At Rancho San Marcos, halfway between Tecate and Ensenada in northwestern Baja California, a 1 km by 5 km group of giant olistoliths of Early Ordovician age occurs within phyllite and metasandstone of Mesozoic(?) age. This group of giant olistoliths is underlain by a mélange of olistolith-derived granule to boulder-size fragments in a foliated, phyllitic matrix. Granitic rocks and andesite/dacite dikes of the Cretaceous Peninsular Ranges arc and batholith intrude both autochthonous and allochthonous rocks. The olistoliths of Ordovician rock are resistant, moderately to well-sorted, blue-gray quartzite; brown, gray, and black, commonly argillaceous, bedded chert; medium to dark gray, finely to coarsely recrystallized, carbonate rock; minor amounts of brown to gray-green metaargillite; and clast-supported cobble conglomerate. North Atlantic and Midcontinent province conodonts from the carbonate rock indicate a medial Arenigian (Early Ordovician) age. Both the Ordovician (allochthonous) and Mesozoic (autochthonous) rocks have undergone low greenschist grade regional metamorphism of Cretaceous age. The quartzite has been openly folded, but argillaceous units are pervasively foliated and isoclinally folded with strikes N30 to N70°W, and dips to the northeast. Field relationships suggest that the mélange is sedimentary, not tectonic, in origin. Emplacement occurred at a time of tectonic unrest during which debris was shed westward off an unstable continental margin into flysch basins. The allochthonous rocks of Rancho San Marcos appear similar in age and lithology to portions of the eugeoclinal Valmy Formation of north-central Nevada. If these strata are correlative, palinspastic reconstruction appears to require large-scale left-lateral displacement. Proposed sinistral displacement on the medial Jurassic Mojave-Sonora megashear, plus northward translation on the San Andreas fault system in the Neogene, would place Valmy-equivalent rocks at roughly the same latitude as San Marcos.

Forty kilometers south of Tecate, Baja California Norte, is an extensive area of pre–medial Cretaceous, greenschist-facies flysch, poorly exposed, and of unknown thickness. These rocks are lithocorrelative with the Triassic(?) French Valley and Julian Schist Formations and the Upper Triassic-Middle Jurassic Bedford Canyon Formation, north of the international border. The flysch type strata of the area consist of tabular, thin- to thick-bedded subarkosic metasandstone interbedded with metapelite. Isotopic studies on detrital zircon suggest a mixed population of discordant Middle to Late Proterozoic zircon and late Paleozoic and Triassic zircon. The flysch strata, herein named the Rancho Vallecitos Formation, is divided into a predominantly sandstone lithofacies and a predominantly shale lithofacies. Vertical sequence patterns and associations of sedimentary structures (graded bedding, outsized clasts, meniscate and diffuse laminae, etc.) indicate that the sandstones of both lithofacies were deposited by high-density turbidity currents. Rare ripple-laminated contourites of the shale lithofacies indicate deposition by northerly and southerly oriented paleocurrents. Vertical sequence patterns of sandstone beds in the sandy lithofacies suggest deposition in the outer part of a submarine fan-like system. Features of the predominantly shale lithofacies suggest a basin-plain depositional environment. Subordinate pebbly mudstone and local olistostrome/mélange deposits of the shaly lithofacies indicate proximity to slope areas. The overall fine-grained, poorly sorted, and matrix-rich character of all the sandstones suggests deposition in a large submarine fan or cone system of unrestricted open ocean basins, characteristically fed by large river/delta systems. Modal analyses of the sandstone framework indicates sediment sources of recycled orogens and possibly craton interior.