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doi:10.2204/iodp.proc.342.106.2014 LithostratigraphyDrilling operations in Holes U1405A–U1405C recovered an ~309 m thick sequence of pelagic sediment of Pleistocene to late Oligocene age, including an exceptionally expanded record of the Oligocene–Miocene transition. The lithology at Site U1405 is divided into two lithostratigraphic units. The upper unit (I) is divided into two subunits, and the lower unit (II) is divided into three subunits (Figs. F5, F6; Table T2). The uppermost ~20 m (Unit I) of the sediment column at Site U1405 is composed of Pleistocene to Pliocene clay with abundant nannofossils, foraminifers, and manganese nodules. Subunit Ia is underlain by the carbonate-free silty clay of Subunit Ib. The underlying ~280 m (Unit II) is lower Miocene to upper Oligocene clay and biogenic ooze characterized by greenish gray color and varying abundances of diatoms, radiolarians, and calcareous nannofossils. Subunit IIb, ~10 m of clayey nannofossil and biosiliceous ooze in the middle of Unit II, is characterized by several distinct layers of lighter colored ooze with a higher nannofossil abundance around the Oligocene/Miocene boundary. Lithostratigraphic units and boundaries are defined by changes in lithology (as identified by macroscopic core description and microscopic 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 abundance of nannofossils, diatoms, radiolarians, and foraminifers (Figs. F7, F8, F9, F10). Lithologic descriptions are based on sediment recovered from Hole U1405A and supplemented with observations from Holes U1405B and U1405C. Unit I
Unit I and its subunits (Ia and Ib) comprise the uppermost 15–20 m of the sediment column at Site U1405. The principal lithology of Unit I is brown to light brownish gray (most common is 10YR 5/3 but includes 7.5YR 5/3, 10YR 6/2, 10YR 6/3, and 2.5Y 6/1) clay and nannofossil ooze with varying abundances of foraminifers and silt-sized quartz. The unit includes (from most to least common): clay with silt, silty clay, nannofossil clay, foraminiferal nannofossil ooze, nannofossil ooze, and nannofossil clay with foraminifers. Manganese nodules ranging from millimeter to centimeter scale are found throughout. Bioturbation is heavy to complete throughout Unit I with no discrete layers or burrows visible. Magnetic susceptibility is higher in Unit I than in Unit II, decreasing gradually over several sections above the unit boundary. The Unit I/II boundary is defined by a color and lithologic change and occurs sharply over ~1 cm. Unit I is divided into two subunits based on the presence (Subunit Ia) or absence (Subunit Ib) of calcium carbonate and calcareous nannofossils and/or foraminifers in smear slide analysis (Figs. F7, F8, F9, F10) and supported by subsequent shipboard measurements of carbonate content (Fig. F5). Subunit Ia
Sand-sized foraminifers are common to abundant in the uppermost ~3 m of the sediment column in all three holes. The uppermost 5.6 m of Hole U1405A is composed of a distinct bed of foraminiferal sand, which is likely the best representation of a true mudline. Holes U1405B and U1405C likely did not recover the uppermost few meters of the sediment column. Foraminiferal abundance decreases downhole, yet calcareous nannofossil abundance remains high for the remainder of Subunit Ia (10.70 m total thickness in Hole U1405A). The boundary between Subunits Ia and Ib is defined as the lowest occurrence of “few” (1%–10%) or greater nannofossils in smear slide analysis. This definition is supported by carbonate content data in Hole U1405A that show a gradual decrease from a peak of 40 wt% in the uppermost foraminiferal sand to <1 wt% in uppermost Subunit Ib. The only sand-sized or larger lithic grains (presumably ice-rafted debris) described in Subunit Ia are three rounded lithic cobbles found in a visibly disturbed interval (likely fall-in) in the uppermost 5 cm of Section 342-U1405C-1H-1. Subunit Ib
