Neogene exhumation histories in the Southern Taranaki Basin

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Sahoo, T.R.; Funnell, R.H.; Bull, S. 2016 Neogene exhumation histories in the Southern Taranaki Basin. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/32 30 p.; doi: 10.21420/G2B880

Abstract: Taranaki Basin is an active margin basin with a sedimentary succession within which is recorded a number of erosion events. Distribution of the unconformities and erosion estimates both in time and space provide valuable information for unravelling the geodynamic setting of the basin’s evolution. As part of the GNS’s 4D Taranaki project, this study integrates well log data and 2D seismic data to estimate the amount of erosion on two major unconformities of Neogene age in the southern Taranaki Basin. Unconformities are identified and mapped in the basin based on biostratigraphic data from wells and analysis of reflection terminations in seismic sections. Apparent erosion amounts were estimated at well locations by comparing normally compacted mudstone porosity-depth (PD) profiles with the mudstone PD profiles for locations affected by exhumation and erosion. Through calibration with the estimated erosion from PD profile analysis, apparently conformable strata were then projected across mapped unconformities on seismic sections to produce erosion estimates along sections. Two major Neogene unconformities, one near the top of the Miocene interval (N50) and the other within the Plio-Pleistocene interval (N82) have been identified in the southern Taranaki Basin. These two unconformities are widespread in the Eastern Mobile Belt region of the basin, whilst the Western Stable Platform region displays a conformable stratigraphic succession. The amount of erosion at the N50 unconformity varies widely from 0 to 3100 m. Erosion is highest on structural highs during the Late Miocene (Tk) to Early Pliocene (Wo) time due to active reverse faulting associated with inversion across the Eastern Mobile Belt. Erosion amounts at the N82 unconformity are higher (> 200 m) in the eastern part of the study area compared to the southern and western parts. Timing of this erosional event is interpreted to be Early Pleistocene (Wc) based on well biostratigraphy. (auth)