| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.314315316.133.2009 BiostratigraphyTwo holes were drilled at Site C0004 on the shallow portion of the megasplay fault system during Expedition 316. Hole C0004C was cored from the seafloor to 134.97 m CSF through the slope sediments into the upper part of the accretionary prism. Hole C0004D was cored from 100.00 to 398.79 m CSF from the upper part of the accretionary prism through its lower boundary into the underthrust slope sediments below. Biostratigraphy was determined for Site C0004 based on examination of calcareous nannofossils, radiolarians, and foraminifers. A few samples were additionally analyzed for diatoms. Calcareous nannofossilsAll core catcher samples plus additional samples from some critical intervals in the vicinity of zonal boundaries were examined for calcareous nannofossils at Site C0004. Calcareous nannofossils from slope sediments are generally abundant and moderately preserved, whereas those from the accretionary prism sediments and the underthrust slope sediments are low in abundance and moderately to mostly poorly preserved (Tables T6, T7). In Hole C0004C, a total of 10 nannofossil biostratigraphic events were recognized (Table T8). Pseudoemiliania lacunosa was absent in Sample 316-C0004C-1H-CC, 0–5 cm (6.27 m CSF) but occurs in Sample 2H-CC, 0–5 cm (16.33 m CSF); therefore, the last occurrence (LO) of P. lacunosa is located between 6.27 and 16.33 m CSF, marking the boundary between nannofossil Zones NN20 and NN19. Zone NN19 was further subdivided by four events: the last consistent occurrence (LCO) and first consistent occurrence (FCO) of Reticulofenestra asanoi and the LO and FCO of Gephyrocapsa spp. large (>5.5 µm) (Table T8). Among these events, the FCO of Gephyrocapsa spp. large (>5.5 µm) (1.46 Ma) was assigned between Samples 316-C0004C-8H-CC, 23.5–28.5 cm, and 9H-2, 94 cm (73.64–75.23 m CSF). The bottom of the FCO of Gephyrocapsa spp. was truncated by a number of late Pliocene–early Pleistocene events:
That is, the lower part of lower Pleistocene Zone NN19 and upper Pliocene Zones NN18 and NN17 were missing in the interval, indicating a significant unconformity (with a time difference of ~1 m.y.) in the interval from 74.44 to 80.87 m CSF, which is compatible with results from magnetostratigraphy in the same hole (see “Paleomagnetism;” Fig. F29). This unconformity, indicated by nannofossil events, can be correlated to the upper boundary of the upper slope deposits, indicated by seismic profile in the same hole. The sediment interval from below 80.87 m CSF to the bottom of Hole C0004C was assigned to middle Pliocene Zone NN16, based on the occurrence of D. surculus. The LO of D. tamalis (2.87 Ma) was identified between Samples 316-C0004C-11H-CC, 0–5 cm, and 12H-CC, 0–5 cm (89.23–98.51 m CSF) (Table T8). In Hole C0004D, reworked nannofossils are common throughout the sediment samples obtained, leading to difficulty in recognizing nannofossil events and zones. For example, middle Pliocene zone markers Discoaster asymmetricus (base marker of Zones NN15–NN14), Ceratolithus rugosus (marker of Zone NN13), and Ceratolithus acutus (rare but important species occurring just below and above the Zone NN12/NN13 boundary) are observed nearly continuously in the upper slope sediment sequences (Table T7). A total of 13 nannofossil biostratigraphic events were recognized for age and depth correlation in Hole C0004D (Table T9; Figs. F30, F31). The upper part of the sediment sequence to Sample 316-C0004D-19H-CC, 0–5 cm (226.98–227.03 m CSF), was assigned to middle Pliocene calcareous nannofossil Zone NN16, based on the co-occurrence of D. surculus (marker of top of Zone NN16) and D. tamalis, as well as on the absence of Sphenolithus spp. and Reticulofenestra pseudoumbilicus (>7 µm) (top marker of Zones NN15–NN14) (Table T9; Fig. F30). Trace occurrences of these two species in a few samples in this interval were considered