Mineralogy and microstructure of pressure-solution cleavage in folded mudstones and siltsones

Abstract

There is abundant microscopic evidence for the existence of pressure solution cleavage in mudstones and siltstones folded during the Acadian orogeny in New Brunswick and elsewhere in the Appalachians. It can occur as s1or s2 cleavage and is recognized by cleavage planes containing a high proportion of insoluble minerals. Various structures and minerals show evidence of shortening and solution. High local compressive stress concomitant with concentration and convergence of cleavage films are directly related to soft sediment structure and symmetrical and asymmetrical microfolds. Reverse microfault failure in silty beds results in polyphase pressure solution producing later cleavage films slightly oblique to and superimposed upon the pre-existing norma lly spaced cleavage films in adjacent mudstone. The degree of solution of the microlithons and silty laminae is controlled by local compressive stress and the mineralogy of the rock and therefore varies within the same rock. Not all of the material that goes into solution is expelled from the rock; a small portion is redeposited elsewhere in the form of pressure shadows and veins.