Longmyndian Supergroup
The Longmyndian Supergroup is a c.6000m thick sequence of Late Precambrian rocks that outcrop between the Pontesford-Linley Fault System and the Church Stretton Fault System in the Welsh Borderland Fault System.[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 are thought to be derived from Uriconian mountains that were formed during the southward subduction of an oceanic plate beneath a continental block (ocean closure). The rocks have since been folded due to fault movements and plunge gently to the south.[1] The Longmyndian rocks were deposited in NE-SW trending faulted rift basins. [1] These were deposited on top of the Uriconian volcaniclastic deposits. [1] Subsequent ocean closure squeezed these rocks together to provide sub-vertical bedding in the synclinal sequence we see in exposed rocks today. As noted below, with the relevant references, the deposited rocks of the Longmyndian show a variety of depositional facies that tell a story of ocean closure. The Longmyndian deposits rest upon Uriconian rocks but different sections are exposed exclusively (as known to date) between the Church Stretton Fault and the Pontesford-Linley Lineament .
Wentnor Group
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]
Stretton Group
The topmost of the Stretton Group, the Portway Formation , consists of braided fluvial deposits with marine incursions.[2] The underlying Lightspout 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]
Dating
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]
References
- ^ a b c d e P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed
- ^ a b c d e f Pauley, J.C., A revision of the stratigraphy of the Longmyndian Supergroup, Welsh Borderland, and its relationship to the Uriconian Volcanic Complex,1991 Geological Journal; v.26; pp167-183
- ^ a b W. Compston, A. E. Wright, P. Toghill, Dating the Late Precambrian volcanicity of England and Wales, Journal of the Geological Society 2002; v.159; pp 323-339