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

Abstract

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 soilcarbonates (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' 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 threedimensional character ,OF the cyclothems.

Sequence Stratigraphy: Methodology and Nomenclature

“…The basic relationship between base-level change and the cycle of aggradation and sedimentation in fluvial systems was first explored by Allen (1974). Modern sequence models have been proposed by Wright and Marriott (1993) and Shanley and McCabe (1994).…”

Rivers through geological time: the fluvial contribution to understanding of our planet

“…From careful recording and various analyses they have been able to recognize the influence of tectonic activity and changes in climate, although the latter was predominantly hot, semi-arid and with seasonal rainfall, giving rise to ephemeral streams with flashy flow regimes. Detailed analyses of sediment facies, palaeosols and palaeogeography, together with comparisons with modern-day dryland river systems, have provided important bases for interpretation (see also Allen, 1974;Channel et al, 1992;Marriott and Wright, 1993;Edwards and Richardson, 2004).…”

A Computational Model to Simulate Groundwater Seepage Risk in Support of Geotechnical Investigations of Levee and Dam Projects

“…This study employs a simple, computational model based on the twodimensional Leeder, Allen, and Bridge (LAB) type alluvial architecture model (Allen 1974(Allen , 1978Bridge and Leeder 1979;Mackey and Bridge 1992) that computes synthetic stratigraphy for a floodplain cross section. The model builds a floodplain cross section based on process-imitating rules derived from fluvial geomorphology and sedimentology that control the processes of floodplain sedimentation rate, avulsion frequency, and sediment compaction (i.e., consolidation).…”

“…Typically, LAB models fix the dimensions of a channel deposit to a static mean value approximate to the average channel-belt width and the bank-full channel depth. In sedimentology research, channel-belt width often is estimated as the maximum amplitude of a meander loop (Allen 1974). The model employed in this study initially sets the channel deposit width to one bank-full channel width, but allows it to increase with time based on a defined lateral migration rate that is approximated as the bank erosion rate.…”

“…Generally, unchannelized flow occurring outside the channel within the floodplain produces lower tractive forces than that occurring within the channel system. This phenomenon creates a spatial disparity in relative sediment grain size within the drainage basin, as fine-grained sediment becomes deposited upon the floodplain surface and relatively coarse-grained sediment becomes deposited within the channel network itself (Allen 1974;Friend et al 1979;Fielding 1986). As the floodplain aggrades, it becomes filled with two significantly different types of deposits: fine-grained floodplain ones composed of clays and silts and coarse-grained channel ones composed of sands and gravels (assuming the full range of grain sizes is present within the fluvial system).…”