North Campsie Pyroclastic Member

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North Campsie Pyroclastic Member (NCPY), Campsie Block, Carboniferous, Midland Valley of Scotland[edit]

The North Campsie Pyroclastic Member is part of the Clyde Plateau Volcanic Formation.

Name[edit]

Previously referred to informally as the North Campsie tephra deposits (BGS, 1993)[1] and associated with the North Campsie Linear Vent System.

Lithology[edit]

The member consists of pyroclastic material deposited largely in tephra cones, as ashfall tuffs and lapilli tuffs and reworked volcaniclastic sedimentary rocks derived from the erosion of coallescing tephra cones. Typically, finely stratified tuffs pass upwards into coarse, unstratified spatter deposits and agglomerates. The lower agglomerates are composed largely of accidental and accessory material, including large blocks (up to 2 m) of basalt, variously weathered and textured smaller lava fragments and rare sandstone boulders. Higher agglomerates tend to be composed of essential tephra and scoria bombs in an upward progression that is typical of tephra cones. Impersistent tongues and lenses of highly vesicular basaltic lava occur locally. Since fragmental rocks that form subaerial tephra cones are indistinguishable from those occupying necks, it is possible that some of the tephra is actually neck agglomerate (Craig, 1980)[2].

Stratotype[edit]

Corrie of Balglass on the north side of the Campsie Fells (NS 586 849 to NS 595 854). 2.5 km of cliffs, reaching a height of up to 200 m reveal an upward succession from tuffs and lapilli-tuffs with accidental and accessory clasts into agglomerates with an increasing proportion of essential scoria. Large blocks (over 1 m) of dolerite are derived from the explosive brecciation of early-formed intrusions within the conduit system. Impersistent tongues of scoriaceous lava increase in abundance upwards until the succession is composed largely of microporphyritic lavas but still with significant beds of pyroclastic rock (Craig, 1980)[2].

Lower and upper boundaries[edit]

Rests unconformably on Inverclyde Group strata of the Clyde Sandstone Formation and overlain conformably or locally disconformably by the Campsie Lava Member of the Clyde Plateau Volcanic Formation.

Thickness[edit]

Unspecified and probably highly variable.

Distribution and regional correlation[edit]

Continuous along lower parts of the northern escarpment of the Campsie Fells, from Canny Tops (NS 551 833) in the west to Gonachan Cottage (NS 629 860), on the south side of the Endrick valley, in the east. Best exposures are in the type area around the Corrie of Balglass (NS 586 849 to NS 595 854) and in precipitous cliffs on either side of the Black Spout (NS 610 863) and extending north-west towards Dunmore (NS 606 865).

Age[edit]

Mid Visean (Arundian to Asbian).

References[edit]

  1. British Geological Survey. 1993. Glasgow. Scotland Sheet 30E. Solid Geology. 1:50.000. (Keyworth, Nottingham: British Geological Survey.)
  2. 2.0 2.1 Craig, P M. 1980. The volcanic geology of the Campsie Fells area, Stirlingshire. Unpublished PhD Thesis, University of Lancaster.