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doi:10.2204/iodp.proc.342.110.2014 LithostratigraphyAt Site U1409, we recovered a sedimentary succession of deep-sea pelagic sediment of Pleistocene to early Paleocene age. Coring in Hole U1409A recovered the full sequence (200.47 m), whereas Holes U1409B and U1409C were terminated just below the Paleocene/Eocene boundary (Cores 342-U1409B-19X [170.57 mbsf] and 342-U1409C-21X [160.47 mbsf]). The sediment of Site U1409 comprises four litho-stratigraphic units (Figs. F4, F5, F6, F7, F8, F9, F10; Table T2). Unit I contains an ~18 m thick sequence of alternating brown to reddish brown Pleistocene to late Pliocene silty clay and nannofossil ooze with varying abundances of foraminifers and diatoms (Figs. F6A, F8, F9, F10) and occasional layers of muddy sand with foraminifers (Figs. F5A, F5B, F11). Unit II comprises light yellowish brown to light brownish gray silty clay to nannofossil clay of Oligocene age (Fig. F6B). The ~17 m thick Unit II sediment contains manganese nodules, patches of disseminated sulfides, and rare concentrations of faint red oxide horizons (Fig. F5C). Unit III is an ~65 m thick sedimentary sequence of middle Eocene to early Oligocene age. The sediment of Unit III typically alternates on 50–100 cm scales between light greenish gray nannofossil clay and carbonate-rich white nannofossil ooze (Figs. F5D, F6C, F6D, F12, F13). The alternating light greenish gray and white sediment of Unit III exhibits longer (~13–18 m) wavelength variability in the color contrast observed between adjacent layers, with some intervals characterized by alternating light greenish gray and slightly lighter greenish gray (rather than white) sediment. Near the top of the ~15 m wide intervals of dampened gray–white color variation in Unit III, we observed decimeter- to centimeter-scale red oxide horizons (Fig. F14). Unit IV is the thickest sedimentary unit (~100 m) and contains lithologies ranging from pinkish white nannofossil ooze with varying abundances of radiolarians and foraminifers to dark brown claystone, siliceous limestone, and chert (Figs. F5E, F5F, F5G, F7B, F7C, F8, F9, F10, F15). The lower Eocene to lower Paleocene sediment of Unit IV is divided into three subunits. Subunit IVa is composed primarily of pinkish white nannofossil ooze with radiolarians (Figs. F5E, F7B, F8, F9, F10). Subunit IVb is a chert-rich interval of highly varied lithologies including pink to dark brown or gray nannofossil ooze to chalk with interbedded chert, siliceous nannofossil limestone, and nannofossil claystone (Figs. F5, F7C, F8, F9, F10, F15). Subunit IVc contains pink to pale gray or pale brown nannofossil chalk (with and without radiolarians) with darker pink to pink-brown color banding, pink oxidative blebs, and mottling (Figs. F5G, F7D, F8, F9, F10). 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) (Fig. F4). The lithologic differences observed between units are primarily attributable to varying abundances of nannofossils, diatoms, radiolarians, and foraminifers (Figs. F7C, F8, F9, F10). Lithologic descriptions are based on sediment recovered from Hole U1409A and refined with observations from Holes U1409B and U1409C. The Unit I/II boundary sits at the sharp contact between the banded and mottled brown to gray Pleistocene- to late Pliocene–age sediment and the underlying, heavily bioturbated, light yellowish brown Oligocene-age silty clay. A sharp contact likewise delineates the boundary between the light yellowish brown Oligocene-age nannofossil clay of Unit II and the interbedded light greenish gray nannofossil clay and white nannofossil ooze of Unit III. The Unit III/ IV boundary is defined by the transition from alternating nannofossil clay and ooze to pinkish white, carbonate-rich nannofossil ooze with foraminifers. Subunit IVb is divided from the overlying and underlying Subunits IVa and IVc by the first occurrence of chert and the last occurrence of siliceous facies. Lithologically, the sedimentary sequence recovered at Site U1409 strongly resembles that of Site U1408, with the exception of Unit IV. Unit IV at Site U1409 is expanded and lithologically diverse in comparison to similarly age sediment at Site U1408. Unit I
