Deposition of some Upper Old Red Sandstone conglomerates in the Clyde area: A study in the significance of bedding

Abstract

Synopsis On the basis of structure and texture, the Upper Old Red Sandstone conglomerates and conglomeratic sandstones of the Clyde area of Western Scotland are divided into four types which represent the accumulations of alluvial fans (mudflow and stream), braided streams, and floodplains. The deposits of alluvial fans show a relationship between bed thickness and maximum particle size, but this relationship is not clear in the case of floodplain deposits. The relationship of the bed thickness to maximum particle size and the nature of the frequency distribution of both maximum particle size and bed thickness are shown to be of considerable importance in determining conglomerate genesis. Stratigraphical sections show an upward reduction in grain size together with a change from fan mudflow to fan stream to braided stream to floodplain deposition. These features suggest the diminishing effects of the source areas through time at any one locality; probably due to erosion and planation of newly emergent fault-bounded mountains. The transition from essentially untrenched mudflow deposits to channel-filling stream deposits and possibly braided stream deposits is regarded as the inevitable change an alluvial fan suffered in the tectonic setting of the Clyde area during Upper Old Red Sandstone times.