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doi:10.2204/iodp.proc.342.104.2014

Lithostratigraphy

The downhole sedimentary progression recovered from Holes U1403A and U1403B reveals a diversity of deep-sea pelagic sediment of Pleistocene to Late Cretaceous (Campanian) age, comprising five lithostratigraphic units (Figs. F4, F5, F6, F7; Table T2).

Lithostratigraphic Unit I is a 3 m thick nannofossil ooze with clay and foraminiferal sand of Pleistocene age. Underlying Unit I is Unit II, which comprises ~70 m of fine-grained siliciclastics composed of clay and clay with silt. Unit II contains no diagnostic microfossil datums but has a preliminary middle to late Eocene age on the basis of magnetostratigraphy for 72.63–46.08 mbsf in Hole U1403A. Unit III is radiolarian clay of Eocene age. Unit IV is defined at the top by the occurrence of relatively carbonate rich sediments and at the base by the first downhole occurrence of chert. The sediment of Unit IV is predominantly nannofossil ooze with clay and radiolarians and clay with radiolarians and is of middle Eocene age. Unit V comprises nannofossil chalk with variable amounts of planktonic foraminifers and clay and interbedded, poorly recovered chert. Unit V is divided into two subunits, Va and Vb, based on the transition to high carbonate–content chalks below the K/Pg boundary.

Lithostratigraphic units and boundaries are defined by changes in lithology (as identified by visual core description and smear slide observations), physical properties, color reflectance (L*, a*, and b*), and biogenic content (calcium carbonate and silica). The lithologic differences observed between units are primarily attributable to varying amounts of nannofossils, diatoms, radiolarians, and foraminifers (Fig. F7). Lithologic descriptions are based on sediment recovered from Hole U1403A and supplemented with observations from Hole U1403B.

Unit I

  • Intervals: 342-U1403A-1H-1, 0 cm, to 1H-2, 150 cm; 342-U1403B-1H-1, 0 cm, to 1H-2, 150 cm
  • Depths: Hole U1403A = 0–3.0 mbsf; Hole U1403B = 0–3.0 mbsf
  • Age: Pleistocene
  • Lithology: clay with nannofossils

The main lithology of Unit I is brown (10YR 5/3 and 7.5YR 4/3) clay with varying abundances of nannofossils, foraminifers, and silt-sized quartz, allowing for the identification of various individual beds of foraminiferal clay, nannofossil clay with foraminifers, and nannofossil ooze with clay and/or foraminifers (Figs. F4, F5, F6). Foraminiferal sand occurs in a discrete 1 cm thick layer at interval 342-U1403A-1H-1, 45 cm. Carbonate content changes (from 0 to 34.6 wt%) are principally driven by the abundance of calcareous nannofossils (Figs. F4, F7). Manganese nodules ranging from millimeter to centimeter scales are found throughout (Fig. F8). One notable nodule at interval 342-U1403A-1H-1, 105 cm, was dragged through the core during splitting, causing significant disturbance. A semirounded, 3 cm diameter, black aphanitic cobble, likely a basalt dropstone, was found at interval 342-U1403A-1H-1, 3 cm. Bioturbation is heavy to complete throughout Unit I with no discrete layers or burrows visible. Magnetic susceptibility is higher in Unit I than in most of Unit II. However, the downhole transition in magnetic susceptibility between these two units takes place much more gradually than the lithologically defined unit boundary, which is defined by the absence of calcareous nannofossils and measurable calcium carbonate.

Unit II

  • Intervals: 342-U1403A-1H-3, 0 cm, through 8H-CC, 43 cm; 342-U1403B-1H-3, 0 cm, through 9H-5, 124 cm
  • Depths: Hole U1403A = 3.0–72.30 mbsf; Hole U1403B = 3.0–70.64 mbsf
  • Age: middle Eocene
  • Lithology: clay

The major lithology of Unit II is brown (10YR 6/2 to 7.5YR 4/3 to 5YR 5/4) to light gray (10YR 7/2) clay. The clay contains quartz and zeolites as minor lithologic components (Figs. F4, F5, F6, F7). Color variations in this unit are largely due to the variable distribution of disseminated Mn oxides and discrete Mn oxide layers. Large (up to 10 cm in diameter) Mn nodules are present at the mudline. Sediment is usually heavily bioturbated or is fully homogenized. Rare occurrences of radiolarians and distinctive pink (2.5Y 7/3) nodules or concretions are increasingly common toward the bottom of the unit, the latter of which are found as discrete blebs 1–3 cm in diameter or in crude layers. In general, magnetic susceptibility in Unit II is low compared to Unit I. The contact between Units II and III is clearly defined by the first downhole occurrence of abundant, well-preserved radiolarians.

