CONTINENT-ATTACHED AND CONTINENT-DETACHED COOL-WATER LIMESTONES IN AN ACTIVELY DEFORMING FOREARC BASIN, HAWKE'S BAY, NEW ZEALAND

Hawke's Bay Basin, on the east coast of New Zealand's North Island, is part of the obliquely-convergent plate boundary through the New Zealand subcontinent. It lies adjacent to where the Pacific plate is being subducted beneath the Australian plate. Hawke's Bay Basin is a forearc basin that has developed since the Late Miocene as a consequence of this interaction and, despite the predominance of a siliciclastic fill of sandstone and mudstone, includes widespread development of shallow-marine limestones of Late Miocene to Early Pleistocene age. These young limestones are cool-water (or temperate) carbonates characterised by common to abundant barnacle debris, and are significant because they accumulated during a time of high siliciclastic sediment input to an actively deforming basin experiencing known eustatic sea-level oscillations.

The limestones can be assigned to examples of “continent-attached” and “continent-detached” carbonate systems, not previously enunciated for temperate settings. These two depositional end members formed along the opposite sides of a tidal current-swept paleoseaway that developed in the Early Pliocene. Limestones of the “continent-attached” realm accumulated along the western margin of the seaway upon a narrow shelf fronting a greywacke hinterland and are typically siliciclastic-rich with common barnacles, ostreids and semi-infaunal bivalves. Carbonates from “continent-detached” settings accumulated towards the eastern side of the seaway on and about thrust fault-controlled paleohighs developed on the inboard margin of an accretionary wedge, are siliciclastic-poor and contain abundant barnacles, pectinids, brachiopods and ostreids.

The paper will compare and contrast the morphodynamic, facies and genetic attributes of the “continent-attached” and “continent-detached” limestones, including their sequence stratigraphic architectures, with the intention of contributing to the rapidly developing knowledge base for shallow-marine cool-water carbonate systems globally.