Site 1261 lies in the shallowest water depth (~1900 mbsl) of the Leg 207 Demerara Rise depth transect. From the two holes at Site 1261, we recovered 68% of the ~478 m cored (Fig. F3). Sediments and sedimentary rocks recovered at Site 1261 range in age from Albian to Pleistocene (see "Biostratigraphy"), but the section was drilled and washed with occasional spot cores from 0 to 237 mbsf. The recovered section is interrupted by at least two major and several minor hiatuses.
Five lithostratigraphic units were recognized at Site 1261 (Table T2). The oldest unit recovered (Unit V) is dominated by quartz sandstone. The superjacent unit (Unit IV) is composed dominantly of laminated calcareous organic-rich claystone (black shale) and laminated chalk/limestone. The youngest three units (Units I–III) recognized are pelagic and are composed dominantly of foraminifers, nannofossils, carbonate debris, diagenetic calcite, 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. Unit II contains a relatively high abundance of diagenetic calcite, with only minor quantities of zeolite. The carbonate content for pelagic Units I and II increases progressively downhole from 10 to 80 wt%. Values then decline through the clay-rich Unit III to ~20–50 wt% carbonate at its base.
Lithostratigraphic units recognized at Site 1261 generally parallel those reported for Sites 1257–1260 (Fig. F4). Differences between the lithostratigraphic divisions in units co-occurring at Site 1261 and other Demerara Rise sites represent differences in the level at which unit boundaries are placed and can be attributed to uncertainty related to short coring gaps and hiatuses of varying length and age. Unit I is considerably thicker at Site 1261 than previous sites because of the recovery of an extraordinarily thick pre-Eocene package. Units II and III were comparatively condensed in relation to sections recovered from previous sites, but the succession of lithologies was similar. Unit IV ranked second in total black shale thickness of all sites (89 m) and represents the longest stratigraphic range of Unit IV at one site, with deposition spanning from the upper Santonian to the lower Cenomanian (see "Biostratigraphy"). Finally, the basal Unit V (quartz sandstone) at Site 1261 is similar to sandstones recovered from Unit V at Sites 1257 and 1259. This unit shows significant variation across all sites and probably represents synrift deposition. Quartz grains of Unit V at Site 1261 are subangular medium sand, implying a relatively little weathering and a relatively proximal setting.
Unit I is composed dominantly of nannofossil ooze, nannofossil clay, and a matrix-supported conglomerate. This unit also includes rare bluish gray clay layers. Pyrite and foraminifers are minor constituents throughout the unit.
Unit I is subdivided into three subunits. Subunit IA is a nannofossil ooze, and Subunit IIB is a nannofossil clay. Spot coring over the interval from 22.5 to 236.9 mbsf does not allow for the nature or placement of the boundary between these subunits to be rigorously determined. Subunit IC is a matrix-supported conglomerate composed of clayey nannofossil ooze (matrix) and nannofossil chalk (clasts). Unit I is distinguished from Unit II by a sharp transition from relatively clay-rich oozes and debris flow deposits of late Miocene age at the base of Subunit IC to the pelagic middle–early Eocene nannofossil chalks and limestones of Unit II.
Subunit IA is a nannofossil ooze with foraminifers that is characterized by few discrete traces and frequent black mottles and streaks. There is a sharp color change from pale yellow/pale brown to olive gray/greenish gray at ~0.25 mbsf. Neither sediment composition nor sediment fabric change is noticeable at this boundary, so 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 Subunit IA (69.70 mbsf) is placed at the base of a coring gap between nannofossil ooze (Subunit IIA) and nannofossil clay (Subunit IB).
Subunit IB is a greenish gray to olive-gray to bluish gray nannofossil clay. The top of this subunit is placed at the first massive clay-rich sediments at the top of Core 207-1261A-4R (Fig. F5). Clay contents were estimated at 40%–50% in smear slides. Nannofossils compose most of the rest of the sediment. Color variations are seen as distinct bluish gray layers found at a 20- to 30-cm spacing superimposed on meter-scale gray to reddish brown color cycles. Blue layers have clay contents of 80%–90%, with the remaining 10%–20% composed of nannofossils. Bioturbation is moderate to pervasive throughout, but only a few discrete Zoophycos and Planolites burrows are observed. Pyrite is millimeter-sized crystals through much of the subunit and occasionally concentrated within burrows. The base of this subunit is marked by a distinct contact with the multicolored matrix-supported conglomerate of Subunit IC at 310.55 mbsf (Hole 1261A).