Shipboard X-ray diffraction (XRD) analyses revealed that the sediment of Unit II is homogeneous with illite, montmorillonite, illite/montmorillonite mixed-layer clay minerals, and quartz as the main components. Minor lithologic components include plagioclase, kaolinite, and chlorite.

Unit III

  • Intervals: 342-U1403A-9H-1, 0 cm, through 12H-CC, 34 cm; 342-U1403B-9H-5, 124 cm, through 14H-CC, 23 cm
  • Depths: Hole U1403A = 72.30–119.14 mbsf; Hole U1403B = 70.64–120.40 mbsf
  • Age: middle Eocene
  • Lithology: radiolarian clay

Unit III is light brown (10Y 5/2) to pinkish gray (7.5Y 6/4) mottled to homogeneous radiolarian clay transitioning to greenish gray (10Y 6/1) and brownish gray (2.5Y 6/2) radiolarian clay in the lowermost 20 m (Figs. F4, F5, F6, F7). Smear slide analyses reveal well-preserved radiolarians, sponge spicules, diatoms, and silicoflagellates (in order of descending abundance) (Fig. F6). Bioturbation is moderate to complete, resulting in a mottled to homogeneous sedimentary fabric, respectively.

Minor lithologic constituents include unusual pink (2.5Y 7/3 to 2.5YR 6/4) nodules that occur throughout this unit in variable abundance. These nodules occur as 1–2 cm thick blebs or in thin (1 cm), distinct layers and are slightly harder than their host sediment. XRD analyses revealed that these nodules consist of an authigenically formed pink montmorillonite (Fig. F9) with minor quantities of carbonate minerals. In Core 342-U1403A-11H, small manganese nodules (<1 cm in diameter) with green halos also occur occasionally.

Unit IV

  • Intervals: 342-U1403A-13H-7, 0 cm, through 17H-CC, 23 cm; 342-U1403B-14H-CC, 23 cm, through 17X-1, 88 cm
  • Depths: Hole U1403A = 119.14–147.80 mbsf; Hole U1403B = 120.40–145.40 mbsf
  • Age: early Eocene
  • Lithology: nannofossil ooze with clay and radiolarians

Unit IV is a ~26 m thick nannofossil ooze with clay and radiolarians. Sediment color varies at the decimeter scale, alternating between pale green (10Y 7/2) and greenish gray (5GY 6/1) and between pale brown (10YR 6/3) and reddish brown (5YR 5/3) (Figs. F4, F5, F6). Reddish brown horizons in the lower part of this unit are radiolarian-dominated oozes and contain fewer calcareous nannofossils than the other sediment of Unit IV.

Carbonate content (up to 70 wt%) is principally contributed by calcareous nannofossils (Figs. F4, F7). Sediment is heavily bioturbated through most of the unit, and a pinkish color (5YR 7/4) characterizes the lowermost 7.6 m of Unit IV. The downhole increase in carbonate content is paralleled by a major increase in a* and b* color data and a distinctive minimum in dry bulk density, declining from ~1.5 to 1.2 g/cm3. Magnetic susceptibility remains low but increases slightly (from ~15 to 35 IU) from the top to the bottom of the unit.

This unit is clearly defined by the presence of biogenic carbonate at its top and by a prominent chert layer at its bottom. The contact with Unit V, as defined by the chert layer, was recovered in Core 342-U1403A-17H. This chert tentatively represents the major seismic reflector at ~145 mbsf identified in site survey seismics.

Unit V

  • Intervals: 342-U1403A-18X-1, 0 cm, through 28X-CC, 29 cm; 342-U1403B-18X-1, 0 cm, through 32X-CC, 12 cm
  • Depths: Hole U1403A = 147.80–242.30 mbsf; Hole U1403B = 145.40–261.76 mbsf
  • Age: early Eocene to Campanian
  • Lithologies: nannofossil ooze with clay, clay, nannofossil chalk with clay and nannofossil chalk; minor lithologies include radiolarian clay, clay with nannofossils, and cherts.

Unit V and its two subunits comprise a mixed sedimentary package of nannofossil chalk with clay, clay, and interbedded cherts. The top of the unit is defined by the first occurrence of chert (Fig. F4). Notably, sediment in this unit also contains the first downhole occurrence of planktonic foraminifers after those observed in Unit I. Prominent hyperthermal events (PETM and Eocene Thermal Maximum 2 [ETM2]) are lithologically well expressed in Unit V, displaying local maxima in magnetic susceptibility. A well-preserved K/Pg boundary ejecta layer and a Campanian–Maastrichtian sequence were also recovered.

