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A wide range of mineralogical and textural variations was observed between the base and the top of marine Tertiary Annot Sandstone turbidites in the Lac d'Allos region of the French Maritime Alps. This variability may be related to the characteristic vertical distribution of grain sizes and shapes within such graded deposits. Relative percentages of most light and heavy minerals and of the primary matrix, as well as factors of sphericity and roundness, are directly related to grain size. Thus the upward decrease in grain size and increase in grain sorting is reflected in a progressive change in rock composition. Samples (43) were collected at 4-cm intervals between the base and top of four well graded turbidites, each less than one meter thick, and these were compared wit spot samples (125) collected from a 250-m flysch section. The range of petrographic variability within a single turbidite is as great as the range of mineralogic and textural variations between the base and the top of the formation. The same minerals are found in each turbidite, indicating that source areas and eroded parent rock materials accumulated nearshore remained the same during the period of this formation's history. However, notable variations between succeeding turbidites were noted. This may be explained by the fact that each turbidity current is characterized by its own specific density and velocity gradient. As a result, differing proportions of particle sizes settled out of each suspension flow over the same spot on the ocean floor. Even though gravity currents are able to transport identical sedimentary materials from nearshore environments into the deeper basin, such quantitative variations should be expected in any formation composed of turbidites.
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