Geology of the red rocks — turbidite association, Wellington peninsula, New Zealand

Abstract

The Wellington peninsula is composed of Torlesse terrane greywacke turbidites with intercalations of red rocks that occur in various combinations of metabasalt, chert, and coloured (red, green, grey) argillite. The red rocks define 2 km-scale folds (the Rimurapa inverted syncline and Evans Bay Syncline) in the Wellington peninsula. The best known example of red rocks crops out at a point called Red Rocks on the coast about 8 km south of Wellington City. Radiolarians in cherts at Red Rocks indicate a Late Permian age, whereas radiolarians from phosphorite and a probable hydrozoan fossil (Heterastridium) in grey argillite of the turbidite sequence indicate a Late Triassic (Carnian-Norian) age. Radiolarians from phosphatic concretions in grey argillite interbedded with turbidites elsewhere in the Wellington peninsula also indicate a Late Triassic age. The fossil ages indicate a 40–50 Ma age difference between the red rocks and the enclosing turbidites. In addition to their older age, the red rocks: (1) are oceanic sediments and intraplate tholeiitic basalts; (2) have conformable contacts with greywacke turbidites and may have a basal bedding fault and depositional top; (3) can be traced for considerable distances along strike, and have a variable lithology and a higher degree of internal deformation than the surrounding greywacke. Two models are proposed to explain the red rocks — greywacke turbidite association: (1) the red rocks are the top part of Late Permian oceanic crust overlain by Late Triassic turbidites that was sheared off by decollement and incorporated into an accretionary prism of a Late Triassic subduction zone; (2) the red rocks are submarine gravity slides of “oceanic” material from Late Permian seamounts as they encroached upon an area of Late Triassic turbidite deposition. They were later incorporated into an accretionary prism as packets of turbidite and red rocks by decollement during subduction. These interpretations are not mutually exclusive, and a critical point is whether the basal contact of the red rocks is a tectonic imbrication surface or primary deposition surface.