Ordovician Snatigraphy of the Florentine Synclinorium, south-west Tasmania

Abstract

The Florentine Synclinorium consti tutes the type area of the Ordovician Junee Group in Tasmania, and the group is herein re-defined according to the formations present in this area. The base on the western side is formed by the Reeds Conglomerate, a unit of siliceous fanglomerate up to l,560 m thick lying conformably above a thick Upper Cambrian sequence on the Denison Range. The laterally-equivalent sandstone unit on the southeastern side is also given formation status (Tim Shea Sandstone). The overlying sequence of marine sandstone and siltstone is designated the Florentine Valley Formation, and is of Late Tremadocian-Arenigian age. A sub-unit of siltstone and limestone occurs in the middle part of the formation in some areas, but is not given formal status pending further mapping. The "Gordon Limestone", subdivided into three formations, becomes the Gordon Limestone Sub-Group. The basal Karmberg Limestone, of Upper Canadian -? Chazyan age, includes a mappable chert-rich unit which forms chert- covered ridges and is designated Wherretts Chert Member. The Cashions Creek Limestone, corresponding to the “Maclurites-Girvanella zone" of earlier reports, succeeds the Karmberg Limestone. Above this, and forming the bulk of the sequence, is the Benjamin Limestone, consisting of three members, viz. Lower Limestone Member, Lords Siltstone Member, Upper Limestone Member. A characteristic coral fauna with Favosites and cateniporines occurs near the top of the latter member, and includes conodonts which suggest an age not younger than Maysvillian. Above the limestone sequence and transitional with the overlying Eldon Group sandstone is a unit of siltstone and fine sandstone designated Westfield Beds. These contain a fauna correlated with the Richmondian, and the fauna in the overlying sandstone also appears to be Late Ordovician.