Geology and petrography of a conglomerate in southern Stewart Island (Note)

Abstract

Abstract The Sisters islets off Port Pegasus consist of coarse conglomerates dipping north at 30°, towards the Rakeahua Granite of Stewart Island from which they were mostly derived. Boulders sampled are microcline granite, locally crushed microcline granite, coarse-grained pink granite probably transitional to pegmatite, protoclastically crushed granite, gneissic biotite tonalite, and epidiorite with gabbroic or doleritic features. The matrix contains granite, gabbroic, and metamorphic fragments and has been zeolitised, with the formation of stilbite and heulandite/clinoptilolite. In the absence of paleontological evidence, the Sisters conglomerate is tentatively correlated with the lower to middle Cretaceous Puysegur Formation of Preservation Inlet and with the Sand Hill Group of probable Upper Jurassic or Cretaceous age, of western Southland.