Coomb Volcanic Formation

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The Coomb Volcanic Formation is a geological strata of volcanic origin that was deposited during the late Neoproterozoic ( Ediacarian ) in southwestern Wales . It contains primitive fossils .

geology

Llangynog lies in the middle of the Coomb Volcanic Formation

The Coomb Volcanic Formation is located in the Llangynog Inlier , a mere 3 square kilometer uprising of the basement in Wales, where the southwest tip of the Wrekin Terran appears on the surface. The contacts to the overburden are either faults or discordances . For example, Lower Cambrian with the Foraminifera ( Psammosphaera and Saccamina ) Allt y Shed Sandstone follows discordantly above the Coomb Volcanic Formation. The Llangynog Inlier, like the Longmyndian Supergroup, is wedged between two east-northeast trending faults - the Pontesford-Linley Fault to the west and the Church Stretton Fault a little further to the east. The Pontesford-Linley Fault forms the border with the Cymru-Terran East Avalonias .

stratigraphy

The bimodal Coomb Volcanic Formation was deposited in the marine environment with a thickness of 1,100 meters. The formation can be divided into two members, the 350 to 400 meter thick Castell Cogan Rhyolite Member lying on the floor and the 700 meter thick Coed Cochion Volcaniclastic Member in the hanging wall .

Overall, the formation consists predominantly of rhyolite and andesitic lavas and tuffs deposited under the sea and intruded by dolerites . Layers of siltstone and sandstone intervene under the extrusive volcanic rocks .

The initial phase of volcanism was of an explosive nature, ash river banks formed. Under somewhat calmer conditions, rhyolite streams then poured out and rhyolite domes formed. Bimodal lavas, ash river banks, smaller intrusions and volcanic plastics then lay over them.

The rhyolites of the Castell Cogan Rhyolite Member are largely silicified and devitrified and show little of their original structure. In some outcrops, flow bands can be seen and pearlite and snowflake structures indicate their glass-shaped origin. The following Coed Cochion Volcaniclastic Member is much more varied and is dominated by volcaniclastic siltstones that were deposited in shallow water. Intermediate are basaltic-andesitic lavas, autobreccias , rare submarine hyaloclastites , rhyolitic pumice and cinder-rich ash river banks.

Emergence

It is assumed that the volcanic rocks originated on an ephemeral chain of islands, which was processed relatively quickly and then leveled. Geochemical studies show that both lavas and tuffs are connected to one another through fractional crystallization. The geochemical signatures indicate both plate interior and subduction and are very similar to the Uriconian Group volcanic rocks in Shropshire .

Age

The Coomb Volcanic Formation has not yet been dated radiometrically. However, their geochemical proximity to the Uriconian Group allows their ages to be narrowed down to 566 ± 2 million years to 560 ± 1 million years.

Fossils

The volcaniclastic siltstones contain a typical late neoproterozoic fauna consisting predominantly of disk-like forms such as Cyclomedusa and Medusinites . Flatly branching worm structures and feeding marks can also be found. The genus Palaeopascichnus delicatus , which belongs to the Ichnozone of Harlandiella podolica, is present as a trace fossil . The body and trace fossils can be found between individual tufa layers. The fossils are exposed in the Coed Cochion Quarry , a former quarry that has now been elevated to the Site of Special Scientific Interest (SSSI).

Individual evidence

  1. ↑ John CW Cope and Duncan McIlroy: On the occurrence of foraminiferans in the Lower Cambrian of the Llangynog Inlier, South Wales . In: Geological Magazine . tape 135 (2) , 1998, pp. 227-229 .
  2. ^ Cope, JCW and Bevins, RE: The stratigraphy and setting of the Precambrian rocks of the Llangynog Inlier, Dyfed, South Wales . In: Geological Magazine . tape 130 , 1993, pp. 101-111 , doi : 10.1017 / S0016756800023761 .
  3. ^ Tucker, RD and Pharaoh, TC: U-Pb zircon ages for Late Precambrian igneous rocks in southern Britain . In: Journal of the Geological Society . tape 148 , 1991, pp. 435-443 , doi : 10.1144 / gsjgs.148.3.0435 .
  4. ^ Cope, JCW: An Ediacaran-type fauna from South Wales . In: Nature . tape 268 , 1977, pp. 624 , doi : 10.1038 / 268624a0 .
  5. ^ Nigel Woodcock and Rob Strachan: Geological History of Britain and Ireland . Blackwell Science, Oxford 2000, ISBN 0-632-03656-7 .