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SITE SUMMARIES (continued)

Site 1222

Site 1222 (13°48.98'N, 143°53.35'W) forms a south-central component of the 56-Ma transect drilled during Leg 199 and is situated ~2° of latitude south of the Clarion Fracture Zone in typical abyssal hill topography. On the basis of regional magnetic anomalies, we anticipated basement age at Site 1222 to be equivalent to C25r or C25n (~56–57 Ma; Cande et al., 1989), slightly older than at Site 1219. At the outset of drilling at Site 1222, our estimate for total sediment thickness was ~115 m; based upon a fixed hotspot model (Gripp and Gordon, 1990, for 0- to 5-Ma Pacific hotspot rotation pole and Engebretson et al., 1985, for older poles) Site 1222 should have been located ~1° north of the equator at 56 Ma and ~4°N at 40 Ma. A nearby gravity core (EW9709-17GC) taken during the site survey cruise recovered more than 5 m of red clay with a late to middle Miocene age on the basis of radiolarian biostratigraphy (Lyle, 2000).

Two holes were drilled at Site 1222. Hole 1222A was drilled to basement, which was penetrated at ~98 mbsf. Hole 1222B recored the APC-suitable portion of the sediment column (to ~60 mbsf). The sediment column at Site 1222 contains a sequence of lithologies significantly different to that seen at other Leg 199 sites. Site 1222 recovered a 97.8-m thick section dominated by clay-rich lithologies and chert. Unit I consists of an ~60-m-thick yellowish brown to dark gray brown clay with zeolites grading to radiolarian clay. A thin interval of diatomaceous clay occurs near 17 mbsf (Sections 199-1222A-3H-2 to 3H-4). Within the diatom ooze is an interval with unusual layers of laminated diatom ooze that contain a nearly pure diatom component. Laminations are mainly white to olive. Diatom frustules within some laminae contain significant pyrite making them appear very dark gray to black. A sharp contact exists at the base of the diatom ooze interval and the underlying clay of Subunit IA. In Hole 1222A, this contact also contains a large (5-cm diameter) botryoidal manganese nodule. Unit II consists of a thin (~2 m thick) dark brown to brown nannofossil clay grading downcore to yellowish brown nannofossil ooze. Very pale brown mottles are common. Nannofossil content (estimated from smear slides) gradually increases from 30% in the upper portion to 80% near the base of the unit. Unit III consists of an ~8-m-thick sequence dominated by clay with iron oxides and zeolites. Unit III is underlain by a thick (>30 m) poorly recovered sequence of massive chert that, in turn, overlies basement basalt.

Radiolarians provide the biostratigraphic framework for Site 1222. Planktonic foraminifers are completely dissolved in all samples investigated. Calcareous nannofossils of early Oligocene age occur in Unit II and of early Eocene age in chalk attached to a piece of chert occur just above basement. This large chert nodule is rimmed with very pale brown nannofossil chalk that provides datumsİindicating an early Eocene age (NP10). The biostratigraphic distribution of radiolarians suggests massive reworking and mixed Pliocene assemblages in the uppermost two and a half cores, between ~0 and 21 mbsf. The following two cores, between ~21 mbsf and 40 mbsf, contain upper and lower Miocene radiolarian assemblages. Two cores between ~40 mbsf and 60 mbsf hold upper and lower Oligocene radiolarian assemblages. Nannofossils are present between ~55 mbsf and 60 mbsf, supporting the radiolarian biostratigraphy. This upper part of the section at Site 1222 appears to contain several breaks in sedimentation. A major hiatus occurs between the lowermost Oligocene and the middle Eocene, having a duration on the order of ~5–6 m.y.

It was possible to construct a continuous spliced record from Site 1222 down to 34.44 mbsf (37.88 mcd), below which it was possible to align most cores from the two drilled holes to each other, but without continuous overlap. Cores 199-1222A-7H and 199-1222B-7H, however, showed good overlap and allowed the splicing of an additional ~13 m from ~59 to 72 mcd.

The paleomagnetic data from lithologic Subunit IA, down to (~40 mbsf) are not interpretable, except for the very top of the Hole 1222A. A large group of ChRM directions show relatively high magnetic inclinations after demagnetization especially in the most clay-rich lithologic units. The cause of the steep inclinations is unclear and might be indicative of unrecognized deformation during the drilling. A preliminary correlation of the magnetic stratigraphy to the GPTS shows that at the very top of the section we were able to identify the bottom of polarity Chron C1n (Brunhes). Stratigraphically downward we identify Chrons from C6Cn to C11n. A long reverse magnetization Chron at the bottom of the section (~67–75 mcd) is thought to represent Chron C12r.

The pore water profiles from Site 1222 primarily reflect minor organic matter degradation and the dissolution of biogenic silica. Most of the dissolved chemical constituents show a lack of gradient with depth and values similar to or slightly higher than seawater.

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