THE NEWPORTE IMPACT STRUCTURE, NORTH-DAKOTA, USA

The 3-km-diameter Newporte structure is located close to the USA-Canad a border in North Dakota, in the Williston Basin. The structure is cur rently covered by about 3 km of various supracrustal rock formations a nd was first studied in the course of hydrocarbon exploration in the l ate 1970s. The structure is situated in Precambrian crystalline baseme nt, which is highly fractured and brecciated. Detailed petrographic an d geochemical studies of rock samples from the Duerre 43-5, the Mott 1 4-34, and the Wisdahl 23-10 drill cores show the presence of three typ es of breccias: a mainly granite-derived breccia, a predominantly (met a)sediment-derived breccia, and a mixed breccia type. Quartz grains (a s well as a few rare feldspar grains) from all three breccia types sho w planar deformation features (PDFs) with up to five sets per grain. M easurements of the crystallographic orientations of the PDFs show pred ominantly (<10(1)over bar3>) (omega) and (<10(1)over bar2>) (pi) orien tations, which are characteristic of shock metamorphism and indicate p eak shock pressures in excess of 12 GPa. The major and trace element c omposition of the target rocks (granitoid and sedimentary rocks) and t he fragmental impact breccias was measured. All three rock types (sedi ments, granitoids, and breccias) show a wide compositional range. Mixi ng calculations were performed and yield results that are in general a greement with the petrographic observations, but the compositional sim ilarity of the target rock components and their wide range in chemical composition make unambiguous mixing calculations difficult. The resul ts of our study confirm the impact origin of the Newporte structure.