Deposits of the sandy braided South Saskatchewan River: implications for the use of modern analogues in reconstructing channel dimensions in reservoir characterisation

Abstract

Data on dunes, unit bars, cross-bar channels and compound bars and their associated deposits are presented from the sand-bed braided South Saskatchewan River, Canada. These data provide a test of widely-used models that relate the scale of formative bedforms to the dimensions of the preserved deposits, and therefore an insight into how such deposits may be preserved over geologic time. The preservation of bedform geometry is quantified using a comparison of the alluvial architecture above and below the maximum erosion depth of the modern channel deposits. This comparison shows that there is no significant difference in the mean set thickness of dune cross-strata above and below the basal erosion surface of the contemporary channel, thus suggesting that relationships between dune deposits and the formative bedform dimensions are likely to be valid from both recent and older deposits. However, theoretical relationships between the distribution of set thicknesses and formative dune height result in slight overestimates of the latter, as well as mean bankfull flow depths derived from these measurements. Unit bars and cross-bar channels are truncated slightly more in thickness below the basal erosion surface and thus can also be used to provide useful estimates of mean bankfull flow depth. Estimates of mean bankfull flow depth derived from dune, unit bar and cross-bar channel deposits were all very similar. This thus suggests that using all these metrics can provide a useful check that all components and scales of the alluvial architecture have been correctly identified. The deposits of unit bars were found to be truncated more in lengths than in width, with unit-bar deposits below the erosion surface being ~50% shorter than those above the erosion surface, and only ~10% of the mean barform length. These findings highlight that the preservation of cross-strata, and thus the paleohydraulic inferences that can be drawn from them, are a function of the ratio of the spatio-temporal scale of the bedforms, notably their size and migration rate, and the temporal scale of aggradation and channel migration. These findings highlight that these factors must 3 be considered when deciding on appropriate length:thickness ratios for the purposes of object-based modelling. Identifying the deposits of compound bars was found to be especially difficult, and hence estimates of channel depth based on this method may be problematic. Likewise, identifying unit bars within core alone is also difficult, and thus where this is the only data available (i.e. there is no outcrop data) it is suggested that dune cross-strata may be more reliable than cross-sets produced by unit bars in their use to estimate formative flow depths.