Elsevier Science Publishers B.V., Amsterdam
Net sediment transport patterns inferred from grain-size trends, based upon definition of "transport vectors"
Shu G a o and Michael Collins
Department of Oceanography, The University, Southampton S09 5NH, UK
(Received September 2, 1991; revised version accepted June 16, 1992)
ABSTRACT Gao, S. andCollins, M., 1992. Net sediment transport patterns inferred from grain-size trends, based upon definition of "transport vectors". Sediment. Geol., 80: 47-60. A new approach to the analysis of the grain-size trends, associated with net sediment transport directions in shallow marine environments, is developed here. Two specific trends, modified from McLaren and Bowles (1985) on the basis of aliterature review, are used to define a grid of "trend vectors". Such vectors are filtered and transformed into "transport vectors". It is suggested that an ordered pattern of the transport vectors represents net transport paths. The orderliness of the derived pattern is examined on the basis of a significance test, using the average length of the transport vectors as a criterion. Both the filteringoperation and the significance test procedure are based upon the assumption that the trend vectors which are not associated with net transport have the same probability of being in any direction. The technique is applied to data from Yangpu Harbour, southern China. The technique can be applied to inner continental shelf and coastal environments. For such an analysis, the area under investigation shouldbe sampled at an interval large enough to reflect the real grain-size trends resulting from net sediment transport, but small compared with the dimensions of the sedimentary environment. The time-scale of the results depends upon the sampling depth. Applicability of the technique is likely to be somewhat limited, where the environment is highly variable over short distances. Furthermore, due toits statistical character, the analysis may fail to identify net transport patterns.
Grain-size distributions of loosely consolidated surficial sediments tend to show significant differences between various locations. Within any sedimentary environment, grain-size trends may be present, i.e. spatial changes in a combination of several grain-size parameters. These trends are inresponse to a variety of processes, such as abrasion, selective transport, and the addition of locally produced sedimentary particles. Any specific grain-size trend which can be related to transport patterns is invaluable, not only in the interpretation of ancient environments, but also
Correspondence to: S. Gao, Department of Oceanography, The University, Southampton SO9 5NH, UK.
in theassessment of sediment movement in modern environments. Consequently, many authors have attempted to identify grain-size trends associated with net sediment transport pathways. In early investigations, the size of the sediment grains was assumed to decrease in the transport direction (Pettijohn et al., 1972). Such down-drift fining is not, however, universally valid. For instance, in the case oflongshore transport on beaches, both fining (Pettijohn and Ridge, 1932; Self, 1977) and coarsening (McCave, 1978; Nordstrom, 1981) trends of material in the transport direction have been observed. Hence, uncertainties exist in defining net transport pathways using a single grain-size parameter. As an alternative approach, the possibility of using a combination of parameters has been investigated. Forexample, McLaren (1981) has de-
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veloped an idealised model to investigate changes in the mean, sorting and skewness associated with transport. This author concluded that successive deposits along the transport path must become better sorted and more positively skewed,...