The geology of the Tahora district, southern Raukumara Peninsula, New Zealand

Abstract

The Tahora District is located in Southern Raukumara Peninsula 50km west-northwest of Gisborne City. It lies on the east side of the axial ranges, forms part of the complex East Coast deformed belt, and for many years has been regarded as a potential petroleum bearing area. The stratigraphy and structure of the Cretaceo-Tertiary sequence exposed in the Tahora District are described and illustrated by maps, and the inferred environments of deposition are discussed. The strata range in age from late Cretaceous to early Miocene. Four formations are recognised in the Cretaceous to lower Eocene part of the sequence and three time stratigraphic units in the Oligocene to lower Miocene part. In upward succession they are: 1) The Karekare Formation of the Matawai Group, a sequence of pelites and minor sandy siltstones of Arowhanan to Mangaotanean age (Late Cretaceous); 2) the Tahora Formation of the Tinui Group, a sequence of pelitic to silty sandstones (Knoll Facies), sandy to gritty siltstones (Mutuera Facies) and interbedded pelitic to silty sandstones and pelites (Houpapa Facies) of ?Piripauan to lower Haumurian age (late Cretaceous); 3) the Whangai Formation of the Tinui Group, a monotonous sequence of claystones, pelites and minor argillites of lower to upper Haumurian age (late Cretaceous); 4) the Wanstead Formation of the Mangatu Group, a sequence of smectitic claystones of Waipawan to Heretaungan age (early to lower middle Eocene); 5) the Whaingaroan-Duntroonian (early to late Oligocene), tentatively placed in the Mangatu Group, a sequence of flysch (Terewhatu and Marewa Facies) conformably overlain by siltstones to pelitic sandstones, and minor coquina limestones and greensands (Maungatapere Facies); 6) the Waitakian (late Oligocene), a sequence of siltstones to pelitic sandstones lithologically similar to the Maungatapere Facies of the Whaingaroan-Duntroonian; and 7) the Altonian (lower Miocene), a sequence of predominantly sandy siltstones with minor turbidites, coquina limestones and greensands. While the Tahora Formation is considered to have little potential as a petroleum reservoir within the area studied, the depositional and structural models developed for the Tahora Anticline suggest the formation may have reservoir potential elsewhere in the subsurface of the region. Three phases of folding have been recognised: The first occurred between Teratan and Piripauan time, the second between Porangan and Runangan time, and the third between Clifdenian and the present day. The orientations of folds of the earliest phase are poorly known but folds of the second and third phases both indicate predominant shortening in a northwest-southeast direction. A minor northeast-southwest component of shortening is also indicated. Documented faults are generally sub-parallel to fold trends and most are probably post-Altonian. Only two faults which predate Whaingaroan deposition are known. The displacement on all major faults can be explained by dip-slip movement but a strike-slip component can not be disproven. Silicic volcanic glass shards occur in all stratigraphic units but are especially common in Tertiary strata. The persistent occurrence and calc-alkaline chemistry of the shards in Tertiary samples suggests that a subduction-related volcanic arc or arcs occurred in the provenance area in early to lower middle Eocene and Oligocene times. The existence of volcanic rocks in the provenance area of Landon to Miocene strata is further supported by the mineralogy of the detrital rock-fragments in those sediments.