Flood deposits in the Hornelen Basin, west Norway (Old Red Sands tone)

Breccias, conglomerates, and sandstones of the Hornelen Basin were deposited as alluvial fan and slope sediments in a tectonic valley which dipped westwards from highlands in the east and southeast. The rocks display various scales of rhythmic sedimentation. Minor rhythms (up to a few metres thick) are characterized by one or more of these lithofacies in upward succession: basal pebbles (A), massive sandstone (B), parallel laminated sandstone (C), cross-bedded sandstone (D) and cross-Iaminated sandstone (E). Each such sedimentary unit is bounded by a thin siltstone interval or by an erosional surface where the top is truncated. These minor rhythms can be related to flood deposition in uniform or decreasing flow-regimes. Megarhythms (usually from a few metres up to hundreds of metres thick) often coarsen upwards. They can be related to progradation of fans and valley floor alluvial wedges in response to levee breaching following pulses along the original fault-bounded escarpments. Very fine sandstones which occur in the finer part of the sandstone megarhythms and el ose to the margins of the basin are probably related to overbank flooding or flooding on distal parts of the alluvial slopes rather than to deposition in permanent lakes.