Unit I is an ~18 m thick succession of Pleistocene to late Pliocene sediment with decimeter-scale interbedded deep reddish brown (2.5YR 4/1) clay, gray (5Y 6/1) silty clay, lighter gray nannofossil ooze with foraminifers, and dark greenish gray (10Y 4/1) diatomaceous nannofossil ooze (Figs. F5A, F5B, F6A, F8, F9, F10). The colors, lithologies, and minor sedimentary components of Unit I are variegated in comparison to the underlying sedimentary succession. For example, Core 342-U1409A-3H (10.60–20.64 mbsf) contains three intervals of muddy sand with foraminifers interbedded among gray nannofossil ooze with foraminifers and silty clay. The three muddy sand intervals are composed of crude to distinct intercalated beds of grayer, relatively foraminifer rich and greener, relatively silt rich sand, occasionally with normal grading (Fig. F11). Corresponding muddy sand beds are found in Core 2H in Holes U1409B and U1409C. The upper ~8 m of Unit I contains discrete layers of small pebbles of ice-rafted debris, whereas the lowermost ~1 m contains rare manganese nodules <1 cm in diameter. Sediment is moderately to completely bioturbated throughout with the exception of the muddy sand layers, which are slightly bioturbated. The Unit I/II boundary is defined by a sharp contact between the brown-, red-, and gray-banded Pleistocene sediment and the underlying yellowish brown Oligocene-age silty clay in Holes U1409A and U1409B. In Hole U1409C, a large 10 cm long manganese nodule obscures the sharp contact used to define the Unit I/II boundary. We place the Unit I/II boundary in Hole U1409C at the top of this manganese nodule given the scarcity of such nodules in Unit I in Holes U1409A and U1409B. In addition to lithology, the sediment of Units I and II is differentiated by physical properties such as magnetic susceptibility and increased yellow reflectance b* and natural gamma radiation (NGR) in Unit II (Fig. F4; see “Physical properties”). Unit I sediment is characterized by the highest, most variable magnetic susceptibilities of the entire sedimentary succession. Magnetic susceptibility values decrease at the Unit I/II boundary. Sediment in the uppermost interval of Unit I (e.g., Cores 342-U1409A-1H through 2H; 0–10.67 mbsf) contain radiolarians and diatoms of varying abundances (Fig. F6A). These siliceous groups are only found in Unit I and Subunits IVa and IVc and serve to further distinguish Unit I sediment from that of the underlying Unit II (Figs. F8, F9, F10). Unit II
Unit II contains ~17 m of light yellowish brown (2.5Y 6/3) to light brownish gray (5Y 6/2) nannofossil clay with abundant centimeter- to decimeter-scale manganese nodules, disseminated black sulfide flecks, and faint red oxide horizons (Figs. F5C, F6B, F8, F9, F10). The uppermost ~3–5 m of Unit II sediment is barren of planktonic foraminifers and nannofossils (see “Biostratigraphy”) and is of uncertain age. At Site U1409, the presence of abundant manganese nodules is unique to Unit II sediment. The Oligocene-age sediment of Unit II is likewise distinctly characterized by a combination of physical properties and exhibits higher b* color reflectance values and gamma ray attenuation (GRA) density values than sediment in other units. In Hole U1409A, Unit II mean values are approximately three and two times higher than those observed in all other units for b* and NGR, respectively. Sharp, erosive contacts define the upper and lower stratigraphic boundaries of Unit II in all holes and coincide with abrupt changes in color, lithology, and the aforementioned physical properties between adjacent units (i.e., the Unit I/II and Unit II/III boundaries). Unit III
