Compaction of clay and shale results in large reductions in volume as pore water is expelled. Preferred orientation of the platy minerals increases with compaction strain and loss of porosity according to the March-Owens model. This relationship has been studied quantitatively by experimentally consolidating kaolinite clay from slurries and analyzing the resulting fabrics with the X-ray pole figure goniometer and scanning electron microscope (SEM). 'Initial' porosity corresponds to the onset of the strain recorded by the preferred orientation; and the values of 0.78 for dispersed slurries and 0.76 for flocculated slurries reflect the electrostatic forces between the clay platelets. 'Initial' porosities of recently deposited fine silt and clay are in the range of 0.60-0.90 and are a function of grain size and mineralogy. Loss of this 'initial' porosity has a large effect on the subsequent development of slaty cleavage. Matrix methods were used to model deformation paths for slates in the Welsh slate belt. Preferred orientation of mica and ellipsoidal shapes of 'reduction' spots were simulated for one locality by loss of a 0.60 'initial' porosity, a 6° tilt of the beds and horizontal shortening involving plane strain. Strain determinations for shales and slates should include the large reduction in volume.
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