Bartestree Quarry

Exposed Units: Bartestree Dolerite, Raglan Mudstone

The Bartestree Dolerite is an intrusion of dolerite into the Silurian Raglan Mudstone Formation along the line of the Neath Disturbance, measuring 13.5m wide where it crops out at the WSW end of the quarry. It is a composite intrusion, having been formed by the sequential injection of three phases of basic material:

First phase

Green dolerite without olivine is seen on the southern extent of the outcrop.

Second Phase

Composed of teschenite, a type of dolerite characterised by the mineral assemblage olivine-analcime (zeolite). This solidified before the intrusion of phase three.

Third phase

Basalt forms the central and major part of the dyke.

The dyke intrudes into the silty mudstones of the Raglan Mudstone Formation, which have undergone contact metamorphism close to the dyke margins, suggesting that the dolerite was intruded into cold country rocks.

Early workers suggested that the dyke may have been related to the latest stages of the Variscan Orogeny and suggested an age of ca. 295 million years as the age of emplacement and consolidation. The dyke has since been dated to between 260+/-20 million years and 186+/-18 million years and has been correlated with other Carboniferous-age dolerite in the Midlands.

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Terminology

Teschenite – A mafic (iron-magnesium rich) igneous rock that cooled underground.

Dyke – A body of igneous rock that has been intruded into the surrounding rocks and has a ‘sheeted’ geometry. This ‘sheet’, cuts across the sedimentary layering in the surrounding rocks.

Variscan Orogeny – A period of mountain building in central Europe, North America and southern England that took place between 380 and 280 million years ago.

References

Fitch, F.J. and Miller, J.A., 1964, ‘The age of the paroxysmal Variscan Orogeny in England’, in W.B. Harland, A. Gilbert Smith, B. Wilcock (eds.) The Phanerozoic Time-Scale: A Symposium, Geological Society Special Publications, London, pp. 159-175.

Fitch, F.J. and Williams, S.C., 1970, ‘Isotopic ages of Carboniferous rocks’, Conference Report of the 6th Congress on International Stratigraphic Geology, pp. 771-789.

Reynolds, S.H. 1908, ‘The basic intrusion of Bartestree, near Hereford’, Quarterly Journal of the Geological Society, vol. 64, 4, pp. 501-511.

March 2011