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doi:10.2204/iodp.proc.342.108.2014 LithostratigraphyThe sedimentary sequence recovered from Holes U1407A–U1407C presents a diverse collection of deep-sea pelagic to shallow-marine (including reefal) sediment of Pleistocene to Early Cretaceous (late Albian) age and comprising six lithostratigraphic units (Figs. F4, F5; Table T2). Unit I is ∼8 m thick and composed of grayish brown to pale brown Pleistocene foraminiferal nannofossil ooze interbedded at the decimeter scale with foraminiferal sand and clay with nannofossils (Fig. F5A). Rock fragments of pebble to cobble size and coarse silt- to sand-sized quartz and amphibole are pervasive. Unit II is a ∼10 m thick sequence of early Oligocene age composed of pale brown clay with nannofossils with disseminated manganese nodules, sulfide patches, and intercalated millimeter- to centimeter-sized sulfide layers (Fig. F5B). Unit III consists of ∼65 m of middle Eocene pale brown to greenish gray nannofossil ooze with foraminifers. Sulfide patches and layers are also present (Fig. F5C). An abrupt downhole change in color from light greenish gray (5GY 7/1) into white (N8) is associated with a significant shift in physical proxies and carbonate content that marks the contact between Units III and IV. Unit IV is a ∼20 m thick sequence of lower Eocene nannofossil ooze with foraminifers (Fig. F5D). Unit V is composed of Paleocene to Albian nannofossil chalk and is divided into two subunits (Va and Vb; Fig. F5E, F5F). Subunit Va consists of ∼60 m of Paleocene nannofossil chalk with foraminifers and radiolarians. Subunit Vb is ∼95 m thick and composed of mainly nannofossil chalk, or nannofossil chalk with foraminifers, of Albian to Paleocene age. Subunit Vb also contains black shale of the Cenomanian/Turonian boundary Oceanic Anoxic Event (OAE) 2 (93.5 Ma). The top of Unit VI is a laminated, partially silicified, and dolomitized calcareous sandstone of late Albian age (Fig. F5G). The remainder of Unit VI was extremely poorly recovered but consists of fossiliferous backreef and lagoonal moldic limestones of Albian age (Fig. F5H). Representative smear slide photographs of all of these lithostratigraphic units are shown in Figures F6 and F7. Lithostratigraphic units and boundaries are defined by changes in lithology (as identified by visual core description and smear slide observations), physical properties including color reflectance (L*, a*, and b*), and biogenic content (calcium carbonate and silica). The lithologic differences observed between units are primarily attributable to varying abundances of nannofossils, diatoms, radiolarians, and foraminifers (Figs. F8, F9, F10). Lithologic descriptions are based on sediment recovered in Hole U1407A and supplemented with observations from Holes U1407B and U1407C. Unit I
Unit I is 7.75–8.14 m thick and consists of multicolored sediment of mainly grayish brown (10YR 5/2), light gray (10YR 7/2), and pale brown (10YR 6/3) foraminiferal nannofossil ooze and secondary reddish brown (5YR 5/3) clay with nannofossils present in decimeter-thick beds (Fig. F5A). Sand-sized foraminifers are locally concentrated into patches or in continuous to semicontinuous beds. Pebble- to cobble-sized rock fragments, coarse silt- to sand-sized quartz, and amphibole are also present. Unit I is distinguished from the underlying nannofossil-rich clay of Unit II by the presence of foraminiferal sand beds and the abundance of foraminifers (Figs. F5A, F5B, F6, F7, F8, F9, F10). The contact between Unit I (Pleistocene) and Unit II (early Oligocene) is characterized by a ~30–40 cm thick yellowish to rusty brown clay rich in disseminated sulfides and manganese oxides. Unit II
Unit II is 3.93–9.90 m thick and consists of very pale brown (10YR 7/3) clay with nannofossils of early Oligocene age. Discrete manganese nodules of varying size (1–5 cm) and patches of disseminated sulfides as thick as 10 cm are present as secondary components within the ochre to black clay (7.5YR 5/8) (Fig. F5B). Unit II is distinguished from underlying Unit III by its relatively minor abundance of carbonate (<25 wt% compared to ~50 wt%; Fig. F4), nannofossils, and foraminifers (Figs. F6, F7, F8, F9, F10). Unit III
