Abstract - During this study we compared the micro-scale properties and fault zone architecture of cm-scale soft sediment faults formed at low (<50 meters), intermediate (300-500 meters) and relatively high (~1400 m) maximum burial depths within interbedded sands, silts and clays from three localities in Denmark. We used Scanning Electron Microscope backscatter imaging and image analysis techniques to quantify the micro-scale properties such as porosity. At shallow and intermediate burial depths, sands, silts and clays are commonly dragged into faults creating composite, layered fault zones. Sands incorporated into fault zones deform by particulate flow causing elongate grains to be re-orientated parallel to the fault zone. Grain scale mixing occurs at the interface between clay smears and adjacent sand layers. At shallow burial depth some fault zones show smearing of sands, but truncation of clay layers suggests that clays can be more brittle than sands at shallow burial depths. At deeper burial depths (<1400 m) and larger offsets, grain crushing occurs along sand-sand contacts and between adjacent slip surfaces causing porosity reduction across slip surfaces. Thicker, more continuous clay smears are more common at deeper burial depths, though grain-scale mixing between clays and sands is still pronounced.
Abstract of talk given to:
Irish Geological Research Meeting, Trinity College Dublin, February 2005.