Characterisation and high resolution sequence stratigraphy of storm-dominated braid delta and shoreface sequences from the Basal Grit Group (Namurian) of the South Wales Variscan peripheral foreland basin

Abstract

Sedimentological studies of Basal Grit successions indicate that they were deposited in braid deltas that prograded into a storm-dominated, shallow-marine, ramp-type environment. High resolution sequence stratigraphy data and concepts have been used to subdivide the strata into fourth-order systems tracts and longer period, third-order sequence sets. Falling stage and lowstand deposits of forced regressions are recorded by a variety of sharp-based/incised sandstone bodies, transgressive phases by thin marine bands with high gamma ray values and highstands by coarsening upward parasequences. Incised, lowstand sandstone bodies are particularly well represented and these have frequently removed underlying highstand and transgressive deposits. Their predominance is considered to be related to high sediment flux and slow basin subsidence, the latter frequently being balanced or outpaced by glacio-eustatic sea level falls resulting in the creation of limited to negative accommodation space. Synsedimentary tectonics have frequently overprinted the effects of glacio-eustatic sea level changes. The complex interplay between these allogenic processes was a direct result of repeated Gondwana glaciations (slow sea level falls) and melt-outs (rapid sea level rises) and the location of the depositional site on the cratonic side of a Variscan foreland basin. The tectono-eustatic depositional model proposed in this case study is considered as an analogue for a number of hydrocarbon-bearing sequences.