Unit III is an ~65 m thick sedimentary sequence composed primarily of interbedded light greenish gray nannofossil clay and carbonate-rich white nannofossil ooze (Fig. F5D, F6C, F6D, F8, F9, F10, F11, F12, F13). The light greenish gray (10Y 7/1 to 5GY 6/1) layers of Unit III are typically one-third thicker than the interbedded white (N 8) nannofossil ooze layers (Fig. F12). Carbonate content varies from ~30 wt% in the greenish layers to ~85 wt% in the white layers. The white nannofossil ooze and light greenish gray clay are further differentiated by physical properties, including color reflectance (L*), magnetic susceptibility, and NGR (Fig. F13). The sharpness of the relative contrast in color between the interbedded light greenish gray and white layers also changes (Fig. F16). Some intervals contain sharply contrasting beds of light grayish green and white (e.g., Sections 342-U1409A-5H-5 through 6H-5 [35.6–46.6 mbsf] and 8H-2 through 9H-2 [59.6–70.61 mbsf]). Other intervals contain subtly contrasting beds of light grayish green and lighter grayish green (e.g., Sections 7H-1 through 7H-6 [48.6–57.62 mbsf] and 9H-3 through 10H-4 [70.61–83.1 mbsf]). Bioturbation is moderate throughout Unit III, with Zoophycos, Planolites, and Chondrites burrows mottling both the light greenish gray and white sediment. Rare green horizons hosting elevated concentrations of glauconite and chlorite occur throughout the light greenish gray beds of Unit III, but these are conspicuously absent from the white nannofossil ooze. The concentration of green horizons appears to be higher in the uppermost portion of the light greenish gray nannofossil clay intervals immediately underlying the white/green contacts and within the ~15 m thick intervals of reduced color contrast (see expanded discussion in “Cyclicity in middle Eocene sediment”). Layered light greenish gray nannofossil clay and white nannofossil ooze of middle Eocene age were also recovered at Southeast Newfoundland Ridge Sites U1407 and U1408 (Unit III in both). Similar but less visually distinct alternations were recovered in Oligocene–Miocene sediment at J-Anomaly Ridge sites, most notably at Site U1405. The cyclical sediment of Unit III at Site U1409 contrasts with that of the previous Southeast Newfoundland Ridge sites (U1407 and U1408) by containing two intervals with decimeter- to centimeter-scale graded red-brown oxide horizons (see red-brown oxides in uppermost portions of Unit III in Sections 342-U1408A-4H-4 through 4H-5 in Fig. F17). The most notable red-brown oxide horizon extends over 50 cm of core (Fig. F14; see also Sections 342-U1409B-6H-4, 70 cm, through 6H-5, 90 cm [51.5–53.2 mbsf], and 342-U1409C-6H-7, 8 cm, through 7H-1, 50 cm [49.38–50.3 cm]) and grades upward from a sharp lower contact overlain by red-brown sediment to a diffuse light reddish brown sediment over ~50 cm. This occurrence of red-brown oxide is found near the top of an interval of reduced color cyclicity, as is a second interval of red-brown oxide in interval 342-U1409A-9H-3, 34–142 cm (70.95–72.03 mbsf and corresponding sections in Holes U1409B and U1409C). A slump in the middle Eocene is indicated by convoluted bedding with centimeter- to decimeter-scale clasts, completely homogenized green nannofossil clay, and contorted bedding in Holes U1409A and U1409C (83.05–86.96 and 84.39–88.09 mbsf, respectively) (Fig. F18). Evidence for slumping is seen at comparable depths in Hole U1409B (e.g., Core 342-U1409B-9H and 10H). However, a fracture in interval 342-U1409B-9H-7, 3–12 cm (83.83–83.92 mbsf), underlain by ~1 m of homogenized green sediment suggests that the majority of slump sediment in Hole U1409B could lie between Cores 9H and 10H. Alternatively, lateral variability in the slump deposit may exist at the scale of tens of meters (e.g., the distance between holes). A second, smaller slump (or unconformity) may also account for the unusually sharp contact between adjacent light greenish gray and white layers in Sections 342-U1409A-12H-2, 342-U1409B-11H-6, and 342-U1409C-12H-2 just above the Unit III/IV boundary. The Unit III/IV boundary is defined by the transition from interbedded gray and white nannofossil clay and ooze to pinkish white, carbonate-rich nannofossil ooze with foraminifers. The Unit III/IV boundary is observed as a contact in Holes U1409A and U1409C and is underlain by a series of closely spaced pink horizons. In Hole U1409B, the Unit III–IV transition likely falls between Cores 342-U1409B-11H and 12H. The Unit III/IV boundary coincides with an abrupt downhole decrease in NGR values from a Unit I–III average of 22 cps to a Unit IV average of ~6 cps. Magnetic susceptibility values also decrease downhole from an average of ~32 SI in Unit III to ~26 SI in Unit IV (Fig. F4; see “Physical properties”). The carbonate content of sediment increases downhole at the Unit III/IV boundary from an average of 38 wt% in Units I–III to an average of 82 wt% in Unit IV. Unit IV
