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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Transport and Deposition of Resedimented Conglomerates: The Cap Enrage Formation, Cambro-ordovician, Gaspe, Quebec
Ian C. Davies, (2) Roger G. Walker
There is no descriptive model for resedimented conglomerates (akin to the Bouma sequence for turbidites). Hence there is no generalized basis for the interpretation of transport and depositional processes. We have therefore attempted to define facies. in superb coastal exposures of the Cap Enrage Formation. The cobble-boulder, and cobble facies are characterized by a lower zone of inverse grading, followed by massive bedding or normal grading. Clast imbrication is very well developed, long axis both parallel to flow and clipping upstream. Stratification is absent. In the fine-grained facies (coarse- and fine-pebble conglomerates, :red graule sandstones), there is no inverse grading, but stratification is dominant. In the lower parts of beds, the stratification is shown up by concentra ions of quartz granules in layers. Higher in the beds, the stratification marks erosional surfaces, and higher still, there are concentrations of granule-rich and granule-poor sandstones.
The inverse-to-normally graded model (characterized by inverse grading) is interpreted in terms of deposition from suspension. The fabric strongly suggests that clasts did not roll on the bed. Clasts were maintained in suspension both by fluid turbulence, and by dispersive pressure between clasts in a highly concentrated zone near the base of the flow.
The graded-stratified model (characterized by stratification) is interpreted in terms of deposit/on from suspension for the lower portion, but with bed load movement becoming increasingly important upward. Calculations suggest that granule layers 10 cm thick and 15 m long may have taken about one half to two hours to emplace. The upper granule and sand layers can alternate through several meters of section implying fluctuating flow velocities on a time scale of perhaps 30 to 120 minutes, but with continuing flow for many 1 hours.
The generality of these two models can only be established by detailed observations in many other areas.
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