Late middle Eocene to late Oligocene radiolarian biostratigraphy of ODP Hole 113-689B of Maud Rise, Southern Ocean

We propose a new biostratigraphic scheme comprising the Eucyrtidium spinosum, Eucyrtidium antiquum (new), Lychnocanoma conica (emended), Clinorhabdus robusta (emended) and Stylosphaera radiosa (emended) Zones, in ascending order, in Eocene to Oligocene sediments drilled on Maud Rise in Southern Atlantic Ocean (Site 689, Ocean Drilling Program Leg 113). The bases of these zones are defined by the lowermost occurrences of E. spinosum, E. antiquum, L. conica, C. robusta and the uppermost occurrence of Axoprunum irregularis (?), respectively. From correlation to the magnetostratigraphic data, the E. spinosum, E. antiquum, L. conica, C. robusta and S. radiosa Zones are assigned to the late middle Eocene through late Eocene (Subchrons C17n2 to C13r), earliest Oligocene (C13n to C11n), late early Oligocene (C11n to C10n2), early late Oligocene (C10n1 to C8r) and latest Oligocene (C8r to C7An), respectively. The four boundary datum levels and supplementary datum levels such as the lowermost occurrences of A. irregularis (?), Dicolocapsa microcephala and Lithomelissa challengerae may be recognized in other ODP sites in the Southern Ocean. The first occurrence of E. antiquum approximates the Eocene-Oligocene boundary in Southern Ocean but the last occurrences of many species such as Periphaena decora, D. microcephala and the Lithomelissa sphaerocephalis group are commonly diachronous between high latitude sites. Two new species, Theocyrtis (?) triapenna and Spirocyrtis parvaturris, are described.

Peak areas and heights for minerals of ODP Leg 113-696B (Table 1)

Seismic data acquired over the eastern shelf and margin of the South Orkney microcontinent, Antarctica, have shown a high-amplitude reflection lying at a sub-bottom two-way traveltime (TWT) of 0.5-0.8 s. There appear to be two causes for the reflection which apply in different parts of the shelf. The more widespread cause of the reflection is a break-up unconformity associated with the opening of Jane Basin to the east. This is clearly seen where reflections in the underlying sequence are discordant. In contrast, in Eotvos Basin and the southeastern part of Bouguer Basin, the high-amplitude reflection in places cuts across bedding and is interpreted to be caused by silica diagenesis. A post-cruise analysis of core samples from Site 696 in Eotvos Basin by X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed the presence of a silica diagenetic front at 520-530 mbsf. The position of the unconformity at this site is uncertain, but probably coincides with a change of detrital input near 548 mbsf. Fluctuations of physical properties related to the depth of the diagenetic front are difficult to separate from those related to the variation of detrital composition over the same depth interval. Correlation of the drilling record with the seismic record is difficult but with a synthetic seismogram it is demonstrated that diagenesis is the probable cause of the high-amplitude reflection. In Bouguer Basin at Site 695 the depth of the high-amplitude reflection was not reached by drilling; however, the reflection is probably also caused by silica diagenesis because of the biogenic silica-rich composition of the sediments cored. The estimated temperatures and ages of the sediments at the depths of the high-amplitude reflections at Sites 695 and 696 compare favorably with similar data from other diagenetic fronts of the world. The high-amplitude reflection in Bouguer Basin is commonly of inverse polarity, possibly caused either by interference between reflections from several closely-spaced reflecting layers, such as chert horizons, or by free gas trapped near the diagenetic front.