Link, Albert George
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...[Show more] ({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 & 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).
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