The composition of a Torlesse conglomerate—Ethelton, North Canterbury

The Ethelton conglomerate (lower Cretaceous) is a distinctive unit of the Torlesse terrane outcropping in the banks of the Hurunui River near Ethelton, north Canterbury. It is about 150 m thick, underlain and overlain by north-east striking, vertically dipping thick-bedded sandstone and mudstone containing detrital plant material. Bedding in the conglomerate is at angles of up to 35° to the enclosing strata. Three hundred pebbles were collected on a grid pattern to establish their physical properties and lithological proportions. Pebbles of lithic feldsarenite similar to, but not identical with, the enclosing sediments are dominant, and are of two types—one dominated by granitic K-feldspar, and the other by plagioclase. Pebbles of quartzarenite and quartz-lutite, rhyolite, and lutite are common. Calcareous concretions, granite, jasper, and quartz-pumpellyite-chlorite-epidote pebbles are prominent, but minor. Size distribution and pebble shapes suggest that sorting and shaping took place in a fluv...