Subunit Va

  • Intervals: 342-U1403A-18X-1, 0 cm, through 26X-5, 12 cm; 342-U1403B-18X-1, 0 cm, through 28X-1, 46.5 cm
  • Depths: Hole U1403A = 147.80–220.62 mbsf; Hole U1403B = 145.40–217.07 mbsf
  • Age: early Eocene to Paleocene
  • Lithology: nannofossil ooze with clay; minor lithologies include clay and chert

Subunit Va comprises an Eocene through Paleocene sequence of brown (7.5YR 5/4 to 7.5YR 7/3) and pale green (10Y 7/2) to light greenish gray (10Y 6/1) nannofossil ooze with clay, with minor very dark grayish brown (10YR 3/2) chert in the Eocene sequence (Figs. F4, F5, F6, F7). Subunit Va is defined at its top by the first downhole occurrence of a chert layer (see “Unit IV”). The contact between Subunits Va and Vb is defined by the K/Pg boundary. Downhole of the Subunit Va/Vb contact is the first occurrence of very carbonate rich chalk (>70 wt% carbonate).

Prominent reddish brown (5YR 5/3-4) and very dark gray (5YR 3/1-2) clay layers are associated with the PETM (discussed below). The Paleocene interval of this subunit is composed of light brownish gray (10YR 6/2) nannofossil ooze and dark yellowish brown (10YR 4/4) clay devoid of carbonate. The K/Pg interval was recovered in both Holes U1403A (interval 342-U1403A-26X-5, 10 cm; 220.60 mbsf) and U1403B (342-U1403B-28X-1, 47 cm; 217.07 mbsf). The 2.5 cm thick event bed is characterized by greenish black (5G 2.5/1) sandy mud (2.5 cm thick) with normally graded green sand-sized spherules (Figs. F10, F11).

Minor lithologies include very dark grayish brown (10YR 3/2) cherts and biosiliceous indurated sediment (porcellanites) with very well preserved Planolites burrows associated with intervals of poor core recovery (Fig. F5E). Carbonate content ranges between 0 and 50 wt%. Magnetic susceptibility and reflectance data display several distinctive maxima and minima for this subunit partly related to well-described changes in magnetic susceptibility through the ETM2 and PETM (Figs. F12, F13). Bioturbation intensity ranges from complete to slight, with minima occurring during the hyperthermal intervals. Rhythmic color variations typically at the decimeter scale between light brownish gray (10YR 6/2) and dark yellowish brown (10YR 4/4) occur during several intervals of the lower part of Subunit Va and are superimposed over primary color changes.

Subunit Vb

  • Intervals: 342-U1403A-26X-5, 12 cm, through 28X-CC, 29 cm; 342-U1403B-28X-1, 46.5 cm, through 32X-CC, 12 cm
  • Depths: Hole U1403A = 220.62–242.30 mbsf; Hole U1403B = 217.07–261.76 mbsf
  • Age: Maastrichtian to Campanian
  • Lithology: nannofossil chalk with foraminifers and clay

The first downhole occurrence of Cretaceous-aged sediment is pale yellow (2.5Y 7/3) to very pale brown (10YR 7/2) nannofossil chalk (Figs. F4, F7, F10). This sediment is characteristic of the ooze–chalk transition; it deforms slightly under pressure but typically fractures. Subtle decimeter-scale rhythmic color variations between pale yellow (2.5Y 7/3) to very pale brown (10YR 7/2) are present throughout Subunit Vb. Sediment color changes episodically to dark gray (10YR 4/1) nannofossil chalk with clay. Bioturbation is well developed in this sediment, with Planolites and Chondrites as the dominant ichnofacies. Centimeter-scale manganese nodules are also present. Calcium carbonate content ranges from 35 to 80 wt%.

Subunit Vb is the only interval at Site U1403 in which planktonic foraminifers occur consistently in higher abundances than in pre-Pleistocene-age sediments. This microfossil group contributes substantially to the calcium carbonate content of the sediment. Calcium carbonate content in this subunit is the highest seen at Site U1403, consistent with observed high dry bulk density values (Fig. F4). The thickness of this subunit varies between the two holes because of the deeper subbottom depths attained in Hole U1403B.

