Jurassic sedimentary mélange and associated facies, Baja California, Mexico

The Middle Jurassic Coloradito Formation of Cedros Island, Baja California, is a fine-grained flysch sequence that includes some coarse debris flow–filled channels. Sedimentary mélange (olistostrome) is widespread, and is composed of sheared black argillite with megablocks of Triassic cherts (shallow-water origin?), Paleozoic limestone, and continental sandstone derived from the north. This unit represents the earliest record of arc-continent interaction in the Baja area.

The Eugenia Formation is locally present in erosional contact above the Coloradito Formation. The unit is a northerly derived channel-fill deposit, in which the lower one-third is dominated by metasedimentary detritus and first-cycle calc-alkaline volcanic detritus dominates above this.

The long axes of clasts in sheared argillite of the Coloradito have orientations parallel to clast imbrication in associated conglomerate channels, suggesting shearing due to downslope sliding. Contrasting deformation styles (sheared versus open folded) of fine-grained Coloradito units may be due to abundance of smectite in the latter, allowing higher water retention and a more plastic style of deformation.

Jurassic sedimentary sequences on Isla Cedros record a complex history of explosive island-arc volcanism (Gran Canon Formation), arc-continent collision (Coloradito Formation), and renewal of magmatic arc volcanism (Eugenia Formation).