CLAOUE-LONG, JON, Australian Geological Survey Organisation, Canberra, ACT, Australia

ABSTRACT: SHRIMP Zircon Dating and the Timing of Phanerozoic Processes

Seismic sections now permit the form of sedimentary basins to be mapped in three-dimensional detail. Through conceptual models that associate characteristic sediment packages with subsidence, sedimentation, sea level change and tectonism, recognition of the sedimentary end product of likely processes has become the main exploration tool in use today. The weak link between processes and sedimentary packages is measurement of the times and rates at which sequences formed.

We are now measuring directly the ages of sedimentary sequences, using the SHRIMP ion microprobe to date zircon crystals in volcanic ashes that are interlayered in both marine and non-marine sequences. Accuracy of about 1% is achieved, a numerical time resolution comparable to that of many fossil zones. SHRIMP's microbeam targets magmatic zones within crystals separately from altered or inherited areas, and so obtains reliable age information in complex samples.

We have focussed on the Palaeozoic, where systematic dating of biozones has resulted in large revisions to Carboniferous time and changes to assumed rates of processes. The Carboniferous Period is about 20% shorter with a younger base (c. 355Ma) and older top (c. 300Ma) than published timescales suggest. Numerical ages also constrain the timing of non-marine facies devoid of diagnostic biozones, such as the Late Palaeozoic glaciation of Gondwana which is shown to begin 20 Ma earlier than previously supposed. SHRIMP zircon dating is now being extended into the Mesozoic.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.