Clastic grain shape and orientation in relation to cleavage in the Aberystwyth Grits, Wales

Lisle, Richard John 1977. Clastic grain shape and orientation in relation to cleavage in the Aberystwyth Grits, Wales. Tectonophysics 39 (1-3) , pp. 381-395. 10.1016/0040-1951(77)90105-6

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Abstract

Samples from a competent greywacke bed within the Aberystwyth Grits at Cwm Tydi, Cardiganshire, reveal a preferred dimensional orientation of clastic grains parallel or subparallel to the cleavage. Plots of grain axial ratio against orientation () indicate that grain-shape changes have occurred as well as alignment of the long axes. These plots are compared with those predicted by a mechanism involving homogeneous strain superimposed on initially randomly-oriented elliptical markers. New reference curves facilitate a straightforward comparison of actual plots with the theoretically-predicted ones. The diagrams show consistently a broader scatter of Ø values than the homogeneous model predicts suggesting that other mechanisms have been operative during the deformation. Textural evidence under the microscope strongly supports a mechanism involving solution of grain contacts lying along distinct “channel-ways” parallel to the cleavage. A simple pressure solution model involving reduction of grain size is considered. The model produces a low degree of preferred orientation but substantial changes of axial ratio thus giving similar, though not identical, diagrams to those obtained from the rocks. The cleavage probably owes its existence less to the preferred orientation of the grains than to the preferred location of the grain contacts along the “channel-ways”.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Earth and Environmental Sciences
Subjects:	Q Science > QE Geology
Publisher:	Elsevier
ISSN:	0040-1951
Last Modified:	04 Jun 2017 02:11
URI:	https://orca.cardiff.ac.uk/id/eprint/10588

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