Some aspects of the sedimentology of pliocene and miocene rocks north of Aorangi range, Wairarapa. sheet N165

Abstract

Three main sedimentary episodes are represented, and are separated locally by angular unconformities. Deposits of the second of the three episodes are divisible locally (in shallow-water facies) into three units with cyclic features. In order of increasing age the sedimentary episodes are as follows (Table 1): (a) Mangapanian - and younger? (Bull Creek Limestone, Upper Mangopari Mudstone, and Greycliffs Formation). (b) Kapitean - Waipipian Unit 1: Upper Haurangi Limestone and basal greensand (bored surface at base). Unit 2: Lower Haurangi Limestone and Makara Greensand (bored surface at base). Unit 3: Clay Creek Limestone (regional angular unconformity at base). (c) Tongaporutuan (Sunnyside Conglomerate and Bells Creek Mudstone). The Kapitean is separated by unconformity from underlying strata in the whole of the area studied, but elsewhere in Wairarapa deep-water deposition was continuous between the Tongaporutuan and Kapitean ages, (Kennett, 1967; Vella and Briggs, in press). The Mangapanian is separated by unconformity from underlying strata definitely at Haurangi Road, and probably over most of the area studied (Figs. 2 and 25). Deposition appears to have been continuous from Waipian to Mangapanian at Mangaopari Stream, and a rather rapid permanent increase in median grain size near the base of the Mangapanian was probably a secondary effect of the event that caused angular unconformity further to the south-west. These main breaks in the sedimentary sequence, marking off the intervals between the main sedimentary episodes, may be entirely due to intermittent tectonic movements. The paleotemperature data for Mangaopari Stream section (Devereux et al. 1970) shows a warming phase commencing shortly below the base of the Upper Mangaopari Mudstone (Mangapanian), following an interval of intense cold. If this warming phase were accompanied by melting of high latitude ice sheets, the Mangapanian transgression might be due to glacio-eustatic sea-level rise rather than to crustal movement. The Clay Creek Limestone represents a widespread transgression in Kapitean time. This transgression might possibly have accompanied a glacio-eustatic sea-level rise, as Kennett (1967) postulated a very cold climate in New Zealand in Kapitean time. Thus the evidence is inconclusive, and supports a tectonic explanation for the main sedimentary episodes, but by no means obviates a climatic explanation. Perhaps the real explanation lies in a combination of tectonic and climatic effects. The three small essentially cyclic sedimentary units in the shallow-water facies of the Kapitean-Waipipian episode, show no evidence of differential tectonic movements. It is likely that the Upper Haurangi Limestone corresponds to the Middle Mangaopari Mudstone. If so the three small sedimentary units are also reflected in the deep-water facies, but are very weakly defined. Minor paleotemperature changes in the Opoitian, and a major one in the Waipipian (Devereux et al., 1970) suggest a possible climatic explanation for the three units, but cannot yet be correlated accurately with the Haurangi Hairpin section. Eustatic effects are expected to be slight in deep-water facies. The cyclic changes at Haurangi Hairpin, on the other hand, could well be explained by periodic fluctuations of sea depth of less than 100 m. Here the evidence favours a eustatic rather than a tectonic explanation. Before any final conclusion can be drawn, further paleoclimatic and paleoecological work will be needed in sections other than Mangao-pari Stream.