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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 46 (1976)No. 4. (December), Pages 913-929

Sedimentology of a Melange: Franciscan of Trinidad, California

K. R. Aalto


A study of layer features within coarse proximal-exotic sediment gravity-flow deposits of the Jurassic(?) Franciscan Formation and Patricks Point metasedimentary unit of the Trinidad quadrangle reveals the following common layer sequence: I--massive fine to medium-grained or inversely graded pebbly sandstone, II--pebbly sandstone or conglomerate with or without preferred clast orientation, III--coarse tail and/or content normally graded pebbly sandstone, IV--diffusely laminated and/or cross-stratified pebbly sandstone, V--massive fine to medium-grained sandstone, and VI--diffuse to sharply laminated fine to medium-grained sandstone which may contain dish and pillar structures. This sequence is developed vertically and laterally during a single flow event. It reflects transitions from rain support by dispersive pressure to fluid turbulence which occur as a sediment gravity-flow progresses downslope and the entrained grain population becomes finer. The sequence may therefore be used to recognize individual flow events in a vertical sequence and to determine proximality of deposition on the inner submarine fan or suprafan.

Franciscan Formation sedimentary rocks were deposited chiefly on an inner fan, whereas those of the Patricks Point unit were deposited on an outer fan downfan from a suprafan lobe. The Patricks Point unit is either allochthonous and of a different age than the Franciscan Formation, or was severed, metamorphosed and emplaced east of the Trinidad Franciscan during melange formation.

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