Geology of the Lake Rotorua area, Nelson

Sheet M29BD covers part of the upper Buller catchment including the Howard River and its tributaries. Each of three physiographic regions consists of mainly one major rock type, and almost all the area was strongly influenced by glacial erosion and deposition. In the southeast, high ranges of Torlesse Supergroup rocks are bounded by the Alpine Fault. In the west the Rotoroa Complex forins lower ranges, with the Hope Range north of the Buller River preserving a dissected Eocene peneplain. In the east, hills and wide valleys are formed of upper Cenozoic deposits, including those resulting from a succession of glacial advances. The precenozoic rocks of the mapped area fall within the Median, Brook Street, and Rakaia terranes. Thick sequences of sediments of the Mt Robert Group, (Torlesse Supergroup) are dominated by quartzofeldspathic sandstone. Marked increase in disruption of bedding and development of schistosity in finer grained rocks lead to textural subzone II in the west. Shearing and crushing increase towards the Alpine Fault in the north, and mylonite bands are well developed in the south to at least 1 km southeast of the fault. Between the Alpine Fault and the inferred Flaxmore Fault, poorly exposed basic igneous rocks and sediments of the Brook Street Volcanics occupy a belt up to 2km wide. The Rotoroa Complex, which occurs over most of the sheet, is redefined to include the Rotoroa and Separation Point suites. The Rotoroa Suite includes layered gabbronorite-anorthosite and metagabbro of the Howard Gabbro (new formation), diorite-quartz diorite-trondhjernite, migmatite, intrusion breccia, and hybrid diorite of the Braeburn Diorite (new formation), and amphibolite and epidiorite of the Kawatiri Amphibolite (new formation). Possibly late middle Jurassic, gabbro-diorite-trondhjemite of the Braeburn Diorite is extensively intruded by Early Cretaceous granitic rocks of the Separation Point Suite. This was accompanied by widespread metasomatism in the more mafic rocks and development of migmatite, intrusion breccia, and hybrid diorite at granite contacts. Layered rocks of the Rotoroa Suite are folded into northeast-trending, broad symmetrical anticlines following the dominant structural trend dictated by intrusion of the Separation Point Batholith. Granite-diorite and granite-gabbro contacts are commonly faulted, and northeast-trending mylonite zones are developed along some major faults. Sandstone and siltstone, some calcareous, of the Paleogene Maruia and Matiri formations extend from the west to cross the map area in a narrowing steep-dipping strip on the south flanks of Mt Murchison and Mt Hope, and arc cut off to the southeast by the Tutaki Fault. Neogene sediments, apart from a small area of Mangles Formation in the extreme northwest, consist of the gravel-dominated Tadmor Group, which either crops out over, or underlies Quaternary deposits, in the northeast of the sheet. The locally-derived Glenhope Formation, which includes clay and lig nite, is overlain with minor unconformity by low-dipping Moutere Gravel derived from southeast of the Alpine Fault. Moutere Gravel is overlain, perhaps conforniably, by till, lake deposits, and gravel-the glacial Porika Formation of late Pliocene age. The middle and late Pleistocene glaciations, together with preceding and intervening interglacial episodes, took place during a time of slow regional uplift northwest of the Alpine Fault and rapid uplift to the southeast. Consequently, erosional landforms dominate the southeastern ranges, and depositional landforms-principally moraines and outwash aggradational surfaces-dominate the Gowan and Howard valleys to the northwest. A complex of tills, lake beds, and outwash gravels make up the Manuka, Harry, Tophouse, Roundell, and Speargrass formations. Late Cenozoic structural development is dominated by uplift east of the Alpine Fault, along which are numerous active traces, and by uplift of the Braeburn and Mt Murchison-Hope ranges.s Meanwhile, the Moutere depression to the east was accumulating sediment. Uplift of the ranges has continued, and the Moutere depression is now rising slowly. These structural elements have northerly trends, and a northeast trend, best shown by the Tutaki Fault, is probably inherited from pre-Cenozoic structure. Alluvial gold mining was patchy and rarely economic. Platinum is recorded from gold claims, and disseminated copper minerals from several fault zones, Radioactive minerals and coal occur sparsely. Gravel is abundant. Some Rotoroa Complex rocks have potential as dimension stone, provided large enough blocks can be located. Earthquake risk, mainly to communications, is similar to that of other areas of southwest Nelson where minor historic earthquakes have occurred. (auth)

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