Devonian Old Red Sandstone sedimentation and tectonic history of Billefjorden fault zone, Spitsbergen

Abstract

Devonian Old Red Sandstone exposed within and west of the Billefjorden fault zone in Dicksonland, central Spitsbergen, consists of up to 3000 m of conglomerate, sandstone, and mudstone that rapidly thin eastward. The Devonian sediments form a single, large-scale fining upward sequence of fluvial, paludal, and marginal marine origin. Low-sinuosity channel processes dominated the lower portion of the sequence; the middle portion was transitional, probably deposited in meandering channels, and by proximal overbank processes on levees and floodplains. The upper portion of the sequence consists of floodbasin and marginal marine sediments. Nodular calcrete zones suggest prolonged weathering and root mottles indicate plant growth. Trace fossils suggest periods of marine and brackish water conditions within the fluvial-appearing sequence. Current directions and facies distributions indicate gradual wearing down of a source area to the south. The sedimentation pattern within the Old Red Sandstone adjacent to the Billefjorden fault zone contrasts with that of known strike-slip fault-bounded basins: there is no repetitive stacking of facies, and there are no thick sequences of conglomerates or breccias; none of the units show coarsening near the fault. Units thin to the east and lack evidence of faulting contemporaneous with deposition. The eastern distal edge of the Devonian basin may coincide with the gently sloping edge of a half-graben. The early to middle Devonian passive basin margin, late Devonian reverse slip on the Billefjorden fault zone, and normal faults controlling the western margin of the Carboniferous depositional basin may coincide because all were controlled by a Caledonide zone of weakness along a pre-Old Red Sandstone fault in Hecla Hoek rocks.