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
2003 New Zealand Journal of Geology and Geophysics  
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
more » ... nvironmentally 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-nannofossilsiliciclastic wackestone and mudstone formed away from G02015; 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. provide the setting for accumulation of more basinward facies of Tikorangi Formation, as occurs in the offshore Maui-1 study well (Fig. 1B) . The intention of this paper is to record the compositional and textural character of the Tikorangi Formation rocks using standard petrographic techniques. Following Dunham's (1962) scheme, a range of petrographically based rock types (petrofacies) for the formation has been identified, including mudstone, wackestone, packstone, mixed packstone/ grainstone, and sparry grainstone. These rock types provide a framework for defining three major skeletally and texturally defined associations or'"megafacies" within the Tikorangi Formation, which are interpreted to have formed in shelfal, foredeep, and basinal environments (Fig. 2) . A reference well for each of the three megafacies is defined, as well as a complete compositional log (devised using correlations between petrographic and geophysical log data) to aid in the
doi:10.1080/00288306.2003.9515016 fatcat:w4pfhu7ignc2ddu7bxzpzsml34