An Interpretation of Trends in Grain Size Measures

ABSTRACT: The mean grain size. sorting, and skewness of a sedimentary deposit are dependent on the sediment grain size distribution of its source and the sedimentary processes of i) winnowing (erosion), ii) selective deposition of the grain size distribution i transport, and iii) total deposition of the sediment in transport. If a source sediment undergoes erosion, and the resultant sediment in transport is deposited completely, the deposit must be finer, better sorted, and more negatively skewed than the source. This trend is referred to as Case I. The lag remaining after erosion, on the other hand, must be coarser, better sorted, and more positively skewed (Case ll). If sediment in transport undergoes selective deposition, the resultant deposit can either be finer (Case IlIA) or coarser (Case nlB) than the source, but the sorting will be better and the skew more positive, Although exceptions to these trends may occur, they suggest that comparison of one sediment must be made with another for the proper identification of the sedimentary process, and therefore it is not possible for a single grain-size distribution to identify the depositional environment. The trends also suggest that the skewness of a grain-size distribution has been widely misinterpreted and implies neither the truncation of one of the tails nor the mixing of more than one mode. Rather, a skewed sediment is the natural result of the sedimentary process. In a system of related environments, these trends can be used to identify both the probable source and the probable deposit and. by inference, the net sediment transport paths among sedimentary deposits. Such an analysis provides a rapid understanding of the sedimentary processes, identifies patterns of erosion and accretion, and may suggest transport processes.