Petrology and origin of the Hervey group, upper devonian, Central New South Wales

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Abstract
The Hervey Group (Upper Devonian), central New South Wales, consists of 10,000 ft of orthoquartzites, protoquartzites, lithic sandstones and quartz siltstones. The predominantly fluvial sequence is subdivided into the Beargamil, Nangar and Cookamidgera Sub‐Groups. More than 500 petrographic analyses of sandstones show that the percentage of unstable rock fragments and poly‐crystalline quartz in the sandstones increases with increase in grainsize. Mineral facies maps drawn from these analyses show that polycrystalline quartz, and quartzose sedimentary rock fragments increase westwards and southwards for all sequences of the Hervey Group, indicating derivation of this detritus from a southern landmass. The distribution of rock fragments and feldspar also indicates that minor source areas existed to the southeast and northeast. The red sediments of the basal Beargamil Sub‐Group belong to an arkosic facies derived from nearby basement highs of granite, acid volcanics, and sedimentary rocks. The sediments become increasingly quartz‐rich upwards in the section indicating that the basement highs were gradually covered. The Nangar Sub‐Group consists of cyclothems of coarse orthoquartzites that are cross‐stratified and grade upwards into finer sandstones with more lithic detritus, and red silt‐stones. These sediments are inferred to have been deposited on a large flood‐plain, and the cyclothems to have originated by offlap deposition of migrating river channels. The siltstones were probably mainly deposited in backswamp areas, large ox‐bow and inland lakes, and mudflats that were frequently exposed to the atmosphere. Sedimentation ceased as the streams became sluggish and the floodplain ceased to sink relative to the southern landmass. The sediments of this last phase consist of the dominantly poorly‐sorted, red sandstones and siltstones of the Cookamidgera Sub‐Group.

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