Unit IV is an ~100 m thick sequence of lower Eocene to lower Paleocene sediment and is divided into three subunits. Subunits IVa and IVc are compositionally quite similar; both are composed of nannofossil ooze (or chalk) with or without radiolarians and/or foraminifers. The ooze–chalk transition spans the lowermost 10 m of Subunit IVa through IVb and provides the primary lithologic distinction between these subunits. Subunit IVb, in contrast, has numerous interbedded cherts and other silicified sediment, which clearly differentiate this subunit from Subunit IVa above and Subunit IVc below. Subunit IVa
Subunit IVa comprises ~25 m of pinkish white (N 8) to very pale brown (10YR 8/3) nannofossil ooze with varying abundances of radiolarians and foraminifers (Figs. F5E, F6A, F8, F9, F10). Decimeter-scale banding between pinkish white (5YR 8/2 to 5YR 8/3), nearly white (N 8 with pink undertones), and light pinkish brown (10YR 8/3) occur throughout most of Subunit IVa. Sediment becomes darker downhole in Subunit IVb, progressing from a light pinkish white (N 8) to a very pale brown in Core 342-U1409A-15H. Heavy bioturbation, evidenced by faint mottling, occurs throughout. Bright pink (2.5YR 8/4) blebs and nodules of montmorillonite are common in a few sections near the top of the subunit (see montmorillonite in Sections 342-U1409A-13H-1, 0 cm, and 13H-2, 35 cm, in Fig. F19). The transition from the lower Eocene pink ooze of Subunit IVa to the underlying Subunit IVb is marked by the first downhole occurrence of chert. Unit IVb
Subunit IVb consists of light pinkish brown (10YR 7/3 to 10YR 8/2) nannofossil chalk with frequent centimeter- to decimeter-scale beds of light brown (10YR 4/3) chert and dark brown (10YR 4/1) silicified sediment (Figs. F5F, F15). Common chert layers slowed drilling, impeded core recovery, and likely contributed to the difference in the thickness of Subunit IVb among holes (~29 m in Hole U1409A, ~36 m in Hole U1409B, and ~36 m in Hole U1409C). Subunit IVb extends from lower Eocene nannofossil Zone NP12 just across the PETM into the latest Paleocene (nannofossil Subzone NP9a). The thickest siliceous sequence in Subunit IVb is found near the base of the subunit and consists of ~45 cm of chert, siliceous nannofossil limestone, siliceous nannofossil claystone, and claystone (Fig. F15). The occurrence of nannofossil excursion taxa (nannofossil Subzone NP9b) within this ~45 cm thick siliceous sequence indicates that it coincides with at least a portion of the PETM. An unconformity coincident with dark brown (10YR 4/1) chert at the top of the silicified PETM sequence separates the overlying, heavily bioturbated, very pale brown (10YR 8/3) nannofossil chalk from the underlying dark brown (10YR 4/1) siliceous nannofossil claystone (see “Biostratigraphy”). The duration of the unconformity at the top of the PETM chert differs between Holes U1409A and U1409B, whereas the contact appears conformable in Hole U1409C (see “Biostratigraphy”). X-ray diffraction (XRD) samples from the lithified PETM interval show evidence of incipient silicification, denoted by a double peak in the diffractogram indicating the presence of tridynamite (20.5° and 21.6°2θ) and possibly also cristobalite (21.9°2θ) (Fig. F20). In contrast, samples from the nannofossil chalk above and the claystone in the center of the PETM interval show a mineralogical background composition with mainly calcite, quartz, and clay minerals (e.g., illite). Upper Paleocene nannofossils are present in mottled light brownish (10YR 6/2) nannofossil claystone immediately below the siliceous nannofossil limestone in the lower portion of the silicified PETM interval (Fig. F15). The lithologic boundary between Subunits IVb and IVc is defined by the last downhole occurrence of silicified sediment in Unit IV. The Subunit IVb/IVc boundary lies ~45 cm below the base of the PETM siliceous nannofossil limestone and is a 2 cm thick chert in Section 342-U1409A-20X-3, 122 cm, and a pair of ~5–10 cm thick siliceous limestones in Section 342-U1409B-18X-6. Drilling was terminated in Subunit IVb in Hole U1409C. Subunit IVb is additionally differentiated from the overlying and underlying sediment of Subunits IVa and IVc by the near-complete absence of radiolarians and diatoms (Figs. F8, F9, F10). Subunit IVc
