Synsedimentary faulting in a Mesozoic deltaic sequence, Svalbard, Arctic Norway - Fault geometries, faulting mechanisms, and sealing properties

Distributary channel sandstones of the Lower Cretaceous Helvetiafjellet For mation and underlying prodelta shales and thin-bedded sandstones of the Upp er Jurassic Janusfjellet Formation exposed along the east coast of Spitsber gen are cut by syndepositional planar and listric faults forming collapse s cars with depths of 35-90 m and widths up to 1.5 km. The fault zones mostly have a dip of 40-60 degrees, a width of up to 2.3 m, and contain fault-par allel 3-5-m-long overlapping sandstone sheets with widths of 5-40 cm, and u p to 2-m-thick fault-parallel sandy mudstones, The intrafault sandstones sh ow fault-parallel banding resulting from differences in detrital clay conte nt and grain size. The banding has been enhanced by selective, late diagene tic quartz cementation of the clay-poor bands. Thin clay laminae, now devel oped into fault-parallel stylolites, occur along the margins of the intrafa ult sandstones. The clay laminae do not emerge from clay layers in the faul t blocks and are not clay smears. The laminae probably formed during faulti ng when fluidization within the fault zones allowed clay particles to move laterally and accumulate along the margins of the fault zones. There is no enhanced cementation or cataclastic deformation within the fault zones. The ability of the fault zones to act as capillary seals or barriers to fluid flow is therefore mostly determined by the clay laminae rimming the intrafa ult sandstones. Fractures filled by quartz and calcite cement containing oil inclusions tha t homogenize at 58-73 degrees C probably were not produced by the syndeposi tional faulting and may have formed during uplift of the area.