STRATIGRAPHY AND ENVIRONMENT

The coloured brachiopods were collected from one of the most recent of a remarkable series of marine terraces that provides excellent exposure of the Quaternary sediments in the Wanganui Basin (Figure 1). Running for over 100 km parallel to the present-day coastline and extending over 20 km inland at elevations of over 300 m, these terraces are interpreted as the product of Quaternary sea-level fluctuations and tectonic uplift (Fleming 1953; Pillans 1990). In essence, the terraces and their overlying coverbeds are inferred to have been formed during relatively warm interglacial periods when rising sea-levels produced marine transgressions that first cut a terrace in the existing Wanganui sediments and then deposited richly fossiliferous marine sediments. Cold glacial phases would have seen the retreat of the sea and a period of non-deposition. Because of rapid tectonic uplift, these terraces have been protected from subsequent destruction, and a series of 12 terraces has been identified dating back to approximately 700,000 ka (Pillans 1990).

The brachiopods were collected from the basal conglomerate that rests on the Hauriri Terrace (Figure 2). The terraces have been dated using a combination of techniques [fission-track dating, radiocarbon dating, amino acid racemization, and a terrace-deformation model - (Pillans 1990)], and the age of the Hauriri Terrace is interpreted as approximately 80 ka. The basal conglomerate would have been deposited pene-contemporaneously with the formation of the terrace which is richly fossiliferous in places. The coloured brachiopods were obtained from a coastal exposure, approximately 250 m east of the mouth of the Wairoa Stream, 7 km from the town of Waverley, South Taranaki (Map Reference N137/170992; Old Series 1inch Map). The fossils are present along a 35-m section of vertical cliff, in a narrow, impersistent, pebbly and conglomeratic bed that has a maximum thickness of approximately 0.2 m at this locality and is situated about 2 m above the present beach level.

The locality that yielded coloured brachiopods is close to or at fossil locality G.S. 4174 of Wanganui Memoir of the Geological Survey of New Zealand (Fleming 1953). The coastal section in this area is predominantly composed of unconsolidated sediments that are subject to rapid erosion, and it is possible that the particular exposure yielding coloured fossils was not available to earlier investigators, and the section will shortly be destroyed by erosion. Two species of brachiopod are listed in the Wanganui Memoir as components of an extensive fauna from this and other fossiliferous localities at the base of what was then known as the Rapanui Formation (Fleming 1953). However the Wanganui Memoir makes no mention of the preservation of original colouration at this locality, and the discovery of a single specimen of the brachiopod Terebratella sanguinea during the present study is a new addition to the faunal list from the basal conglomerate of the Rapanui Formation. Subsequent work (Pillans 1990) has led to a subdivision of the Rapanui Formation, sensu Fleming 1953.

The fossils were deposited at or below low-tide level off an open eroding coast, and many of the species present are characteristic of present-day eroding coastlines in New Zealand (Fleming 1953). These species, including the brachiopods, are presumed to be indigenous because they are largely intact or cleanly broken and show no signs of abrasions or other damage consistent with significant transportation. A subset of the species recorded are more characteristic of offshore silty and sandy habitats, and these abraded and encrusted shells are thought to have been transported into the area of deposition. A third discrete set of fossils is apparent at this locality--massive oysters and other thick-shelled molluscs that are believed to have been eroded from the gently-dipping fossiliferous Upper Waipipi Shellbed that formed the substrate on which the terrace was formed and the Hauriri sediments were deposited (Fleming 1953). The faunal list compiled by Fleming (1953) includes over 150 different species, the great majority being molluscs, from the several different localities then assigned to the Rapanui Formation.