Heavy mineral variations in mid-Carboniferous deltaic sandstones: Records of a pre-depositional sediment history?

Abstract

Sandstone composition is influenced by multiple factors, including acidic weathering, occurring during storage in the hinterland, prior to deposition. This study aims to better understand and constrain how the nature and duration of such pre-depositional factors might impact the final sediment composition. Mid-Carboniferous deltaic sandstones from the Clare Basin, western Ireland, for which depositional environments and provenance are well constrained, are the target of this study. Conventional heavy mineral analysis and specific heavy mineral ratios, such as the apatite–tourmaline index are utilised to examine these phenomena. Relatively high apatite–tourmaline index values observed in channelised sandstones contrast with lower values seen in sandstones associated with mouth bar and interdistributary bay facies. These variations are not linked to changes in provenance and thus potentially indicate differences in weathering intensity due to variable duration of alluvial storage. These changes are probably linked with shorter hinterland residence time in the channelised than in mouth bar and interdistributary bay sandstones. Variations are seen in the rutile-zircon index without any clear link with facies. These fluctuations could be ascribed to variable supply from a source, which is relatively rich in rutile but poor in zircon and apatite. Despite the apatite component in these sandstones being partially derived from recycled sources, the apatite–tourmaline index stills appears to hold information on the last sedimentary cycle.