Origins of clay-rich strata in Cenozoic paleochannel deposits: an example from Suttor Formation, Queensland

Abstract

Cenozoic sediments cover much of Queensland’s landscape and record changes in climate and flora that evolved in tandem with the tectonic evolution in eastern Australia, associated with uplift and erosion events arising from Late Cretaceous sea-floor spreading in the Coral Sea, leading to the passive margin we see today. In eastern Australia, incised and potentially fault-bounded valleys developed on the pre-Cenozoic landscape and were variably infilled at different times by siliciclastic sediments, carbonaceous muds and peat, and basaltic lava flows. This study examines a 100 m-deep drill core through one such siliciclastic-dominated paleochannel incised into upper Permian coal measures on the western limb of the Bowen Basin. Palynological analysis, core logging, hyperspectral logging of the core, X-ray diffraction (XRD), SEM–EDS and U–Pb zircon geochronology were used to investigate the age and origins of these clay-rich deposits. Palynological analysis confirmed that the strata belong to the upper Paleogene and Neogene Suttor Formation, a time when Australia’s climate has been interpreted to shift from wet and warm to a more arid setting. Core logging revealed that the sequence in the studied channel began as a breccia/alluvial fan with chaotic angular clasts of Permian laminated siltstone, tuff and coal floating in disturbed poorly lithified sand with evidence of soft sediment deformation, possibly initiated by localised faulting. These clasts were continually reworked and broken down into a clast supported pebble conglomerate capped by a series of claystone with intermittent carbonaceous claystone and lignite, interpreted to have formed in a lacustrine environment that could be localised on the floodplain (oxbow) or more regional in extent. Above this, and interpreted from palynology as corresponding to a climatic shift to more arid conditions, are well sorted but subrounded sugar sands and intermittent clays with abundant iron staining (?paleosols) interpreted to represent sandy ephemeral channels. These are overlain by extremely weathered undated basalts. The hyperspectral logging of the core and XRD studies of the white claystone samples indicate that they are kaolinite-rich, but observations under SEM–EDS noted that their honeycomb structure mimicked that of smectite. The presence of abundant zircon suggested the possibility of syndepositional volcaniclastics, but U–Pb zircon geochronology recorded an age of 272–254 Ma (middle to late Permian). Hence local recycling of the tuffaceous smectite-rich coal measures within the incised alluvial system, assisted by intense weathering under a warm and wet climate, and potentially acid leaching from the organic-rich layers was the origin of the kaolinite-rich clay strata. KEY POINTS Clay-rich Cenozoic paleochannels in the northern Bowen Basin belong to the upper Paleogene and Neogene Suttor Formation, based on palynological study. The clay-rich strata are developed by local recycling of middle to late Permian tuffaceous smectite-rich coal measures. The interpretation based on palynology and clay mineralogy confirm that Australia experienced a shift from wet and warm to a more arid climate during the upper Paleogene and Neogene.