The stratigraphy and geologic history of the late Neogene sequence within the Ngarara Stream-Huangarua River region, southeastern Wairarapa, New Zealand

Abstract

This study is of Pliocene to Recent marine and terrestrial sediments in a 35 km2 area, 10 km southeast of Martinborough, in the southeastern Wairarapa, New Zealand. The greater than 800 m thick sequence comprises six formations, which in order of decreasing age are the; Mangaopari Mudstone, Greycliffs Formation, Pukenui Limestone, Hautotara Formation, Te Muna Formation and Ahiaruhe Formation. The >4.5 to 2.4 Ma Mangaopari Mudstone consists of at least 250 m of massive blue-grey mudstone, with an interbedded tephra, the Missing Tuff (4.5 - 4.1 Ma) and an interbedded 6 m thick turbidite sandstone, the lateral equivalent of the Bridge Sandstone Member at Mangaopari Stream 9 km to the southeast. The deposition of the Mangaopari Mudstone began in water 400-500 m deep forming a claystone with no macrofossils and outer shelf to upper slope benthonic foraminifera, shallowing to 200-250 m in the upper part of the formation, which is a siltstone containing scattered middle to outer shelf macrofossils and benthonic foraminifera. The Greycliffs Formation consists of 250 to 350 m of predominantly poorly sorted sandy mudstone with minor fine sandstone, deposited between 2.4 to ca. 2.0 Ma. The depositional depth of the formation shallows up section from ca. 250 m to ca. 50 m, reflecting a regional shoaling over the 400 Ka depositional period. The Pukenui Limestone is an approximately 100 m thick alternating sequence of coquina limestone units, and muddy sandstone and mudstone units, deposited between ca. 2.0 Ma to 1.82 Ma. The limestone beds consist of inner shelf, mainly mollusc shells with medium to coarse sandy allochems. The lowermost limestone bed, which records the onset of calcareous sedimentation varies laterally, from a shoreface deposit in the northeast of the study area to a deeper, slightly above wave base (ca. 50 m depth, Atkins, 1995) deposit elsewhere. The sandstones and mudstones are predominantly muddy sandstones deposited at 30-60 m depths (depth varies laterally and stratigraphically) although in the northeast towards the base of the formation they consist of supratidal, aeolian and estuarine sediments. The Hautotara Formation is a glacio-eustatic controlled cyclic sequence, between 18 to 75 m thick of marine, marginal marine and non marine sediments, deposited between 1.82 to 1.64 Ma. A total of thirteen facies were identified within four sections through the formation, with the most common facies being well sorted beach sand, massive conglomerate and estuarine mudstone. Glacio-eustatic high stand deposits shallow from depositional depths of about 40 m at the base of the formation to sea level at the top, recording a gradual regression from marine to non marine conditions. The 1.64 to <0.88 Ma Te Muna Formation, which ranges from <20 to >96 m thick, is a cyclic fluvial conglomerate-lacustrine mudstone sequence, which contains rare interbedded tephras and sandstones. The conglomerates range in thickness between 9 to >25 m, increase in thickness and become more poorly sorted up section, and are significantly thicker than the 3 to 5 m thick interbedded mudstone units. The conglomerates consist of greywacke and argillite clasts that were deposited by braided rivers on a laterally extensive subsiding flood plain, with rivers that flowed in a consistent northwest to southeast direction. The 420 to 310 Ka Ahiaruhe Formation overlies the five older formations along an erosional angular unconformity, of up to 60°. The formation consists almost entirely of southwards derived greywacke and argillite alluvial conglomerates, with minor loess and lacustrine mudstone. Ahiaruhe conglomerates contain significantly larger clasts than conglomerates of the underlying Hautotara and Te Muna formations. Deformation in the study area since the Pliocene has been controlled by five major northeast striking structures, which are from east to west; the Ngarara Syncline the Windy Peak Anticline, the Windy Peak Monocline, the Huangarua Syncline and the Huangarua Fault. Lithostratigraphic evidence within the Pukenui Limestone indicates that tectonism in response to regional shortening commenced with deposition of the Pukenui Limestone (2.0 Ma) and was initially confined to the fold pair the Windy Peak Anticline and the Ngarara Syncline in the east. Subsequently, the locus of upper crustal strain moved westwards with folding commencing on the Windy Peak Monocline <1.64 Ma and on the Huangarua Syncline <1.4 Ma. Uplift of the northeast -trending Windy Peak Range initiated in the south of the study area in the vicinity of Windy Peak (474 m) and propagated northeast.