The microstructure of UK mudrocks

The microstructures of Jurassic and Cretaceous mudrocks reflect their environment of deposition and the processes which occurred during their burial. One measurement of mudrock microstructure is particle long axis orientation. Preferred particle orientations are measured by analysis of SEM images of broken mudrock surfaces. Strong preferred particle orientations are observed in unweathered material with high quantities of illite. They are observed across a range of burial depths within Cretaceous and Jurassic materials. No preferred orientation is observed where a material is weathered, or where large quantities of silt are present. Moderate preferred orientations are observed where materials are carbonate rich such as with the Blue Lias, or are shallowly buried such as with the London Clay. Strong preferred particle orientations formed through particle rearrangement are created at greater burial depths than the London Clay samples (200m) and at shallower depths than the Gault Clay samples (500m). Microstiuctures form in response to each event in a mudrocks history. The final structure of a mudrock is dependent on all of the events in its history. The relative effect of each event is dependent on its intensity of its application to the mudrock. Each event has the possibility of destroying structures formed by previous events. For example burial microstructures often replace depositional structures. Particles will align by rearrangement as much as possible during initial consolidation. Further alignment can occur by the process of pressure solution at a much greater depth. Fluids rich in chemical ions may deposit cements in the pores of mudrocks as they flow through them. The growth of these cements can make further alignment of clay mineral particles impossible