Global Worry at Local Quarry

Ercall Quarries north-east of the Wrekin and south of Telford is an SSSI (Ercall Local Nature Reserve, Shropshire Wildlife Trust). One of the Saturday field trips at the OUGS Symposium 2000 (at Telford) went to this series of quarries. The advice that wet weather gear, walking boots and hard hats were essential was indeed well put. Please let me use the poor weather as a poor excuse for the poor quality of my photos.

Quarry 1

OUGS Symposium delegates study exposures of rhyolite lavas. These Uriconian Volcanics are of Precambrian age (ca. 566 Ma). Rhyolites are felsic volcanic rocks with a composition similar to granites. The presence of rhyolite suggest that this sequence was formed near a subduction zone, probably in an island arc.

In Quarry 2 you see granophyre intruded into the rhyolites. Granophyre is a relatively fine-grained granitic rock, relatively rich in alkali feldspar, which gives it a pinkisk colour. Granophyre has the same composition as dacite, i.e. darker than granite/rhyolite. (Dacite is also found in Uriconian Volcanics, this was demonstrated on the OUGS Symposium 2000 Field Trip on Monday). The granophyre has been dated at ca. 560 Ma.

	E = South Britain = East Avalonia = West Avalonia = Gondwana
Position of Cadomian arc and Southern Britain at the end of Precambrian?

At the end of the Precambrian, begin of the Cambrian (544 Ma) subduction seems to have stopped (no volcanic or metamorphic rocks of Cambrian age are found in Southern Britain), and erosion took over.

Unconformably on the igneous rocks lies the Wrekin Quartzite (lower Cambrian). In fact it is not quite metamorphosed, but rather a quartz-rich sandstone or more correctly a quartz arenite. At the base it is a conglomerate with fragments of the Uriconian Volcanics. The black line on the picture marks the unconformity with the pink granophyre to the left and the Wrekin Quartzite to the right.
Quarry 2: Granodiorite/Wrekin Quartzite
Quarry 2

The quartzite shows some wonderful ripple marks, so at this time a shallow sea was flooding over the area.

Wrekin Quartzite/Lower Comley Sandstones
On top of the Wrekin Quartzite (down left) lies the Lower Comley Sandstones (top right), a greenish micaceous sandstone. Its green colour is caused by the mineral glauconite, the formation of which is entirely restricted to marine environments. The sea has taken over. We seem to have a transgression (or subsidence of the earlier island arc).

There seems to have been a world wide transgression in the Cambrian, maybe with a sea-level rise as much as 200-300 m. E.g. the Baltoscandian craton (Baltica) was transgressed in the early Early Cambrian, and here the sea-level rose all through the Cambrian to reach a maximum in the Early Tremadoc. It has been suggested that the breaking up of the continents in this period was the major cause.

By Ole Nielsen

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