The Longmyndian Supergroup is a c.6000m thick sequence of rocks that outcrop between the Pontesford-Linley Fault System and the Church Stretton Fault System in the Welsh Borderland.[1] The supergroup consists of two major geological groups, the Stretton Group and the overlying Wentnor Group.[1] The rocks are a generally regressive sequence from basinal facies to clastic sedimentation.[2] The rocks have since been folded due to fault movements and plunge gently to the south.[1]
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The Wentnor Group is made up of the fluvial deposits of the Bridges Formation and is the topmost of the supergroup.[2] This is underlain by the braided deposits of the Bayston-Oakwood Formation which completes the Wentnor Group.[2]
The topmost of the Stretton Group, the Portway Formation, consists of braided fluvial deposits with marine incursions.[2] The underlying Lighspout Formation is interpreted as a fluvio-deltaic environment, this is succeeded by a similar facies in the Synalds Formation.[2] Turbidite facies are prevalent in the Burway Formation and the deep marine basinal environments continue toward the base of the supergroup within the Stretton Shale Formation. The Ragleth Tuff Formation completes the Stretton Group and the supergroup sequence.[2]
Within the strata are bentonite and lapilli tuff horizons that allow for zircon U-Pb dating and noted within the Batch Volcanics of the Synalds Formation, toward the base of the Stretton Group, a date is returned of 566+/-2.9Ma.[3] The Lightspout Formation toward the top of the Stretton Group returned a date of 555.9+/-3.5Ma.[3]