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Upward coarsening sedimentation units and facies lineages, Old Red Sandstone, Scotland

ABSTRACT Proximal braided stream deposits of the Old Red Sandstone of central Scotland are characterised by a variety of upward coarsening units formed during downstream migration gravel bars. Transitional variations in thickness, grain size and structure of these units, and their associated sediments are used to establish facies and paths of facies change (lineages). Three facies lineages are recognised which are interpreted as recording the downstream changes the structure of sediments of three different types of braided river systems. Lineages differ from each other in proportion of sandstone, channel depth and the structure of upward coarsening units: lineage 1 represents alluvium formed in a river system which produced texturally mature gravels and which increased in depth downstream; lineage 2 comprises sediments deposited in shallow channels which become shallower downstream; lineage 3 represents a downstream change from conglomerate to sandstone and from an unchannelled to achannelled regime. Stream depth and ratio of sand to gravel exert fundamental controls on the structure of alluvium. Thick (thickness >50 m) sections showing vertical changes in clast size often comprise single facies lineage. Thick upward coarsening sequences are found with lineages 2 or 3 and upward fining with lineage 1 or 3.