COMPACTION FABRICS OF PELITES - EXPERIMENTAL CONSOLIDATION OF KAOLINITE AND IMPLICATIONS FOR ANALYSIS OF STRAIN IN SLATE

Compaction of clay and shale results in large reductions in volume as pore water is expelled. Preferred orientation of the platy minerals in creases with compaction strain and loss of porosity according to the M arch-Owens model. This relationship has been studied quantitatively by experimentally consolidating kaolinite clay from slurries and analyzi ng the resulting fabrics with the X-ray pole figure goniometer and sca nning electron microscope (SEM). 'Initial' porosity corresponds to the onset of the strain recorded by the preferred orientation; and the va lues of 0.78 for dispersed slurries and 0.76 for flocculated slurries reflect the clectrostatic 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 developme nt of slaty cleavage. Matrix methods were used to model deformation pa ths for slates in the Welsh slate belt. PTeferred orientation of mica and ellipsoidal shapes of 'reduction' spots were simulated for one loc ality by loss of a 0.60 'initial' porosity, a 6-degrees tilt of the be ds and horizontal shortening involving plane strain. Strain determinat ions for shales and slates should include the large reduction in volum e.