Stratigraphy and structural setting of the Mid-Cretaceous Olvidada Formation, Baja California Norte, Mexico

Prebatholithic clastic and minor carbonate and volcanic rocks are exposed along the 30th parallel of eastern Baja California Norte, Mexico. This formation has an apparent thickness of over 6,000 m and is subdivided into three informal members. The lower member (200 m) is comprised of quartz-rich, boulder-pebble conglomerate and sandstone, volcaniclastic sandstone, and minor limestone interpreted to have been deposited in a shallow-marine setting. The middle member (5,000 m) gradationally overlies the lower member, and consists of rhythmically thin-bedded, and laminated, black chert, fine-grained sandstone, and shale interpreted to represent a slope to basin-plain environment. The upper member (850 m) rests unconformably on the middle member and is composed of cobble conglomerate, sandstone, shale, and vesicular andesite. The sequence contains angular granules of black chert probably reworked from the middle member. The upper member is interpreted to have been deposited in a nonmarine environment. The age of the section is interpreted to be Aptian-Albian (possibly as young as Turonian) based on stratigraphic and intrusional relationships and nerineid gastropods from the lower member. The sequence is interpreted to have been deposited in a continental margin back-arc basin, contemporaneous with the Aptian-Albian Alisitos volcanic arc complex exposed 30 km to the west. The section was subsequently deformed into near-vertical, steeply plunging isoclinal folds, and metamorphosed to greenschist-facies grade.