Subunit IC is composed of a matrix-supported conglomerate, interpreted as a debris flow (Fig. F5B). The matrix is clay rich with well-preserved nannofossils and well-preserved but heavily fragmented foraminifers. The clasts consist of nannofossil chalk of late Miocene age (Fig. F5C). The top of this subunit is placed at a distinct contact between the olive-gray to greenish gray nannofossil clay of Subunit IB and the multicolored matrix-supported conglomerate of Subunit IC at 310.55 mbsf (Hole 1261A). The subunit is intensely slumped, folded, and faulted (e.g., Cores 207-1261A-15R and 19R). The intervals that appear to be less disturbed are characterized by moderate to heavy bioturbation, with distinct dark coloration of backfilled Zoophycos and Planolites burrows. These more stable sedimentary intervals possess distinctive sharp, thin, very dark green layers (Cores 207-1261A-16R and 17R), which may represent redox boundaries in the sediment. Large clasts of nannofossil chalk are located in Sections 207-1261A-14R-6, 15R-1, 15R-6, 17R-6, and 19R-5 through 19R-CC. These chalk clasts are moderately bioturbated with discrete white mottles. The base of Subunit IC is placed at a sharp contact of the matrix-supported conglomerate with the underlying middle Eocene calcareous chalk of Unit II at 369.27 mbsf.
Unit II is composed of calcareous chalk containing nannofossils, foraminifers, diagenetic calcite, and clay, which grades downcore to limestone. Cyclic alternations (20–50 cm thick) between white to light greenish gray and greenish gray calcareous chalks are a striking feature of this unit (Fig. F6A). These color cycles appear to be caused by a slightly higher clay content in the darker intervals than in the lighter intervals and may have been enhanced by diagenesis. Foraminifers and nannofossils are poorly preserved in this subunit and range in abundance from 15%–30% to 10%–15% in the light and dark intervals, respectively. Diagenetic sparry calcite content is relatively high but variable (15%–50%). Clay and zeolite are also present. Sediment in Section 207-1261A-33R-4 (around the P/E boundary) displays a distinct reddish brown hue (Fig. F6) and is characterized by elevated clay content (up to 90%).
Rarely interbedded within color couplets are short ~5- to 10-cm intervals of indurated siliceous calcareous chalk. Bioturbation is heavy to common throughout Unit II. The darker intervals have a wispy anastomosing subhorizontal fabric with prominent Zoophycos burrows, and the lighter intervals are mottled with few distinct burrows (Fig. F6). Mottles, blebs, and streaks are common and seem to be associated with burrows. The base of Unit II (Hole 1261A [491.70 mbsf]) is defined by a transition to the more clay-rich calcareous chalk of Unit III. This transition is gradational, and the boundary is placed at the base of the red and green clay layer corresponding to the biostratigraphically defined P/E boundary.
Unit III is composed of nannofossil chalk with clay and claystone with nannofossils. Planktonic foraminifers, calcite debris (both biogenic and diagenetic), pyrite, barite, and zeolites are minor constituents in portions of the unit. Dominant colors are shades of greenish gray that grade to dark gray at the base. Distinct to subtle color alternations on a decimeter scale occur. The upper contact with Unit II is placed at the base of a distinct 20-cm-thick greenish gray and red clay layer that corresponds to the biostratigraphically recognized P/E boundary (see "Biostratigraphy"). Unit III is easily distinguished from Unit IV by its lack of laminations, low organic carbon, and greenish gray color. The base of Unit III is placed at the first downhole occurrence of high total organic carbon (TOC)-laminated strata (Fig. F6). Unit III is subdivided into two subunits at the K/T boundary. The K/T boundary coincides with a downhole shift to clay from carbonate as the major lithologic constituent. No ejecta layer was recovered at Site 1261, and the boundary occurs at Sections 207-1261A-37-CC and 207-1261B-2R-3, 78 cm (see "Biostratigraphy").
