Neogene stratigraphy and geological history of the Wainuioru Valley eastern Wairarapa, New Zealand

Abstract

Detailed mapping of thick uplifted marine sediments of Neogene age within the Wainuioru Valley, distinguishes 21 lithstratigrahic units of which 13 (those marked with an asterisk) are described for the first time in this report. In order of decreasing age they are; ANNEDALE GROUP, including Takiritini Formation; KUAMAHANGA GROUP*, including Ngaumu Mudstone* with basal Tattons Sandstone Member* and interbedded Waimangu Sandstone Member*, Karamu Sandstone and Bankview Mudstone* with Pahuri Turbidite Member*; PALLISER GROUP, including Bells Creek Mudstone with Ngatahuna Tuff Bed*, Halloween Tuff Bed* and Ririwai Sandstone Member*; ONOKE GROUP, including Makara Greensand, Morland Mudstone* with Beetham Sandstone Member* and Brancepeth Tuff Bed*, and Mangaopari. Mudstone with basal Oruatamore Tuff Member*. Neogene sediments represent facies ranging from shell-rich inner shelf sandstones to massive very fine grained middle bathyal mudstones. Sediments contain rich microfaunal assemblages which enable a series of 12 foraminiferal zones and subzones to be recognized. In order of decreasing age they are; G. miozea miozea Zone (LP1); G. peripheroronda Zone (Sc-ES1); G. miozea conoidea Zone (MS1-LS1); T. gladizea Zone (Sw); G. dehiscens Zone (BTt-MTt), including the B. quadrilatera Subzone (BTt) and B. pohana Subzone (Ltt-MTt); N. pachyderma Zone (LTt-ETk); G. sphericomiozea Zone (LTk); G. crassaformis Zone (LWo-MWo); G. inflata Zone (LWo-Wp); G. puncticuloides Zone (Wp); G. crassula Zone (Wh). Deformation of Neogene strata within the Wainuioru Valley has been controlled by vertical movements on faults trending north-east, east-south-east, east-north-east, and north. North-east striking, high-angle faults are generally upthrown to the west and contribute to the north-westerly dip of strata; there is no evidence of strike-slip movement on these faults, if any, since the middle Miocene. ESE and ENE striking faults are downthrown to the north and control the distribution of major facies belts within the Wainuioru. North striking low-angle reverse faults are relatively young structures and are associated with recent dextral strike-slip movement on the ENE trending Kaumingi Fault. Folding is relatively minor in extent and is largely restricted to localized deformation associated with faulting. Neogene sediments within the Wainuioru Valley represent the uplifted and dissected margin of the Wairarapa forearc basin, onlapping with complexly deformed rocks of the Coastal Range structural high. Development of the Wainuioru portion of the basin began in the late Altonian as a narrow trough adjacent to the emerging Brocken Range and propagated towards the west during the Southland Series, allowing shelf sediments to be deposited over much of the Wainuioru Valley. A marked increase in basin size, initiated during the late Waiauan led to a transgressive sequence being deposited during the Taranaki Series, until the initiation of basin inversion during the early Opoitian. The Wanganui Series is represented by a regressive sequence, culminating in the Hautawan with the deposition of the inner shelf Pukenui Limestone along the western margin of the Valley.