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SUMMARY

A clastic deposit is represented by several diagrams. C-M diagrams in which C is the one-percentile, M the median of the grain-size distribution, characterize the coarsest fractions of the samples. F-M, L-M and A-M diagrams in which F, L and A, respectively, are the percentages by weight in the samples of grains finer than 125, 31 and 4 μ characterize the finest fractions. As a group, these diagrams form the “grain-size image” of a deposit.

The above parameters were selected because they are closely related to transport and deposition mechanisms. As a consequence, the grain-size image gives fairly precise information on hydraulic conditions under which a deposit is formed.

A large number of deposits were compared by means of their grain-size images. This comparison shows that some texture characteristics are related to the environment of deposition while others are controlled by basin mobility. Mobility should, therefore, be considered when comparing sediments of different basins and particularly recent and ancient sediments.

Relationships which exist between certain sizes of clastic grains and the most probable deposition mechanism are used to classify clastic sediments by subdividing them into types indicative of their genesis.

Grain-size types can be represented on maps and cross-sections. Grain-size facies maps of ancient formations represent lateral variations of texture and of hydraulic conditions across all basin environments and do not necessitate a reconstruction of these environments, which, often, is questionable. These maps, sell suited to show and project sedimentary trends, complete conventional isopach and sand/shale ratio maps. Integrated with ecological determinations and sedimentary structure descriptions, grain-size facies maps are a means of environment analysis.