Relative intensity of tectonic events revealed by the tertiary sedimentary record in the North Wanganui Basin and adjacent areas, New Zealand

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Abstract
The Tertiary sediments in the North Wanganui Basin are referred to 16 formations that range in age from Oligocene to Pliocene and have a combined maximum thickness of about 7500 m. Marginal marine and shallow marine sediments dominate the sequence. Bulk variations in lithology, thickness, sedimentation rate, carbonate content, texture, mineralogy, and other properties for the stratigraphic succession are traced on sedimentary logs. The resulting curves are explained in terms of the relative importance of tectonic activity in controlling sediment properties and sedimentation patterns in the region. Contemporaneous basement faulting has variably influenced the distribution and character of each of the Tertiary formations. From the sedimentary logs the relative intensity of tectonic movements with time in the North Wanganui Basin and adjacent areas is suggested to be as follows: Oligocene (38-24 m.y. )-very low; early Lower Miocene (24-20 m.y.)-high; late lower Miocene to Middle Miocene (20-11 m.y.)low to moderate; Upper Miocene (11-5 m.y.)-high; Pliocene (5-2 m.y.)-very high. The sudden increase in tectonic activity at the Oligocene-Miocene boundary may be related to initiation of predominantly dextral transcurrent movement on the Alpine Fault and the establishment of the Indian-Pacific plate boundary in its present form through New Zealand. The Upper Miocene-Pliocene tectonic event is referred to the Kaikoura Orogeny and is correlated with the change in the pole of rotation for motion between the Indian and Pacific plates at about 10 m.y. B.P., after which compressional tectonics and accelerated uplift characterised the Alpine Fault sector of the New Zealand region.