Lithostratigraphic unit summary

At Site U1403, the uppermost 2 m of the sedimentary sequence is Pleistocene nannofossil ooze with clay, foraminifers, and radiolarians (Unit I). These sediments are underlain by a 69 m thick sequence of clay with silt and manganese nodules, barren of any siliceous or calcareous microfossils (Unit II). The upper half of this unit remains undated at the time of this report, whereas the lower half is dated as middle to late Eocene on the basis of magnetostratigraphy (see “Paleomagnetism”). Unit III comprises 38 m of middle Eocene radiolarian ooze with clay, and Unit IV is 28 m of Eocene nannofossil ooze with interbedded clay layers. The lowermost 95 m (Unit V) at Site U1403 is composed of nannofossil ooze, chalk, and clay with interbedded cherts. The occurrence of cherts necessitated XCB coring and resulted in poor recovery compared to the overlying units.

The low and variable carbonate content at Site U1403 suggests that the site was close to the CCD for much of its Cenozoic history, and incursions of carbonate reflect the local history of the CCD. Notably, calcium carbonate content increases 4 m above the Paleocene/Eocene boundary (as identified by biostratigraphy and magnetic susceptibility), which may be evidence of a carbonate compensation overshoot, or deepening of the CCD (Dickens et al., 1997), during the recovery interval of the PETM. A similar increase in carbonate content occurs above the ETM2, suggesting that this site was situated at an ideal depth to capture the magnitude and duration of such CCD fluctuations (Fig. F4).

The majority of the Paleocene is carbonate poor, suggesting a paleodepth below the local CCD. By contrast, sediments below the K/Pg boundary are predominantly calcareous chalks, suggesting that Site U1403 was at or above the CCD during Late Cretaceous time.

Possible record of the late Eocene Chesapeake Bay impact event

In Cores 342-U1403A-6H and 342-U1403B-6H, we observed prominent, 30 cm thick, bluish green (5G 6/1) clay layers that are associated with a significant excursion in magnetic susceptibility to low values and local maxima in natural gamma radiation (NGR) (Fig. F14). Smear slides reveal euhedral feldspar crystals, suggesting that this bed may represent a volcanic ash layer. Alternatively, paleomagnetic data suggest a late Eocene age consistent with the timing of the late Eocene Chesapeake Bay bolide impact event.

Paleocene/Eocene Thermal Maximum and Eocene Thermal Maximum 2

Magnetic susceptibility at Site U1403 in the interval spanning the early Cenozoic hyperthermal events (the PETM at ∼55.9 Ma and ETM2 at ∼54.1 Ma) shows relatively high amplitude variations (Fig. F12). High-frequency magnetic susceptibility oscillations display a cyclic pattern, with a prominent long-wavelength magnetic susceptibility cycle occurring at ~20 mbsf, bounding the two events. Both the ETM2 and PETM show higher magnetic susceptibility values, whereas magnetic susceptibility variations within the interval (163–173 mbsf) are smaller in amplitude (~20 IU). Sediment is greenish gray (10Y 7/1) in this interval (Fig. F12), possibly indicating anoxic sedimentary conditions and thus fewer ferrimagnetic minerals incorporated in the sediment. Magnetic susceptibility shows no correspondence with carbonate content. The onset of the PETM is placed at the base of Core 342-U1403A-21X, which corresponds to the first occurrence of PETM-excursion radiolarians (Figs. F12, F13).

Cretaceous/Paleogene boundary event

A K/Pg boundary section was identified at Site U1403 on the basis of lithostratigraphic and biostratigraphic observations (see “Biostratigraphy”). A normally graded spherule-rich ejecta bed was recovered in both holes drilled at this site (Figs. F10, F11). The base of the spherule layer occurs at intervals 342-U1403A-26X-5, 12 cm (220.62 mbsf), and 342-U1403B-28X-1, 46.5 cm (217.07 mbsf). Both boundary sections display the same general lithostratigraphic sequence, including from bottom to top:

  1. Moderately bioturbated splotchy pink and white-gray latest Cretaceous chalks with abundant, diverse Late Cretaceous nannoplankton assemblages;

  2. A very lightly bioturbated white-gray layer, ~0.5 cm thick, with a sharp basal contact immediately below;

  3. A ~0.5 cm thick bed of green sand- to silt-sized impact spherules overlain by a ~0.5 cm thick bed of light greenish gray chalk with abundant calcispheres and early Paleocene biostratigraphic markers; and

  4. A distinctly bioturbated pink (5YR 7/4) bed topped by light brown (7.5YR 6/3) chalk containing early Danian planktonic foraminifers and nannoplankton.

The K/Pg boundary in Hole U1403B appears less disturbed by drilling than in Hole U1403A, as evidenced by the presentation of distinct brown-tan color cycles in the upper Maastrichtian and relatively little deformation of the spherule bed in Hole U1403B and an undisturbed fine white-gray layer underlying the impact spherules and less fracturing of K/Pg boundary clays.