Modern and ancient Zygochlamys debicatula shellbeds in New Zealand, and their sequence stratigraphic implications

The scallop Zygochlamys delicatula is an indicator species for a carbonate shellground facies which occurs on the sediment-starved outer shelf and upp er slope around southern New Zealand. The modern distribution of Z. delicat ula is restricted to areas where sea summer surface temperatures are less t han ca. 15 degrees C. Its occurrence in Late Pliocene and Pleistocene strat a in North Island has, therefore, been taken to indicate a northward-expand ed range during former glacial intervals. We describe the sequence stratigr aphic setting of examples of the Z. delicatula assemblage from the modem ea stern South Island shelf, and from the Plio-Pleistocene Wanganui and Mangao pari Basins in North Island. We show that the Z. delicatula fauna occurs in both glacial and interglacial sediments, and that its development requires the concurrence of cold-water conditions and terrigenous sediment starvati on. These conditions occur most commonly at the transition between the tran sgressive and highstand systems tracts (i.e. during rapid post-glacial sea- level rise, and probable warming temperature). However, for palaeo-location s below the contemporaneous lowstand shoreline, Z. delicatula can also occu r at the transition between the highstand and lowstand systems tracts (i.e. during rapid post-interglacial sea-level fall, and probable cooling temper ature). (C) 1998 Elsevier Science B.V. All rights reserved.