Geology of the Forgotten River area : sheet E39AC, scale 1:50,000 (Flat map only)

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Bishop, D.G. 1994 Geology of the Forgotten River area : sheet E39AC, scale 1:50,000(flat map). Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences geological map 15 28 p. + 1 fold. map

Abstract: The Forgotten River map area is a mountainous region of the Southern Alps comprising five geological terranes which now outcrop in contiguous, subparallel strips separated by major faults. To the northwest of the Alpine Fault a small segment of the Early Paleozoic Buller terrane is represented by metagreywackes of the Greenland Group, with associated granitic material. Immediately southeast of the Alpine Fault is the Brook Street terrane - a volcano-plutonic arc assemblage of Early Permian age. Brook Street rocks are separated from another Lower Permian assemblage, the Maitai terrane, by a narrow strip of Jurassic sedimentary rocks of the Murihiku Fault system. The Maitai terrane comprises a basement of Red Mountain Ultramafites - a strip of Lower Permian oceanic crust. The Red Mountain Ultramafites consist of peridotite and serpentinised peridotite overlain by a sequence of gabbros, sheeted dikes, metavolcanic rocks, and hyaloclastic breccia. These are in turn overlain by a sequence of Mid to Upper Permian sediments. The latter is metamorphosed to lawsonite-albite-chlorite facies. The Maitai terrane is bounded in turn on the east by the Livingstone Fault, east of which are schistose rocks of the Caples terrane which form part of the Haast Schist Zone. The Haast Schist Zone itself is an amalgamation of the Torlesse and Caples terranes. The contact between the Crescent Formation and the Mystery Pelite may correspond to the Torlesse-Caples suture. A mineral spring occurs just beyond the head of the Barrier Flats. Metallic minerals include nickel, awaruite, native nickel, chromite, and copper. Non-metallic minerals include asbestos fibre, talc magnesite, and nephrite. Quaternary deposits fall into the main categories of glacial deposits and post-glacial valley infillings. Extensive morainic deposits are found only in the northwest. Valley fill material includes alluvium and terrace gravel, fans, rockfall, lake, and swamp deposits. The largest glaciers in the mapped area are the Andy, which has the extensive Olivine Ice Plateau as its neve, and the Joe. Accretion and amalgamation of the terranes probably did not occur until the Tertiary, after deposition of the Paleogene Annick Group. It appears to have been achieved, at least in part, by movements on the Alpine, Hollyford, and Livingstone faults. Subparallel strands of the Hollyford Fault System in particular have resulted in remarkably continuous, long narrow strips of some map units. Recent tectonism is concentrated on the Alpine Fault, although a number of ridge rents and possible adjustment faults exhibiting recent activity are associated with both the Hollyford and Livingstone faults. A high level of risk exists from large, local earthquakes. (auth)

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