Depositional controls and characteristics of subaqueous bedded volcaniclastics of the Lower Devonian Snowy River Volcanics

The Mt Johnson Member represents a relatively deep, quiet marine enclave within the largely subaerial, Lower Devonian Snowy River Volcanics. It is intersected by seven diamond drill holes, allowing a detailed analysis of the constituent sediment sequences. "Normal" sedimentation is represented by a mudstone facies which consists of interbedded black mudstones and turbiditic siltstones. However, this fine-grained deposition was periodically interrupted by the emplacement of thick, amalgamated volcaniclastic mass-flow sequences. Six mass-flow sequences are defined, and two types can be recognized according to their overall bedding thickness and grainsize profile. Upward thinning and fining sequences are characterized by thickly bedded basal deposits of juvenile, magmatically fragmented pyroclastic debris, emplaced as cold, water-supported mass-flows. These are interpreted to be essentially syn-eruptive, arising either from flow transformation as pyroclastic flows ingested water upon entering the sea, or from immediately post-eruptive slumping of unreworked pyroclastic debris ponded at the shoreline. These juvenile deposits are overlain by intervals of thinner bedded mass-flows, which are of a similar provenance, but are considerably depleted in fines. The second type of mass-flow sequence, the complex bedding thickness sequences, has a basal zone of bedded, fines-depleted mass-flow deposits, with juvenile deposits in the mid to upper portions. These mass-flow sequences are interpreted to have arisen either from volcanic eruptive events which did not initially deposit primary pyroclastic debris subaqueously, or from epiclastic events which eroded previously erupted unconsolidated volcaniclastic debris. The depletion of fines in much of the volcaniclastic debris within the Mt Johnson Member indicates that it was subjected to a degree of epiclastic reworking. Since this cannot have occurred in the deep, quiet marine host environment, it indicates an external source for the parental magmatic eruptions. The eruptions probably occurred in an adjacent subaerial hinterland drained by an alluvial/fluvial/deltaic or fan-deltaic system. This system would have been a natural site for ponding and storage of freshly erupted debris and would have allowed the complex interaction between eruptive and epiclastic events which appear to have been responsible for the mass-flow sequences.