Previous   |    Next

doi:10.2204/iodp.proc.333.105.2012

Biostratigraphy

Preliminary analysis of the core catcher samples from Holes C0012C (Sections 333-C0012C-1H-CC to 14H-CC), C0012D (Sections 333-C0012D-2H-CC to 13H-CC), and C0012E (Sections 333-C0012E-1X-CC to 3X-CC) reveals assemblages of calcareous nannofossils (Table T4). The biostratigraphic events recognized in Holes C0012C, C0012D, and C0012E are reported in Table T5. All these biostratigraphic events are identified from the analysis of calcareous nannofossils. Accordingly, the sediments drilled in Holes C0012C and C0012D have a Pleistocene to late Miocene age. Hole C0012E sediments deposited on the basaltic basement have a middle Miocene age.

Species abundance is generally common to rare. Sediments throughout the holes contain warm-water genera such as the calcareous nannofossil genus Discoaster.

Calcareous nannofossils

Preliminary examination of all core catcher samples from Holes C0012C, C0012D, and C0012E obtained well- to poorly preserved nannofossils that occur in common abundance in most of the samples. In the upper part of the sequence, species diversity is comparatively higher and preservation is better. However, severe dissolution occurred in several stratigraphic intervals, leading to coccolith-barren intervals or very poor occurrences (Table T4). Most of the zonal markers of Martini’s zonation (1971) and Raffi et al. (2006) have been identified in the sedimentary sequence. The nannofossil occurrences in cores from Holes C0012C, C0012D, and C0012E are listed in Table T4.

The youngest assemblages in these holes belong to Pseudoemiliania lacunosa (Zone NN19) and are present in Sample 333-C0012C-1H-CC with Gephyrocapsa parallela (sp. 3). The last consistent occurrence of Reticulofenestra asanoi, which defines the middle part of Zone NN19, is observed between Samples 333-C0012C-1H-CC and 2H-CC. The last occurrence (LO) of Discoaster brouweri is recorded between Samples 333-C0012C-2H-CC and 4H-CC and is poorly constrained because this coincides with an interval of poor nannofossil preservation. The bottom of Zone NN16, marked by the LO of Reticulofenestra pseudoumbilicus, is observed between Samples 333-C0012C-4H-CC and 5H-CC. The LO of Amaurolithus spp., which indicates the early Pliocene (4.5 Ma), is found in Samples 333-C0012C-7H-CC and 8H-CC. Discoaster quinqueramus and Discoaster berggrenii, which indicate upper Miocene Zone NN11 (5.59–8.52 Ma), are found in Samples 333-C0012C-9H-CC to 333-C0012D-12H-CC. The paracme end (Raffi et al., 2006) of R. pseudoumbilicus is recorded between Samples 333-C0012C-14H-CC and 333-C0012D-4H-CC. The interval of Samples 333-C0012E-1X-CC to 2X-CC, which is characterized by the occurrence of Cyclicargolithus floridanus, is assigned to the upper part of Zone NN6 (between 12.037 and 13.532 Ma). The LO of C. floridanus in Expedition 333 holes is inferred to occur somewhere in the interval between Samples 333-C0012D-13H-CC (179.997 mbsf) and 333-C0012E-1X-CC (504.205 mbsf). However, because the interval between the base of Hole C0012D (180 mbsf) and the top of Hole C0012E (500 mbsf) was not cored during Expedition 333, we cannot provide more information on the horizon of the LO of C. floridanus. Note that previous nannofossil examination during Expedition 322 reports that the LO of C. floridanus was found between 284.86 and 293.04 mbsf in Hole C0012A (Expedition 322 Scientists, 2010).

Sedimentation rates based on biostratigraphy

The sedimentation rates in Holes C0012C, C0012D, and C0012E based on calcareous nannofossils are shown and compared with paleomagnetism results in Figure F28. The apparent sedimentation rates are very low (0.82–1.23 cm/k.y.) between 0 and 37.5975 mbsf (0 and 3.79 Ma). The sedimentation rate increases (3.87 cm/k.y.) in the interval between 37.5975 and 65.0615 mbsf (3.79 and 4.50 Ma). A very low apparent sedimentation rate (0.83 cm/k.y.) is observed in the interval from 65.0615 to 74.065 mbsf (4.5 to 5.59 Ma). This may suggest the presence of a hiatus near the base of a slumped package inferred from structural observations (primarily bedding dips, see “Structural geology”) indicating deformation between 14 and ~85 mbsf. The interval from 74.065 to 128.479 mbsf (5.59 to 7.122 Ma) is characterized by a relatively high sedimentation rate of 3.55 cm/k.y. The lowest sample (333-C0012E-2X-CC, 510.43 mbsf) shows an age older than 12.037 Ma.

The intervals with very low apparent sedimentation rates may be due to possible time gaps in the upper and middle parts of the sequence during the Pleistocene. Taking into account constraints from paleomagnetism and structural observations, an age gap between Samples 333-C0012C-2H-CC and 3H-CC, corresponding to the top of a slumped package, is suspected. Another age gap is inferred between Samples 333-C0012C-8H-CC and 9H-CC. A possible hiatus associated with a chaotic interval from 81.63 to 86.11 mbsf is also suggested from magnetostratigraphy (see “Paleomagnetism”).

Top of page   |    Previous   |    Next