Subunit IIIA consists of light greenish gray to greenish gray mottled nannofossil chalk with clay and clayey nannofossil chalk. Carbonate contents are generally between 40 and 60 wt% (Fig. F3). Zeolites, quartz, calcite debris (diagenetic and biogenic), and pyrite are common as minor constituents. Subtle to distinct alternations between light (relatively carbonate rich) and dark (relatively clay rich) color bands occur on a decimeter scale. Burrow mottling is pervasive. Discrete traces including Zoophycos, Planolites, and Chondrites are common and often filled with black monosulfide minerals. Subunit IIIA is distinguished from Subunit IIIB by its generally higher calcium carbonate content and lower clay content. The contact is placed at the K/T boundary, which is roughly coincident with a downcore shift from carbonate to clay as the major lithologic constituent.
Subunit IIIB consists of greenish gray to dark gray claystone with nannofossils and minor amounts of claystone with foraminifers and nannofossils and clayey limestone with foraminifers. Pyrite is common, and barite is present as millimeter- to centimeter-sized nodules. Light–dark color alternations occur with a 20- to 50-cm spacing and represent changes in the relative abundance of clay and carbonate. Bioturbation is heavy to pervasive, but darker intervals often have a subhorizontal fabric that may reflect primary lamination. Zoophycos, Planolites, and Chondrites are often ringed by diagenetic halos. The lower portion of Unit III is less bioturbated, more clay rich, and darker in color than other parts of the unit. Unit III is distinguished from Unit IV by its lack of laminations, low organic carbon content, and greenish gray color. The lower contact is placed at the first downhole occurrence of laminated black calcareous claystone with organic matter (Fig. F6).
Unit IV ranges from black, finely laminated claystone with organic matter to olive-gray laminated clayey limestone and clayey chalk with nannofossils. The top of the unit is defined by a relatively sharp shift from the bioturbated greenish gray clayey calcareous chalk of Unit III above to the laminated claystones below (Fig. F6).
The two major lithologies in Unit IV, laminated claystone and laminated nannofossil chalk and limestone, show cyclic alternations and gradational contacts over a centimeter to decimeter scale (Fig. F7). There are two modes of cyclicity observed. Fine-grained laminated olive-gray chalks and limestones of 20–50 cm thickness are found every few meters in the top of Unit IV interbedded with organic-rich claystones that contain phosphoritic stringers, fecal pellets, and inoceramids. In the middle of Unit IV, coarse-grained calcite-cemented foraminifer sands are present (Sections 207-1261A-46R-5, 83 cm, to 48R-1, 140 cm; and 207-1261B-12R-1, 0 cm, to 12R-4, 40 cm). These coarser stringers alternate with the organic-rich claystone over 20- to 50-cm scale for an interval of up to 11 m and occur in the fine-grained olive-gray laminated limestone cycles described above. At the base of the unit, the sediments still show cyclic variation but become generally lighter in color, reflecting an increase in carbonate content (Sections 207-1261A-49R-1, 80 cm, to 50R-3, 23 cm; and 207-1261B-13R-2, 0 cm, to 14R-4, 48 cm). The exception to this trend is the black laminated bed found immediately above Unit V in interval 207-1261A-50R-3, 15–32 cm.
Intervals 207-1261A-45R-2, 1–45 cm, to 207-1261B-9R-4, 121–153 cm, contain coarse-grained massive beds with glauconite, fecal pellets, diagenetic calcite, and phosphatic nodules and stringers that extend in burrows into the underlying sediment. These beds are similar to lithologies found in the black organic-rich claystones at Sites 1257–1260 and also appear as a distinctive natural gamma ray (NGR) and magnetic susceptibility peak (see "Physical Properties"). Additionally, moderately to pervasively bioturbated centimeter-scale beds with coarser sediment containing fecal pellets and fish debris are a minor lithology.
