Ariel Bank off Gisborne—an offshore late Cenozoic structure, and the problem of acoustic basement on the East Coast, North Island, New Zealand

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Abstract
Continuous seismic reflection profiling has shown that morphological highs off the East Coast, North Island, are structure-controlled and may have older sedimentary rock exposed on the sea bottom. Samples with plentiful microfossils taken simultaneously with seismic profiling across Ariel Bank prove a mid-Miocene and possibly older age for the core of a tight anticline, and an age around the Plio-Pleistocene boundary for outward-dipping beds on its western and eastern flank. Correlation between bottom samples and seismic profiles suggest that two well-marked unconformities in the syncline west of Ariel Bank correspond to (1) a regional unconformity between the Miocene and the Pliocene, and (2) an unconformity of more local character in the uppemost Pliocene. Thick Pleistocene deposits in this syncline may have been derived from the east. Acoustic basement in seismic sections lies within the sedimentary sequence around the Lower Miocenes-Lower Tertiary interval. Very thick sediments known to exist in the Eocene to Aptian sequence and also their basement (probably Jurassic), are effectively concealed from seismic detection. It is suggested that seismic energy is dissipated and, therefore, no deeper penetration achieved, because of the presence of a thick series of under-compacted Maastrichtian to Eocene shales, which are under high pressure because of their sealed-in water and gas content.