Chronostratigraphy of sediment core CRP-3 from the Ross Sea, Antarctica

An 823 m thick glaciomarine Cenozoic section sitting unconformably on the Lower Devonian Beacon Supergroup was recovered in CRP-3. This paper reviews the chronostratigraphical constraints for the Cenozoic section. Between 3 and 480.27 mbsf 23 unconformity bounded cycles of sediment were recorded. Each unconformity is thought to represent a hiatus of uncertain duration. Four magnetozones have been recognised from the Cenozoic section. The record is complex with several 'tiny wiggles'' recorded throughout. Biostratigraphical or Sr ages, which could be used to link these magnetozones to the magnetic polarity time scale are restricted to the upper 190 m of sediment. Two diatom datums (Cavitatus jouseanus at 48.9 mbsf and Rhizosolenica antarctica at 68.60 mbsf), together with five Sr-isotope dates derived from molluscan fragments taken from between 10.88 and 190.29 mbsf indicate an early Oligocene (c. 31 Ma) age for this interval. The appearance of a new species of the bivalve ?Adamussium at about 325 mbsf, suggests that the Oligocene age can be extended down to this level. This confirms that the dominantly reversed magnetozone (RI), recorded down to about 340 mbsf, is Chron C12r. The ages imply high sedimentation rates and only minimal time gaps at the sequence boundaries. Below 340 mbsf there are no independent datums to guide the correlation of the magnetozones to the magnetic polarity time scale. However, the absence of in situ dinocysts attributable to Transantarctic Flora, if not a result of environmental control, limits the age of the base of the hole to between c. 33.5 and 35 Ma.

(Table 1) Age of Cretaceous seamounts in the Western Pacific

Dynamics of the Pacific Plate is recorded in the systematic variation of location and the 40Ar-39Ar age of seamounts in the Western Pacific from 120 to 65 Ma ago. The seamounts are grouped into three linear zones as long as 5000 km. The seamounts become younger in the southeastern direction along the strike of these zones. Correlation between age and location of seamounts allows to divide the history of their formation into three stages. Rate of seamount growth was relatively low (2-4 cm/yr) during the first and the third stages within intervals of 120-90 and 85-65 Ma, whereas during the second stage (90-85 Ma), the seamounts were growing very fast (80-100 cm/yr). In the midst of this stage, at ~87 Ma ago, magmatic activity increased abruptly. Dynamics of seamount building is in good agreement with (1) pulses in development of the Ontong Java, Manihiki, and Caribbean-Colombian oceanic plateaus; (2) age of spreading acceleration in the mid-Cretaceous; and (3) a short period when the Izanagi Plate ceased to exist and the Kula Plate was formed. Variation in seamounts' age and location are in consistence with the hypothesis of diffuse extension of the Pacific Plate in course of its motion with formation of impaired zones of decompression melting. Direction of extension (325°-340° NW) calculated from the strike of seamount zones is consistent with the path of the Pacific Plate (330° NW) in the Late Cretaceous. Immense perioceanic volcanic belts were formed at that time along the margin of the Asian continent. The Okhotsk-Chukchi Peninsula Belt extends at a right angle to the compression vector. Three stages of this belt's evolution are synchronous with the stages of seamount formation in the Pacific Plate. Delay in origination of the East Sikhote-Alin Volcanic Belt and its different orientation were caused by counterclockwise rotation of the vector of convergence of oceanic and continental plates in the mid-Cretaceous. At the same time, i.e. 95-85 Ma ago, volcanic activity embraced the entire continental margin and tin granites were emplaced everywhere in the Eastern Asia. This short episode (90+/-5 Ma) corresponds to the mid-Cretaceous maximum of compression of the continental margin, and its age fits well a culmination in extension of the Pacific Plate.