to be due to reworking. The frequent to common occurrence of R. pseudoumbilicus (>7 µm) between the interval from 228.80 to 256.79 m CSF (Samples 316-C0004D-20H-CC, 0–5 cm [228.77–228.82 m CSF], through 25R-1, 79 cm [256.79 m CSF]) marked the short middle interval of Hole C0004D as upper lower Pliocene Zones NN15–NN14. In this interval, Sphenolithus spp. (LO just above the top of Zone NN15) occurs frequently to commonly. D. asymmetricus (base marker of Zones NN15–NN14 with first occurrence [FO] at 4.13 Ma) occurs continuously in this interval (Table T7), suggesting the sediments at the bottom of this interval are younger than 4.13 Ma (Fig. F30). Sample 316-C0004D-25H-CC, 0–5 cm (257.72–257.75 m CSF), was barren of nannofossils. The interval from below this sample to 292 m CSF (Sample 316-C0004D-33R-1, 20 cm) lacked frequent occurrences of Sphenolithus spp. and R. pseudoumbilicus (>7 µm); however, this interval had occurrences of D. surculus and D. tamalis and therefore was assigned to Zone NN16. The age of the bottom of this interval was estimated between 3.65 and 2.87 Ma, based on the absence of Sphenolithus spp. (LO = 3.65 Ma) and the presence of D. tamalis (LO = 2.87 Ma) (Table T7). There is an age gap between Zones NN15–NN14 sediments and underlying Zone NN16 sediments in the interval between ~256.79 and 260.69 m CSF (Fig. F30). The reentry of Zone NN16 below sediments of Zones NN15–NN14 suggests a sequence reversal or disturbance of normal sediment sequences caused by faulting. The location of this age reversal is consistent with the top of lithologic Unit III, the fault-bounded unit (see “Structural geology”). The determination of zonation or subdivision for the interval from Samples 316-C0004D-33R-CC, 0–5 cm (296.06–296.11 m CSF), through 36R-CC, 0–5 cm (307.47–307.52 m CSF), was impossible during the cruise because samples observed contained a mixture of Pleistocene and Pliocene nannofossils in various degrees or were barren of nannofossils. Sediments within this interval are fractured and brecciated (see “Structural geology”). The lower part of the sediment sequence of Hole C0004D to the bottom of the hole (from Sample 316-C0004D-37R-1, 17 cm [310.16 m CSF], to the bottom of the hole) was assigned to Pleistocene Zone NN19 because of the frequent to common occurrence of various sized Pleistocene Gephyrocapsa spp., such as Gephyrocapsa spp. large (>5.5 µm) and Gephyrocapsa spp. medium I and medium II. The FO of Gephyrocapsa spp. large (>5.5 µm) (1.560–1.617 Ma) can be placed between Samples 316-C0004D-40R-CC, 4.0–9.0 cm (325.43–325.48 m CSF), and 41R-CC, 0–5 cm (330.37–330.42 m CSF). This further provides an estimation of the age for the top of the lowest sequence of lithologic Unit IV: between 1.46 Ma (FCO of Gephyrocapsa spp. large [>5.5 µm]) and 1.560–1.617 Ma, indicating the lower part of Hole C0004D is in the range of lower Zone NN19 (Fig. F30). The occurrence of Zone NN19 below both Zone NN16 and the zone containing a mixture of Pleistocene and Pliocene nannofossils implies a significant age reversal. This event is in good agreement with the lower boundary of the thrust fault suggested by seismic reflection profile and LWD in the same hole and a fractured/brecciated zone of Unit III (see “Structural geology” and “Lithology”). The age difference for the age reversal is ~2.05 m.y. (Fig. F30). Other microfossil groupsRadiolarians, diatoms, and foraminifers occur in variable abundances and preservation throughout the upper part of the hole (Unit I and Subunit IIA). Sediments retrieved from the lower part (Subunit IIB to Unit IV) contain only rare microfossils. Radiolarians and diatoms are reworked or are barren. Although all core catchers were processed and radiolarians were checked throughout the hole, only the upper part appeared to be reliable for biostratigraphic analysis (Fig. F31). RadiolariansRadiolarians are common to abundant and moderately to well preserved in