New Zealand Journal of Geology and Geophysics abstracts
Petrogenesis of diachronous mixed siliciclastic-carbonate megafacies
in the cool-water Oligocene Tikorangi Formation, Taranaki Basin, New Zealand
Steven D. Hood
Campbell S. Nelson
Peter J. J. Kamp
Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
Abstract The Oligocene (Whaingaroan-Waitakian) Tikorangi
Formation is a totally subsurface, lithostratigraphically complex, mixed
siliciclastic-limestone-rich sequence forming an important fracture reservoir
within Taranaki Basin, New Zealand. Petrographically the formation comprises
a spectrum of interbedded rock types ranging from calcareous mudstone to
wackestone to packstone to clean sparry grainstone. Skeletal and textural
varieties within these rock types have aided in the identification of three
environmentally distinctive megafacies for the Tikorangi Formation rocks-shelfal,
foredeep, and basinal. Data from these megafacies have been used to detail
previous conclusions on the petrogenesis and to further refine depositional
paleoenvironmental models for the Tikorangi Formation in the central eastern
Taranaki Basin margin.Shelfal Megafacies 1 rocks (reference well Hu Road-1A)
are latest Oligocene (early Waitakian) in age and formed on or proximal to
the Patea-Tongaporutu-Herangi basement high. They are characterised by coarse,
skeletal-rich, pure sparry grainstone comprising shallow water, high energy
taxa (bryozoans, barnacles, red algae) and admixtures of coarse well-rounded
lithic sand derived from Mesozoic basement greywacke. This facies type has
previously gone unrecorded in the Tikorangi Formation. Megafacies 2 is a
latest Oligocene (early Waitakian) foredeep megafacies (formerly named shelfal
facies) formed immediately basinward and west of the shelfal basement platform.
It accumulated relatively rapidly (>20 cm/ka) from redeposition of shelfal
megafacies biota that became intermixed with bathyal taxa to produce a spectrum
of typically mudstone through to sparry grainstone. The resulting skeletal
mix (bivalve, echinoderm, planktic and benthic foraminiferal, red algal,
bryozoan, nannofossil) is unlike that in any of the age-equivalent limestone
units in neighbouring onland King Country Basin. Megafacies 3 is an Oligocene
(Whaingaroan-Waitakian) offshore basinal megafacies (formerly termed bathyal
facies) of planktic foraminiferal-nannofossil-siliciclastic wackestone and
mudstone formed away from redepositional influences. The siliciclastic input
in this distal basinal setting (sedimentation rates <7 mm/ka) was probably
sourced mainly from oceanic currents carrying suspended sediment from South
Island provenances exposed at this time.Tikorangi Formation rocks record
the Taranaki Basin’s only period of carbonate-dominated sedimentation across
a full range of shelfal, foredeep, and basinal settings. Depositional controls
on the three contrasting megafacies were fundamentally the interplay of an
evolving and complex plate tectonic setting, including development of a carbonate
foredeep, changes in relative sea level within an overall transgressive regime,
and changing availability, sources, and modes of deposition of both bioclastic
and siliciclastic sediments. The mixed siliciclastic-carbonate nature of
the formation, and its skeletal assemblages, low-Mg calcite mineralogy, and
delayed deep burial diagenetic history, are features consistent with formation
in temperate-latitude cool waters.
Keywords petrogenesis; limestone; shelf-to-basin facies;
Tikorangi Formation; Oligocene; Taranaki Basin; reservoir; cool-water carbonate
G02015; Received 28 February 2002; accepted 18 March 2003; online publication
date 10 September 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 387-405
0028-8306/03/4603-0387 $7.00/0 © The Royal Society of New Zealand
2003
PDF file of entire paper: Print-quality (6366K)
| screen-quality (1515K)
This year's abstracts |
Journal home page |
All abstracts |
Publishing home page
