New Zealand Journal of Geology and Geophysics abstracts

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Sedimentology and petrography of mass-emplaced limestone (Orahiri Limestone) on a late Oligocene shelf, western North Island, and tectonic implications for eastern margin development of Taranaki Basin

CAMPBELL S. NELSON
PETER J. J. KAMP

Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton 2001, New Zealand

HARVEY R. YOUNG

Department of Geology
Brandon University
Brandon
Manitoba R7A 6A9, Canada

Abstract  The Te Kuiti Group in North Wanganui Basin, North Island, New Zealand, of Oligocene - earliest Miocene (Whaingaroan-Waitakian) age, is dominated by calcareous siltstone, calcareous sandstone, and skeletal limestone. Exposures in the southwestern corner of the basin at Awakino Tunnel are distinctive because, compared with elsewhere, the group is generally thicker (>300 m), has strong dips (25-45deg.E), exhibits an up-section decrease in the amount of dip, and the capping Orahiri Limestone includes several thick (up to 3 m) mass-emplaced units containing a variety of 1-10 cm sized calcareous lithoclasts of older Te Kuiti Group rocks. Petrographic and d¹⁸O and d¹³C data suggest that the source deposits of these lithoclasts were cemented at relatively shallow subsurface burial depths (100-500 m) before their uplift and erosion. The lithoclasts so produced were rounded by abrasion in shoal water, often bored profusely by pholad bivalves, and sometimes encrusted by coralline algae, before being periodically mass-emplaced from west to east onto a shelf accumulating coeval Orahiri Limestone lithofacies now in the vicinity of Awakino Tunnel. Pressure-dissolution during subsequent burial provided the main source of calcite cement in the host Orahiri Limestone, mainly at moderate burial depths of 500-1000 m, according to d¹⁸O data.

The source region for the lithoclasts probably lay west of Awakino Tunnel and corresponds to the southern part of the basement Herangi High, which otherwise separates North Wanganui Basin from Taranaki Basin, but must have been submarine and accumulating Te Kuiti Group-equivalent calcareous facies during the early Oligocene (Early Whaingaroan, 36-32 Ma). Uplift of this depocentre was accompanied by synsedimentary eastward tilting of the Te Kuiti Group strata already deposited immediately east of Herangi High, contributing to the dips now measured at Awakino Tunnel. Inversion and tilting of the high began in the Late Whaingaroan, after 32 Ma ago, concomitant with the onset of rapid subsidence along eastern Taranaki Basin margin directly west of Herangi High. Uplift continued throughout the Duntroonian (28-24 Ma), when erosion and mass-emplacement supplied the abundant calcareous lithoclasts in the Orahiri Limestone, and into the Waitakian (24-22 Ma), when erosion possibly expanded onto parts of the shelf, stripping out sections of the overlying Otorohanga Limestone, the topmost formation in the group. The uplift and partial emergence of Herangi High is viewed as a topographic response to the initiation of basement overthrusting from the east along the Taranaki Fault Zone. The resultant loading evolved a carbonate foredeep in eastern Taranaki Basin during the Duntroonian-Waitakian. This late Oligocene phase of deformation developed in a mildly compressive regime, which corresponds to a time of proto-plate boundary development through New Zealand that preceded propagation of the continuous and more localised present plate boundary through the country at 22-23 Ma. The latter was marked in both North Wanganui Basin and Taranaki Basin by rejuvenated, more active basement overthrusting, the influx of copious amounts of terrigenous sediment, and the termination everywhere of extensive limestone and Te Kuiti Group deposition.

Keywords  Te Kuiti Group; Orahiri Limestone; Oligocene; Awakino Tunnel; North Wanganui Basin; Taranaki Basin; Taranaki Fault; limestone; mass emplacement; petrography; diagenesis; oxygen and carbon isotopes; tectonics

New Zealand Journal of Geology and Geophysics, 1994, Vol. 37: 269-285

0028-8306/94/3703-0269 $2.50/0 (c) The Royal Society of New Zealand 1994

PDF file of entire paper: medium quality (6385K); (scanned from paper original: notes about this process)

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