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Abstract: High-Frequency, Late Pliocene Marine Cyclothems from Wanganui Basin, New Zealand: A Model for High-Resolution Sequence Stratigraphic Analysis of Shelf Sequences at the Outcrop

Tim R. Naish, Peter J. J. Kamp

Late Pliocene (c. 2.6-1.7 Ma), cyclothemic marine depositional sequences crop out spectacularly as a southward-dipping (4°-9°), c. 900 m-thick succession in cliffs of the Rangitikei River Valley, Wanganui basin, New Zealand. Twenty sedimentary cycles or sequences occur within the succession, each of which contain a remarkably similar lithofacies architecture and correspond in duration to the 41 ka obliquity-driven orbital rhythm.

Sequences are bounded by erosional unconformities (ravinement surfaces superposed on sequence boundaries) and comprise transgressive (TST), highstand (HST), and forced regressive (FRST) systems tracts. Two "end-member" styles of depositional sequence are recognised in the succession, "asymmetrical sequences" and "symmetrical sequences". Symmetrical sequences comprise a thick TST (5-30 m) reflecting accumulation in the sediment-satiated near shore to innermost shelf sand prism, whereas asymmetrical sequences contain a thin TST (<2m) reflecting stratigraphic condensation on the sediment-starved inner to mid-shelf. All sequences contain: (i) a thin HST (10-20 m) that comprises an inner to mid-shelf aggradational siltstone facies assemblage deposited under static relative sea-level conditions during the eustatic highstand and early fall; and (ii) an overlying thicker FRST (10-60 m) that comprise an inner shelf to shoreface progradational sandstone lithofacies assemblage deposited under conditions of rapidly falling eustatic sea-level.

Five types of condensed deposit shellbed (3 of them after Kidwell, 1991), which are associated with key stratal discontinuities, elucidate the architecture of each sequence by defining the position of the sequence boundary and ravinement surface (condensed onlap shellbeds), downlap surface (condensed backlap and shellbeds), and marine flooding surfaces (condensed flooding surface shellbeds). The recognition of condensed shellbeds associated with stratal terminations in shelf sequence enables the seismic sequence approach to be more easily applied to sequences in outcrop: the shellbeds are surrogates for seismic reflectors.

The chronology for the succession is based on isothermal plateau fission track ages of rhyolitic tephra, existing paleomagnetic data, and biostratigraphic data, and allows correlation of the sequences with Oxygen Isotope Stages 100-58. Through the integration of the late Pliocene oxygen isotope-derived eustatic sea-level model with the basin subsidence history, we have extracted a record of relative sea-level change for the succession, which closely approximates faunally-derived paleobathymetric changes and explains the observed upsection variations in facies architecture of the Rangitikei Valley succession.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France