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Description
Recent field mapping near La Olvidada barite prospect, at the 30th parallel in northeastern Baja California, has delimited a 125 square km exposure of medial Cretaceous (Barremanian-Turonian), pre-batholithic, upper greenschist to lower amphibolite metasedimentary rocks. These rocks, collectively referred to as the metasedimentary rocks of La Olvidada, are exposed in a seven km thick section characterized by northwest-striking bedding and foliation, steep northeasterly dips, and isoclinal folding. This section consists of an unconformable basal sequence of quartz-rich metaconglomerate, metaquartzite, phyllite, and marble overlain by rhythmically interbedded and laminated flysch-like carbonaceous metachert and quartz-mica phyllite. The basal sequence is interpreted to have been conglomerate, sandstone, and limestone deposited on a subsiding continental shelf, whereas the flysch-like rocks are interpreted as pelagic bedded chert and turbidite sandstone deposited predominantly in the basin plain facies. A lens of metaconglomerate, metaquartzite, and minor metaandesite enveloped by flysch-like rocks is also exposed in the section. These rocks are interpreted as conglomerate and calcareous sandstone deposited in a channelized facies of a submarine fan or flank talus deposited adjacent to a basin floor high. Andesite was probably erupted from local submarine vents. Clastic provenance and paleocurrent evidence imply that a cratonal source terrane was situated southwest of the depositional basin. The metasedimentary rocks of La Olvidada crop out continentward of Aptian-Albian age metavolcanic rocks (Alisitos Formation) which have been interpreted as a fossil volcanic arc. This tectonic geometry suggests that the rocks of La Olvidada were deposited in a back-arc setting. An alternate hypothesis explains the present position of the arc by Late Cretaceous northwest translation, which dislocated a segment of a western mainland Mexico arc (Sonoran) and placed it seaward of the rocks of La Olvidada.
