The geology of the Kekerengu-Waima River district, North-East Marlborough

Abstract

The stratigraphy and structure of the Kekerengu-Waima River district are described in the text and presented in a geological map and cross sections, on a scale of 1:15840. An interpretation of the geological history of the area is given in terms of the sedimentation and tectonic evolution. The area has a distinctive and varied stratigraphy but is most notable for its unusual structure. The rocks of the Kekerengu-Waima River district consist of a Jurassic and Lower Cretaceous undermass, unconformably overlain by an Upper Cretaceous and Tertiary cover. The undermass is a regularly deformed northwards-plunging core over which the covering strata are wrapped in a complex plunging anticline, named the Ben More Anticline. The cover is much less indurated than the undermass, although only 6 to 10 million years separates the undermass from the oldest formation of the cover. Different lithofacies of the same age occur within the cover which has a stratigraphic thickness of approximately 11,000 feet and ranges in age from Raukumara (Teratan) to at least the top of the Southland (Waiauan). No break is known between the two stratigraphic units of the undermass (Greywacke and Good Creek Formation) although the relationship between them remains an outstanding stratigraphic problem. The undermass is fundamentally a turbidite sequence but includes grain flow and submarine fan deposits. Collectively it ranges in age from Oteke to Clarence (Ngaterian). The cover is a conformably sequence of 13 different formations, one of which (Amuri Limestone) is divided into 7 members. The main structural features are two major dextral faults (the Kekerengu and Clarence Faults) and a block, named the Ben More Block, between them. Within the Ben More Block, several more or less parallel faults and folds, confined to the Upper Cretaceous and Tertiary cover, swing in an arc around an anticlinal core composed of the Good Creek Formation. The arcuate pattern contrasts with the trend of structures throughout north-east Marlborough and perhaps with that throughout the whole zone of strike-slip faulting in the middle part of New Zealand. The trend of all structures in the cover swings completely across the trend of the basement structures. The break between basement and cover is in part, an unconformity and, in part, a complex fault, the two merging into each other. In general deformation in the area becomes more complex upwards through the sequence, not simpler. The structural pattern is explained by a tectonic décollement which is postulated to have moved south-westwards along the basement/cover unconformity, in the eastern half of the Ben More Block. The décollement is attributed to dextral displacement of the rigid basement, by the Kekerengu Fault. Eventually the covering décollement slab was itself ruptured and displaced dextrally by the fault, giving rise to the observed offset of three miles.