Stratigraphy and sedimentology of the Caples terrane of the Thomson Mountains, northern Southland, New Zealand

Abstract

The Thomson Mountains west of Lake Wakatipu are underlain by sediments of the volcaniclastic Caples terrane, lying east of the Dun Mountain Ophiolite Belt and gradational into the western margin of the Haast Schist Zone. Although traditional formation-mapping methods are difficult to apply in the area, five formations have been differentiated. The Caples sediments can be divided into eight subordinate lithofacies. Bold Peak Formation is lithologically diverse, but dominated by quartz-poor, very thick bedded, coarse lithic sandstone. Upper Peak Formation consists of thin-bedded quartz-lithic sandstone and siltstone and is finer grained than other formations. Kays Creek Formation is a distinctively red-and green-coloured unit of quartz-deficient thick-bedded coarse volcanogenic sandstone. Momus Sandstone and Mt Campbell Formations are lithologically identical and consist of monotonous sequences of quartzose flysch-like medium sandstone. Bodies of Kays Creek-like lithologies occur within other formations. Of the eight subordinate sedimentary lithofacies recognised, some are restricted to one formation while others are common to several. Each lithofacies has a distinctive range of bed form and thickness, grain size, and sedimentary structures. Lithofacies recognised are black mudstone, red and green mudstone, thin-bedded siltstone, thin-bedded sandstone, massive sandstone, conglomerate, flysch-like sandstone, and a volcanogenic association. Most of these can be matched by similar lithofacies in the Mutti-Ricci Lucchi classification of submarine fan facies. Paleontologic evidence from the area is sparse, and only Atomodesma, in limestone clasts in conglomerates and in a volcanogenic association, is diagnostic. The Caples terrane is considered to be of late Permian to early Triassic age. The Caples terrane is interpreted as a submarine fan complex that formed in a trench-slope and possibly trench-floor environment.