AAPG Foundation 2019 Grants-in-Aid Projects

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Sub-seismic scale architectural characterisation of incised valley-fill successions in the Upper Carboniferous strata of the UK offshore: Insights from the Spireslack Sandstone, Scotland.


Incised valley-fill are strongly linked with allogenic controls and often correspond to base-level falls. They can form stratigraphic traps and are common within the Carboniferous succession of the UK. Whilst seismic-scale geometries can be constrained, the sub-seismic internal architecture of incised valley-fill successions are poorly understood. This work characterises the architectural elements of the Spireslack Sandstone and its sequence stratigraphical context. The study uses photogrammetry with LiDAR and sedimentary logging. A facies to architectural scale bounding surface analysis allows for sequence stratigraphical interpretation. The architecture includes shelf and shoreface margin sands and large to decimetre- scale barforms. Results also reveal several parasequence scale cycles dominated by flaser bedded sandstones, grading up into more continental facies. Palaeosol facies near the top, indicate limited aggradation and sediment bypass of the shelf. Erosionally overlying is the Spireslack Sandstone, deposited as a high sediment load, low-sinuosity, fluvial system grading into more tidally dominated transgressive deposits. Carboniferous incised valley-fills are dominated by basally eroding, high sediment load, homogenous fluvial strata, displaying low-sinuosity. During transgression, the fluvial system retrogrades depositing increasingly heterogeneous strata, overlain by estuarine mudstones, providing a potential intraformational seal. This work has significant implications for predicting the sub-seismic internal geometries of Carboniferous incised valley-fills of the Southern and Central North Sea, offshore UK. Further work will use the Spireslack incised valley-fill to interpret further successions of the onshore and offshore Upper Carboniferous strata of the UK.