3, 55 cm, through 20X-4, 52 cm; 207-1257B-6R-1, 0 cm, through 17R-1, 17 cm; and 207-1257C-1R-3, 95 cm, through 11R-1, 3 cm

Depths: Hole 1257A: 76.85–174.50 mbsf; Hole 1257B: 78.70–174.40 mbsf; and Hole 1257C: 85.95–178.00 mbsf

Thickness: 97.65 m

Age: late Paleocene–early Campanian

Lithology: nannofossil chalk and foraminifer nannofossil chalk

This unit is mainly composed of nannofossil chalk with variable amounts of planktonic foraminifers and clay. Zeolite is common in the deposits, and siliceous microfossils are rare to absent. The upper contact is placed at the base of a gravity flow deposit at the P/E contact, which approximates the level below which clay content increases and zeolites are more common than siliceous microfossils (see "Site 1257 Smear Slides"). The base of Unit III is placed at the upper surface of a 5-cm-thick laminated limestone that overlies the organic-rich claystones and siltstones of Unit IV. Unit III is subdivided into two subunits based on differences in the abundance of foraminifers and carbonate content. The contact is placed at the level of a slump that coincides with a hiatus that represents much of the Maastrichtian and Paleocene.

Subunit IIIA

Intervals: 207-1257A-10X-3, 55 cm, through 16X-CC, 60 cm; 207-1257B-6R-1, 0 cm, through 12R-7, 60 cm; and 207-1257C-1R-3, 95 cm, through 7R-5, 15 cm

Depths: Hole 1257A: 76.85–140.70 mbsf; Hole 1257B: 78.70–145.50 mbsf; and Hole 1257C: 85.95–145.15

Thickness: 66.8 m

Age: late Paleocene–middle Paleocene

Lithology: nannofossil chalk and redeposited nannofossil chalk and limestone

Subunit IIIA mainly consists of light greenish gray to greenish gray mottled to homogeneous nannofossil chalk (Fig. F5). Carbonate content generally ranges between 30 and 50 wt% (Fig. F4). The carbonate fraction is dominated by nannofossils, with planktonic foraminifers representing a relatively minor component. Zeolites are consistently present, and siliceous microfossils (radiolarians and sponge spicules) were observed only near the contact with Unit II. Fish debris was observed in trace amounts in several samples.

The P/E boundary falls in the upper portion of Subunit IIIA. It is constrained to within a ~2.5-m interval in Sections 207-1257C-1R-3, 125 cm, through 1R-CC (see "Biostratigraphy"). A 3-cm-thick green clay layer in Sample 207-1257C-1R-5, 48–51 cm, is the most likely candidate for a lithologic expression of events at the boundary. Unfortunately, this interval was not recovered in Holes 1257A or 1257B. Further, although a mottled fabric is still apparent, drilling disturbance seems to have shattered the rocks below the putative boundary. Otherwise, burrow mottling is present throughout Subunit IIIA and discrete traces assigned to Zoophycos and Chondrites are common. Most discrete traces are darker than surrounding sediments, but lighter white and tan traces are also present. A high abundance of Chondrites and Zoophycos traces filled with darker material is seen in Cores 207-1257A-14X and 207-1257B-11R.

The upper contact of Subunit IIIA is gradational and is placed at the base of a slump (Fig. F3) that approximates the level where the abundance of siliceous microfossils decreases and zeolites appear in smear slides. The lower contact of Subunit IIIA is placed at the base of a gravity flow deposit in Hole 1257A that corresponds to the contact between early Maastrichtian and middle Paleocene deposits (Fig. F5). The latter deposit is clast supported and contains angular pieces of greenish gray chalk and white limestone up to 75 cm across. The clasts exhibit internal burrowing, but no burrows seem to penetrate matrix and clasts in the deposit. The clasts and matrix examined in this interval contain Paleocene microfossils, as does the superjacent pelagic chalk. Pelagic chalk below the slump, on the other hand, contains only Cretaceous fossils (see "Biostratigraphy").

