Studies in fluviatile sedimentation: Implications of pedogenic carbonate units. Lower Old Red Sandstone, Anglo‐Welsh outcrop

Carbonate units occur in varying numbers in the fine‐grade members of the alluvial fining‐upwards cyclothems present in the Lower Old Red Sandstone of the Anglo‐Welsh outcrop. They closely resemble contemporary soil‐carbonates (calcretes) and hence show that the local Siluro‐Devonian climate was relatively hot with a comparatively low seasonal rainfall. Compared with contemporary calcretes, the units suggest that sites on the alluvial plains were denied river‐borne sediments for periods each in the general order of 10 4 years. The pedogenic interpretation of the carbonate units, developed in the light of the behaviour of rivers today, leads to alternative models for the geomorphology of the Siluro‐Devonian alluvial plains and for their gross subsurface structure. Geomorphologically, the plains at any instant presented depositionally active and inactive areas in juxtaposition. Under some circumstances the relief was provided only by alluvial ridges. Under others, relief was afforded by valley sides and perhaps river terraces, in addition to ridges. Knowledge of the character of the Lower Old Red Sandstone in vertical sequence leaves as plausible three alternative gross structures for the alluvial pile beneath the plains. They differ chiefly in the lateral variability and connectedness of the palaeosols and place hitherto unavailable limits on the three‐dimensional character of the cyclothems.