Longmyndian Supergroup: Difference between revisions
No edit summary |
No edit summary |
||
| Line 1: | Line 1: | ||
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]].<ref name="multiple">P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed</ref> The supergroup consists of two major geological groups, the [[Stretton Group]] and the overlying [[Wentnor Group]].<ref name="multiple"/> The rocks are a generally regressive sequence from basinal facies to clastic [[sedimentation]].<ref name="multiple4">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</ref> 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.<ref name="multiple"/> |
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]].<ref name="multiple">P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed</ref> The supergroup consists of two major geological groups, the [[Stretton Group]] and the overlying [[Wentnor Group]].<ref name="multiple"/> The rocks are a generally regressive sequence from basinal facies to clastic [[sedimentation]].<ref name="multiple4">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</ref> 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.<ref name="multiple"/> |
||
| ⚫ | The Longmyndian rocks were deposited in NE-SW trending faulted rift basins. <ref name="multiple"/> These were deposited on top of the [[Uriconian]] volcaniclastic deposits. <ref name="multiple"/> 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’’’. |
||
| ⚫ | The Longmyndian rocks were deposited in NE-SW trending faulted rift basins. <ref name="multiple"/> These were deposited on top of the Uriconian volcaniclastic deposits. <ref name="multiple"/> 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 |
||
==Wentnor Group== |
==Wentnor Group== |
||
The [[Wentnor Group]] is made up of the [[fluvial]] deposits of the |
The [[Wentnor Group]] is made up of the [[fluvial]] deposits of the ‘’’Bridges Formation’’’ and is the topmost of the supergroup.<ref name="multiple4"/> This is underlain by the braided deposits of the ‘’’Bayston-Oakwood Formation’’’ which completes the [[Wentnor Group]].<ref name="multiple4"/> |
||
==Stretton Group== |
==Stretton Group== |
||
The topmost of the [[Stretton Group]], the |
The topmost of the [[Stretton Group]], the ‘’’Portway Formation’’’, consists of braided [[fluvial]] deposits with marine incursions.<ref name="multiple4"/> The underlying ‘’’Lightspout Formation’’’ is interpreted as a fluvio-deltaic environment, this is succeeded by a similar facies in the ‘’’Synalds Formation’’’.<ref name="multiple4"/> 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.<ref name="multiple4"/> |
||
==Dating== |
==Dating== |
||
Within the strata are [[bentonite]] and [[lapilli]] [[tuff]] horizons that allow for [[Uranium-lead dating|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.<ref name="multiple2">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</ref> The |
Within the strata are [[bentonite]] and [[lapilli]] [[tuff]] horizons that allow for [[Uranium-lead dating|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.<ref name="multiple2">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</ref> The ‘’’Lightspout Formation’’’ toward the top of the [[Stretton Group]] returned a date of 555.9+/-3.5Ma.<ref name="multiple2"/> |
||
== References == |
== References == |
||
Revision as of 10:10, 16 November 2012
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