Abstract - Few studies have investigated the geometry and origin of sand smear formation along faults despite their direct implications for leakage of fluids along fault planes. By contrast, clay smears have been the subject of much research in the past decade. This study presents examples of different types of sand smears along shallow burial faults in unconsolidated, interbedded sands, silts and clays from three different localities along Danish coastal cliff sections. The study highlights the critical role of burial depth and host rock rheological properties, in particular sands and clays, on the geometry and persistence of related smears.
(i) Extensive sand smears occur along faults with mm to tens of cm offsets in 13.000 yr old late glacial deposits along the west coast of northern Jutland at Nr. Lyngby. Maximum burial depth of these sediments is less than 50 meters. The sand smears have formed by particulate flow of grains and generate zones of high permeability along fault planes. Micro-scale properties include evidence of grain reorientation, without grain crushing. By contrast, clay layers tend to behave in a brittle manner, reflecting the competency inversion between sands and clays for shallow burial faults.
(ii) Miocene tidal deposits at Jensgaard on the east coast of southern Jutland show similar fault zone properties including continuous sand smears and competency inversion between sands and clays along faults with up to 10 cm displacements. Again, some grain reorientation has occurred but there is no evidence for grain breaking or cataclasis. The generation of sand smears contrasts with the sometimes brittle nature of clay rocks, though clay smears are also a feature of these faults.
(iii) By contrast, faults formed in Jurassic tidal deposits at Galgeløkke on the south coast of the Island of Bornholm, with an estimated maximum burial depth of c. 300 meters, show porosity reduction and cataclasis in sand smears along shear bands despite the unconsolidated nature of these sediments. Grain reorientation and deformation induced mixing of clays with sand grains are additional deformation characteristics. The distinguishing feature of the Bornholm faults is the ubiquitous development of clay smears.
This study has shown that burial depth and host rock rheological properties are the controlling factors in the formation of sand and clay smears. Very shallow burial depths at the time of faulting, provides continuous, non-cataclastic, sand smears accompanied by brittle deformation of clays with poor clay smear development. For deeper burial depths, cataclastic shear bands within sandstones develop in association with clay smears.
Abstract of poster presented at:
Tectonic Studies Group Annual Meeting, Durham, January 2004