Trench-sedimentation versus accreted submarine fan—an approach to regional-scale facies analysis in a Mesozoic accretionary complex: “Torlesse” terrane, northeastern North Island, New Zealand

Abstract

Abstract The Waioeka subterrane of the North Island, New Zealand, considered as the youngest part of the Torlesse terrane, is interpreted as a Late Mesozoic accretionary complex at the Gondwana margin. These basement rocks are characterized by sedimentological and petrological monotony, scarcity of bio- and lithostratigraphic markers, and tectonic complexity due to accretion in an oblique subduction regime and possible post-accretionary strike-slip overprint. A modified approach to facies analysis is presented which allows the recognition of distinct characteristics of a turbidite system, and which reduces the number of possible interpretation schemes, despite the lack of lateral correlation of sedimentary sequences. In addition, criteria such as complexity of the pattern of facies distribution, position of slumping of oligomict material, and proximal-to-distal distribution of Bouma sequences and conglomerate fabrics, are suggested to distinguish between two end-members of possible pre-accretionary depositional environments: (a) a trench-depositional environment; and (b) a submarine fan on a broad basin plain. The Waioeka subterrane is characterized by mud/sand- to sand-rich sediments, consisting dominantly of very thick sandstones (facies B), and alternating sandstone–mudstone sequences (facies C2), and only minor conglomerates (facies A) and silty/muddy deposits (facies D). Overall sedimentological features suggest a depositional setting beyond a significant lower slope break (base of slope), at the lower reaches of a turbidite system, with sediment supplied by a small number of large point-sourced channels. Five defined facies associations are interpreted as major and minor midfan-type channel deposits, lobe and lobe-/fan-fringe facies, and/or trench-axial channel facies, and levee/overbank facies. Proposed criteria favour a trench-depositional environment, rather than an earlier postulated submarine fan setting on a basin plain subsequently accreted at a convergent margin.