Geology of the mid Rangitata Valley

Fittall, Alan Matthew.

An area lying on the west side of the middle reaches of the Rangitata River is mapped and described. Basement Torlesse terrane rocks,·cretaceous conglomerate and Mount Somers Volcanics, an infaulted block of Tertiary sediments, and Quaternary Glacial outwash and alluvium are all present in the area.
The basement Torlesse terrane rocks are subdivided into two facies, massive sandstone facies and siltstone facies for mapping purposes. The massive sandstone facies dominates throughout the area. The massive sandstone facies consists of sequences dominated by thick bedded massive sandstones with interbedded siltstone, occasionally graded, amalgamated and showing water escape structures. The massive sandstones are inferred to be of sediment gravity flow origin, deposited from sandy high density turbidity currents. The siltstone facies is subdivided into laminated siltstone and graded siltstone subfacies. These thin bedded laminated or graded siltstone sequences are treated as mappable where they are more than 10m thick. The siltstone beds are inferred to have been deposited from dilute turbidity currents.
Measured sections are interpreted in terms of depositional environment. In terms of a submarine fan model the massive sandstone facies is inferred to be upper mid-fan distributory channel deposits and the siltstone facies interchannel deposits.
Mineralogically the sandstones are lithic - arkoses and arkoses. The Torlesse terrane sediments are first cycle erosion products. The detrital mineralogy and the internal structures of quartz grains indicate a volcano-plutonic arc source terrane. Development of cleavage is well documented in these sandstones.
A Middle Triassic to Permian-? Carboniferous age range is assigned to these rocks using petrographic trends defined by MacKinnon (1980).
Three phases or folding are recognised, an initial macroscopic phase, Which is folded into a subregional isoclinal syncline which in turn is gently folded. An axial planar cleavage is developed in the isoclinal subregional syncline. A regional fault, inferred to have both Rangitata Orogeny and Kaikoura Orogeny movement, passes through the area.
The Torlesse terrane basement has been subject to prehnite-pumpellyite facies metamoiphism locally reaching the pumpellyite-actinolite facies.
Overlying the Torlesse on an angular unconformity is a lense of conglomerate, inferred to be fluviatile in origin. Overlying this conglomerate and lying directly on the Torlesse terrane basement by onlap, is the Mount Somers Volcanics consisting or a lower sequence of andesitic flows and an upper sequence or rhyolitic flows. Differentiation trends indicate that fractional crystallization was acting on the magma. The large volume or rhyolites and the presence or quartz xenoliths out or equilibrium with the surrounding magma may indicate assimilation or basement rocks as an important process. Xenoliths in the andesite also contain an anomalously Al-rich orthopyroxene.
A transgressive sequence or Tertiary sediments is infaulted on the eastern border or the area. A zone of very low sedimentation is present within the upper parts or the Tertiary sequence.
Glacial outwash also adhering to the slopes in some places, and alluvium cover the valley floors.

142 leaves : ill., maps ; 30 cm.

1982Fittall

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