1. Background and rationale
Towards a better understanding of sand delivery to passive margin basins
Martin Nauton-Fourteu and Shane Tyrrell
Earth and Ocean Sciences and Irish Centre for Research in Applied Geosciences (iCRAG),
National University of Ireland Galway, Ireland
HC4.1PhD2
Background and rationale
The ultimate aim of this study is to move towards a better understanding of the processes controlling the sediment composition in passive margin sedimentary basins.
Among all the different processes which can
occur, this project focusses on those
which take place prior to deposition,
such as weathering, recycling,
mixing or sorting
– summarised in figure (right).
Such modifications of the sediment
take place during transport and/or
intermediate storage which can be
strongly influenced by sea-level
fluctuations. In this study, we are trying
to understand how such modification
processes might be recognised in the detrital
record. This could have important implications
for predicting reservoir quality.
Conceptual Models
Previous work has highlighted how detrital mineralogy in sedimentary rocks varies due to the processes which modify sediment during transport, storage and burial. The conceptual models, presented below, aim to illustrate ways in which several factors can interact or combine to alter detrital mineralogy. Some of these are related to changes in source while others relate to modification (e.g. during storage).
Previous work
Hypothesis 1
Hypothesis 2
Apatite is prone to dissolution during weathering, whereas tourmaline is far more resistant. This index therefore allows us to investigate variations in weathering (potentially during storage). Morton et al., 2012, linked this variation to sea level change, but could this be related to source changes?
This hypothesis emphasizes the fact that, for a constant source, variations observed in the bulk detrital signal should be caused by processes occurring between erosion and deposition, during transport and/or storage (e.g. weathering, recycling, mixing or sorting)
This example shows bulk detrital signal modified either by the addition of recycled material or by a change of source. It illustrates another potential modification of the sediment composition during transport
* Single grain provenance implies techniques such as U-Pb geochronology of zircon or apatite and Pb isotopes in K-feldspars, where applicable
** The bulk detrital signal indicates the detailed petrographic analyses of both framework and heavy mineral components
Methodology
We will test these conceptual models by interrogating bulk detrital signals and single grain provenance data in well- constrained sequences in the Clare Basin (right). These data will be acquired through:
Detailed logging and sampling of coastal outcrops in Co. Clare
Detailed petrographic analyses of framework and heavy mineral components – SEM, EDS, RAMAN and Cathodoluminescence (iCRAG TCD)
Provenance analyses: U-Pb geochronology of zircon and/or apatite, Pb isotopes in K-feldspar, where applicable
The combination of these methods should enable us to:
Identify variations of heavy mineral indices or framework components
Link these variations to specific sedimentary facies and relate these to sea-level fluctuations and to previous storage
Investigate if the observed variations are related to a change in provenance
Future Work
To test the above models and hypotheses, we are utilising the well-constrained sequences of the Mid-Carboniferous (Serpukhovian-Bashkirian) Clare Basin. These show a depositional record influenced by glacio-eustatic sea-level fluctuations, and the composition of the sedimentary rocks has potentially been differentially altered by prolonged storage phases.
The Ross Formation and Tullig Cyclothem are the immediate targets for research. The Ross Formation represents the early, deep water fill of the basin, with turbidites and slope sequences, whereas the later Tullig Cyclothem is shallower in nature and dominated by deltaic and associated environments.
Logging and sampling has begun at specific locations along the coast of Co. Clare (left). Detailed sedimentological interpretation will allow us to understand the facies and the depositional environments, such that we can link these to the detailed compositional data obtained with our methodological approach.
After Best and Wignall, 2016 and Pointon et al., 2012
Contact :
This publication 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.
References:
Best, J. L., and Wignall, P. B., 2016, A Field Guide to the Carboniferous Sediments of the Shannon Basin, Western Ireland, Wiley-Blackwell
Morton, A., Mundy, D., and Bingham, G., 2012, 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, v. 487, p
Pointon, M. A., Cliff, R. A., and Chew, D. M., 2012, The provenance of Western Irish Namurian Basin sedimentary strata inferred using detrital zircon U-Pb LA-ICP-MS geochronology: Geological Journal, v. 47, no. 1, p