New Zealand Journal of Geology and Geophysics abstracts
Late Pliocene (2.8 -2.4 Ma) cyclothemic shelf deposits, Parikino,
Wanganui Basin, New Zealand: lithostratigraphy and correlation of
cycles
AVON P. MCINTYRE
PETER J. J. KAMP
Department of Earth Sciences
The University of Waikato
Private Bag 3105
Hamilton, New Zealand
Abstract The Wanganui River valley in the vicinity of
Parikino in western Wanganui Basin, New Zealand, contains a late Pliocene,
c. 400 m thick, southward dipping (c. 5deg.) cyclothemic marine
succession. Thirteen sedimentary cycles or cyclothems occur within the
succession. The lower two sandstone-dominated cycles are assigned to the
Paparangi Group, which includes a renamed formation (Pitangi
Sandstone) and a newly defined formation (Moukuku). The overlying
Okiwa Group is emended, its base being relocated to the base of the
Wilkies Shellbed. This group contains seven new formations, in ascending order:
Whauteihi, Whakaihuwaka, Tirotiro, Parikino,
Whariki, Shaw, and Wickham Formations. Most formations
contain one or more unconformity-bounded cyclothems, which comprise a lower
thin (0.2-1.5 m thick) coquina (shellbed) member, an overlying siltstone
member, and an uppermost sandstone member. The distribution of each formation
is presented on a new geological map for the area, and their lithologies are
shown on stratigraphic columns. Grainsize analysis on a suite of (200) closely
spaced sample sites highlights the sedimentary cyclicity and the gradational
change from siltstone into sandstone in most cycles. Combined with the depth
paleoecology implied by the macrofauna and benthic foraminifers, regular
changes in water depth from mid-shelf to shoreface paleoenvironments are
inferred for each cycle.
The cyclothemic succession is correlated with the late Pliocene part of the
oxygen isotope stratigraphy for ODP site 846 (eastern equatorial Pacific),
based on correlation of the Hautawa Shellbed with Stage 98. The lowermost cycle
(1) in the Parikino section accumulated between the peaks of Stages G11 and
G10, and the uppermost cycle (13) accumulated during Stages 92 and 93. The
one-to-one match between cyclothems and the isotope stage couplets means that
the cyclothems have the same frequency as the isotope curve, which, for the
late Pliocene, corresponds to the dominant 41 000 yr Milankovitch
orbital rhythm. It follows that the cyclicity results chiefly from repetitive
glacio-eustatic sea-level changes. Significantly, the first cyclothem is aged
c. 2.8 Ma, which precedes what is commonly regarded as the first
major Northern Hemisphere continental glaciation (Stage 100, 2.54 Ma) by
0.26 m.y., but coincides with the first occurrence of significant
ice-rafted debris in core from the Arctic Barents Sea.
Keywords cyclothems; Wanganui Basin; lithostratigraphy; new
stratigraphic names
New Zealand Journal of Geology and Geophysics, 1998, Vol. 41: 69-84
0028-8306/98/4101-0069 $7.00/0 (c) The Royal Society of New Zealand
1998
PDF file of entire paper: medium quality (1638K); (scanned from paper original: notes about this process)
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