Unit III consists of ~65 m of very pale brown (10YR 8/2) to light greenish gray (5GY 6/1) nannofossil ooze with foraminifers of middle Eocene age (Figs. F5C, F6C). A notable gradational change in color from very pale brown (10YR 8/2) into light greenish gray (5GY 7/1) occurs between 43.2 and 43.5 mbsf (Fig. F4). Greenish gray (5GY 6/1) mottling and layering and subtle color banding of light greenish gray (5GY 7/1) occur within host greenish gray (5GY 7/1) nannofossil ooze (Fig. F5C). Millimeter-scale clay layers with ochre Fe oxide stain and altered glauconite are also commonly observed. Black sulfide patches and layers are occasionally present throughout the unit. Bioturbation is moderate to heavy with discrete Planolites and Zoophycos burrows. Unit III is distinguished from underlying Unit IV by an abrupt downhole change in color from the distinctive light greenish gray (5GY 7/1) sediment to white (N8) sediment (Fig. F4). Additionally, Unit IV is characterized by an increase in the abundance of biosiliceous components compared to Unit III, in which these components are absent or only rarely present (Figs. F8, F9, F10). Unit IV
Unit IV is ~20 m thick and consists of white (N8) nannofossil ooze with foraminifers of early Eocene age. The prominent change in color from overlying Unit III to the white of Unit IV is associated with a significant increase in carbonate content, from ~40 wt% in Unit III to >80 wt% in Unit IV (Figs. F4, F11). Other physical properties such as magnetic susceptibility and natural gamma radiation (NGR) also noticeably shift across the Unit III/IV boundary (Fig. F11). The lower part of Unit IV is composed of very pale brown (10YR 8/2) nannofossil ooze with radiolarians to pink (7.5YR 8/3) radiolarian nannofossil ooze with brown (10YR 5/3) mottling. The poorly recovered interval between the base of this lithostratigraphic unit and the upper boundary of Unit V (Fig. F4) contains chert in all three holes. The thickness and number of individual chert layers as well as the nature of interbedded sediment is unknown as a result of poor recovery in this interval. Unit V
Unit V and its two Subunits, Va and Vb, comprise an ∼155 m thick succession of nannofossil chalk and nannofossil chalk with foraminifers of Paleocene to Albian age (Fig. F4). The boundary between Subunits Va and Vb is defined by the rarity or the absence of radiolarians in Subunit Vb (Figs. F8, F9, F10). Subunit Vb includes a prominent interval of zeolitic claystones with organic matter (black shale) that are described in detail in “Cenomanian/Turonian boundary interval and Oceanic Anoxic Event 2.” Unit V is distinguished from underlying Unit VI by a change from pelagic sediment to neritic, laminated, and partially silicified and dolomitized calcareous sandstone (grainstone and framestone). Subunit Va
Subunit Va is an ~45–60 m thick sequence of white (N8) to pinkish white (7.5YR 8/2) nannofossil chalk with foraminifers and radiolarians and nannofossil chalk with radiolarians of Paleocene age. Chert fragments and black patches of sulfides are present occasionally. Bioturbation is moderate to heavy with some well-preserved Zoophycos burrows. Secondary lithologies within Subunit Va include laminated siltstone beds, one of which displays a 3 cm thick basal division of an inversely graded bed overlain by normally graded sediment (Fig. F12). This graded bed is overlain by an 11 cm thick siltstone layer with horizontal to inclined laminations. The comparably minor bioturbation and the sedimentary structures in this bed suggest that it is an event bed, potentially from a current-resuspended density flow. Physical proxies (color reflectance parameters, NGR, and magnetic susceptibility) show low variability in Subunit Va compared to the underlying Subunit Vb (Fig. F4). Carbonate content within Subunit Va is lower than in Subunit Vb (see “Geochemistry”). The rarity or absence of radiolarians in Subunit Vb marks the transition from Subunit Va to Vb (Figs. F8, F9, F10). Subunit Vb
Subunit Vb is predominantly greenish white (10Y 8/1) and white (N 8) to pinkish white (7.5YR 8/2) nannofossil chalk and nannofossil chalk with foraminifers with disseminated patches and layers of sulfides and spans a 86.79–95.07 m thick sequence from the Albian to Paleocene. Secondary lithologies within Subunit Vb include laminated claystone with nannofossils, laminated to massive zeolitic claystone with organic matter, and chert and radiolarian sand (Fig. F7). Two dark gray to black sequences of nannofossil- and organic-rich claystone with black to dark green color alternations (Figs. F13, F14, F15) are associated with the Cenomanian–Turonian OAE 2 (~93 Ma). A detailed description of this interval in all three holes drilled is provided in “Cenomanian/Turonian boundary interval and Oceanic Anoxic Event 2.” Macroscopic fossils include an aptychus, inoceramid shell fragments, and belemnites in Cores 342-U1407A-28X and 29X. In Section 342-U1407B-27X-CC, Subunit Vb is distinguished from underlying Unit VI by a change from calcareous chalk to laminated and partially silicified and dolomitized calcareous sandstone (grainstone and framestone). Unit VI
