Exposed coasts vs sheltered bays: contrast between New Zealand and Japan in the molluscan record of temperature change in Plio–Pleistocene cyclothems

Abstract

Cyclothems in the Omma Formation, on the Japan Sea coast of central Honshu, each contains two ecostratigraphic datums: the appearance and disappearance datums of warm-water molluscs. Cold-water molluscs occur both below (in early TST) and above (in late HST and possibly RST) warm-water ones (which occur in late TST and early HST). In contrast, no New Zealand cyclothems in the main Plio–Pleistocene basins contain both cold- and warm-water faunas in one cycle. Cyclothems in the Petane Group, central Hawke's Bay and in the Castlecliff section, Wanganui Basin, contain entirely eurythermal and warm-water molluscs throughout. Cyclothems in South Wairarapa have entirely eurythermal and cold-water molluscs throughout (the subantarctic Zygochlamys delicatula fauna), even in HSTs of some cycles. This contrast demonstrates the importance of exposure of the deposition site to strong coastal currents for the maximum faunal expression of temperature change. The Omma Formation was deposited in an open site on the Japan Sea coast, where currents transported larvae of a wide variety of molluscs northwards and southwards in response to temperature oscillations. The main New Zealand cyclothemic successions were deposited in large embayments, sheltered from the influence of coastal currents. The maximum faunal expression within one cyclothem of Plio–Pleistocene temperature change can be expected in only those few deposition sites that satisfy the optimum requirements: deposition of reasonably complete sequences (at least TSTs, HSTs and RSTs preserved) in an exposed coastal site influenced by strong currents from both the north and south, on the inner shelf, in the temperate regions (between ca. 30° and 60°, north and south), near the north–south oriented coast of a large continent.