| Computer Code: |
JAPT |
Preferred Map Code: |
JPF |
| Status Code: |
Full |
| Age range: |
Frasnian Age (DR)
— Frasnian Age (DR) |
| Lithological Description: |
Dominated by pale to mid-grey greenish mudrocks which are characteristically featureless apart from restricted zones of bioturbation. Locally the slates are finely laminated and higher in the succession interbanded purple slates occur. Fine- to-medium grained sandstones with either sheet or lenticular geometries are irregularly interbeded with the slates. Thin sandstone sheets are common; they have sharp planar or gently undulatory bases and graded or laminated tops. Intraformational matrix-supported conglomerates - in beds up to 2 m thick - are irregularly developed in the formation. At Jacket's Point [SX 0368 8338] and elsewhere, fine-to-medium grained matrix-rich lithic and crystal tuff with local agglomerate and grey and brown laminated slaty mudstone occur in irregularly-bedded units up to 10 m thick. |
| Definition of Lower Boundary: |
The Jacket's Point Formation occurs only at the base of the Bounds Cliff Succession, a sequence that crops out between Port Issac and Westdowns. The base of the Succession is a thrust fault which places the Tredorn Slate Formation on top of it [SX 0351 8341], so that no stratigraphic relationships at the base of the Jacket's Point Formation are known. The thrusted contact between the Jacket's Point Formation and the Tredorn Slate would be marked in the field by an abrupt transition from grey-green mudrocks with fine to medium grained sandstones (Jacket's Point Formation) to greenish grey quartz-chlorite-mica slate with lenses of bioclastic limestone and dolomite beds (Tredorn Slate Formation). |
| Definition of Upper Boundary: |
Conformable with overlying Harbour Cove Slate, but complicated by faulting at the southern end of the section (around [SX 0178 8128]). The transition from Jacket's Point Formation stratigraphically upwards in to the overlying Harbour Cove Slate Formation is marked by an increase of purple slate beds upwards. |
| Thickness: |
100 m (but incomplete sequence). |
| Geographical Limits: |
North Cornwall; along coastal section north of Bounds Cliff [SX 0178 8128]; inland approximately 3.5 km, to the village of Benbole [SX 0610 8170]. |
| Parent Unit: |
Not Applicable (-)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Along coast for approximately 3 km north of Bounds Cliff. Selwood and Thomas, 1986. |
| Reference(s): |
| Selwood, E B, Thomas, J M, Williams, B J, Clayton, R, Durning, B, Smith, O and Warr, L N, 1998. Geology of the country around Trevose Head and Camelford. Memoir of the British Geological Survey, sheets 335 and 336 (England and Wales). |
| Selwood, E B and Thomas, J M, 1986. Upper Palaezoic successions and nappe structures in north Cornwall. Journal of the Geological Society of London, Vol.143, 75-82. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