There are several possible ash layers in Unit IV that can be correlated between Holes 1261A and 1261B (e.g., intervals 207-1261A-42R-5, 55–58 cm, and 207-1261B-7R, 138–140 cm). These are homogeneous gray clay layers that have a waxy texture and are devoid of nannofossils. Cores from the two holes at this site also contain several blue diagenetic calcite fans (cone-in-cone structures) (Fig. F7A, F7B) precipitated in a black claystone matrix (intervals 207-1261A-47R-1, 86–91 cm [missing in Hole 1261B], and 48R-4, 63–81 cm, which may correlate with interval 207-1261B-12R-6, 127–142 cm).
Unit V consists of gray coarse-grained quartz sandstone with calcite cement. The sandstone is heterogeneous with white and black speckles, and the unit is moderately biscuited. The top of the unit is placed at a sharp color and lithologic boundary (Fig. F7) and the unit extends to the bottom of Holes 1261A and 1261B. The quartz grains are subangular. Shell material and ammonite fragments are minor sedimentary components. Silty clay intercalated with the sandstone in interval 207-1261B-16R-1, 93–113 cm, has yielded a possible calcareous nannofossil age of late Albian–Cenomanian. The base of Site 1261 (Sample 207-1261B-16R-1, 141–147 cm) contains a limestone rich in oysters of the genus Aucellina.
The oldest interval recovered at Site 1261 is a quartz sandstone unit (Unit V) of late Albian–Cenomanian age. These rocks may represent synrift deposition. Unit V at Site 1261 appears to be correlative to Unit V at other sites. However, poor age control at all sites limits the rigor of this correlation. Site 1261 may represent a more proximal or high-energy site of sedimentation than the shallow-marine silty calcareous claystones at Site 1260, the claystones with organic matter at Site 1258, and the tidal, possibly calcareous, siltstones of Site 1259.
From the late Cenomanian to Santonian, high productivity and low bottom water oxygen levels, resulting in the preservation of large volumes of organic and phosphatic matter, prevailed at Site 1261. This phase is recorded in the sediments by the dark laminated calcareous claystones and laminated chalks/limestones of Unit IV. Organic carbon concentrations in Unit IV approach 16 wt% TOC, with the organic matter of marine origin (see "Organic Geochemistry"). Zeolites (clinoptilolite) and rare radiolarians suggest siliceous microfossils were a common component of the sediment. Carbonate values are highly variable (5–95 wt%) and in part reflect diagenetic dissolution and precipitation of inorganic calcite.
Open marine conditions and oxic bottom waters were established by the late Campanian, as indicated by the heavily bioturbated pelagic marls of Unit III. The sharp contact between the bioturbated claystones of Unit III and the laminated shales, clayey nannofossil chalks, and limestones of Unit IV in Hole 1261A reflects a hiatus. A glauconite-rich interval in Hole 1261B, however, is consistent with the transition being recorded by a highly condensed interval. Furthermore, the trace fossils and sedimentary fabric may indicate that bottom waters were relatively dysoxic in the late Campanian and early Maastrichtian.
The K/T boundary interval and associated impact ejecta layer are missing from Holes 1261A and 1261B. Hole 1261B gives a latest Maastrichtian age immediately below Sample 207-1261B-2R-3, 78 cm, and an early Danian age immediately above, implying a short hiatus. The P/E boundary seems to be relatively complete at Site 1261. A 3-cm-thick greenish gray clay layer and superjacent 12-cm-thick reddish brown clay layer in Hole 1261A fall in an interval that contains the P/E boundary (see "Biostratigraphy"). The lack of carbonate in this layer is consistent with sedimentary changes expected during the Paleocene/Eocene Thermal Maximum (PETM).
Above the Eocene calcareous chalks, an extensive Miocene debris flow was cored, which exhibited intense slumping, folding, and faulting. Overlying the debris flow, there is a ~290-m-thick sequence of late Miocene and Pliocene nannofossil clay, although much of this interval was not cored. The Pliocene sediments are capped with a 25-cm veneer of Pleistocene nannofossil ooze.