Defining unconformities in Proterozoic sedimentary basins using detrital geochronology and basin analysis—An example from the Mount Isa Inlier, Australia

Abstract

Detrital zircons from sedimentary rocks below and above a major regional Palaeoproterozoic unconformity (the ∼1690–1670 Ma Gun unconformity of the Mount Isa Inlier) were dated using U–Pb SHRIMP techniques to test whether calculated maximum depositional ages could be used to identify this ∼20 my boundary, and determine which facies provide the best opportunity to identify this surface. We show that samples taken in a landward position directly above the regional unconformity surface from fluvial or incised valley-fill facies contained significant numbers of ∼1675 Ma ages, suggesting that these settings are the most useful for recording maximum depositional ages close to the actual age of deposition. Likewise, the large numbers of ∼1680 Ma ages in basinal turbidite deposits indicate that this facies may also be a good target for determining maximum depositional ages close to depositional ages. Integrating this new detrital zircon data with the magmatic record from the ∼1670 Ma Carters Bore Rhyolite, Sybella Granite and unnamed igneous horizons permits correlation between basin development and magmatism in the Mount Isa Inlier. Magmatic inflation associated with intrusion of the Sybella Granite led to uplift in the Western Succession associated with the Gun unconformity, with erosion and recycling of this hinterland leading to transportation of this material eastward into the deeper water parts of the basin represented by the Soldiers Cap Group.