The Geology of the Lower Miocene Whakataki formation, Eastern Wairarapa

Abstract

The Whakataki Formation exposed north of Castlepoint in eastern Wairarapa consists of inner trench slope flysch deposits of lower Miocene age. Strata were deposited in a reverse-fault controlled basin formed at the same time as basement thrusting of the present North Island accretionary wedge within the East Coast Deformed Belt. Deposition commenced within the basin when subduction began beneath New Zealand in latest Oligocene to earliest Miocene times. Sedimentation within the basin was by turbidity currents and coarse grained sediment gravity flows. A lateral facies change from proximal in the south to distal in the north along the axis of the basin is identified by a decrease in average grain size (decreasing sand to shale ratios), turbidite unit thickness and average flow regime as represented by Bouma sequences. Processes allied to fan sedimentation were slumping and down-slope sediment creep within the feeder channels supplying the basin, with slumping also occurring in several more distal areas of the basin Sandstones are predominantly quartz-rich sublitharenites and lithic greywackes. However, coarser grained conglomerates contain mostly sedimentary lithic fragments of upper Cretaceous and lower Tertiary suites. The two feeder channels identified north of Mataikona River and west-southwest of Castlepoint have marked differences in sedimentary provenance. Coarse grained conglomerates exposed near Castlepoint have a substantial content of Torlesse Supergroup clasts and micritic calcite peloids. In contrast, the feeder channel north of Mataikona River largely contains detritus derived from the adjacent upper Cretaceous - lower Tertiary melange. Individual sediment gravity flow units show differing parent rock assemblages, indicating a variety of local sediment sources. The sequence north of Mataikona River also shows an episode of channel abandonment followed by the resumption of distal turbidite sedimentation. Basement comprises imbricately thrusted, lower Cretaceous Torlesse Complex rocks. The overlying Neogene sedimentary cover of the Whakataki Formation has since been drape folded into an assymetric syncline with ongoing basement thrusting. Conjugate fault sets indicate that the principle stress direction is approximately east-west, or normal to the Whakataki Fault which is a major northeast-southwest trending structural feature of the region. Thereby implying, that this part of the East Coast Deformed Belt exhibits compression with little or no strike slip component. The Whakataki Formation is similar in terms of tectonic setting and predominating paleocurrent direction when compared to later Neogene flysch basins and recent offshore basins within the East Coast Deformed Belt.