SEDIMENTARY RESPONSES TO SUBAERIAL FELSIC VOLCANISM FROM THE LATE DEVONIAN-EARLY CARBONIFEROUS NORTHERN MACALISTER SYNCLINORIUM, SOUTHEASTERN AUSTRALIA

Active sub-aerial volcanism has the capability to rapidly alter both t he topographic and drainage characteristics of a landscape, and thus f undamentally influence resulting sedimentary facies. Relationships bet ween sedimentation and volcanism are explored in this paper, via an in vestigation of the stratigraphy and early depositional history of the Upper Devonian volcano-sedimentary units of the northern Macalister Sy nclinorium, east-central Victoria. Complex interfingering relationship s exist between sub-aerial felsic volcanic successions (Rose River Vol canics) and alluvial, fluvial and lacustrine sedimentary units (the Bi ndaree and Howitt Spur Formations). A depositional model is presented for these units, in which the Rose River Volcanics, an outflow (ignimb ritic plateau) facies of the Tolmie caldera complex to the north, co-e xisted with a series of volcaniclastic alluvial fans and freshwater la kes. A dacitic volcanic centre (Refrigerator Gap Dacite) appears to ha ve developed somewhat separately to the south, in the Jamieson River a rea. These lower successions of the northern Macalister Synclinorium r ecord an episode of sedimentation in close proximity to an active fels ic volcanic terrain, and in a landscape of significant topographic rel ief. The conglomerates of the Bindaree Formation, in particular, recor d the supply of abundant felsic volcanic detritus via high-gradient pr oximal alluvial stream systems. Comparisons can be made with sedimento logical processes operating during deposition of overlying sedimentary successions, where influences by primary volcanic activity were less important. Well defined channel geometry successions and a diversifica tion in clast and grain types within these younger units indicate ongo ing headward erosion into metasedimentary basement rocks, and depositi on within lower-gradient, higher-order stream systems.