Subunit Ib is silty clay ranging in color from light yellowish brown (10YR 6/4 and 10YR 6/6) to pale brown (10YR 6/3). Manganese occurs as centimeter-scale nodules and also as submillimeter-scale flecks commonly concentrated in distinct centimeter-scale dark layers. Silt-sized sediment is mostly rounded to subangular quartz, but feldspar, mica, and heavy minerals including zircons are also present in low abundances. No sand-sized or larger lithic grains were identified by visual core description in Subunit Ib. The contact between this yellowish brown sediment and the distinctively greenish gray sediment in underlying lithostratigraphic Unit II is readily apparent (Fig. F5). In Holes U1405A and U1405C, the Unit I/II boundary occurs as a very sharp contact. However, in Hole U1405B, a two-step color change is observed. Between the pale brown (10YR 6/3 to 2.5Y 6/2) of Subunit Ib and the distinctive greenish gray (5GY 6/1 to 5GY 5/1) of Unit II is a distinct 20 cm layer of gray (2.5Y 6/1) carbonate-free silty clay. Based on smear slide analysis, the silty clay of this gray intermediate interval is assigned to Subunit Ib. Unit II
Unit II and its subunits (IIa, IIb, and IIc) comprise an ~280 m thick sequence of clay with abundant calcareous nannofossils, diatoms, and radiolarians. Commonly no component makes up >50% of total smear slide abundance. As with all lithologic names, the major lithology indicates the most abundant component, the preceding modifier indicates abundant (>25%) components, and the succeeding modifier indicates common (10%–25%) components. For example, a “radiolarian nannofossil ooze with clay” describes an ooze in which nannofossils are the most abundant component (but not necessarily >50%), radiolarians are abundant (>25%), and clay is common (10%–25%). The terms “biosiliceous ooze” and “clayey biosiliceous ooze” are used to denote sediment in which the combined biosiliceous components (i.e., diatoms + radiolarians) are more abundant than any other component (e.g., a sediment composed of 10% clay, 30% calcareous nannofossils, 30% radiolarians, and 30% diatoms would be called a nannofossil biosiliceous ooze with clay). These terms are distinguished from, for example, “radiolarian ooze,” which would imply radiolarians alone as the most abundant component. Under this terminology, the most common major lithologies in Unit II are clay, biosiliceous ooze, and nannofossil ooze. Note that the assignment of lithology in the visual core descriptions is done primarily on the basis of smear slides, without the benefit of carbonate content data, which are acquired later. Comparison of smear slide–based lithologic names with carbonate data suggests that core description tends to overestimate the nannofossil component and underestimate the clay component. In Unit II, most lithologies described as nannofossil ooze contain 20–40 wt% carbonate. Unit II is characterized by distinctive greenish gray biosiliceous ooze. The most common Munsell description is 5GY 5/1 (greenish gray) in Subunits IIa and IIb and 10Y 4/1 (dark greenish gray) in Subunit IIc. Throughout the unit, however, there are slight centimeter- to meter-scale alternations between the dominant lighter green (5GY 5/1) intervals and darker greenish gray (10Y 4/1 and 5GY 4/1) intervals that appear brownish by comparison. Submillimeter-scale black sulfide flecks and centimeter-scale sulfide mottles are present throughout Unit II. Occasional darker green (5G 4/1) layers and mottles contain a greater abundance of glauconite in smear slide analysis. Bioturbation is heavy, with individual burrows rarely visible, but centimeter-scale color alternations and distinct horizons of lighter, nannofossil-rich sediment (see “Subunit IIb”) are preserved. The overall lithologies of Subunits IIa and IIc are similar. However, Subunit IIb is distinguished from the overlying and underlying subunits by the presence of intervals of high nannofossil abundance nearly contemporaneous with the biostratigraphically defined Oligocene–Miocene transition (Figs. F11, F12, F13). Subunits IIa and IIc are not devoid of carbonate but generally lack the clearly defined lighter units seen in Subunit IIb. Color reflectance (L*) data in Hole