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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.

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