The stratigraphic development of the Yass Basin, New South Wales
Creators: name: Link, Albert George
Publisher: name: The Australian National University
Year: 1971

description: Five groups, previously known as the Yass Laidlaw, and Hume Series (Brown, 1941), are recognised in the Yass Basin succession. They are, in order of oldest to youngest, the Yass, Hattons Corner, Booroo Ponds, Barambogie, and Downing Groups. The Yass Group, of earliest Ludlovian age, consists of mainly terrestrial sandstones and mudstones (O'Briens Creek Sandstone) overlain by supratidal-subtidal limestones (Cliftont~ood Limestone). The Hattons Corner Group comprises thick crystal tuffs ({Willow Bridge Tuff Hember), fossiliferous limestones (Euralie Limestone Member), and reworked tuffs (Excursion Creek Sandstone Hember) of the Laidlaw Formation plus coralline limestones (Gums Road Limestone tlember) , tuffaceous sandstones (Tullerah Sandstone member), interbedded mudstones and limestones (Bo,.,spring Limestone l1ember), fossiliferous mudstones (Barrandella Shale Nember), and corallinc-crinoidal limestones (Hume Limestone Member) of the Silverdale Formation. Four facies and eight microfacies have been described from the Bowspring, Barrandella Shale, and Hume Limestone Members. Lagoonal rock types of these members are sparingly fossiliferous massive micrites, reefal types contain mainly stromatoporoid-tabulate biosparites, and fore reefal types consist largely of mudstones and rugosan biomicrites. The conformably overlying Booroo Ponds Group is subdivided into the Black Bog Shale, which contains dark grey shales and mudstones (Lower Nudstone Nember), fossiliferous siltstones (Yarwood Siltstone Member), and graptolitic shales and mudstones (Upper Shale Hember); and the Rosebank Shale, which has fossiliferous marlstones (Rainbow Hill Marl Member), shales and mudstones (Shale Member). The Barambogie Group consists of interbedded turbiditic siltstones and muds tones (Cm-1ridge Siltstone) overlain by medium sandstones and mudstones with lenses or algal limestone (flmside Fornation) containing the early Gedinnian conodonts, Spathognathodus remscheidensis Ziegler and Icriodue woschmidti Ziegler. Poorly sorted conglomerates (Sharpeningstone Conglomerate) together with andesitic (Pilleuil Andesite), rhyolitic (Waynes Knob Rhyolite) and ignimbritic (Vale Beder Ignimbrite) rocks make up the Bmming Group. Sixty-one species of conodonts, twenty-one algae, thirteen stromatoporoids, forty-one corals, and the brachiopod Aliconchidium yassi St Joseph have been described from the carbonate horizons. The dominant control of sedimentation appears to have been tectonic. Against a general background of slow continuous basement subsidence, there were several short punctuations of more rapid subsidence. Models for the recognition of tectonically controlled sedimentation are erected. The source Yassoria of the terrigenous sediments in the Yass, Hattons Corner, and Booroo Ponds Groups was situated in a northeasterly direction from the present basin: the sources (Naoyassoria I , II.l &amp; III) for the Barambogie and Bowning Groups were located to the southwest. Several cycles of sedimcntation related to tectonic pulsations and volcanicity are noted and models for the recognition of dominant transgressive and regressive phases during cyclic sedimentation are presented. The geologic history of the Yass Basin has been portrayed in six major stages of development: (1) Volcanicity (?pre Ludlovian) rapid accumulation of coarse crystal tuffs [Douro Volcanics]; (2) Fluviatile and marine flat deposition (? pre earliest Ludlovian to early early Ludlovian) - transgression and deposition of interbedded sandstones , mudstones , and limestones [Yass Group); (3) Shelf carbonate development and periodic volcanicity (early early Ludlovian to late middle Ludlovian) - mild transgression during the periods of non-volcanic activity and the formation of lagoonal, reefal, and fore-reefal sediments [Hattons Corner Group]; (4) Deposition of off-shore terrigenous fines (late middle Ludlovian to late late Ludlovian) - transgression and deepening of the water resulting in argillaceous sedimentation [Booroo Ponds Group]; (5) Accumulation of turbiditic siltstones and the deposition of shallow water sandstones and mudstones (Pridolian to early Gedinnian) - mild regression followed by the shoaling of the basin resulting in the eventual development of algal limestones [Barambogie Group); (6) Formation of coarse conglomerates and volcanicity (early Gedinnian to ? post early Gedinnian) - major regression, the accumulation of thick conglomerates , and the extrusion of andesitic and rhyolitic lavas [Bowning Group).