U1405A show cyclic variations throughout Unit II (Figs. F5, F6, F11, F12, F14). Power spectrum and filtering show, in particular, two prominent cyclicities of wavelengths of 16.4 and 4.3 m throughout the lower Miocene, which may reflect an orbitally induced control on sediment composition. X-ray diffraction (XRD) (Fig. F15) analyses show significant abundances of the clay minerals chlorite, illite, and kaolinite throughout Unit II, suggesting relatively consistent contributions of each clay mineral. However, thorough quantification of the clay mineral assemblage would require analysis of the micrometer size fraction at much higher resolution than shipboard sampling allows. Calcite occurs in some XRD analyses and not others, consistent with carbonate data and smear slide analysis. Several microfaults were found throughout Unit II, in most cases pyritized along the fault surface. Magnetic susceptibility in Unit II is lower and more stable than in the overlying Unit I. Subunit IIa
The lithology of Subunit IIa is clay with varying abundances of several biogenic components: diatoms, radiolarians, nannofossils, and sponge spicules. Individual lithologies are classified variously as (from most common to least common) clay, clay with biosilica, biosiliceous clay, clay with nannofossils, nannofossil clay, clayey biosiliceous ooze, clayey nannofossil ooze, biosiliceous ooze with clay, and nannofossil ooze with clay. The most common color is greenish gray (5GY 5/1) but ranges to other shades of greenish gray (10GY 5/1 and 10GY 6/1) and dark greenish gray (5GY 4/1). Subunit IIb
The general lithology of Subunit IIb is clay with varying contributions of diatoms, radiolarians, and calcareous nannofossils, similar to the overlying and underlying lithostratigraphic Subunits IIa and IIc. Subunit IIb, however, contains four prominent light-colored intervals (Layers I–IV in Fig. F11). Each of these layers is characterized by higher carbonate content, higher calcareous nannofossil abundance, and paler color (5GY 6/1 or 10Y 5/1) than the rest of Unit II (5GY 5/1 to 5GY 4/1). The stratigraphically highest of these layers, Layer I, occurs in Cores 342-U1405B-16H and 342-U1405C-16H (Figs. F11, F12) but is not present in Hole U1405A, perhaps falling in a core gap. The top of Layer I marks the lithologic contact between Subunits IIa and IIb. In Core 342-U1405C-16H, Layer I contains high carbonate content (peaking at 35 wt%) and abundant calcareous nannofossil Braarudosphaera. The upper contact of Layer I in Core 342-U1405C-16H is sharp, a feature common among the four prominent light-colored layers observed in Subunit IIb. The upper contact of the corresponding Layer I in Core 342-U1405B-16H is gradational, which is different than in Core 342-U1405C-16H. Calcareous nannofossil biostratigraphy constrains the Oligocene/Miocene boundary to immediately above the top of the second light-colored interval (Layer II) in Section 342-U1405A-19H-6, 48 cm (175.68 mbsf). Calcareous nannofossil abundance by smear slide analysis is higher in this interval (abundant to very abundant), and color is a lighter greenish gray (5GY 6/1) than underlying and overlying strata (5GY 5/1). Carbonate content peaks in Core 342-U1405A-19H at 35 wt% within the lightest colored interval, as compared with <10% in the overlying and underlying darker strata (Fig. F11). The light color of this interval in Holes U1405A and U1405B has a sharp upper contact and a gradational lower transition on a decimeter scale (Fig. F12). This repeated stratigraphic pattern may reflect a carbonate accumulation event with gradual onset and abrupt termination or erosional truncation. Conversely, it may reflect a clay influx event with abrupt onset and gradual waning. The upper contact in Hole U1405C is obscured by drilling disturbance. At ~5–8 m below Layer II, another similar light-colored nannofossil-rich interval (Layer III) occurs in all three holes in Core 20H near the Chron C6Cn.3n/C6Cn.2r boundary. Carbonate measurements in Cores 342-U1405A-20H and 342-U1405B-20H document an increase similar to the one seen in Layer