Gullet Main Quarry

Exposed Units: Malverns Complex

Conservation Status: Local Geological Site

Much of the Precambrian sequence is exposed in the high vertical faces of the north side of Gullet Quarry. The Malverns complex is very varied at this locality with a variety of rock types exposed. The Malverns Complex country rock is dominated by highly deformed diorite. The diorite itself has been intruded by several generations of intrusive igneous rocks, including microdiorite dykes, quartz-feldspar pegmatite and pink granite veins.  Gneissic textures are particularly well developed at the western end of the site, where small scale folding of the foliation is visible. Mica schists are also present and have an inferred metasedimentary origin. The Malvernian rocks are also intruded by mineralised veins containing the minerals calcite, epidote and haematite, which occurs on most joint and fracture surfaces throughout the quarry.

The Malverns Complex at Gullet Main Quarry is intensely deformed. Almost every boundary between different rock types has been sheared. A prominent NE-trending foliation dips steeply to the south-east, and is gneissose in places. This gneissose fabric is accentuated by intrusions of thin pink granite veins parallel to the fabric. These have been broken up in places, forming boudinage structures. The intense shearing affects all lithologies in the Malverns Complex, including the late microdiorite dykes and so occurred after the Precambrian. It is likely that some deformation is related to the Variscan orogeny; however it is also possible that the area has been affected by even younger deformation associated with the East Malvern Fault.

The contact between the Malverns Complex and the Wyche Formation is seen in the upper part of Gullet Quarry and this is described in more detail in the entry for Gullet Top Quarry.

Terminology

Gneissic fabric – A deformation fabric seen in metamorphic rocks that is characterised by prominent mineral banding

Schist – A deformation fabric in metamorphic rocks characterised by alignment of platy minerals, such as micas.

Metasedimentary – Term used to describe a sedimentary rock that has undergone metamorphism

Mylonite – An extremely fine-grained rock that has undergone intense deformation. These usually occur within fault zones where the deformation is most intense.

Boudinage – A structure that is formed when a rock is sheared so intensely that it is pulled apart. The resulting pieces resemble a string of sausages.

Photos

View of the main face at Gullet Main Quarry.

Near vertical, pale granitic pegmatites in the Malverns Complex

Malverns complex diorite showing a steeply-dipping gneissic fabric defined by thin, white bands of the minerals quartz and feldspar and pink veins of granite pegmatite.

References

Barclay, W.J., Ambrose, K., Chadwick, R.A., and Pharaoh, T.C., 1997, ‘Geology of the country around Worcester’, Memoirs of the British Geological Survey, London.

Bullard, D.W., (1989) Malvern Hills – ‘A student’s guide to the geology of the Malverns’ Nature Conservancy Council.

Carney, J.N., Horak, J.M., Pharaoh, T.C., Gibbons, W., Wilson, D., Barclay,W.J., Bevins, R.E., Cope, J.C.W. & Ford, T.D. (2000), ‘Precambrian Rocks of England and Wales’, Joint Nature Conservation Committee, Peterborough.

March 2011