Unconfined alluvial flow processes: recognition and interpretation of their deposits, and the significance for palaeogeographic reconstruction

Palaeogeographic interpretation of the sedimentary rock record depends on correct recognition from the preserved evidence of the processes responsible for transporting and depositing the sediment. This in turn depends on robust knowledge transfer from previous workers, and the successful exchange of ideas between workers requires consistent use of a well-defined vocabulary. We have identified serious breakdowns in all these interpretation steps in the case of terrestrial unconfined flow and its deposits, and these failures are leading to unreliable environmental and climatic interpretation. This is significant because such alluvial deposits commonly form a majority of the rock record of continental environments.
Working from the basic principles of geomorphology and fluid dynamics, we have undertaken a wide-ranging analysis of the nature of out-of-channel flow and from this make predictions about the characteristics of its deposits. We identify the range of possible locations and conditions that lead to the development of unconfined flow, review the processes operating in each case, and examine the range of lithological features that can be produced by these processes. This allows us to evaluate the reliability of the criteria claimed for identification of out-of-channel flow deposits, and examine how our new insights might alter palaeoclimatic and palaeogeographic reconstructions published previously by others.
The sedimentary record of unconfined flows is much more diverse and complex than usually portrayed. The received wisdom that the record of unconfined flow consists solely of upwards-fining thin beds produced from shallow waning flows is shown to be flawed. A wide range of lithofacies are possible, and the variation in both flow steadiness and uniformity need to be taken into account. The previously published criteria for recognition of flows of this type are not diagnostic of process or location; unconfined flow deposits cannot reliably be identified from grain size or bed thickness. Similar lithofacies may develop in a wide range of geomorphic and climatic settings because the deposits solely reflect the local flow conditions and sediment availability. We recommend that the terms ‘sandflat’ and ‘sheetflood’ should not be used in sedimentological accounts because there is no longer a safe informal usage for either; contradictory application of these terms, and lack of robust definitions, is leading to significant misunderstanding of palaeogeography and process.
Our analysis should improve reconstruction of past terrestrial environments because it reveals more clearly the true variety of possibilities for the occurrence of unconfined flow and the resultant deposits. Enhanced understanding of the inherent uncertainties, and realization of the wider range of plausible alternative explanations, should help resolve apparent contradictions with independent indicators of climate or geographic position.