II in Core 342-U1405A-19H, but the increase in carbonate content is of even greater amplitude (peak of 56 wt% in Core 342-U1405B-20H in Fig. F11). Much of this increase in carbonate in both intervals (Layers II and III) seems to be driven by calcareous nannofossil abundance, in particular Braarudosphaera, which is generally not observed with the exception of the four carbonate-rich intervals in Subunit IIb (i.e., Layers I–IV). The light-colored interval in Core 342-U1405A-20H has the same sharp upper and gradational lower contact observed in Layer I of Hole U1405C and Layer II of Holes U1405A and U1405B (Fig. F12). In contrast, the upper contact appears more gradual in Holes U1405B and U1405C, occurring over ~5 cm. The highest carbonate content and lightest color in Layer III of Hole U1405B occurs ~10 cm below the onset of the gradational color change (178 mbsf). Layer IV is observed in Cores 342-U1405A-23H and 342-U1405B-22H and is the thinnest (10–20 cm) of the four light-colored layers. Because of the gradational nature of the lower contact of Layer IV, the placement of the lithologic contact between Subunits IIb and IIc is assigned based on the first darker Munsell color description, ~1 m below the lowermost peak in carbonate. Subunit IIc
The lithology of Subunit IIc is overall very similar to that of Subunit IIa, consisting of alternating layers of mostly clay with varying contributions of biogenic components. Individual beds are classified variously as clay, clay with biosilica, biosiliceous clay, clay with nannofossils, nannofossil clay, clayey biosiliceous ooze, clayey nannofossil ooze, biosiliceous ooze with clay, nannofossil chalk, and nannofossil ooze with clay. The color is on average slightly darker than in Subunit IIa. The most common color is dark greenish gray (5GY 4/1), although the greenish gray (5GY 5/1) characteristic of Subunit IIa is also common. The lower 65 m of Subunit IIc is fractured and biscuited by XCB coring in Cores 342-U1405A-27X through 33X (241.90–307.03 mbsf). The lithology of the XCB-cored interval consists of biosiliceous nannofossil ooze with clay (10Y 4/1) and nannofossil claystone (5GY 4/1). Sand-sized lithic grains were observed in Sections 342-U1405A-20H-4, 23H-1, 33X-2, and 33X-4, predominantly within the 63–150 µm sieve size fraction. Angular silt-sized quartz crystals, often showing conchoidal fractures, and various lithic fragments were found. The composition and size of these late Oligocene siliciclastics is very similar to those observed in the earliest Oligocene of Site U1404. Lithostratigraphic unit summarySite U1405 sediment is composed of ~20 m of Pliocene to Pleistocene aged clay and nannofossil ooze (Unit I) overlying an expanded sequence of late Oligocene to early Miocene biosiliceous ooze and clay (Unit II). Carbonate contents vary considerably within the average range of 0 to 25 wt% throughout most of Unit II, with a few extreme values of up to 56 wt% suggesting that Site U1405 was very near or just above the CCD throughout most of the late Oligocene to early Miocene. Several light-colored beds in Subunit IIb document conspicuous fluctuations of carbonate content around the Oligocene/Miocene boundary, possibly the result of CCD fluctuations, carbonate production, and/or sea level, among other causes. However, the large range in carbonate content throughout Unit II and subtle, cyclical oscillations in color reflectance data suggest that perhaps these carbonate content fluctuations may be present throughout the stratigraphic range of Unit II at Site U1405, albeit undetected by the coarse resolution of shipboard carbonate sampling. Lithostratigraphic Unit II at Site U1405 can be correlated to the upper part of Unit II at deeper water Site U1404, which also recovered a distinctive greenish gray unit of Oligocene–Miocene age of overall similar lithologic composition but with less carbonate content. The time-correlative equivalent of Unit II at Site U1405 at the deepest Expedition 342 Site U1403 is completely barren of carbonate. |
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