New Red Sandstone Floodplain and Piedmont Sedimentation in the Hebridean Province, Scotland

Abstract

ABSTRACT A sequence of coarse clastic sediments of New Red Sandstone age in western Scotland was probably deposited by alluvial fan (mudflow, streamflood and braided stream), playa and floodplain (low and high-sinuosity channels) processes. Within the alluvial fan group the mudflow deposits are usually laterally persistent, flatbedded paraconglomerates. Individual mudflow units only rarely show basal erosion surfaces. The streamflood deposits consist of laterally impersistent orthoconglomerates and coarse sandstones which often fill channels as large or very large-scale cross-stratified sets. The braided stream deposits are usually sandstones and fine-grained, well-sorted orthoconglomerates with an abundance of trough cross stratification and intraformational pebbles. The braided stream conglomerates, unlike the mudflow and streamflood conglomerates, show no significant relationship between maximum particle size and bed thickness. The playa deposits show alternations of thin, mudcracked mudstones and laminated or rippled sandstones. The deposits of ephemeral, low-sinuosity floodplain rivers are identified as relatively thin, coarse-grained fining-upwards cyclothems in which both fine and coarse members are laterally extensive and there is little lateral variation in structure, thickness or lithology. There is usually a caliche profile capping the fine member. In contrast, thicker, finer-grained, structurally and lithologically more complex cyclothems probably accumulated from high-sinuosity floodplain systems. Alluvial fan (piedmont) sequences show a textural and structural evolution from mudflow up through streamflood to braided stream deposits. The time trend through the laterally equivalent floodplain sequences is from low to high-sinuosity and less ephemeral stream systems. The piedmont trend probably reflects a geomorphic maturing of the basin margins while the floodplain trend suggests an additional climatic change towards less arid conditions. Either trend is consistent with conformably overlying marine (Jurassic) deposits.