Provenance study and structural aspects of the Hawkdun Range, Home Hills area

Ulrich, Samuel H. (Samuel Hamish)

The Torlesse basement predominantly consists of arkosic sandstone and argillite with minor conglomerate beds, typical of a clastic turbidite sequence. The sandstones are relatively immature. Modal analysis of the sandstones gave an average Q:F:L ratio of 33:50:17, Lv /L ratio of 0.83 and P /F ratio of 0.74. Correlation of these characteristics with previous work give~ a Triassic age. Pebble-sized clasts from a Torlesse conglomerate consist of: sedimentary clasts (50%), mainly autocannibalised older Torlesse-like rocks, with some quartz rich sandstones; volcanic clasts (11 %), mainly rhyolite and dacite with rare basic/intermediate clasts; metamorphic clasts (8%), mainly metasedimentary clasts and some high grade gneisses/ cataclastic granitoids; quartzose clasts (11 %); chert (13%), some containing radiolarians; and plutonic clasts(7%) mainly granite. Provenance information gained from sandstone and conglomerate analysis indicate a continental arc setting with some older plutonic roots exposed, associated with the autocannibalistic reworking of older Torlesse-like sediments, which have been uplifted and eroded following metamorphism to prehnite-pumpellyite fades. The Manuherikia Group sediments of Early Miocene age were deposited unconformably upon the peneplained surface of the Torlesse basement in a braided river and lacustrine environment. They consist of quartz pebble conglomerate, quartz sands and muds, and carbonaceous shales and !ignites. Measured sections in and along the Hawkdun Range are used to demonstrate the structure and deformation of the range. The deformation along the Hawkdun Range front occurs as a zone of distributed deformation upon numerous small faults, most less than 1metre wide. The movement on these faults is predominantly thrust to WSW with associated en-echelon strike-slip movement on approximately N-S striking faults. The faults are orientated similarly to the bedding within the Torlesse basement and are probably controlled by the bedding. Home Hills is characterised by an asymmetric anticline trending NE. Two models are proposed to model its formation. Evidence for recent growth of the anticline to the NE come from air gaps of abandoned streams upon the slopes of Home Hills. Uplift of the Hawkdun Ranges and the growth of Home Hills have formed an asymmetric basin between them where Manuherikia Group sediments are preserved.

vi, 69 p. : ill. (some col.), maps (some col.) ; 30 cm.

1994Ulrich

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