The Mid Pleistocene Waiomio Shellbed (Castlecliffian, c. 550–600 ky), Wanganui Basin, New Zealand

Abstract

A new outcrop of the Waiomio Shellbed is described from the west bank of the Rangitikei River, near Porewa. The shellbed comprises cross-bedded pebbly-shell conglomerate containing a rich intertidal assemblage of estuarine (Macomona liliana, Cominella glandiformis, Austrovenus stutchburyi) and shore face (Paphies subtriangulata, Mactra discors, Spisula aequilatera, Dosinia (Phacosoma) subrosea) Mollusca. This site contains previously undescribed units, and the names Waiomio Formation (which includes the basal Waiomio Shellbed) and Reu Reu Formation are introduced. The excellent exposure demonstrates the position of the Waiomio Shellbed within a complete cyclothem, facilitating interpretation in terms of sequence stratigraphic concepts. A basal disconformity (sequence boundary) truncating a palaeosol horizon (lowstand systems tract), is overlain by the Waiomio Formation (transgressive systems tract), and is succeeded by the Reu Reu Formation, comprising an in situ shellbed (condensed section) and massive siltstone (highstand systems tract). The complete sequence is compared with its probable equivalent in the Wanganui coast section, which correlates with oxygen isotope stage 15.