HIGH-PRECISION RADIOMETRIC AGES FROM THE NORTHERN SYDNEY BASIN AND THEIR IMPLICATION FOR THE PERMIAN TIME INTERVAL AND SEDIMENTATION-RATES

Three pyroclastic samples that bracket the coal-bearing Permian System of the northern Sydney Basin have been dated using the zircon U/Pb and hornblende K/Ar methods. The Matthews Gap Dacitic Tuff Member, situated 170 m below the base of the Permian System, gives a best estimate of 309 +/- 3 Ma. Its age correlates well with the Paterson Volcanics which suggests that the immediately overlying clastic sediments are equivalent to the Seaham Formation. The Awaba Tuff, which at the sample locality is located 50 m below the top of the Permian System, gives a best estimate of 256 +/- 4 Ma. An intervening horizon, the Thornton Claystone fo the Tomago Coal Measures, gives a best estimate of 266 +/- 0.4 Ma. The ages indicate an earlier beginning (approximately 299 Ma BP), an earlier termination (approximately 255 Ma BP) and slightly longer duration (44 +/- 13 Ma) of the Permian System in the Hunter Valley than previously suggested. Sedimentation rates of approximately 65 m/Ma, calculated from proximal sequence thicknesses, are only half the rate calculated from the closest maximum thicknesses. Both are considerably lower than previously quoted rates. The estimated time interval of 10 Ma between the Thornton Claystone and the Awaba Tuff is more than twice the length of time previously attributed to accumulation of the combined Tomago and Newcastle Coal Measures.