Coring in Hole 1140A recovered 234.52 m of sediment overlying submarine pillow basalts with minor dolomitized nannofossil chalk beds. The lower Oligocene to middle Miocene sediment consists of diatom nannofossil ooze, silty diatom ooze, foraminifer-bearing nannofossil ooze, and nannofossil chalk. The uppermost sample (183-1140A-1R-1, 25 cm) contains a mixed assemblage of middle Miocene and Pleistocene diatoms. All samples reported here fall below that interval. Several samples within Section 183-1140A-1R-1 contain well-preserved Pliocene-Pleistocene diatoms that were reworked as a result of severe drilling disturbance (Shipboard Scientific Party, 2000b). Stratigraphic distribution of diatoms is listed in Table T1. Key biohorizons are tabulated in Table T2 along with their average depth of occurrence. Diatoms are abundant in Cores 183-1140A-1R through 9R and 20R through 22R, based on shipboard examination of core catcher smear slides. These intervals were selected for more detailed analysis.
Samples 183-1140A-1R-1, 25-26 cm, through 9R-CC are early Miocene in age. Samples 183-1140A-1R-1, 25-26 cm, through 2R-CC, 17-22 cm, are included in the Nitzschia denticuloides Zone. Core recovery for Cores 183-1140A-1R through 2R was 68%. The first occurrence (FO) of N. denticuloides defines the base of the zone, and the top of the zone is placed at the FO of Denticulopsis praedimorpha. D. praedimorpha was not present in any of the examined samples, indicating that the top of the section is within the N. denticuloides Zone. The FO of N. denticuloides is assigned an age of 13.51 Ma (Shipboard Scientific Party, 1999). The diatom assemblage consists mainly of Actinocyclus ingens, Actinoptychus senarius, Coscinodiscus marginatus, Denticulopsis hustedtii, few Nitzschia species, Triceratium macroporum, and Thalassiothrix longissima.
Samples 183-1140A-3R-1, 100-101 cm, through 4R-2, 25-27 cm, are placed in the D. hustedtii-Nitzschia grossepunctata Partial Range Zone. Core recovery over this interval was low, at 23%. The bottom of the D. hustedtii-N. grossepunctata Zone is defined by the FO of D. hustedtii, with an age of 14.17 Ma (Shipboard Scientific Party, 1999). The diatom assemblage does not contain T. macroporum or any Nitzschia species but contains Actinocyclus curvatulus, Coscinodiscus monicae, Crucidenticula ikebei, and Denticulopsis lauta, which were not present in the overlying zone.
A. ingens var. nodus was not recognized in this sequence and N. grossepunctata was sporadically present, preventing the recognition of the A. ingens var. nodus Zone or the N. grossepunctata Zone. Samples 183-1140A-4R-CC through 6R-2, 100-102 cm, are therefore placed into an undefined zone that spans the A. ingens var. nodus Zone, N. grossepunctata Zone, and the A. ingens-Denticulopsis maccollumii Zone. It is possible that the A. ingens var. nodus, N. grossepunctata, and A. ingens-D. maccollumii zones were not recognized because of a disconformity, but it is more likely that they were not recognized because of the extremely poor core recovery (5%) throughout this interval. The bottom of this section is marked by the first common occurrence (FCO) of A. ingens. The diatom assemblage consists of A. ingens, A. senarius, C. marginatus, D. hustedtii, D. maccollumii, and T. longissima.
Samples 183-1140A-6R-CC through 7R-2, 26-26 cm, were placed in the D. maccollumii Zone. The bottom of this zone is marked by FCO of D. maccollumii. Through this interval there are occurrences of A. senarius, C. marginatus, Hemiaulus spp., Rocella praenitida, Cavitatus jouseanus, and T. longissima.
Samples 183-1140A-7R-CC through 8R-CC were placed in the Crucidenticula kanayae Zone. The base of this zone is marked by the FO of C. kanayae. The C. kanayae Zone spans ~1 m.y. but is limited to only two samples, as the core catcher sample was the only sample recovered from Core 183-1140A-8R. Sample 183-1140A-7R-CC also marks the lowest observed occurrence of A. curvatulus. A. senarius, C. marginatus, and T. longissima are common throughout this interval.
Samples 183-1140A-9R-1, 25-27 cm, through 9R-CC were placed in the Thalassiosira fraga "c" Subzone. The base of this gap zone (a negatively defined zone, based on the absence of key species, after Edwards, 1971) is indicated by the last occurrence (LO) of T. fraga, and the top of the zone is indicated by the FO of C. kanayae. These samples were placed into the subzone based on the absence of T. fraga. Samples below this interval through Sample 183-1140A-20R-1, 25-26 cm, were not examined because shipboard analysis of core catchers through this interval yielded scarce diatoms.
Samples 183-1140A-20R-1, 25-26 cm, through 22R-CC are early Oligocene in age and represent the C. jouseanus Zone and Rocella vigilans "a" Zone. Core recovery averaged 80% through this three-core interval. Samples 183-1140A-20R-1, 25-26 cm, through 22R-4, 25-26 cm, represent the lower Oligocene R. vigilans "a" Zone based on the presence of R. vigilans variety "a" (<40-µm form), whose FO and LO demarcate the nominate zone. The top of this zone is unclear, as samples between Cores 183-1140A-20R and 9R were not examined in this study. Other representative diatoms through this zone are A. senarius, Asteromphalus oligocenicus, Coscinodiscus asteromphalus, C. marginatus, Ethmodiscus rex, Hemiaulus pungens, Pseudorocella barbadensis, Pseudotriceratium chenevieri, Sceptroneis propinqua, Stephanopyxis turris group, T. longissima, and Xanthyopyxis oblonga.
The lower Oligocene C. jouseanus Zone ranges from Sample 183-1140A-22R-4, 100-101 cm, through 22R-CC, the lowermost sample analyzed for this study. The diatom assemblage differs from the R. vigilans "a" Zone in the absence of R. vigilans variety "a" and Cestodiscus pulchellus. Preservation declines rapidly from Sample 183-1140A-22R-6, 24-25 cm, to 22R-CC (Table T1). C. jouseanus is absent from the lowermost sample and rare in the two overlying samples, yet the base of the C. jouseanus Zone is not placed within the study set because of the paucity and highly dissolved state of the diatoms in the lowermost samples. Analysis of core catcher samples from 183-1140A-23R-CC through 25R-CC revealed few non-age-diagnostic diatoms; therefore, samples below 183-1140A-22R-CC were not analyzed for this study.
Linear sedimentation rates (LSRs) were calculated based on seven diatom bioevents spanning the studied interval (Table T2). LSRs vary from 0.95 to 1.8 cm/k.y. with an average of 1.26 cm/k.y. (Fig. F3; Table T3). The sediment accumulation rate is poorly constrained by diatom datums from ~30 to 45 mbsf. This is the interval of our combined A. ingens var. nodus, N. grossepunctata, and A. ingens-D. maccollumii zones, where core recovery was only 5%. With such limited core recovery of rotary cored sediments, it is difficult to determine whether there is a disconformity present or whether the ambiguity is a result of the poor core recovery. Assuming a constant linear sedimentation rate throughout the studied interval, the bottommost sediments recovered (Section 183-1140A-25R-6) would be ~32.8 Ma, representing the lower Oligocene Rhizosolenia oligocaenica "c" Zone. This agrees with the shipboard assignment of an earliest Oligocene age to these sediments based on calcareous nannofossil data (Shipboard Scientific Party, 2000b).