Geology of the middle part of the Clarence Valley

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Reay, M.B. 1993 Geology of the middle part of the Clarence Valley. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences geological map 10 144 p. + 1 map

Abstract: An area of 670 km2 of the middle part of the Clarance valley was studied for Institute of Geological & Nuclear Sciences geological map 10. The area forms part of two major fault blocks tilted northwestward between the Hope-Kekerengu, Clarence, and Awatere faults at the southern end of the East Coast Deformed Belt. Basement consists of structurally-complex Torlesse Supergroup submarine fan sediments of Barremian (mid lower Cretaceous) age divided into lithozones A, B, and C. Several lithotypes are recognised within the lithozones. Submarine fan-delta sediments of the Split Rock Formation (Motuan [Early Cretaceous]), comprising the Tentpoles Conglomerate, Black Rock, and Cold Stream members, locally overlie Torlesse rocks with strong angular unconfomiity. The Wallow Group (Ngaterian [mid Cretaceous]), consisting of the Warder, Bluff Sandstone, Gridiron Volcanics, and Willows formations, overlies Torlesse Supergroup, and locally Split Rock Formation, with marked regional unconformity. Basalt and trachybasalt lava flows of the Gridiron Volcanics overlie the nonmarine Warder Formation and form a wedge within the shallow marine Bluff Sandstone over most of the area. The volcanics are locally overlain by the nornmarine, epiclastic Willows Formation. A yet-undefined and described sedimentary unit ''Hapuku formation'' of Mangaotanean to Teratan (mid Late Cretaceous) age locally overlies the Bluff Sandstone. The Wallow Group is overlain by the Seymour Group comprising shallow marine sediments of the Paton, Herring, and Brarich Sandstone formations of Piripauan to Haumurian (latest Cretaceous) age. The group rests on underlying units with strong unconformity over most of the area. The Seymour Group is overlain by the Muzzle Group - a suite of dominantly calcareous, but including siliceous, quartzose, glauconitic, and igneous, rocks. Constituent units are the Mead Hill, Amuri Limestone (including the Teredo Limestone and Fells Greensand members), and Grasseed Volcanics formations of Haurnurian to Runangan (Late Cretaceous to late Eocene) age. The Muzzle Group is disconformably overlain by the Motunau Group consisting of the Cookson Volcanics, Weka Pass Stone, Waima Formation, and Great Marlborough Conglomerate formations of Whaingaroan to Lillburnian (late Oligocene to mid Mocene) age. The Funnel Outlier contains a condensed middle Cenozoic sequence, probdbly transported into the area by transcurrent faulting. Pleistocene glacial outwash gravels are preserved in terrace remnants adjacent to the Clarence River. Reworked periglacial debris forms terraces in tributary streams, and numerous scree and alluvial fan deposits occur throughout the area. Several types of faulting and folding relate to active margin, followed by divergent margin, and transpressional tectonism during the Rangitata II and Kaikouru orogenies. Slope instability results from accumulation of scree, slopewash, and solifluction debris. Rockfalls, rockslides, slumps, slides, earthflows, and mudfl ows are common. Economic minerals include limestone, aggregate, and insignificant quantities of coal, chalcopyrite, and amethystine quartz. Warm and cold water springs are present. The geological history of the area is outlined. Appendices describe the petrology of Torlesse Supergroup sandstones, coal from the Torlesse Supergroup and Warder Formation; and heavy mineral assemblages. (auth)

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