The Geology along the South-East Flank of the Rimutaka Mountains, New Zealand

Abstract

The geology of the south-east flank of the Rimutaka Mountains is dominated by the presence of a major fault, which generally forms the contact between the older, isoclinally folded and sheared greywacke of the ranges and the younger sedimentary formations of the western side of the Wairarapa depression. The effects of movement along this fault are manifested in the warping and tilting of these sedimentary beds. Displaced erosional surfaces now at elevations of up to seven hundred feet, cut on rocks of the Pounui Formation of probable Pleistocene Age, indicate the recency of major tectonic activity. That movement along the Wairarapa fault has not ceased is indicated by eye-witness accounts of the displacement occurring during the Wellington earthquake of 1855. The effects of this and earlier recurring movement are expressed in steps running across alluvial fans which have formed since the uplift of the Rimutaka Range, and in scarplets and earth cracks at the base of higher, older scarps. Movement along the fault has not been evenly distributed. Some evidence of transcurrent, strike-slip displacement can be found. In a few localities the more recent movement seems to have been a reversal of earlier trends, and some faulting has occurred in the valley sediments up to a mile away from the main fault trace. The coastal portion of the Wairarapa depression shows, both by a number of marine terraces uplifted hundreds of feet, and on the western side of the valley by raised beaches, that it is rising more-or-less simultaneously with the mountain block. This rising tendency can be traced about eight miles inland to a point where erosional surfaces suddenly arch downward and disappear beneath the alluvium of the present river valley. The hinge line of warping can be drawn approximately at the foot of these downwarped surfaces, perpendicular to the trace of the Wairarapa fault. Northward from this hinge line to Featherston, the floor of the depression seems to be synclinally warped transversely to the line of the fault, creating a hollow which has become flooded to form Lake Wairarapa. Other changes in drainage resulting from recent warping also can be found along the flank of the range. In spite of the evidence of uplift along the coast, the present shoreline is one of submergence. A rise of sea level of greater magnitude than the uplift of the land is indicated. It is suggested that this rise of sea level marks the time of the post-glacial Flandrian transgression. Continued uplift of the land since that date has not been able to overcome a rapid tempo of marine erosion and the shoreline has been cliffed and eroded back several miles since the end of the eustatic flooding, although there is evidence that the sea cliffs bordering Palliser Bay west of Lake Onoke have become relict thought the building of a wide shingle beach at their base. A bay head bar three miles long extends from this beach at the foot of the cliffs eastward across an arm of Palliser Bay to cliffs on the eastern side, impounding Onoke Lake.