Subunit IIIB

Intervals: 207-1257A-16X-CC, 60 cm, through 20X-4, 52 cm; 207-1257B-12R-7, 60 cm, through 17R-1, 17 cm; and 207-1257C-7R-5, 15 cm, through 11R-1, 3 cm

Depths: Hole 1257A: 140.70–174.50 mbsf; Hole 1257B: 145.50–174.40 mbsf; and Hole 1257C: 145.15–178.00 mbsf

Thickness: 33.8 m

Age: early Maastrichtian–early Campanian

Lithology: foraminifer nannofossil chalk

Subunit IIIB mainly consists of light greenish gray to greenish gray nannofossil chalk, with carbonate content ranging from 40 to 60 wt% (Fig. F4). The top of Subunit IIIB is placed at the base of the gravity flow deposit containing Tertiary microfossils that rests unconformably on early Maastrichtian rocks. The base of Subunit IIIB is placed at the upper surface of a 55-cm-thick laminated limestone that overlies the organic-rich claystones and siltstones of Unit IV. The basal contact of Unit III is sharp, and the limestone is included in Unit IV because it is laminated.

Subunit IIIB is similar to Subunit IIIA, but is, on average, more carbonate rich and contains more foraminifers. Bioturbation is pervasive throughout and varies between darker intervals with a subhorizontal fabric dominated by Zoophycos and lighter intervals with a more uniform and subtle mottling. The lowest meter of Subunit IIIB (Sections 207-1257A-20X-3, 75 cm, through 20X-4, 52 cm, and 207-1257C-10R-CC) is rich in glauconite. The abundances of both foraminifers and nannofossils decrease in the lower portions of Subunit IIIB. On the other hand, pyrite, both disseminated and concentrated in burrows, increases in abundance in the lower portions of Subunit IIIB. On shorter length scales, changes in pyrite abundance seem to be correlated with changes in the style of bioturbation. Pyrite is most abundant in darker intervals with subhorizontal burrows and less abundant in lighter intervals that are pervasively mottled. Alterations on a 20- to 30-cm scale between these two fabrics occur throughout. Siliceous microfossils are absent except in the middle part of the subunit.

Unit IV

Intervals: 207-1257A-20X-4, 52 cm, through 26X-1, 0 cm; 207-1257B-17R-1, 17 cm, through 25R-2, 112 cm; and 207-1257C-11R-1, 3 cm, through 15R-CC, 39 cm

Depths: Hole 1257A: 174.50–226.80 mbsf; Hole 1257B: 174.40–217.60 mbsf; and Hole 1257C: 178.00–226.20 mbsf

Thickness: 52.3 m

Age: Santonian–middle Cenomanian

Lithology: calcareous claystone with organic matter, clayey chalk with organic matter, and limestone

Unit IV primarily consists of dark olive-gray to black (5Y 3/2 to 5Y 3/2) finely laminated calcareous claystone with organic matter and clayey chalk with organic matter (Fig. F4). The unit shows well-developed submillimeter-scale laminations and has a strong petroliferous odor. There are decimeter-scale rhythmic color variations between dark olive gray and black throughout (Fig. F6A). Carbonate content ranges from 40 to 60 wt% in dominant lithologies and as high as 78 wt% in individual carbonate-rich layers (Fig. F4). Carbonate constituents include nannofossils, foraminifers, and shell fragments. Total organic carbon (TOC) ranges from 1.5 to 15.7 wt% in dominant lithologies. Rock-Eval pyrolysis analyses indicate Type II organic matter, which is consistent with a marine origin. Sample 207-1257C-14R-1, 33 cm, contains fish scales. Inoceramids and other macrofossils are present, especially in lighter-colored intervals. Olive (5Y 4/2) laminated limestone up to 30 cm in thickness, gray (5Y 5/1) and black (N2) chert nodules, and phosphatic and calcareous nodules with foraminifers and nannofossils occur in minor amounts. The phosphatic and calcareous nodules are seen as millimeter- to centimeter-scale white blebs on core surfaces.

Unit V

Intervals: 207-1257A-26X-1, 0 cm, through 31X-CC, 57 cm; 207-1257B-25R-2, 112 cm, through 27R-CC, 22 cm; and 207-1257C-15R-CC, 39 cm, through 16R-CC, 18 cm

Depths: Hole 1257A: 226.80–284.70 mbsf; Hole 1257B: 217.60–227.30 mbsf; and Hole 1257C: 226.20–235.90 mbsf

