Heavy mineral variations in deltaic sandstones: towards a better understanding of pre-depositional sediment history?

Processes such as weathering, hydraulic sorting or mixing can occur during transport and intermediate storage, preferentially affecting certain minerals due to varying relative stability (e.g. quartz vs feldspar) and thus modifying the sediment composition. This study investigates bulk sandstone composition and heavy mineral variations to provide information on the transport/storage history experienced by the sediment prior to deposition. The apatite-tourmaline index (ATi) is believed to be controlled by chemical weathering as during intermediate storage and transport, apatite grains should be more easily dissolved than tourmaline¹. It is expected that, for a long transport and/or a prolonged storage phase, the apatite-tourmaline index (ATi) will be relatively lower than for a sediment with a shorter residence time in the system^2,3. It follows that the extent and duration of any storage phase on the shelf or on the floodplain must be closely related to sealevel fluctuations.

This project investigates the mid-Carboniferous Clare Basin, western Ireland, where glacio?eustatic sea-level fluctuations influenced the deposition of deltaic cyclothems. The Serpukhovian-Bashkirian Tullig Cyclothem has been logged and sampled at three locations on coastal outcrops. At two of the field localities, the deposition shows a delta progradation with a succession of prodelta mud, sandy mouth bar, interdistributary bay and channelised sand deposits. The third location is interpreted as a transgressive shelf sand-body. Petrographic analysis reveals the sandstones at all localities are mineralogically and texturally mature. Heavy mineral analyses show clear changes in heavy mineral indices throughout the deposition. ATi values are higher for the channelised sandstones than for the interdistributary bay or the mouth bar deposits. This could be explained by variations in transport and/or storage that the sediment experienced prior to deposition, with potentially a more direct input of sediment through channels than the more mixed interdistributary bay or mouth bar deposits. Such heavy mineral variations could be due to a change of provenance. For this reason, source characterisation will be investigated through a multi proxy approach using U?Pb geochronology of apatite and zircon.

This presentation has emanated from research supported in part by a research grant from Science Foundation Ireland (SFI) under Grant Number 13/RC/2092 and is co-funded under the European Regional Development Fund and by PIPCO RSG and its member companies.

1. Morton, A. C. & Hallsworth, C. Identifying provenance-specific features of detrital heavy mineral assemblages in sandstones. Sedimentary Geology. 90, (1994). 2. Morton, A. C. & Hallsworth, C. R. Processes controlling the composition of heavy mineral assemblages in sandstones. Sedimentary Geology. 124, (1999). 3. Morton, A. C., Mundy, D. & Bingham, G. High-frequency fluctuations in heavy mineral assemblages from Upper Jurassic sandstones of the Piper Formation, UK North Sea: Relationships with sea-level change and floodplain residence. Geological Society of America Special Papers. 487, (2012).