Deformation by Overburden of a Coarse Quartzite Conglomerate

The conglomerate of the Pass Peak Formation, a coarse Eocene basin-edge deposit in western Wyoming, is crushed though not by tectonic forces. Each cobble is in point to point contact with up to 10 other cobbles within a poorly consolidated micaceous sand matrix. Crushing appears to be due to stress concentration at contact points. Three degrees of deformation have been defined: (1) faint contact marks on the cobble surfaces where the surface polish has been destroyed and a rim of better lithified matrix formed; randomly oriented and distributed fractures cutting a few grains within the cobble; (2) throughgoing fractures, often parallel to one another with intervening complex networks of minor fractures, not necessarily starting or ending at a contact mark; (3) reduction of grain size by crushing up to 60% around contact marks; contact marks are enlarged and bundles of fractures form a complex network connecting these contact marks. Simple calculations show that the 700 m of overburden over the outcrops studied were sufficient to cause failure. A probable scenario is initial contact at minute contact marks which subsequently crush and enlarge as the overburden increases; the contact marks enlarge until the load is supported. No pressure solution was observed in these rocks. However, the crushing process within the Pass Peak may be a common precursor to deformation at higher temperature and pressure. This crushing has the effect of increasing the permeability and decreasing the grain size, especially in the vicinity of contact marks which may aid less brittle processes.