Deltaic sediments, Upper Triassic Torlesse Supergroup, Broken River, North Canterbury

Publisher Website
Google Scholar
Abstract
A small block (8.4 km2) of Upper Triassic Torlesse Supergroup rocks is mapped and described. Three sedimentary facies and one probably volcanogenic facies are recognised. They are interstratified and grade laterally into one another. Very thick-bedded sandstone occurs in wedging units up to 450 m thick. It consists mostly of massive, moderately sorted, medium-fine sandstone with scattered zones of parallel lamination, gently inclined lamination, and ripple cross-lamination. Penecontemporaneous black mudstone clasts occur widely. The facies is unfossiliferous. Thin-bedded siltstone forms units up to 95 m thick. It consists of very thin and thin beds of parallel-laminated, small-scale ripple cross-laminated, and deformed cross-laminated, moderately to well sorted coarse siltstone or silty very fine sandstone, alternating with homogeneous or mottled black mudstone. The worm tubes Titahia and Torlessia and feeding trails are widespread. Black mudstone occurs in units up to 150 m thick. It is characterised by discontinuous lamination and dense fine mottling. Very thin interbeds of laminated and cross-laminated fine sand and bivalve shell debris occur at widespaced intervals through the mudstone. Interstratified chert and shale are regarded as volcanogenic. Only one small unit is known in the area. Bedded sandstone, thin-bedded siltstone, and black mudstone are interpreted to be the products of the distributary-mouth bar, the outer delta front or distal bar, and interdistributary bay, respectively, of a coastal delta. The paleoecologic and paleogeographic implications of this deduction with respect to the Torlesse Supergroup as a whole are discussed.