Unit VI is composed of ~32 m of very poorly recovered carbonate reef lithologies from Holes U1407A and U1407B. The top of Unit VI is laminated sandstone that contains fine sand–sized clasts of carbonate, quartz, dolomite, and occasional belemnites (Fig. F5G). Slightly deeper in Unit VI is coarse sand cemented by Fe-Mn oxides (interval 342-U1407A-31X-CC, 17–18 cm; Fig. F16A, F16B). The sediment recovered in Sample 342-U1407A-32X-CC, 0–3 cm, is a pelleted carbonate mudstone (Fig. F16C, F16D). Large benthic orbitolinid foraminifers were found in Section 342-U1407A-33X-CC, allowing a late Albian age assignment (Fig. F16E, F16F). Section 35X-CC contains molds of gastropods, bivalves, corals, and echinoderm spines. This section also includes a packstone that thin section analysis revealed to be a pel-bio-micrite in the Folk classification (Folk, 1959) (Fig. F16C, F16D). A reef platform facies for these lithologies is consistent with the surface geometry indicated by seismic data. Lithostratigraphic unit summaryThe lithostratigraphy of Site U1407 comprises a pelagic to neritic sedimentary sequence of Pleistocene to Early Cretaceous (late Albian) age. Pleistocene sediment is ~8 m thick and includes foraminiferal nannofossil ooze intercalated with decimeter-thick beds of foraminiferal sand (Unit I). This unit overlies ~10 m of early Oligocene clay with nannofossils (Unit II). Units III and IV are of early to middle Eocene age and comprise an ~85 m thick sequence of nannofossil ooze with foraminifers. An abrupt downhole increase in calcium carbonate from ~43 to ~93 wt%, together with prominent shifts in other physical properties (color reflectance L*, magnetic susceptibility, and NGR) mark the boundary between these two units. A correlative increase in carbonate content is also seen at Sites U1403–U1406. Unit V is composed of 155 m of nannofossil chalk of late Paleocene to Albian age. A prominent black shale horizon (zeolitic claystone with organic matter) associated with the Cenomanian/Turonian boundary OAE 2 (~93 Ma) was recovered from all three holes drilled at Site U1407 and is described in detail below. Finally, Unit VI is ~32 m thick and composed of nannofossil chalk capped by belemnite-bearing, fine-grained sandstone with horizontal laminations. Below, coarse-grained Fe-Mn oxide–coated sand is found in a sequence with poorly recovered shallow-water carbonates. This reef sediment is of Albian age, as inferred from the occurrence of the large benthic foraminifer Orbitolina. Directly above this sandy succession at Section 342-U1407A-31X-6, 35 cm (and above), thermocline-dwelling foraminifers like Rotalipora are observed, suggesting at least an upper bathyal water depth at this level. Cenomanian/Turonian boundary interval and Oceanic Anoxic Event 2The geologic record of the Cenomanian/Turonian boundary transition is characterized by widely distributed organic matter–rich black shale deposition and 13C enrichment in carbonate and organic carbon that together define OAE 2 (e.g., Sageman et al., 2006; Jenkyns, 2010). Drilling operations at Site U1407 recovered an OAE sequence in all three holes, defined on the basis of lithology (organic matter–rich zeolitic claystones or black shale) and calcareous nannofossil biostratigraphy (see “Biostratigraphy”). Significant differences in the lithostratigraphic sequence and thickness of beds exists between Holes U1407A, U1407B, and U1407C (Fig. F13), partly because of drilling disturbances and mass wasting indicated by slump features (Fig. F15). Thicknesses of beds in all of the OAE descriptions that follow will represent the maximum bed thickness observed. The downhole OAE 2 lithostratigraphy (Fig. F13) comprises a 9.95 m thick sequence that includes a greenish white nannofossil chalk with foraminifers interbedded with dark gray nannofossil claystone bands (maximum thickness in Hole U1407C is from 231.45 to 241.45 mbsf) that transitions to a 0.95 m thick interval of white to light gray nannofossil chalk and dark gray to black claystone bands (maximum thickness in Hole U1407C is from 230.30 to 231.25 mbsf). The greenish white and white nannofossil chalk is overlain by organic matter–rich, submillimeter-scale laminated to massive black shale with interbedded dark gray claystone with disseminated submillimeter pyrite framboids. The black shale ranges