Investigation of the shape, scale and distribution of irregular zones of fragmental rocks of the Mawson Formation at Coombs Hill to determine the precesses of volcanic vent evolution

The shape, scale and distribution of irregular zones of fragmental rocks within the overal chaotic deposits of the lower 300m of the Mawson Formation at Coombs Hills was determined. The chaotic deposits are thought to represent an unusually large composite volcanic conduit, in which subterranean jets of debris constitute an important transport mechanism. Geological mapping at scales varying from 1:20,000 to 1:1,000 was completed using aerial photographs, handheld GPS and laser ranging survey equipment. Extensive data on the size shape and location of the fragmented zones was collected. The mixed rocks within different zones were quantitatively characterised by gridded outcrop clast counting. The relationship among zones and surrounding rock was described. The range of contact relationships among contacts between different rock zones within the vent cojmplex, in terms of sharpness, angularity or curvature and scales of irregularity was established. Four subtypes of structureless tuff breccia and lapilli tuff were distunguished. These zones and accompanying clastic dikes, Beacon rafts, pyroclastic raftes, clastic dikes and basalt intrusions were systematically described, catalogued, photographed and sampled. General and thematic maps were prepared for interpretation of volcanic processes. Around 140 samples were collected from the bedrock and analysed by thin section and geochemical purposes. From the thin sections, quantitative clast-shape and vesicle population data, including vesicle volume and interconnectedness, was determined by image analysis on SEM-generated photographs.