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Fining-Upward Cycles: Problems and Prospects: Abstract
Fining-upward cycles are regarded as one of the more popular and powerful tools of environmental reconstruction. This simple feature, though it has led to an enormous increase in our understanding of ancient sediments, also has had many disastrous consequences in terms of interpretation and understanding. Several factors cumulatively led to the misunderstanding of the fining-upward model. There is a good deal of confusion in the usage of the terms ‘sequence’, ‘cycle’ and ‘cyclothem’. The difference in the significance of ‘pattern’, ‘repetition of the pattern’ and the ‘time taken’ or ‘thickness involved’ for the pattern is occasionally not clearly spelt out. The classic fining-upward cyclothem of Allen (1965, 1970), which was regarded as a hallmark of the meandering stream deposit, has been challenged by Reading (1987). Questions which remain to be answered include: a) is lateral accretion the only mechanism for the deposition of the coarse grained member of the fining-upward cyclothem? b) why the fine grained member, which was thought in Allen’s model to be the result of deposition from suspension during overbank flooding, should uniformly fall in line with the grain size variation trend of the coarse grained member?
Fining-upward sequences, without strict adherence to cyclicity, and with or without systematic structural variation have been reported from a wide variety of environments, including alluvial fan, braided steam, distributary channel, tidal flat, tidal inlet, tidal creek, washover channel in a transgressive barrier system, and submarine fan-basin plain system. It would be unwise to put them all under a common umbrella ‘fining-upward cycle’, since (a) all may not be strictly cyclic, and (b) more than one process may be responsible for the observed textural variation. It is necessary to isolate strictly cyclic patterns from non-cyclic patterns and also to differentiate, in the case of cyclic patterns, short-term periodicities from long term periodicities. Superficially similar cycles or sequences may be differentiated on the basis of (a) associated variation in sedimentary structure, (b) careful facies and paleocurrent studies, (c) average thickness range, (d) rate of change of grain size with depth/thickness, and (e) number of beds versus thickness plot. For a better interpretation, beyond empirical deduction, it is necessary to establish the precise cause of grain size variation within a sequence and the periodicity of the cycles.
Acknowledgments and Associated Footnotes
1 Department of Applied Geology, Indian School of Mines, Dhanbad 826004, India
Copyright © 2009 by the Canadian Society of Petroleum Geologists