Sedimentary structures, texture and paleoenvironment of the Hinuera Formation

Abstract

The Hinuera Formation is the extensive Upper Pleistocene terrestrial deposit of alluvial origin, underlying the plains of the Hamilton Basin and the Hauraki Lowland. The textures and sedimentary structures of ten stratigraphic sections are studied in detail, and indicate uniformity of the Hinuera Formation on a regional scale. Four dominant lithotypes occur: rhyolitic and pumiceous gravelly sands, quartz sands, pumice silt, and rhyolitic sandy gravels. A variety of primary and post-depositional structures is present, and four types of cross-stratification (Rho, Nu, Epsilon and Sigma) and three types of horizontal stratification are described. Textures indicate rapid deposition in an environment of flucuating high turbulat energy. Relative energies of the different sedimentary structures are suggested from the relations of structures to texture. Sedimentary structures are also related to flow regeime. Six facies are erected for the Hinuera Formation on the basis of lithology, texture, sedimentary structures, and flow reigeme, which indicate deposition by a braided river system. Deposition was contemporaneous with intense volcanic activity in the Central Volcanic Region, which combined with the cold, wet climate of a glacial period, led to the supply of large amounts of sediment to the river systems of the region, and aggradational fans were constructed in the Hauraki Lowloan and the Hamilton Basin. The six facies are incorperated into a suggested physiographic model of the Hinuera Formation.