Mud‐Dominated Storm Deposits From A Lower Carboniferous Ramp

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Abstract
The Lower Carboniferous Pen‐y‐Holt Limestone of South Wales comprises about 300m of interbedded wackestones and lime mudstones.The wackestones are interpreted as relatively distal ‘turbidite‐like’ storm‐generated deposits and the lime mudstones as background deposits. The storms had a periodicity of about one per 9000–18000 years. They were deposited in a deep‐ramp carbonate environment at least 20–30km from the ancient shoreline and in about 100m water depth, and therefore probably below wave base. The ramp is estimated to have had an average slope angle of 0·5–1·0 degree.Unlike other previously described carbonate or siliciclastic storm deposits, the Pen‐y‐Holt Limestone storm deposits are totaly mud‐supported and generally lack internal sedimentary structures, yet contain large bioclasts such as crinoid ossicles. The simultaneous deposition of lime mud and crinoid ossicles from a storm‐generated turbidity current is hydrodynamically untenable. Thin‐section evidence however, suggests that the lime mud may have originally been deposited as peloids which have since been largely destroyed during diagenesis. Peloids and crinoid ossicles, it is suggested, could have been transported by the same current.