Prospect formation : sedimentology, stratigraphy & significance : late miocene-pliocene syntectonic sediments of the Te Anau Basin, western Southland, New Zealand

Manville, Vernon.

The late Miocene to Pliocene Prospect Formation forms the topmost non-glacial unit in the central Te Anau basin, with its distal correlatives extending south into the northern Waiau basin. The lower part of the formation consists of a number of coarse-grained deltaic systems which prograded across the originally marine Te Anau basin from tectonically uplifted source areas to either side. The upper part of the formation is dominated by gravelly . braided stream deposits. Fieldwork has identified a number of fluvial lithofacies assemblages, including massive conglomerate, cyclothemic sands, and coal-bearing sequences. Several petrofacies, based on the provenance signature of the sediments are recognised in addition. Five stratigraphic members have been defined within the Prospect Formation. The Prospect Formation is grouped with its distal equivale~~, the Rowallan Sandstone and Te Waewae Formation into a single lithostratigraphic group, named here the 'Wilderness Group'. Analysis of over 200 km of seismic lines from the central Te Anau basin has identified the base of the formation, in addition to two seismic members and a major internal reflector between them~. Structure contours on the base of the Prospect Formation derived from the '( seismic data permit the construction of a stratigraphic correlation chart foi,:Unit, allowing the relative st~atigraphic positions of widely dispersed outcrops to be constrained. The maximum thickness of the formation is estimated at in excess of 3.5 km. The Prospect Formation is largely derived from three $eparate and petrographically distinct basement terranes, the Fiordland Complex, the Caples Terrane, and the Takitimu Group (southern Brook Street Terrane). Analysis of clast populations from gravels and the point-counting of sands from throughout tp.e Prospect Formation demonstrates variations in - the relative contributions of these source areas in both time and space. Palaeocurrent data confirm the pattern of sediment transport and dispersal systems indicated from provenance data, with a southwesterly directed major fluvial system sourced from the Caples Terrane and an easterly directed fluvial system sourced from Eastern Fiordland. These systems unite in the southern Te Anau basin and flow south as a single trunk system through the neck between Fiordland and the Takitimu Mountains. The age of the Prospect Formation is constrained by internal pollen dates and the age of the youngest underlying marine sediments. Pollen dates cluster into two groups: an older, latest Miocene (Tongapurutuan-Kapitean) group, an:d a younger, Pliocene (WaipipianNukumuruan) group. Palynofloral assemblages also indicate palaeoclimatic conditions during the period of Prospect Formation deposition, with a deterioration from a warm temperate climate in the late Miocene to a cool or cold temperate climate in the Pliocene. Studies of clay minerals in Prospect Formation sands indicate that most of the clay fraction is detrital in origin, although authigenic smectite is common in Caples-derived sands iii ,I :: ,'I, and minor authigenic kaolinite is found in Fiordland-derived material. Diagenetic carbonates are more diverse, with several generations of dolomite and high-Mg calcite cements developed in parts of the basal marine Prospect Formation. Calcite cements of various morphologies developed in the fluvial members of the Prospect Formation suggest a seasonal climate with periods of aridity in the basin. A single occurrence of an authigenic zeolite mineral, heulandite, is recorded. The Prospect Formation is interpreted to be a syn-tectonic deposit related to the rapid uplift of the basement blocks surrounding the depositional basin. The Te Anau basin is one of several structurally controlled basins in the Western Southland area, adjacent to the IndoAustralian/ Pacific plate boundary in the southern South Island of New Zealand. The area is composed of a mosaic of tectono-stratigraphic basement terranes and their bounding major fault systems, accreted to the Palaeozoic and Mesozoic margin of Gondwana. Changes in the convergence vector across the adjacent plate boundary ca~sed by progressive southeastward migration of the relative pole of rotation during the mid to late Miocene, produced variations in the regional tectonic regime in Western Southland and re-activation of these old fault systems. The position of the continental Challenger Plateau adjacent to the Fiordland Complex crusta! block during the late Miocene, coupled with an oblique convergence vector across the plate boundary adjacent to Western Southland caused Fiordland to rotate clockwise as it was forced northwards along a restraining curve in the Alpine Fault plate boundary. This transferred a component of dextral compressive strain into the more easily deformed Western Southland area to the east, re-activating pre-existing major fault systems. The Caples Terrane was uplifted by a combination of reverse motion on the Livingstone Fault and distributed shortening within the terrane, with the Fiordland Complex uplifted between the Te Anau and Alpine Faults. Theintervening Te Anau basin subsided due to a combination of relative motion and the geometry of the basin-bounding fault systems forming a releasing bend at the basin's southern end. The coupling of large volumes of coarse-grained sediments derived from source area uplift and the creation of accommodation space in the adjacent basin resulted in the deposition of the Prospect Formation. Continued northward movement of the Challenger Plateau on the Alpine Fault during the last 3-5 million years has shifted the locus of maximum deformation north, uplifting the Southern Alps;, and ending deposition of the Prospect Formation in Western Southland. However, eversion of the Te Anau basin and east-west shortening across it, begun in the Pliocene, continues, and the area is still tectonically active.

1 v. : ill. (some col.), maps ; 30 cm.

1995Manville

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