Distributary channel sandstones of the Lower Cretaceous Helvetiafjellet Formation and underlying prodelta shales and thin-bedded sandstones of the Upper Jurassic Janusfjellet Formation exposed along the east coast of Spitsbergen are cut by syndepositional planar and listric faults forming collapse scars with depths of 35-90 m and widths up to 1.5 km. The fault zones mostly have a dip of 40-60°, a width of up to 2.3 m, and contain fault-parallel 3-5-m-long overlapping sandstone sheets with widths of 5-40 cm, and up to 2-m-thick fault-parallel sandy mudstones. The intrafault sandstones show fault-parallel banding resulting from differences in detrital clay content and grain size. The banding has been enhanced by selective, late diagenetic quartz cementation of the clay-poor bands. Thin clay laminae, now developed into fault-parallel stylolites, occur along the margins of the intrafault sandstones. The clay laminae do not emerge from clay layers in the fault blocks and are not clay smears. The laminae probably formed during faulting 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 intrafault sandstones.

Fractures filled by quartz and calcite cement containing oil inclusions that homogenize at 58-73°C probably were not produced by the syndepositional faulting and may have formed during uplift of the area.

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