Geology of the St. Bathans area, Central Otago.

Fiddes, Craig, 1976-

Mesozoic basement rocks of the St Bathans area comprise four distinct lithologies within the Torlesse terrane greywacke to Otago Schist metamorphic transition zone, separated by regional scale north-west striking faults. Four main fault blocks are characterised by textural grade as 1, 2b, 3a, and 3b, and correspondingly increase in metamorphic facies from prehnite-pumpellyite to greenschist. The mineralogy of textural zone 1 rocks correlates well with greywackes of known age deposited during the Upper Triassic. Analysis of trace and major element composition indicates that all basement rocks originated as sediment derived from a continental arc or active margin. Significant differences in the geochemistry of textural zone 1 compared with Qther basement blocks suggests a slightly different origin for this lithology. Cenozoic terrestrial cover strata unconformably rest on a regional sub-planar erosion surface. Basal quartz sandstones and conglomerates of the fluvial Dunstan Formation . characterise a variety of sedimentary facies including high energy valley, braid plain, and marginallacustrine deposits, the latter grading into and interdigitating with Bannockburn Formation lacustrine mud and siltstones. Pliocene to Pleistocene Wedderburn and Maniototo Conglomerate sediments overlie earlier strata either conformably or unconformably, and mark the onset of late Cenozoic tectonic activity. A sequence oflate Quaternary river gravels form terraces unconformably overlying older strata adjacent to rivers and streams flowing through the area. This Cenozoic sequence provides good markers for subsequent deformation, such as fault offset and folded strata. Multiple episodes of deformation are recorded in the geology of this area, with Mesozoic shear zones of the Stranraer and Blue Lake Faults being reactivated and deforming Cenozoic sediments. Orientations of faults in Cenozoic strata suggest that these major regional structures have operated as an aggregate of many smaller structures, and are not necessarily continuous along strike. Ductile deformation is observed as folding of both Cenozoic and Mesozoic strata, creating the St. Bathans and Home Hills - Pennyweight Hill- Blackstone Hill- Raggedy Range antiforms, and the Fiddlers Flat synform. The St. Bathans area appears to be a structural "step-over" zone, with activity focussed on the Stranraer Fault system to the east of Isolated Spur Fault, and on the Blue Lake Fault system to the west of Pennyweight Hill. Horizontal Quaternary terraces however, indicate minimal Late Quaternary deformation in the St. Bathans area.

67 [9] leaves : ill. (some col.), maps ; 30 cm.

1997Fiddes

Abstract Only

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