Sequence analysis and depositional models of crinoidal limestones, Permian Yessabah Limestone, Manning-Macleay Basin, eastern Australia

Updated: 2018-04-20

The Permian Yessabah Limestone, a thick crinoidal limestone unit, appears abruptly in the stratigraphic succession over a large area of the Hastings Block in northeastern New South Wales, disconformably overlying clastic rocks with sharply contrasting facies associations. The formation is part of a thick depositional sequence that began with a major sea-level fall at the end of the Carboniferous. The sea-level fall produced a major basinward shift in facies causing rejuvenation of streams and the deposition of braided-stream gravels in the onshore areas while a mass-movement association, the Parrabel Beds, was deposited offshore in the deep basin (the lowstand systems tract). The Yessabah Limestone was deposited as part of the transgressive systems tract in a shallow-marine high-energy environment as sea level again began to rise. The highstand systems tract then prograded over the Yessabah Limestone in the form of a distal turbidite association , the Warbro Formation and associated units, as sea level reached its maximum. The growth of both crinoids and bryozoa was prolific; their two distinctive faunal assemblages alternately dominated the sea floor, producing thick units of well-washed crinoid-rich limestone followed by a bryozoan-rich unit. As sea level rose, conditions became unsuitable for carbonate deposition and the area was again invaded by clastic sediments, forming a distal turbidite association that prograded across the limestones as part of the highstand systems tract.