Constellaria

Abstract: During the late Darriwilian (late Middle Ordovician), and again during the Gisbornian, Eastonian and Bolindian (Upper Ordovician), an extensive, tropical, carbonate platform, the Tasmanian Shelf, formed in the western half of Tasmania. This comprises predominantly shallow, marine carbonates of the Gordon Group. Almost continuous successions in the Florentine Valley (about 1200 m) and along Mole Creek (about 1000 m) have a rich invertebrate fauna, including bryozoans. Bryozoan assemblages with abundant cryptostomes and trepostomes range through the Late Ordovician. Diverse generic assemblages contain rhabdomesines, ptilodictyids, amplexoporines, eridotrypines, rhombotrypines, monticuliporoids and constellariids. Rhombotrypines are well represented throughout the higher parts of the sequence and, in places, dominate the shallow, suspension-feeding assemblages to the exclusion of other genera. The assemblages are similar to those of the central and eastern United States and include a constellariid closely similar to Constellaria, a distinctive genus of the Middle-Upper Ordovician of the United States. The upper Bolindian bryozoans have a low specific and generic diversity and are characterised by a large number of large individuals, typical of the upper Richmondian of North America. The overlying siltstones and sandstones in Tasmania are correlated with the Gamachian (uppermost Ordovician) and the lower Llandoverian (Silurian). The depositional history of the Tasmanian Middle-Upper Ordovician and Lower Silurian reveals closely similar patterns of major global sea-level fluctuations. The Darriwilian was a time of generally low sea levels with the Cashions Creek Limestone and Standard Hill Formation representing late Darriwilian sea-level fluctuations that flooded the area. During the Gisbornian, the Lower Member of the Benjamin Limestone and the Ugbrook, Sassafras Creek and Dogs Head Formations were deposited in deeper shelfal conditions during higher sea-level highstands and are similar to the Blackriveran of North America. After an unconformity, clastic beds of the Lords Siltstone (and Mole Creek interval) represent a major transgression with many smaller depositional cycles; the lower Eastonian rocks are similar to the Rockland and Kirkfield beds of North America. The succeeding Overflow Creek Formation was deposited during the remainder of the Eastonian and the first half of the Bolindian. Sea levels were relatively high and sea-level fluctuations were relatively minor during most of the Eastonian, but had greater amplitudes during the first half of the Bolindian. A major unconformity and erosional interval during the middle Bolindian separates weathered, often karstified, recrystallised, dolomitic, cherty and generally poorly fossiliferous limestones below, from three or more depositional sequences above in the later Bolindian that have well preserved sediments and fossils and supplied most of the bryozoans. In areas away from the Florentine Valley and Mole Creek, erosion at this unconformity has removed as much as several hundred metres of sediment. Above this unconformity in the Mole Creek area, the Den Formation includes the three or four upper Bolindian depositional sequences and these also form the upper beds of the Upper Member of the Benjamin Limestone in the Florentine Valley. The depositional patterns are similar to those in the North American Central States area. The succeeding Westfield beds in the Florentine Valley and the unnamed siltstones in the Mole Creek area have few limestones and are poorly exposed. These deposits are consistent with global conditions in most areas near the Ordovician-Silurian boundary where cool to cold temperatures associated with maximum glaciation are thought to have limited carbonate production.