the upper part of the sequence in Hole C0004C (until Section 316-C0004C-7H-CC). In this interval (Unit I and Subunit IIA), three biozones were identified. The appearance of Stylatractus universus in Sample 316-C0004C-2H-CC assigns the upper interval to the Botryostrobus aquilonaris Biozone. The association of Lamprocyrtis heteroporos and Pterocanium praetextum in Sample 316-C0004C-6H-CC was assigned to the Eucyrtidium matuyamai Biozone. This zone can be extended to the base of lithologic Unit I, although diagnostic radiolarian taxa are not available to confirm this. Deeper samples (Units II and III) contain rare and nondiagnostic specimens or are barren. In this interval, preservation is moderate to poor and spongeous and fragments of highly silicified radiolarian shells dominate the assemblages, which are often undiagnostic for radiolarian zonation. Preliminary results on the preservation of radiolarian shells (highly silicified and spongeous specimens) seem to show their preservation is significantly different in the four lithologic units (Fig. F31). Unit I contains undamaged shells with only a few broken specimens, whereas shell preservation varies between well preserved and poorly preserved in Subunit IIA and Unit IV. Only a few poorly preserved radiolarian shells were found in Subunit IIB. In Unit III, radiolarians are well preserved and even fragile shells are found. Although a more detailed analysis (e.g., scanning electron microscope [SEM] imaging and opal diagenesis data) is needed and more samples have to be checked, there is an obvious difference in preservation among these four lithologic units, probably because of the surrounding chemical composition (e.g., silica in pore water, sediment content) and the burial depth. DiatomsFor this site, diatom analysis was attempted only in Unit I and in a few critical intervals as reference for nannofossil determinations in Units II and III. Diatoms in Unit I appeared to be well preserved, although breakage is common. Most of the retrieved diatoms were undiagnostic for zonation purposes, although the LO of Nitzschia reinholdii was recognized in the interval between Samples 316-C0004C-2H-CC and 3H-CC. Throughout the hemipelagic interval, diatom assemblages are composed of coastal (e.g., Actinoptycus senarius) and open-ocean species, mainly from warm-water environments. Thalassionema nitzschoides and N. reinholdii are ubiquitous taxa within their biostratigraphic interval, and Coscinodiscus radiatus, a large taxa characteristic of warm-water environments, was systematically found within the radiolarian size fraction (>63 µm). Below Sample 316-C0004C-9H-CC, diatoms (>63 µm) are absent or strongly reworked. SummaryUnit II and III prism sediments in the megasplay fault system are of early–middle Pliocene age (from Zone NN16 to Zones NN15–NN14). Sediments above and below the prism are Pleistocene sequences (upper Pleistocene sediments above the prism and lower Pleistocene sediments below it), based on nannofossil data from Holes C0004C and C0004D. Plots of age and depth for upper Pleistocene slope sediments in Hole C0004C were combined with results from magnetostratigraphy in the same hole (see “Paleomagnetism;” Fig. F29). Three significant age gaps were recognized in the shallow portion of the megasplay fault system. The unconformity (with time difference of ~1 m.y.) at ~78–79 m CSF in Hole C0004C indicates the boundary of the slope sediments and the top of Unit II (Fig. F29). The second age gap occurs in Hole C0004D in the interval between lower–middle Pliocene Zones NN15–NN14 and the reentry of middle Pliocene Zone NN16 (from about >256.79 to <260.69 m CSF). The most significant age gap and reversal occurs in Hole C0004D between middle Pliocene Zone NN16 and Pleistocene Zone NN19 and can be correlated to the lower boundary between Units III and IV. The time difference for this age reversal was estimated as ~2.05 m.y. (Fig. F30). |
|||
