Middle–Upper Ordovician conodonts from the Gunningbland area in central New South Wales with implications for regional correlations

Abstract

This study documents Middle to Late Ordovician conodont faunas primarily from the Billabong Creek Formation exposed in the Gunningbland area located west of Parkes and northwest of Forbes in central New South Wales. Forty-four identifiable conodont species recovered from 105 limestone samples in this area form the basis of the most complete biostratigraphic succession in shallow-water facies known through this interval from Australia. Four conodont biozones are recognised, extending from the middle Darriwilian Histiodella holodentata–Eoplacognathus pseudoplanus and Eoplacognathus suecicus biozones, through the upper Darriwilian Pygodus serra Biozone, to the Pygodus anserinus Biozone spanning the uppermost Darriwilian to basal Sandbian interval. A lower to middle Sandbian carbonate gap coincides with an unzoned interval, followed by the successively younger Belodina compressa, Phragmodus undatus and Taoqupognathus blandus conodont biozones in the upper Sandbian to lower Katian. The T. blandus Biozone directly correlates with the detailed conodont biozonation established in Katian limestones of the Molong Volcanic Belt further east. These age determinations provide much improved precision for correlation within Phases 2 and 3 in the mineral-rich Macquarie Volcanic Province. Analysis of conodont biofacies data supports an enhanced understanding of the geological evolution of the Macquarie Volcanic Province by interpreting the interplay between volcanic activity and carbonate deposition.

KEY POINTS

1. Conodont studies in the Billabong Creek Formation of the Gunningbland area reveal the only known biostratigraphic succession in Australia that extends continuously from the middle Darriwilian to basal Sandbian.

2. Three Late Ordovician (late Sandbian to early Katian) conodont biozones in the upper Billabong Creek Formation correlate precisely with carbonates of the Molong Volcanic Belt to the east.

3. Revised stratigraphic definitions of the Billabong Creek and Gunningbland formations are provided.

Keywords:

• Conodonta
• Ordovician
• biostratigraphy
• biofacies
• Junee–Narromine Volcanic Belt
• Lachlan Orogen
• New South Wales

Acknowledgements

This study has been supported by the GSNSW in collaboration with the Australian Museum. Conodont samples were processed at the Paleontology Lab of the GSNSW at Londonderry with assistance of Gary Dargan and Zoe Wyllie (former staff members) including acid leaching, residue separation and picking of the conodont samples. Stewart Watson is thanked for his assistance to draft Figures 1 and 2. Jodie Rutledge assisted in checking and converting of the coordinates of the samples that were collected by using old map grid references. SEM images were prepared in the Electron Microscope Unit of Macquarie University with assistance from Sue Lindsay. Two anonymous referees are thanked for their careful and constructive reviews of the manuscript. This is a contribution to IGCP Project 735 (Rocks and the Rise of Ordovician Life) and IGCP Project 668 (Equatorial Gondwana History and Early Palaeozoic Evolutionary Dynamics). Yong Yi Zhen and Ian Percival publish with the permission of the Executive Director, Geological Survey of NSW.

Disclosure statement

No potential conflict of interest was reported by the author(s).