Jump to navigation
"Middle" Cretaceous metasedimentary rocks of La Olvidada northeastern Baja California, Mexico
Phillips, James Richard
Gastil, R. GordonMiller, Richard H.McArthur, David S.
155 pages, 2 maps
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.
Plate 1. Geologic map of the metasedimentary rocks of La Olvidada and vicinity. Plate 2.Columnar section of the metasedimentary rocks of La Olvidada.
San Diego State University
Master of Science (M.S.) San Diego State University, 1984
Peninsular Ranges batholithPlutonic rocksMetasomatismFossils
North America - Mexico - Baja California - La Olvidada
northlimit=30.054167; westlimit=-114.783333; eastlimit=-114.633333; southlimit=29.945833
© 2015 SDSU Library & Information Access. All Rights Reserved.