Olistoliths and debris flow deposits at ancient consuming plate margins: an eastern Australian example

Slabs of radiolarian chert up to 400 m long, and smaller blocks of limestone, andesite, basalt and siltstone are imbedded within conglomerates and sandstones of the Wisemans Arm Formation in northeastern New South Wales. Abrupt termination of individual masses, their distribution and orientation, the lack of continuity of stratification from the masses into surrounding rocks, and the presence of boulders, pebbles and sand-sized grains of identical rock types as constituents of the surrounding matrix, indicate that the masses are allochthonous, but of sedimentary origin. The matrix rocks are ill-sorted, devoid of internal sedimentary structures, and were deposited from debris flows. The olistoliths were transported within these flows. Chert olistoliths were lithified at the time of their emplacement. They were detached along bedding surfaces during periods of heightened tectonic activity that also triggered the generation of debris flows. The Wisemans Arm Formation is separated by a major fault system from a magmatic arc-fore arc basin sequence, and is more intimately related to, although also in fault contact with, rocks inferred to be deformed oceanic crust. Components of the formation were derived from both these sources at the start of or during subduction, and accumulated in the immediate vicinity of a major consuming plate boundary.