Stratigraphy and regional setting of the Cliefden Caves limestone group (Late Ordovician), central‐western New South Wales

Abstract

The 460 m‐thick Cliefden Caves Limestone Group is the oldest and best exposed, extensive body of limestone in N.S.W. Its Late Ordovician faunas and floras are comparatively well preserved and it has some excellent silicified horizons, especially in the upper part of the sequence. In the nominal type area at Cliefden Caves, the limestone has been subdivided into three formations—the Fossil Hill Limestone (lower), the Belubula Limestone (middle) and the Vandon Limestone (upper). Only the Belubula Limestone remains undifferentiated. The Fossil Hill Limestone has been further subdivided into six members (Gleesons, Kalimna, Wyoming, Taplow, Dunhill Bluff and Transmission Limestone Members, respectively), and the Vandon Limestone into two members (Trilobite Hill and Mount Lewin Limestone Members). Each of these subdivisions is defined herein, with a designated type section and a description of its content and distribution.

In late Gisbornian to early‐middle Eastonian time, the Cliefden Caves Limestone Group formed in shallow waters (intertidal?‐subtidal) of a subsiding, offshore, predominantly volcanic island (at least in part the Molong High), initially as fringing deposits, and later in a mantling, bank‐like accumulation. Details of the nature of environmental changes during the depositional history of the limestone are discussed. In a period of relative quiescence the carbonates accumulated at a rate which, for the most part, closely matched the rate of subsidence. However, at least three relative transgressive‐regressive depositional cycles can be recognised through the sequence, with a range of environmental changes from inshore‐restricted to more offshore, open‐shelf conditions. These transgressive/regressive phases may be interpreted as sea‐level fluctuations, each of probably less than a million years duration. It is estimated that throughout its depositional history the Cliefden Caves Limestone Group accumulated at rates varying from about 30 to 90 m per Ma.

In terms of regional setting, the late Ordovician carbonate deposits on both the eastern flanks (Cliefden Caves Limestone Group and equivalents) and the western flanks (Bowan Park Group and correlatives) of the Molong High formed in broadly similar conditions. However, the timing of events differed, with sedimentation commencing in a slightly later stage of the initial transgression and continuing to keep pace with subsidence for longer in the western Bowan Park sector. Carbonate deposition apparently ceased in the eastern Cliefden Caves section, not so much because of more rapid, localised rate of subsidence, but because it lay in a more exposed, ocean‐facing position and was subjected at an earlier stage to backwearing of the outer margin of the carbonate platform. This period of back‐wearing was followed, in the late Eastonian, by a phase of pelagic mud sedimentation with associated limestone breccias derived by slumping from the remaining carbonate platform to the west, and then, in the Bolindian, by a major phase of volcanic activity.

Key words:

• Ordovician
• stratigraphy
• limestone
• New South Wales
• depositional environments
• geological history