The role of tectonism in sequence development and facies distribution of Upper Oligocene cool-water carbonates: Coromandel Peninsula, New Zealand



The Torehina Formation is part of a cool-water carbonate succession of Oligocene age in New Zealand that crops out on the Coromandel Peninsula, North Island. It contains two major transgressive sequences that record successive onlap of a once emergent landmass. The first sequence records marine flooding of non-marine to marginal marine fan delta/estuarine facies, followed by deepening upward and formation of a low-energy, deep (100+ m) muddy carbonate ramp. The capping sequence boundary is characterized by differential uplift and varies considerably in its character over a small (9 km2) area, varying from a burrowed glauconitic firmground to an erosional hardground to an undulatory marine contact to a palaeokarst with < 25 m relief. Sequence 2 sediments, which overlie the palaeokarst with minor (< 10°) angular unconformity, are clayey, marine (offshore) siltstones, whereas open-marine limestones of equivalent age overlie the other boundary types with no apparent angular discordance. The siliciclastics could either represent lowstand channel deposits or may define interbank deposits contemporary with adjacent carbonates. Palaeogeographic restriction of palaeokarst and sequence 2 siliciclastics identifies a structural corridor oriented strike-parallel to the adjacent Harauki Graben, which began to develop by this time. Palaeogeographical differences in the character of the basal limestone facies of sequence 2 also occur. These differences identify variation in accommodation during initial stages of deposition imposed by previous differential movement of fault blocks. As a result, relatively warm-water (20 °C) Amphistegina-bearing limestones in one area contrast with co-existing deeper water, silty foraminiferal (benthic > planktic), echinoderm and bivalve limestones in another. This variation disappears upsection, which suggests that initial bathymetric differences were eliminated with renewed rise in sea level, yielding deeper water inner-shelf sediment facies followed by the accumulation of still deeper, but higher energy, outer-shelf bivalve and bryozoan facies. The sequence architecture of the Torehina Formation is controlled by tectonism, both long-term subsidence and short-term differential uplift. This arose as a result of increasing tectonic activity throughout proto-New Zealand during the Late Oligocene. In such a system, local and regional variation in tectonism among adjacent basins can impose subtle to marked differences in the timing of sequence boundaries and the character of basin-fill patterns.