A non-marine Lower Cretaceous rift-related epicIastic volcanic unit in southern Australia: the Eumeralla Formation in the Otway Basin. Part II: fluvial systems

Relationships, thicknesses, and palaeocurrent and other sedimentary data applying to facies associations in Eumeralla Formation outcrops in the eastern Otway Basin distinguish at least three discrete depositional systems (A-C). Each system is characterised by high-energy fluvial flows in broad channel tracts. The multistorey sandstone bodies of system A, between Cape Otway and Apollo Bay, are up to 70 m thick and contain varying proportions of basement-derived quartzose gravel and sand intermixed with mainly volcaniclastic sand. Interchannel siltstones also up to 70 m thick separate the sandstone bodies. Palaeocurrents in system A have an overall southerly trend. This system is interpreted to represent deposition in a medial alluvial fan to proximal braided-stream system. System B occurs around Moonlight Head, and may extend southeast to Rotten Point. It is characterised by multistorey sandstone bodies up to 14 m thick separated by siltstones of similar thickness which locally contain thin coal beds, rooted horizons, and reddened soil profiles. It lacks basement-derived gravel. Palaeocurrents trend north-easterly to northwesterly. The sediments of system B accumulated on a medial to distal braid plain. Facies associations and fluvial architecture of system C, seen in outcrop north of Skenes Creek, resemble those of system B, from which it is distinguished by consistently northwest palaeocurrent vectors, a basement-derived gravel component, and the absence of debris flows and volcanic pebbles. System C also represents deposition on a braid plain or in a braided-river system. The three depositional systems are accommodated in a model for the Eumeralla Formation which suggests that its volcanic detritus was derived largely from infrarift volcanic complexes in the axial parts of the Otway rift basin, which during the Aptian-Albian lay to the south of the present coastline. A volcaniclastic apron spread northwest to northeast across the basin (system B). Elevated basement blocks shed quartzose detritus into flanking alluvial fans, the more distal parts of which mixed with volcaniclastic detritus (systems A, C). The onset of axial volcanism in the Aptian may have displaced a former westerly axial drainage towards the northern basin margin (system C).