Abstract

Due to its flatness, the transport and depositional regimes of mica are of particular interest in paleohydraulic interpretations of sediments. The present study examines the hydraulic equivalence of mica and quartz in a graded turbidite from the Capistrano Formation (Miocene-Pliocene) of California. Measurements of settling rates of mica grains from the deposit were obtained, and these served as the basis for the derivation of equations for the prediction of mica settling rates as a function of the flatness of the mica plate as well as its diameter. One such relationship applies only to small grains in the Stokes range while a second applies to the full range of mica sizes. It was also determined that when mica is sieved the grains pass diagonally through the square sieve openings so that the intermediate axial diameter corresponds closely in size to the sieve diagonal. The results of these analyses permit more refined determinations of mica grain sizes and quantitative examinations of the hydraulic equivalence of mica and quartz. Although the mica in the turbidite is consistently coarser than the quartz, as expected for hydraulic equivalence, this detailed analysis indicates that they are not precisely equivalent, the mica having lower settling rates than the quartz deposited at the same time.

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