Diatoms recovered from the reference section, Site 1039, represent a nearly continuous stratigraphic record from the Quaternary Fragilariopsis doliolus Zone to the lower Miocene Crucidenticula nicobarica Zone (Table T2). Reworked diatoms are frequently present in the Quaternary diatomaceous sediments where numerous turbidites and sandy ash layers are observed (Kimura, Silver, Blum, et al., 1997). This is particularly true of Rhizosolenia praebergonii var. robusta, which has a last occurrence near the Pliocene/Pleistocene boundary, making resolution of the Nitzschia reinholdii Subzone A/B boundary difficult. Furthermore, the poor preservation of diatoms in silty clay units that are present between depths of approximately 85-150 mbsf prevents placement of the Nitzschia marina Subzone A/B boundary. Siliceous nannofossil ooze and chalk are present between approximately 150 and 420 mbsf, above a gabbro intrusion (Kimura, Silver, Blum, et al., 1997), and diatoms are common to abundant and show moderate to good preservation. This interval spans the Nitzschia jouseae to C. nicobarica zones (Table T2). Recognition of many of the subzone boundaries within the Thalassiosira convexa through Thalassiosira yabei zones is hampered by the slow age-depth rate (6 m/m.y.) estimated for the lower Pliocene through upper Miocene section (Kimura, Silver, Blum, et al., 1997).
Drilling at Site 1040 penetrated the sediment wedge in the toe of slope, through the décollement (371 mbsf) and the underthrust sedimentary section (Fig. F2). The Quaternary F. doliolus Zone is recognized between Samples 170-1040A-4X-CC and 9X-CC, but the majority of samples analyzed above the décollement surface are barren of diatoms (Table T3). Where diatoms are present, they are few to rare and consist of poorly preserved frustules of Actinoptychus senarius and other long-ranging benthic taxa (Table T3). The age of this sedimentary wedge (0-371 mbsf) is estimated to be Pliocene/Pleistocene, based on scattered nannofossil and planktonic foraminiferal data from the cores (Kimura, Silver, Blum, et al., 1997). Below the décollement surface, beginning with Sample 170-1040C-23R-2, 124-125 cm (373.74 mbsf), and continuing to Section 170-1040C-50R-6 (638.28 mbsf), diatoms are consistently present. The abundance, preservation, and recognition of the Quaternary through middle Miocene diatom zones is similar to that observed at Site 1039, including the lack of differentiation of the N. marina through T. yabei zones because of the slow age-depth rate between approximately 483 and 487 mbsf (Table T3).
Diatoms continue to show variable states of abundance and preservation in the cores recovered at Site 1041 (Table T4). Some age-diagnostic taxa are present and suggest a Pleistocene to possibly a late Miocene age for the cores (up to Nitzschia miocenica Zone)(Table T4), but the cores are generally of low quality with poor recovery and extensive biscuiting common. Significant reworking of middle and early Miocene diatoms is observed.
The upper Pleistocene F. doliolus Zone (0-0.62 Ma) is recognized down to 4.01 mbsf. Below that, although Pleistocene and Pliocene taxa are present (Nitzschia fossilis, N. reinholdii, T. oestrupii, R. praebergonii var. robusta), their abundance and states of preservation are too variable to zone the cores from 4.01 to 208.67 mbsf with any confidence.
It is possible that the upper Miocene T. convexa Zone (5.12-6.55 Ma) to N. miocenica Zone (6.55-7.27 Ma) is present between depths of 213.02 and 273.24 mbsf, where diatoms are more common and better preserved, allowing for improved recognition of datums (Table T4). This is based on the presence of T. convexa, N. miocenica, and Thalassiosira praeconvexa in cores from this interval. However, the persistence of common T. oestrupii through this interval, which has a first occurrence (B) at 5.63 Ma, could mean that some of the late Miocene taxa are reworked.
Poor core recovery also characterized coring at Site 1042, with the recovery of mostly silty clay and sandy units, including sandy limestone breccia. Samples from cores recovered at Site 1042 are barren or show few diatoms. When the abundance of diatoms is common, preservation is poor, preventing detailed diatom biozonation of the cores (Table T5). Rare and poorly preserved diatoms in Samples 170-1042A-1R-CC (49.93 mbsf) through 3R-CC (156.26 mbsf) did not yield age-significant taxa. In Samples 170-1042A-4R-2, 48-49 cm (203.44 mbsf), through 7R-2, 48-50 cm (231.18), diatom abundance ranges from common to few and diatoms are poorly to moderately preserved. The samples contain a mix of N. reinholdii, T. convexa var. aspinosa, T. praeconvexa, and N. miocenica (last occurrence = 6.07 Ma), suggesting an age of late Miocene, correlating to T. convexa or N. miocenica zones. Diatoms observed in Sample 170-1042B-4R, 73-75 cm (334.01 mbsf), are few and moderately preserved but are suggestive of the middle Miocene Coscinodiscus lewisianus Zone. Denticulopsis hustedtii, C. nicobarica, C. lewisianus, and Cestodiscus pulchellus are characteristic of the C. lewisianus Zone that ranges between 12.86 and 14.03 Ma. Given the lithologic variations at this site, it is possible that a significant amount of material is reworked or not in place (Kimura, Silver, Blum, et al., 1997). Unfortunately, diatoms recovered from Samples 170-1042B-5R-2, 27-28 cm, through 8R-CC (Table T5) are only rare and very poorly preserved, preventing an age assignment for this part of the site.
Diatoms are variable in abundance and preservation at Site 1043 but show distinctive characteristics above and below the décollement surface (~150 mbsf). Table T6 shows the abundance and zonal distribution of diatom assemblages from this site. Sediments present above the décollement surface are silty clays similar to the wedge sediments recovered at Site 1040. However, it is possible at Site 1043 that several faults are present in the wedge sediments and that diatomaceous material from the downgoing plate has been accreted in fault slices (Kimura, Silver, Blum, et al., 1997). Diatoms present in Samples 170-1043A-2H-2, 124-125 cm (10.74 mbsf); 13X-2, 49-50 cm, through 13X-4, 49-50 cm (111.49-114.49 mbsf); and 15X-CC (130.31 mbsf) are all characteristic of the Pleistocene F. doliolus or N. reinholdii Zone. Species such as F. doliolus, T. oestrupii, Azpeitia nodulifer, N. marina, and N. fossilis indicate that thin intervals of upper Pleistocene diatomaceous clay has been faulted into the slope wedge (Kimura, Silver, Blum, et al., 1997).
Within Core 170-1043A-17X at approximately 150 mbsf, the décollement surface is present. Below the décollement, Samples 170-1043A-17X-CC through 30X-CC are characteristic of the Pleistocene through upper Miocene diatom zones recognized at the reference section (Site 1039) and the section underthrust at Site 1040 (Table T6).