Late Miocene paleogeography and flora of north-western Wairarapa, New Zealand

Abstract

<p>The Late Miocene flora and paleogeography of north-western Wairarapa were determined by examining and sampling the Upper Miocene marine and non-marine deposits of the Mangaoranga Formation. This formation unconformably overlies Mesozoic greywacke basement in areas of north-west Wairarapa and contains the oldest sediments preserved immediately overlying basement in this area. Little work has been carried out previously to fully understand the depositional history of the formation or the surrounding vegetation cover. Thus, the present study is intended to improve interpretations of the Late Miocene paleogeography and flora of north-western Wairarapa. The strata of the Mangaoranga Formation were examined at three locations in north-western Wairarapa: at Mangaoranga Stream (Eketahuna), Central Mangaone Road (Eketahuna) and Mauriceville. For paleogeographic reconstructions, the strata were described at Mangaoranga Stream and subsequently correlated to strata at the Mauriceville and Central Mangaone Road sites. The formation was sampled several times at each site for palynological analysis. Additionally, samples available for pollen analysis from the Mt Bruce and Alfredton areas were also examined. The results of these analyses, in conjunction with mummified leaves, aided reconstructions of the Late Miocene vegetation cover. Fission-track analysis on apatite grains from Torlesse basement immediately below the formation was also undertaken, providing evidence for the cooling (and hence exhumation) and subsequent burial history of the basement strata. The results of the apatite fission-track analysis suggest that exhumation of the basement strata above the apatite closure temperature (110ºC) occurred between 36 – 25 Ma. The basement strata were subsequently exhumed at rates of 0.36 – 0.20 mm/yr or 0.28 – 0.16 mm/yr until exposed above sea level by about 11 Ma. Between 11 and 7 Ma, sedimentation of the Mangaoranga Formation occurred. First, northward-flowing braided rivers deposited conglomerate (sm1) in half-grabens. At the Mauriceville and Mangaoranga Stream sites, a large co-seismic lake developed, leading to the sedimentation of interbedded sandstone and mudstone (sm6). The lake persisted for around 95 ky and was often flooded. Eventually, the lake shallowed, and rivers flowed back across the area. The region was subsequently submerged as a marine transgression occurred, leading to the sedimentation of the upper three members of the Mangaoranga Formation (sandstone (sm3), siltstone (sm4) and mudstone (sm5)). Water depths in north-western Wairarapa reached a maximum of 600 ± 300 m by about 8 to 7 Ma. The results of the floral investigation indicate that areas of significant relief were present in north-western Wairarapa during the Late Miocene (possibly up to, or just over, 900 m above sea level). These areas were occupied by cool temperate beech (Nothofagus fusca type) forests, with minor components of Phyllocladus, Podocarpus spp. and Coprosma spp. On low-lying areas, warm temperate beech (Nothofagus brassi type) forests were common, which often contained Laurelia novaezelandiae and Dacrycarpus dacrydioides in areas with impeded drainage and, in areas with better drainage, Dacrydium cupressinum type. In coastal areas, woodland forests of Metrosideros spp. and Casuarinaceae spp. were common. Although no new direct information on the history of north-western Wairarapa between the latest Early Cretaceous and Middle Miocene was determined in this study, the apatite fission-track results suggest that little to no sedimentation occurred in the region between 36 – 25 Ma and 11 Ma, as cooling of the basement strata as a result of uplift and erosion occurred over this time.</p>