in thickness from 15 cm in Hole U1407C to 44 cm in Hole U1407A. Estimates of the true thickness of the OAE 2 black shale at Site U1407 are complicated by significant drilling disturbance. The black shale is overlain by 26 cm of laminated greenish gray nannofossil claystone and dark gray radiolarian sand (best expressed in Hole U1407B at 231.92–232.18 mbsf) and 107 cm of greenish white nannofossil chalk with interbedded chert and silicified limestone with radiolarians (best expressed in Hole U1407A at 229.56–230.64 mbsf) (Figs. F13, F14). In Holes U1407A and U1407B, a striking transition to 3.6 m of pink to pinkish gray foraminiferal nannofossil chalk occurs. The overlying pinkish brown clayey nannofossil chalk is likely to be of middle Turonian age on the basis of the first occurrence of Lithastrinus septenarius (Fig. F13). Shipboard nannofossil stratigraphy from Holes U1407A and U1407B (see “Biostratigraphy”; Fig. F4) indicate a relatively complete yet highly condensed OAE 2 sequence. Elsewhere, the last occurrence of Corollithion kennedyi, present at 232.6 and 232.8 mbsf in Holes U1407A and U1407B, respectively, occurs after the initial rise in marine δ13C that defines the initiation of OAE 2 (Sageman et al., 2006). This placement suggests that the OAE 2 interval begins in the underlying nannofossil chalks and that black shale deposition at Site U1407 lags onset of the global increase in the fractional burial of organic carbon. The first occurrence of Quadrum intermedium occurs in the greenish white nannofossil chalk ~80 cm above the black shale (Fig. F13) and is within the δ13C excursion plateau that defines the heart of OAE 2. The termination of OAE 2 elsewhere (Sageman et al., 2006) occurs between the first occurrence of Q. intermedium and the first occurrence of Quadrum gartneri in the early Turonian and is tentatively placed between Q. intermedium and Q. gartneri datums at the transition from greenish white to pink chalk at 229.60 mbsf in Hole U1407A (Fig. F4). The lack of a well-developed pink chalk in Hole U1407B suggests the presence of a hiatus or slumping at 230.64 mbsf. The color transitions and lithologies seen at Site U1407 are very similar to Cenomanian/Turonian boundary sequences from the Umbria-Marche Basin of Italy. The greenish white to pink nannofossil chalk is reminiscent of the Scaglia Bianca and Scaglia Rossa limestones that bound the Bonarelli horizon (Arthur and Premoli Silva, 1982). The black shale in Holes U1407A and U1407B comprises a “doublet” separated by dark gray claystone (Fig. F14). The presence of radiolarian sand, chert, and silicified limestone with radiolarians at Site U1407 is also very similar to Italian sequences. However, the position of the black shale relative to C. kennedyi suggests that the black shale deposition at Southeast Newfoundland Ridge occurred later than in the Tethys Ocean. Early Paleocene to early Campanian stratigraphic inversions and unconformitiesDrilling at Site U1407 recovered a biostratigraphically complete upper to lower Paleocene sequence (nannofossil Subzone NP9a to Zone NP4). The Paleocene sequence is truncated by a series of unconformities and stratigraphic inversions beginning with a chert layer separating the Danian (Zone NP4) from the Campanian (Zone UC16) (see Danian–Campanian unconformity in interval 342-U1407A-22X-CC, 5–30 cm, in Fig. F17). This unconformity is underlain by younger Danian strata (Zone NP2) that are in turn terminated by an abrupt contact between the Danian and the upper Maastrichtian (Zone UC20) in Section 342-U1407A-23X-3. Nannofossils in Section 23X-CC are indicative of the lower Campanian (Subzone UC14d/UC15a boundary). We did not date the relative ages of lithostratigraphic contacts observed between Sections 342-U1407A-23X-3 and 23X-CC. Litho- and biostratigraphic evidence from Holes U1407B and U1407C suggest a comparable number of unconformities and hiatuses between holes, although variation clearly exists with regard to the thickness and presence of some intervals. In Hole U1407C, we identified a third unconformity between the upper Maastrichtian and lower Campanian in Section 342-U1407C-20X-1. The stratigraphically complex interval spanning the Danian to lower Campanian is likely not fully resolved by shipboard biostratigraphy, given a number of undated lithostratigraphic contacts observed in all three holes. The presence of deformational structures in Hole U1407B (Figs. F15, F17) suggests local slumping as the cause of the observed stratigraphic complexities. |
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