Cold-water carbonate deposition in a high-latitude, glacially influenced Permian seaway (Southern Carnarvon Basin, Western Australia)

摘要

The Lower Permian Callytharra Formation is a conspicuous interval of limestone and variably fossiliferous siltstone within an otherwise poorly fossiliferous and terrigenous clastic dominated succession, which records the acme and waning stage of the late Paleozoic ice age. The facies, macrofauna and stable isotope geochemistry of these rocks were examined to develop a depositional model for the formation and to examine the significance of the rocks in a context of Gondwanan glaciation. The dominant macrofossils—bryozoans, brachiopods and crinoids—comprise a calcitic, fully heterozoan assemblage typical of cold-water carbonate systems. A cold-water setting is supported by oxygen isotope data from well-preserved brachiopod shells, which indicate depositional temperatures of598C. Facies and their distributions are consistent with deposition in a mixed carbonate–siliciclastic ramp-like system that developed in a low-energy interior seaway that received localised fluvial input. Carbonate accumulation was most widespread at the onset of Callytharra deposition, during a time of rising sea-level that corresponded temporally with the mid-Sakmarian retreat of major ice sheets. Differential subsidence along the basin axis limited subsequent limestone deposition to areas that were relatively shallow and protected from siliciclastic input. The limited distribution of limestones overall demonstrates the sensitivity of these slow-growing, cold-water carbonate communities to terrigenous influx. Their presence, therefore, is significant in that they indicate times when siliciclastic sedimentation is impeded and, thus, very low sedimentation rates.