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Abstract
A detailed sedimentological and mineralogical study has been undertaken of the Early Permian Koogah Formation which is located a little to the northeast of Wingen in the Hunter Valley, NSW, with the principal objective being a better understanding of the conditions responsible for development of this unusual and indeed possibly unique sedimentary unit. From the outset it was realised that the formation comprises two distinctive and to some extent contrasting facies: (a) that forming the lower part, consisting essentially of kaolinite clayrocks or flint clays up to 75 metres thick; and (b) that constituting the upper section, composed almost entirely of quartz-lithic sandstones and conglomerates. Nevertheless coal seams that are generally lenticular and non-commercial are common to both facies. The Lower Koogah Formation is underlain by the Werrie Basalts, a two kilometre thick lava pile composed of fractionated alkaline basalt flows. The topmost flows are altered due to chemical weathering and several alteration profiles exist. Analysis of vertical sequences within the Lower Koogah Formation enabled four lithofacies to be distinguished: Sub-unit I consists of massive kaolinite clayrock with relict volcanic textures and is a palaeosaprolite: Sub-unit II is composed of xenoclasts, aggregates and particles of kaolinite and, in places, kaolinite clasts containing gibbsite and boehmite, all of which are derived from Sub-unit I and the lithofacies is interpreted as proximal mudflow breccias: Sub-unit III contains siliceous pebbles of incongruent roundness and is interpreted as proximal debris-flow conglomerates: Sub-unit IV is a varied stack of kaolinite clayrock beds with sedimentary structures and interbands of coals, red-beds and ironstones and interpreted as the mid-to distal-fan accumulations of alluvial fans. Local lakes, interlobe swamps and vegetation colonies occupied the surface of the fans. The Upper Koogah Formation consists of conglomerates, sandstones, siltstones, shales and coals with minor interbeds of kaolinite clayrocks and are typical coal measures deposited in a floodplain setting. The body of evidence favours an allochthonous origin for the bulk of the kaolinite clayrocks in the Koogah Formation.