At Site 1203, the first 300 m of pelagic sediment was drilled ahead with a center bit and the lower sedimentary section was RCB cored before reaching basalt at 457 mbsf. At nearby Site 883 (15.5 nmi), drilled during Leg 145 (Rea, Basov, Janecek, Palmer-Julson, et al., 1993), the whole sedimentary section was cored by advanced piston corer/extended core barrel, to which reference is made for younger sediment not recovered at Site 1203.
The recovery in Hole 1203A was low, owing to the type of drilling used (RCB) and the possible occurrence of sandy intervals that washed out of the core barrel. Therefore, it is not possible to conduct a full-scale comparison of the sediment recovered from Site 1203 with that from Site 883. But the main features of the lithologies reported from Site 883 are recognizable at Site 1203. Three major sedimentary units, Units I, II, and III, early Eocene to late Miocene in age, have been identified at Site 1203, for a total of ~157.56 m. Units I and III have each been divided into subunits, IA and IB and IIIA and IIIB (Fig. F2), on the basis of downcore change in carbonate content (Table T2). These units overlie a fourth lithologic sequence, basaltic basement. Farther downhole, short sequences of interbedded sediment of mainly volcaniclastic origin are present; these are described in "Physical Volcanology and Igneous Petrology". The upper part of the Hole 1203A cores (Unit I) consist of pelagic sediment composed of siliceous and calcareous oozes (Cores 197-1203A-1R to 9R). In Subunit IA, alternating nannofossil and diatom ooze beds are present in varying proportions, characterized by gradational contacts. Nannofossil and diatom abundance is complementary and accounts for nearly all of the sediment. Below, in Subunit IB, diatom abundance decreases until this component virtually disappears in Unit II, which is calcareous chalk. Close to the Subunit IB/Unit II boundary (Section 197-1203A-10R-1), large sponge spicules and other siliceous microfossils (e.g., radiolarians and silicoflagellates) are abundant. Vesicular fragments of unaltered volcanic glass are present toward the top of Unit II.
The disappearance of diatom-bearing sediment marks the Subunit IB/Unit II boundary. In Unit II, nannofossil and calcareous chalk beds contain clay as a minor component. Foraminifers are also present, both as fragments and preserved silt-sized tests, thus forming nannofossil foraminifer ooze (see "Site 1203 Smear Slides"). Unit III contains firm sandy silt beds directly overlying basement volcanic rock with a peak in the authigenic carbonate content (up to 60-70 modal% of calcite, dolomite, and, possibly, other carbonates) at its base (Subunit IIIB). At the topmost part of Subunit IIIB, red and brown clay deposits are present.
The calcareous taxa Thoracosphaera spp. and foraminifers are present throughout the units, although they are commonly more abundant (i.e., as much as 19% at 455.8 mbsf [Section 197-1203A-17R-2] and 21% at 331.4 mbsf [Section 197-1203A-4R-CC]). At some levels, authigenic rhombohedral calcite was observed where nannofossils are less abundant (i.e., Sections 197-1203A-4R-1, 70 cm [329.7 mbsf], and 197-1203A-4R-CC, 7 cm [331.5 mbsf]).
Further detailed description of cores are provided in the barrel sheets (see "Site 1203 Core Descriptions").
Unit I (300.0-388.9 mbsf) consists of nannofossil ooze and diatom ooze beds, with diatom abundance diminishing downhole. This unit consists of core sections characterized by alternation of two end-member lithologies (i.e., nannofossil ooze and diatom ooze) along with a hybrid sediment type containing approximately equal amounts of siliceous and calcareous microfossils. We refer to the hybrid type as nannofossil-diatom mixed ooze. A drastic reduction in diatom abundance occurs in Cores 197-1203A-3R and 7R (Fig. F2).
Unit I has been divided into two subunits, Subunits IA and IB, based on the downhole variation in carbonate content (Table T2), which matches estimated nannofossil content from smear slides (Fig. F2) (see "Site 1203 Smear Slides") and sediment color.
Subunit IA contains nannofossil-diatom mixed ooze, nannofossil ooze, and diatom ooze. The diatom component can vary greatly over a short core interval (10%-90%), but it is consistently at a relatively high level down to the bottom of Core 197-1203A-5R. Compositional variations within and between the different types of ooze result in distinct light green and grayish green bedding and lamination throughout Subunit IA. Large gaps in recovery are present in Cores 197-1203A-5R and 6R (which recovered only 23 cm in the core catcher), and therefore we were not able to exactly place the boundary between Subunits IA and IB. The best approximation for the Subunit IA/IB boundary is at the bottom of Core 197-1203A-5R, where there is a change in the downhole trend of carbonate abundance along with the presence of distinct alternation of diatom-rich vs. nannofossil-rich sediment. Specifically, diatom ooze containing 88%-97% diatoms is more common in Subunit IA (Fig. F2) than in Subunit IB, and in the latter, nannofossil ooze dominates with the exception of a middle Miocene peak in diatom abundance (i.e., 97% at 371.4 mbsf).
