The sedimentology and environment of deposition of the Mallacoota Beds, eastern Victoria

Abstract

Abstract The Mallacoota Beds of Ordovician age in eastern Victoria include: (1) a succession of greywackes, sandstones, siltstones and shales that show a variety of sedimentary structures and bedding features, including graded bedding, lateral continuity of beds, convolute lamination, slumps, flute casts, load structures and all divisions of the Bouma sequence; (2) radiolarian cherts and black shales, representing pelagic and hemipelagic deposition. These former radiolarian‐rich oozes, which occur between turbidites, have been diagenetically converted to bedded and nodular chert, whereas thick (>100 m) ribbon‐chert sequences probably accumulated in areas such as topographic highs, away from dilution by turbidite deposits. Six separate lithofacies can be defined, and bed thickness analysis indicates the presence of thinning and/or fining‐upward facies sequences and thickening and/or coarsening‐upward facies sequences, suggesting the variable proximity of channel axes with time. Palaeocurrent measurements indicate that the turbidity currents flowed in a northerly direction at the moment of deposition. The source‐rocks for the sequence are thought by the authors to be the Cambrian and Precambrian sediments of the Ross Orogen of eastern Antarctica.