Upper Quaternary Geology of the Rangitikei Drainage Basin, North Island, New Zealand

Abstract

River terraces developed during the last 250,000 years, and the loess, tephra, and dune sand beds that mantle them have been mapped in the drainage basin of the Rangitikei River, North Island, New Zealand. The ages of the terraces and mantling beds have been estimated, and from the ages, rates of tectonic deformation within the basin have been estimated. Inferences about the climates and weathering regimes over the last 250,000 years, based on the geomorphic and sedimentary patterns, and on loess morphology, have been made. Fourteen sets of river terraces have been mapped: Kakariki, Onepuhi, Rewa, Bulls, Ohakea, Vinegar Hill, Rata, Putorino, Porewa, Cliff, Greatford, Marton, Burnand, and Aldworth, in order of increasing age. The major terrace sets: Ohakea, Rata, Porewa, Greatford, Marton, Burnand, and Aldworth are in part aggradational. They are considered to have formed during episodes of cold climate which devegetated and caused erosion of the axial mountain ranges above about the 900 m contour, and hence caused overloading of rivers draining the ranges. The terrace alluvium is gravel, predominantly derived from greywacke sandstone of Mesozoic age. Argillites, sandstones, mudstones, and limestones that outcrop in the basin do not survive river transport. The gravels become finer and more rounded in a downstream direction but do not change shape. The loss of size without change of shape is considered to be mainly the result of cleaving of boulders along joint planes. The maximum size of boulders is thus not due to selective sorting, and is not a reliable guide as to relative paleogradients of different aged flood plains. Except that the Aldworth alluvium is notably finer, contains more volcanic rocks, and appears to have been largely derived from more northerly source areas than that of younger terraces, all terrace alluvium is remarkably similar. Progressively older terraces have progressively steeper gradients, and older terraces are locally folded and faulted. The steepening of gradients can be most simply explained by steady tilting in a downstream direction about zero isobases situated some 20 km inland from the present coastline. Eight loess beds: Ohakea, Rata, Porewa, Greatford, Marton, Burnand, Aldworth, and Waituna, have been mapped. In most places they have been largely derived from flood plains of major rivers, although locally they have been partly derived from the sides of nearby gullies. All are non-calcareous. In areas with present day mean annual rainfalls of less than 1,000 mm the Ohakea loess is an olive brown or olive grey fine sandy loam through to clay loam; in wetter areas it is a yellowish brown fine sandy loam through to silty clay loam. In areas with present day mean annual rainfalls of less than 1,100 mm, older loess beds are olive grey, and are finer textured, Porewa and older loess beds being plastic clays. In wetter areas older loess beds trend towards lower chromas and values and redder hues than for Ohakea - Greatford having a modal colour of dark brown (7.5YR 4/4). Like the loess in drier areas, the older beds are finer textured - Greatford and older beds are poorly plastic clays. The morphology of the loess beds thus depends not only on age, but also on environment of weathering - mainly the frequency and severity of wetting and drying cycles. Two especially prominent rhyolitic tephra marker beds have been recognised, Aokautere Ash dated at 20,000 ± 1,000 years B.P., and Mount Curl Tephra dated at 230,000 ± 30,000 years B.P. Aokautere Ash outcrops two-thirds of the way down the Ohakea loess. Mount Curl Tephra overlies and immediately post-dates the Aldworth loess. Three sets of coastally derived dune sands have been recognised: the Waitarere - Motuiti - Foxton phase set, the Mount Stewart, and the Brunswick. The Waitarere - Motuiti - Foxton phases are 5,000 or fewer years old, and overlie the Ohakea loess. The Mount Stewart Dune Sand overlies and immediately post-dates the Greatford loess. The Brunswick Dunesand overlies and immediately post-dates the 230,000 year old Mount Curl Tephra. Each of the dune sands is considered to have been deposited during an interglacial high sea level stand. The upper Quaternary glacial and interglacial stages for New Zealand have been adopted in the present study using the relationships between climate episodes, and coastally derived dune sands, loess, and aggradation alluvium. The Waitarere - Motuiti - Foxton phase dunes have been deposited during the Aranuian Interglacial Stage. The Mount Stewart Dune Sand is considered to have been deposited during the Oturian Interglacial Stage, and the Brunswick Dunesand during the Terangian Interglacial Stage. The glacial stages have been subdivided into substages of relative cold and warmth. Names for the substages of relative cold being derived from the names of the loess beds and associated terrace sets. The Otiran Stage has been subdivided into the Ohakean, Apitian, Ratan, Orouan, and Porewan Substages. The Waimean Stage has been subdivided into the Greatfordian, Huntervillian, Martonan, Rangatiran, and Burnandian Substages. The Waimaungan Stage has been subdivided into the Aldworthian, Kiwitean, and Waitunan Substages. Apart from the radiometric dates available for the tephra beds and dune sands, the Onepuhi terrace set is known to be c. 1,800 years old and the Rewa set c. 1,800 to c.3,000 years old. Using available dates of the tephras alluvium and dune sands, together with relative thicknesses and degrees of weathering of loess beds, and overseas dates of the interglacial high sea level stands, the major river terraces and associated loess beds are estimated to have been deposited during the following time intervals: Ohakea 12,000 to 24,000 years B.P.; Rata, 30,000 to 40,000 years B.P.; Porewa 70,000 to 80,000 years B.P.; Greatford 120,000 to 125,000 years B.P.; Marton 130,000 to 140,000 years B.P.; Burnand 170,000 to 180,000 years B.P.; Aldworth 230,000 to 240,000 years B.P. From the ages and tilts of terraces at least the lower part of the basin has been tilting in a downstream direction at an average rate of 61 nrad year-1 over the last 250,000 years, double the rate indicated by dips of lowermost Quaternary marine strata in the basin. Uplift rates in the basin increase upstream to about 4 mm year-1 in the middle part of the basin, and probably more within the mountain ranges in the headwaters region of the basin. Faulting has resulted in differential vertical movements within the basin of the order of 0.1 mm year-1. Faulting and tilting rates appear to be about ten times less than rates in the Wairarapa region of the southern North Island, an area traversed by the active transcurrent faults of New Zealand. Judged by the geomorphic and sedimentary patterns, climates during cold climate episodes are considered to have been as much as 5°C to 6°C colder than those of the present day, with slightly lower rainfalls, and fewer high intensity rainstorms. The climates during cold climate episodes can be most simply explained by assuming that during the cold climate episodes, the paths of the anticyclones which dominate the present day weather pattern were displaced northwards by some 7° of latitude relative to their present day paths.