Abstract

Sandstones and shales of the Lower Permian Mackellar Formation conformably overlie glacigenic sediments and are conformably overlain by braided stream deposits. They record the filling of a large fresh to brackish water post-glacial inland sea by fine-grained turbidites (silt, fine sand) deposited in prograding channel-overbank systems. Upward-coarsening sequences 10 to 25 m thick are capped by broadly cross-cutting channels a few metres deep and hundreds of metres wide. Sediment-laden streams entering the inland sea from the adjacent braidplain delta delivered sediment directly to the turbidite systems; Gilbert-type deltas and distributary mouth bar deposits are rare or absent.

Fine-grained submarine fans and turbidite systems typically lack upward-coarsening trends and are characterized by a few stable channels. The Mackellar turbidites illustrate that upward-coarsening sequences recording progradation by channel-overbank complexes form in fine-grained turbidite systems and demonstrate that these systems can be characterized by numerous shifting channels.

Fine-grained turbidite systems adjacent to and conformably overlain by sandy braidplain delta deposits may be characteristic of the filling of large post-glacial lakes or inland seas. Continental glaciation produces large quantities of poorly sorted sediment. After glaciation, streams reworking the glacial deposits transport fine-grained sediment to the inland sea, leaving behind the coarser sand on the braidplain.

Introduction

Melting of Permo-Carboniferous ice sheets in Gondwana formed lakes and caused widespread marine incursions onto the continent in response to sea-level rise (e.g. LeBlanc Smith and Eriksson, 1979; Tankard et al., 1982; Banks and Clark, 1987; Domack, 1988; Martini and Rocha Campos, 1991; Redfern, 1991).