| Computer Code: |
GWYN |
Preferred Map Code: |
notEntered |
| Status Code: |
Pending Upgrade |
| Age range: |
Arenig Series (OR)
— Caradoc Series (O4) |
| Lithological Description: |
The Gwynedd Supergroup was introduced by Woodcock (1990), whose conception was of an Ordovician megasequence bounded by basin-wide unconformities, one approximating to the base of the Arenig Series, the other to the base of the Ashgill. The Arenig to Caradoc sequence which unconformably overlies the Tremadoc Series is mainly composed of silty mudstones and siltstones, with local, impersistent sandstone intercalations, and throughout the area it is overlain by black graptolitic mudstones of the Nod Glas Formation and its equivalents. Within this sedimentary sequence several major volcanic divisions are emplaced. In approximate upward succession they are the Rhobell, Aran, Llewelyn, Snowdon and Llanbedrog Volcanic groups. The local intercalation of these groups, together with their wedging-out laterally, cause many problems in correlation, in the hierarchical status of individual formations, their terminology and synonymy. |
| Definition of Lower Boundary: |
Unconformably overlies Tremadoc rocks. |
| Definition of Upper Boundary: |
none recorded or not applicable
|
| Thickness: |
none recorded or not applicable
|
| Geographical Limits: |
none recorded or not applicable
|
| Parent Unit: |
Not Applicable (-)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| none recorded or not applicable |
| Reference(s): |
| Woodcock, N H. 1990. Sequence stratigraphy of the Palaeozoic Welsh Basin. Journal of the Geological Society of London, Vol. 147, 537-547. |
| Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula. British Geological Survey Research Report, RR/99/08. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
