Sedimentary cyclicity in the marine Pliocene-Pleistocene of the Wanganui basin (New Zealand): Sequence stratigraphic motifs characteristic of the past 2.5 my

Earth's climatic history since 2.5 Ma has been controlled by Milankovitch v ariations in the planetary orbit, comprising alternate periods of glaciatio n and interglaciation with a dominant frequency of 41000 yr. Concomitantly, eustatic sea level has fluctuated 70 to 130 m, causing rapid transgression s and regressions of the shoreline across the world's continental shelves, The resulting sedimentary record is cyclothemic,:each cyclothem correspondi ng to a single climate and sea-level cycle. The Wanganui basin, New Zealand , contains a 2-km-thick, almost complete, composite record since isotope st age 100 (ca, 2.5 Ma) in the form of 47 superposed cyclothems of shelf origi n. Each cyclothem corresponds to-an unconformity-bound stratigraphic sequen ce, and typically contains a transgressive systems tract, sometimes a mid-c ycle shell bed, a high-stand systems tract, and sometimes a regressive syst ems tract. No advantage accrues from using transgressive-regressive units r ather than cyclothems and/or sequences in description of the succession. Si x basic sequence motifs represent deposition in locations between the shore line and offshore shelf, i.e., the Hawera, Birdgrove, Turakina, Seafield, C astlecliff, and Rangitikei moths, A seventh, the Nukumaru moth (which inclu des dominant coquina limestone), represents deposition in shallow-water are as of reduced terrigenous sediment on the flank of the basin. The sequence motifs represented in any section change systematically in sympathy with ba sin-scale changes in subsidence and sediment supply. In contrast with the 4 1000 year length of individual glacio eustatic sequences, these basin-wide tectonic cycles have a periodicity of many hundreds of thousands to a few m illion years, i.e., that of third- or,fourth-order sequences of the Exxon t ype. This, coupled with the restriction of strongly cyclothemic sediments t o geological periods of known glacio-eustasy (Permian-Carboniferous, Plioce ne-Pleistocene), suggests that tectonic subsidence cycles rather than glaci o-eustasy are the driving forces behind the development of the third- and f ourth-order unconformity-bound sequences that are reported to occur through out the stratigraphic record.