ABSTRACT

The fining-upwards cyclothems repeated through thick stratigraphic sections in the Old Red Sandstone (Devonian) of Britain and the Appalachian region of North America comprise a maximum of six major facies. The cyclothems when complete consist each of a lower coarse member and an upper fine member, between which the facies are distributed. An analysis of facies transitions is made to arrive at an understanding of the composition and variability of these cyclical deposits. The coarse members, which as regards the variety of facies and numbers of facies representatives are more variable than the fine members of the cyclothems, are shown to have been formed through processes of lateral deposition in streams with sinuous talwegs. A quantitative physical model, partly based on general hydr ulic principles and partly empirical, is advanced to show how the different facies sequences observed in the coarse members can be understood in terms of the chief variables involved in river flow. Specifically, the model reproduces the vertical patterns of grain size and sedimentary structure observed in the coarse members. The model also shows how the coarse members may be interpreted in terms of stream power and channel sinuosity.