Pliocene-Pleistocene depositional units on the continental slope off central New Zealand: Control by slope currents and global climate cycles

Abstract

An analysis of 13 unconformity-bounded, Pliocene-Pleistocene-age seismic units within the down-slope thickening late Cenozoic section of the northwest Chatham Rise slope, east of New Zealand, indicates an episodic history of mid-bathyal current erosion and deposition. The seismic stratigraphy, which is constrained by biostratigraphic dating of sediment cores, indicates that erosion began in the mid-Pliocene and was most widespread in the Late Pleistocene, when several regional-scale erosion surfaces were developed. The regional extent of the older erosion surfaces differs from the previously mapped pattern of oblique-to-slope, en echelon, scour channels and associated sediment drifts which are related only to the five youngest depositional units (< 0.25 Ma). The eight youngest seismic units (8 to 1) are Late Pleistocene in age and have an average periodicity of about 57–75 kyr. The previous four seismic units (13 to 9), deposited between late Early Pliocene and Late Pleistocene, have a longer frequency of about 750 kyr. All of the erosional or non-depositional unconformities between the 13 seismic units resulted from major velocity changes in the northward flow of intermediate depth water over the 580-m deep Mernoo Saddle, between the Chatham Rise and New Zealand. Therefore, the sedimentary units and their intervening unconformities have a different origin to sequences in the Vail/Exxon conceptual sea-level model of sequence stratigraphy. Nevertheless, the paleoceanographic fluctuations causing the seismic units are related to high-amplitude Plio-Pleistocene glacial-interglacial climatic oscillations superimposed on the late Cenozoic subsidence of Mernoo Saddle. This interpretation is consistent with: (1) the similar periodicity of the Late Pleistocene seismic units with that of high-order (40 and 100 kyr) glacio-eustatic sea-level cycles; (2) the modern physical oceanography in the region; (3) a recent phase of reduced erosion during the Holocene; and (4) the inferred subsidence history of the region.