Subunit IB is characterized by alternating white nannofossil to light gray nannofossil chalk and siliceous microfossil sediment. Distinct Zoophycos trails are present (Fig. F3A). It is marked by an increase in the carbonate content and high nannofossil and variable diatom content (12%-77%). At the top of Core 197-1203A-10R, olive-colored (2.5Y 5/3) sediment indicates input of clay and organic-rich debris in a meter-thick interval.
The Subunit IB/Unit II boundary is located at Section 197-1203A-10R-2, 19 cm (388.9 mbsf), and is characterized by a gradational and bioturbated contact between white (2.5Y 8/2) siliceous nannofossil chalk and light brownish gray (2.5Y 6/2) nannofossil chalk (Fig. F3B). This boundary signifies the beginning of the diatom deposition at 388.9 mbsf during the late Oligocene and corresponds to the Subunit IIIB/IVA boundary at 652 mbsf established at Site 883 (Leg 145) (Rea, Basov, Janecek, Palmer-Julson, et al., 1993).
Unit II consists mainly of pinkish gray to pale red nannofossil chalk and calcareous chalk (with minor clay and foraminifers) that changes to pinkish white or white nannofossil chalk downhole. Intervals of darker olive-colored sediments (e.g., interval 197-1203A-13R-1, 0-100 cm) reflect dilution by influx of opaque minerals, feldspar, and organic debris. Horizontal burrows are filled with organic-rich chalk containing pyrite (Fig. F3A). Carbonate abundance is consistently high throughout the unit.
The Unit II/Subunit IIIA boundary is at Section 197-1203A-16R-2, 66 cm. At this level (447.3-457.6 mbsf), we observe the termination of the cyclic deposition of Fe-rich clay and calcareous silty clay in organic-rich(?) intervals.
Unit III is a sandy-silty sequence directly overlying the basement volcanic rock. Nannofossil chalk remains the dominant sediment type but is accompanied by intervals enriched in clay, Fe oxide, pyrite, and organic debris (see "Site 1203 Smear Slides"). The nannofossil and calcareous chalk (generally finely laminated and bioturbated) are interbedded with authigenic 2- to 5-cm-thick beds of stiff red clayey Fe oxide bands (i.e., Section 197-1203-17R-3 [456.9-457.6 mbsf]) and silty sand (interval 197-1203A-17R-2, 49-61 cm). At the contact with basement, this unit contains a dark brown interval (7.5YR 5/3) that almost entirely consists of calcite-dolomite silty sand.
This subunit exhibits bioturbation, and distinct orange Zoophycos trace fossils are common. Several erosional contacts and reddish brown to pink silt and clay beds indicate an oxidating environment and alternation between relatively high- to low-energy depositional settings. The Subunit IIIA/IIIB boundary (at Section 197-1203A-17R-2, 51 cm [456.3 mbsf]) (Fig. F3C) is characterized by a gradational contact between pink (7.5YR 7/4) calcareous nannofossil chalk and reddish yellow (7.5YR 6/6) sandy silt nannofossil chalk.
Subunit IIIB is a sandy-silty clay nannofossil chalk with a high Fe oxide content. The subunit is thoroughly bioturbated, with distinct Zoophycos trails indicating oxygenated bottom conditions. Erosional contacts and sandy beds at the bottom of the sedimentary sequence and finely laminated beds and high contents of biogenic carbonate debris and dolomite formation indicate variable fluid velocities and oxygen availability at the time of deposition. Several depositional cycles are indicated by a sequence of beds with distinct colors between pink, reddish yellow (7.5YR 7/6), and light brown (10YR 6/4), which contains high concentrations of Fe oxides (30%-60%).
Despite incomplete coring and only partial recovery of cored intervals at Site 1203, we were able to observe many similarities to the sedimentary section completely cored at nearby Site 883 (Rea, Basov, Janecek, Palmer-Julson, et al., 1993). In particular, the Site 1203 Unit I/II boundary at 388.9 mbsf, marking the onset of diatom production in late Oligocene time, is equivalent to the Subunit IIIB/IVB boundary at 652 mbsf at Site 883. Such a drastic change in sediment type has been related to changes in deepwater circulation patterns (Woodruff and Savin, 1989) revealed in detail at Leg 145 sites (Rea, Basov, Janecek, Palmer-Julson, et al., 1993). It is also worth noting that in Hole 1203A, the beginning of silica deposition (i.e., radiolarians, well-preserved silicoflagellates, and large sponge spicules at 402.1 mbsf [Section 197-1203A-11R-4, 61 cm]) occurred earlier (early Oligocene) than the onset of diatom deposition (Section 197-1203A-10R-2, 20 cm [388.9 mbsf]; early Miocene). However, we set the Unit I/II boundary at the beginning of diatom production and deposition rather than at the first occurrence of silica deposition for purposes of comparison with previous findings at Site 883. A significant difference is the presence of vitric ash layers in the lowermost 150 m at Site 883, which are absent at Site 1203. However, a dramatic increase in opaque minerals and Fe oxyhydroxide clay in Subunit IIIB may be weathering products from these ash layers or exposed basement volcanic rock.