Sedimentary cyclicity in the marine Pliocene-Pleistocene of the Wanganui basin (New Zealand): Sequence stratigraphic motifs characteristic of the past 2.5 m.y.
Earth’s climatic history since 2.5 Ma has been controlled by Milankovitch variations in the planetary orbit, comprising alternate periods of glaciation and interglaciation with a dominant frequency of 41 000 yr. Concomitantly, eustatic sea level has fluctuated 70 to 130 m, causing rapid transgressions and regressions of the shoreline across the world’s continental shelves. The resulting sedimentary record is cyclothemic, each cyclothem corresponding to a single climate and sea-level cycle. The Wanganui basin, New Zealand, contains a 2-km-thick, almost complete, composite record since isotope stage 100 (ca. 2.5 Ma) in the form of 47 superposed cyclothems of shelf origin. Each cyclothem corresponds to an unconformity-bound stratigraphic sequence, and typically contains a transgressive systems tract, sometimes a mid-cycle shell bed, a highstand systems tract, and sometimes a regressive systems tract. No advantage accrues from using transgressive-regressive units rather than cyclothems and/or sequences in description of the succession. Six basic sequence motifs represent deposition in locations between the shoreline and offshore shelf, i.e., the Hawera, Birdgrove, Turakina, Seafield, Castlecliff, and Rangitikei motifs. A seventh, the Nukumaru motif (which includes dominant coquina limestone), represents deposition in shallow-water areas of reduced terrigenous sediment on the flank of the basin. The sequence motifs represented in any section change systematically in sympathy with basin-scale changes in subsidence and sediment supply. In contrast with the 41000 year length of individual glacioeustatic sequences, these basin-wide tectonic cycles have a periodicity of many hundreds of thousands to a few million years, i.e., that of third- or fourth-order sequences of the Exxon type. This, coupled with the restriction of strongly cyclothemic sediments to geological periods of known glacio-eustasy (PermianCarboniferous, Pliocene-Pleistocene), suggests that tectonic subsidence cycles rather than glacio-eustasy are the driving forces behind the development of the third- and fourth-order unconformity-bound sequences that are reported to occur throughout the stratigraphic record.