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doi:10.2204/iodp.proc.314315316.210.2012

Results

Range charts of age-diagnostic nannofossil species for Holes C0004C, C0004D, and C0008A are given in Tables T2, T3, and T4.

Site C0004

In Hole C0004C, a total of 14 nannofossil biostratigraphic events were recognized (Table T5). Three inconsecutive nannofossil zones were distinguished: Pleistocene Zones NN21 and NN19 and middle Pliocene Zone NN16, indicating the presence of unconformities between Zones NN21 and NN19 and between Zones NN19 and NN16.

A total of 16 nannofossil events were recognized for age and depth correlation in Hole C0004D (Table T6). The upper part (from the top of the hole to 255.27 mbsf) was assigned to Zones NN16, NN15–NN14, and NN13 of early–middle Pliocene age. The middle part of this hole (259.53–291.05 mbsf) was characterized by two Zone NN16 intervals sandwiched by a Zone NN15–NN14 interval. The lower part of Hole C0004D (from 312.16 mbsf to the bottom of the hole) was assigned to early Pleistocene Zone NN19.

The biostratigraphic results for Site C0004 are correlated to lithology, log interpretation, and core recovery, and are summarized in Figure F4A. This correlation suggests that Units II and III defined for the prism sediments in the megasplay fault system are of early–middle Pliocene age (Zones NN16, NN15–NN14, and NN13), whereas sediments above and below the prism are Pleistocene age (i.e., upper Pleistocene sediments lie above the prism and lower Pleistocene sediments lie below it). The lowermost sediments at this site are <1.67 Ma, according to the presence of Gephyrocapsa spp. medium I (>3.5–<4 µm); however, the presence of Zone NN19 sediments below Zone NN16 sediments at the bottom of the hole as well as the presence of multiple nannofossil zone repetitions and age reversals implies significant disturbances of the sedimentary sequence in the area, likely due to accretionary prism formation and subsequent disruption.

Site C0008

For Hole C0008A, 17 nannofossil events were recognized (Table T7). The upper part (from the top of the hole to 241.78 mbsf) is Pleistocene age (Zones NN21 and NN19), whereas the lower part is middle–late Pliocene age, containing Zones NN18, NN17, and NN16.

The correlation of these zones with lithology, log interpretation, and core recovery for this hole are summarized in Figure F4B. The data are interpreted to indicate that most of Unit I is of Pleistocene age (Zones NN21 and NN19), and the lowest part of Unit I and Unit II are of late Pliocene age (Zones NN18, NN17, and NN16). The bottom sediments in this hole are <3.65 Ma, according to the absence of Sphenolithus spp.

Biostratigraphic correlation between sites

Figure F4 shows the correlation of zones and important nannofossil events between Site C0004 and Hole C0008A. The correlation of two Pleistocene nannofossil events, first occurrence of Gephyrocapsa spp. large (≥5.5 µm, 1.56 Ma) and last occurrence Calcidiscus macintyrei (>11 µm, 1.60 Ma), between Site C0004 and Hole C0008A indicates most of Unit II (75.94–312.16 mbsf) (Tables T4, T5) at Site C0004 is heavily faulted and that the faulting probably occurred roughly between 1.60 and 1.56 m.y. The faulted interval at Site C0004 can be further subdivided into an upper part (75.94–255.27 mbsf) with a normal and relatively intact and continuous zone sequence (Zones NN16, NN15–NN14, and NN13), and a lower part (259.53–291.05 mbsf) characterized by common faults, missing intervals, disruptions, and age reversals. Although further investigation would be required to more completely delineate these different intervals and to illuminate the detailed nannofossil sequence, the data can be interpreted to represent the influence of (1) uplift of the accretionary prism (the upper part of the faulted section) and (2) disruption of the uplifted prism by splay faulting or out-of-sequence thrusting (the lower, more intensely disrupted part of the faulted section). This sequence of events parallels that interpreted based on Strasser et al. (2009).

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