Subunit IVc, a lower to upper Paleocene sedimentary succession, was recovered primarily in Hole U1409A. In Holes U1409B and U1409C, drilling was terminated just below the Paleocene/Eocene boundary. Subunits IVa and IVc are lithologically quite similar, despite spanning the ooze–chalk transition. Sediment in the uppermost portion of Subunit IVc is light brownish gray (10YR 6/2) and contains common to abundant radiolarians, rare diatoms, and centimeter-scale pink diagenetic blebs. The downhole sedimentary succession includes a transition from nannofossil chalk with radiolarians to nannofossil chalk and several color transitions from light brownish gray (10YR 6/2) to very pale brown (10YR 7/3) to pink (5YR 8/3) to light bluish gray (10Y 8/1). Diagenetic crosscutting pink blebs characterize the brownish gray to light brownish gray sediment, along with faint traces of mottling. Lowermost pink and bluish gray sediment of Subunit IVc is relatively homogeneous. Drilling was terminated in Hole U1409A when relatively silicified sediment slowed drilling and resulted in poor core recovery. Lithostratigraphic unit summaryFour lithostratigraphic units were identified in the ~200 m thick sedimentary succession of deep-sea sediment recovered at Site U1409. Unit I contains alternating brown to reddish brown Pleistocene silty clay and nannofossil ooze with varying abundances of foraminifers and diatoms and occasional layers of muddy sand with foraminifers. The Unit I/ II and II/III boundaries are erosive contacts. Unit II is heavily bioturbated, light yellowish brown Oligocene-age silty clay to nannofossil clay containing manganese nodules, patches of disseminated sulfides, and rare concentrations of faint red oxide horizons. Unit III contains alternating beds of light greenish gray nannofossil clay and white nannofossil ooze, with some intervals containing oxide horizons and reduced color contrast between adjacent beds. Unit IV contains lithologies ranging from pinkish white nannofossil ooze with varying abundances of radiolarians and foraminifers to dark brown claystone, siliceous limestone, and chert. The middle Eocene to lower Paleocene sediment of Unit IV is divided into three subunits. Subunits IVa–IVc contain
The lithostratigraphy of Site U1409 strongly resembles that of Site U1408 in terms of appearance, lithology, and sedimentary succession for Units I–III and, to a lesser degree, Unit IV. We observed four lithostratigraphic commonalities among all sites drilled to date on the Southeast Newfoundland Ridge (Sites U1407–U1409). They include
The observed cyclicity in middle Eocene sediment is discussed in greater detail below. Cyclicity in middle Eocene sedimentThe middle Eocene sediment at Site U1409 exhibits strong cyclicity defined by alternating white nannofossil ooze with foraminifers and greenish gray nannofossil clay from the top of Unit II (Core 342-U1409A-5H) to Core 9H and from Core 11H to Section 342-U1409A-12H-4. The frequency of the cycles is consistent with astronomical forcing. One hypothesis is that clay dilution with astronomical periodicity is the main factor driving these cycles (Fig. F11). Green glauconite/chlorite bands are commonly seen 10–30 cm beneath the white ooze layers. These glauconite bands overprint the burrowing in the green clay-rich layers (Fig. F13), but can be cut by burrows that transport white ooze from above. This observation suggests that the glauconite layers are early diagenetic features that are roughly coeval with the deposition of the white ooze layers. We speculate that the distinct green glauconite bands represent times of relatively slow deposition caused by relatively low dilution by clay (see the “Site U1408” chapter [Norris et al., 2014e] for further discussion on the origin of the green layers), consistent with the clay-dilution hypothesis for the observed green–white cyclicity. |
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