Abstract

Braided fluvial deposits in the Pennsylvanian Morien Group contain an unusual association of erosional features at contacts between sandstone and mudstone units, which represent flat-lying to vertical channel margins. Mudstone intraclasts up to 2 m in diameter form lensoid lags at channel bases and are interpreted as bank collapse rubble. Flutes on vertical channel walls, channel floors and intraclasts indicate active flow, whereas rills on steep surfaces resulted from downslope drainage over the edges of banks or mudclasts. Delicate plumose patterns on vertical fracture surfaces, modified by flow, formed during cracking of the muds by desiccation and bank collapse. Scoured desiccation cracks are common on flat-lying surfaces, where flow-parallel linear scours 50 cm deep may have exploited orthogonal desiccation cracks. The channel-margin features described here require deposition and subsequent reworking of cohesive fines on high topographic levels of a fluvial system, due to alternating periods of flooding and subaerial exposure. Braided and ephemeral river systems with variable discharge are likely to provide the most suitable setting for these features.

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