Sedimentology of Pleistocene and recent ironsands, West Coast of the North Island, New Zealand: a study of grain size distributions of some recent ironsands and the sedimentology of a Pleistocene and recent ironsand deposit at Waikato North Head

Abstract

Samples of titanomagnetite ironsands from beaches and dunes on the west coast of the North Island, between Mokau and Castle-cliff, were sieved as total samples and density separates in an attempt to find grain size parameters for reliable enviromental recognition. However, the methods used were found unsatisfactory due to complications introduced by the complex mineralogy, transporting processes and source of the sands. In association with this study the rich titanomagnetite ironsand deposit at Waikato North Head, at present being mined by N.Z. Steel Ltd., is described in terms of sedimentary structures and other features. Of the three formations exposed, only sands from the top two, the Entrican Dune Member of the Mitiwai Formation and the Waiuku Blacksand Member of the Hood Formation, are mined. Sands, silts and clays of the Awhitu Formation, the third and oldest formation, are exposed only on the floor of the mine. Kear (1962; 1965) considered that the Awhitu Formation was deposited in the Early Pleistocene and the Hood Formation in the Mid Pleistocene, whereas the Mitiwai Formation is Late Holocene in age. On the floor of the eastern part of the mine the top of the Awhitu Formation is represented as a fossiliferous dark-bluish grey mud probably of alluvial overbank, flood basin, or soil origin. In the western part of the mine the top of the formation is exposed as an ash overlying pale green silty sand. The Waiuku Blacksand Member of the Hood Formation, as exposed in the mine, consists of a series of alternating cross-laminated and horizontally laminated units, which in some cases are separated by units of mud. The very large scale of the sets of steeply inclined cross-lamination (up to 12 meters thick), their directional, grain size, and internal sedimentary structural features suggest an aeolian origin. Some of the horizontally laminated units are topsets associated with the foresets of the cross-laminated units, whereas others are of alluvial or beach back-shore origin. If the latter is the case, marine transgressions are recorded at heights of approximately 21 and 46 meters above present mean sea level. Mud units may represent soils, interdune swamps and lagoons or alluvial over-bank and flood basin deposits. The Entrican Dune Member of the Mitiwai Formation consists of large migrating dunes which were fixed in this area by the planting of a forest in 1935. Internally they are composed of sets of very large scale, steeply inclined, cross-lamination and large scale sets of horizontal lamination or low angle cross-lamination.