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doi:10.2204/iodp.pr.320T.2009 LithostratigraphyTwo holes were drilled at Site U1330 (Fig. F2). Hole U1330A was drilled without coring (washed) to ~100 mbsf where two RCB cores were recovered from the interval of ~103 to 123 mbsf. Hole U1330B was cored continuously to a maximum depth of 91.2 mbsf. Sediments at Site U1330 are characterized by varying proportions of calcareous nannofossils and planktonic foraminifers with minor amounts of siliceous microfossils (Table T5). The stratigraphy recovered from both holes can be described as a single lithostratigraphic unit (Fig. F7). Note that Core 320T-U1330B-10H was not cut during Expedition 320T but was reserved for core flow demonstration for Expedition 320 scientists. From visual inspection of the whole-round sections and comparison with Site 807 results (Kroenke, Berger, Janecek, et al., 1991), this core is assumed to be part of the same lithostratigraphic unit as the preceding nine cores. Description of Unit 1
Unit 1 consists predominantly of light gray to white nannofossil ooze with foraminifers, locally grading to foraminifer nannofossil ooze. The latter lithology dominates the uppermost core (320T-U1330B-1H), which is noticeably more brownish in color, perhaps owing to the presence of terrigenous material and organic matter. Traces of siliceous microfossils (radiolarians, silicoflagellates, and diatoms) are present throughout. The entire unit is characterized by moderate to slight burrowing with common darker burrow-mottled horizons. Prominent diagenetic features include color banding and burrow halos in shades of green and purple. Locally burrows are partly pyritized, forming hard "bits" in the otherwise soft ooze. Black burrow features without visible framboidal pyrite in smear slide may contain submicroscopic sulfides such as greigite (see Musgrave et al., 1993). InterpretationSite U1330 is a reoccupation of ODP Leg 130, Site 807, which is situated near the Equator (~3°N) in a basement graben feature of the Ontong Java Plateau, a location thought to have protected it from bottom current activity as pelagic sediment accumulated (Kroenke, Berger, Janecek, et al., 1991). The single unit designation (Unit 1) for Expedition 320T core is consistent with that of Site 807 (Kroenke et al., 1991) where Unit IA extended from Holocene/Late Pleistocene calcareous ooze at the sediment/water interface to the ooze–chalk transition in Miocene (~10.4 Ma) sediments at 293 mbsf. This transition is approximately the timeframe (~10–12 Ma) when the site crossed the equator (Kroenke, Berger, Janecek, et al., 1991). As at Site 807, pelagic sediment cored at Site U1330 contains very little terrigenous input except perhaps in the uppermost few meters of the section. No pyroclastic debris was observed. Sediment was deposited above the lysocline (Kroenke, Berger, Janecek, et al., 1991) in a zone of perhaps lower than expected organic carbon content suggesting low surface water productivity compared to other equatorial sites (Stax and Stein, 1993). The color bands are fairly early diagenetic (redox) features in that they are present ~10 m below the sediment/water interface. According to Lind et al. (1993), the purplish bands reflect the distribution of disseminated fine-grained iron sulfide and the green bands are enriched in Fe- and Al-bearing silicates. The latter may be clays derived from volcanic ash as suggested by Lind et al. (1993); however, we saw no evidence for this in smear slides of these laminae. Furthermore, Kroenke et al. (1993) only noted distinct ash layers at Site 807 below the section cored at Site U1330. |
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