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Sub-Seismic Scale Architectural Characterisation of Incised Valley-Fill Successions in the Upper Carboniferous Strata of the United Kingdom Offshore: Insights From the Spireslack Sandstone, Scotland


Incised valley-fill successions are strongly associated with allogenic controls and often correspond to base-level falls. They have the potential to form stratigraphic traps and are common within the Carboniferous succession of the UK. Whilst seismic-scale geometries and spatial and temporal distributions can be constrained, the sub-seismic internal architecture of incised valley-fill successions are comparatively poorly understood.

This work characterises the architectural elements of the Spireslack Sandstone and its sequence stratigraphical context. The study utilises both UAV and terrestrial photogrammetry, in conjunction with LIDAR data over 3 km and sedimentary logging over the ~150 m section. A facies and architectural scale bounding surface analysis is conducted to determine the depositional environments comprising the succession. The resulting depositional environments are placed in a sequence stratigraphical context. The architecture of the succession is complex in nature, and includes shelf and shoreface margin sands, large-scale fluvial barform geometries and decimetre scale tidal barforms. Results from sequence stratigraphical analysis reveal several parasequence scale cycles dominated by flaser bedded sandstones, that grade upward into more continental facies. Near the top of the logged succession, palaeosol facies show significant maturity indicating a period of limited aggradation and sediment bypass of the shelf. These are erosionally overlain by the Spireslack Sandstone, which was deposited as a high sediment load, low-sinuosity, fluvial system grading into more heterolithic fluvial strata and tidally dominated transgressive deposits. Carboniferous incised valley-fill successions are dominated by basally eroding, high sediment load, homogenous fluvial strata that show simple low-sinuosity geometries. During transgression the fluvial system retrogrades depositing increasingly heterogeneous deposits which are overlain by estuarine mudstones, providing a potential intraformational seal. This work has potentially significant implications for predicting the sub-seismic internal geometries within the underexplored Carboniferous incised valley-fill successions of the Southern and Central North Sea, offshore UK. Further work will look to utilise the Spireslack incised valley-fill succession to interpret further successions of the onshore and offshore Upper Carboniferous strata of the UK.