Thickness: 57.90 m

Age: middle–late Albian

Lithology: silty calcareous claystone to clayey carbonate siltstone

Unit V consists of dark greenish gray (5G 4/1) silty calcareous claystone to clayey carbonate siltstone. The rocks of Unit V exhibit a discontinuous, subhorizontal wavy fabric defined by millimeter-scale calcite-rich and calcite-poor layers (Fig. F6C). This fabric is tentatively attributed to dissolution of primary carbonate, mobilization, and preferential recrystallization into more carbonate-rich intervals. Carbonate content ranges from 24 to 45 wt%, with most being undifferentiated calcite crystals in the thin undulatory layers. Quartz, shell material, pyrite nodules (Fig. F6E), and trace amounts of nannofossils are minor constituents. TOC ranges between 0.4 and 0.8 wt%. Gray calcareous sandstone intervals are found in Core 207-1257B-26R and Sections 207-1257C-15R-CC and 16R-1. Thin section analyses show subangular detrital quartz grains surrounded by sparry calcite cement. Drilling disturbance (biscuiting) is common in all holes but is pervasive in Hole 1257A.

The oldest interval recovered at Site 1257 is a unit dominated by silty calcareous claystone and clayey calcareous siltstones (Unit V) of middle–late Albian age. These rocks lie below Reflector C (see "Seismic Stratigraphy" in "Background and Objectives"), which represents the trace of an angular unconformity on Demerara Rise, and, thus, Unit V is interpreted as a synrift depositional sequence. The lithology is quite homogeneous. Shells are common but trace fossils are rare, and no physical sedimentary structures were observed. The abundance of terrigenous material and common shells and the presence of subangular quartz sandstones in this interval suggests a relatively proximal, shallow-water setting. However, the site was apparently deep and/or sheltered enough that physical sedimentary structures were not formed. Alternatively, diagenetic overprinting may have obscured subtle depositional features. Regardless, the relatively high organic carbon content and the absence of lamination or fossil indicators of high productivity suggest high sedimentation rates.

From the late Cenomanian to Santonian, high productivity and low bottom water oxygen levels prevailed at Site 1257. This phase of deposition is recorded by the ~50 m of sediment dominated by the dark laminated calcareous claystones and laminated limestones of Unit IV. Unfortunately, the contact with Unit V was poorly recovered, but it seems to correspond to a hiatus. Organic carbon concentrations in Unit IV approach 15 wt% (Fig. F4), and abundant zeolites suggest siliceous microfossils were a common component of the sediment. Carbonate values average ~50 wt% (Fig. F4) through the interval and include nannofossils, foraminifers, shells, and carbonate debris. Despite the virtual lack of bioturbation and benthic microfossils, inoceramids are common, especially in the lower portions of the unit, attesting to this taxon's tolerance for marginal habitats. Layers of nonprismatic shells (i.e., not inoceramids) may represent other similarly tolerant taxa, brief oxygenation events, or pelagic taxa (e.g., ammonites) that lived in the overlying water column. Phosphatic lenses may represent coprolites. If this identification is correct, their high abundance, coupled with the high abundance of fish remains, suggests some combination of low sedimentation rates, high preservation potential for phosphatic material, and many fish.

Open-marine conditions and oxic bottom waters were established by 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 at Site 1257 may reflect a hiatus, but the glauconite-rich interval in the lowermost portion of Subunit IIIB is consistent with the transition being recorded by a highly condensed interval. Further, the trace fossils and abundance of pyrite may indicate that bottom waters were relatively dysoxic in the late Campanian and early Maastrichtian. However, variations in sedimentary fabric and minor sedimentary components suggest that conditions at the seafloor and/or overlying water column fluctuated on geologically short timescales.

The P/E boundary seems to be relatively complete in Hole 1257A. As noted above, a 3-cm-thick clay layer in Hole 1257C 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. Recovery of this boundary in Hole 1257C is fortuitous, as a gravity flow deposit is present ~2.5 m above the clay layer and the interval was not recovered in either Hole 1257A or 1257B.

Gravity flow deposits and temporal gaps are common in the pelagic record at Site 1257. In addition to the mass flow deposit above the P/E boundary, the Cretaceous/Tertiary (K/T) and E/O boundaries are represented by hiatuses and/or gravity flow deposits. The dominance of Paleocene fossils in the matrix and clasts of the flow deposit between Cretaceous and Tertiary pelagic intervals demonstrates that this deposit is not a direct manifestation of events occurring at the K/T boundary. At least five other mass flow deposits were recognized at Site 1257. The apparent instability of the slope may reflect local conditions related to the steep topography and its position near the edge of Demerara Rise. Recovery of more complete sections at other Demerara Rise sites would support the interpretation that failures at Site 1257 are local features.