Tracing regional unconformities in intracratonic basins with high-precision zircon CA-TIMS geochronology

Abstract

New high-resolution ID-TIMS geochronology of zircon from tuff horizons reveals a regionally traceable temporal hiatus of ∼ 3–10 Ma in sedimentation along the contact between the Middle-Upper Jurassic Walloon Coal Measures (WCM) and the Upper Jurassic Springbok Sandstone (SS), Surat Basin, Australia. Undecompacted sedimentation rates of 15–65 m.Ma−1 in the WCM drop to apparent sedimentation rates of 0 to 7 m.Ma−1 near the WCM-SS transition, rising again to 15–51 m.Ma−1 during SS deposition. U-Pb geochronology identifies a hiatus – either a cessation in sedimentation or an erosional event defining a regional unconformity – not discernable from sedimentary features, highlighting the need for high-resolution geochronology in defining key bounding surfaces in sequence stratigraphy of intracratonic basins. Determining the exact position of lithological contacts and characterizing petrophysical properties of units below and above the WCM-SS transition are particularly important in quantifying connectivity between the gas-bearing units below with regional aquifers above the unconformity. As exploration for coal-seam and shale gas in intracratonic sedimentary basins expands globally, identification of major stratigraphic transitions through high-resolution geochronology will help assessing the economic potential and environmental vulnerabilities of such systems.