PALAEOHYDRAULICS REVISITED - PALAEOSLOPE ESTIMATION IN COARSE-GRAINEDBRAIDED RIVERS

Methods for estimating palaeoslope from fluvial deposits have been ava ilable for some time, but new data and improved understanding of the r elevant physical processes afford the possibility of improving existin g methods, and the emerging field of quantitative stratigraphy provide s a new context for the results. Here we focus on deriving palaeoslope estimates for coarse-grained fluvial deposits. These estimates can be used in basin analyses to constrain the magnitude of the slope change necessary for a given deflection of palaeocurrents, to constrain temp oral and spatial variation in basin subsidence rate, and to provide a surface datum for use in sediment-backstripping calculations. The algo rithm we derive to estimate palaeoslope applies to rivers that self-ad just through variations in channel width to maintain a temporally and spatially averaged bed shear stress equal to some constant multiple of the critical shear stress for initial motion of bed sediment. Data fr om modern coarse-grained rivers with minimal bank cohesion and form re sistance suggest that this boundary shear stress is equal to about 1.4 times the critical shear stress for movement of the median-sized clas t of the surface layer. The key sedimentological criteria for recognit ion of systems appropriate for this type of analysis are: (1) field re lations suggesting that channel banks formed in effectively noncohesiv e gravel (i.e. free of clay-size sediment and plant roots); (2) absenc e of significant volumes of dune-derived cross-stratification and (3) absence of indicators of extremely rapid, flash-flood-type deposition. The basic input data for a palaeoslope calculation are spatially aver aged estimates of palaeodepth and median grain size. The most importan t aspect of data collection is that the depth and grain-size estimates should be determined independently by random sampling over the whole outcrop. Joint analysis of data from appropriate modern rivers and of errors associated with palaeodepth and grain-size estimates indicates that in coarse-grained braided-river deposits, palaeoslope can be esti mated to within a factor of 2.