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.

Abstract Tertiary marine mollusks in sediments associated with radiometrically dated volcanic rocks provide improved resolution for correlating upper Oligocene to Pliocene sediments in the Gulf of California and its northern extension, the Salton Trough of California. The megafossils discussed in this report came from the following formations: San Gregorio Formation, El Cien Formation, Isidro Formation, Trinidad Formation, Salada Formation, San Ignacio Formation, Boleo Formation, San Marcos Formation, Tirabuzón Formation (= Gloria Formation of Wilson, 1948), Carmen Formation, Marquer Formation, Imperial Formation, Ferrotepec Formation, and unnamed deposits at San Felipe, Isla Tiburón, Rancho el Refugio, and the La Mira basin, Michoacan. Paleoshorelines are identified by littoral mollusks whose faunal affinities are with the Tertiary Caribbean and Pacific-Panamic provinces. By the late middle Miocene (13 Ma), the sea extended to Isla Tiburón, Sonora, where it supported a distinctive molluscan fauna. Before the late Miocene (6 Ma), some of the same mollusks lived as far north as the Salton Trough, as seen from Imperial Formation megafossils at San Gorgonio Pass, California.

Abstract In Mexico, Ordovician sedimentary rocks are exposed in the states of Baja California, Sonora, Chihuahua and Oaxaca, comprising approximately 30 stratigraphic successions ranging from Lower to Upper Ordovician. The ages of the sequences have been established primarily by utilizing conodonts and graptolites, which have also allowed us to differentiate between platform and oceanic basin environments. The State of Sonora has the most complete Ordovician stratigraphic sequences, ranging from Tremadocian to Hirnantian. The deposits in Baja California are Floian in age, while the sequences of Chihuahua range from Sandbian to Katian, and the deposits in Oaxaca are Tremadocian. The Ordovician deposits of northern Mexico (Baja California, Sonora, and Chihuahua) present a palaeogeographic relationship to the North American craton, mainly owing to faunal interspecific affinities, while the southern deposits (Oaxaca) are controversial owing to the high degree of endemism of the faunas; however, they show affinity with Gondwana, Baltica and Avalonia, with a possible insular origin. The biotic assemblages of the Ordovician of Mexico include a variety of taxa, including algae, poriferans, corals, bryozoans, brachiopods, molluscs, trilobites, echinoderms, graptolites and conodonts as predominant elements. Despite many years of field studies in Mexican Ordovician localities, biostratigraphic correlations are as yet insufficient and incomplete or are based on limited interpretations. Thus, the Ordovician biostratigraphic data from Mexico compiled in the present paper have great potential and significant value. The advancement in the knowledge of the Ordovician biostratigraphy of Mexico will contribute to a major understanding of the relationships with the Ordovician System to a continental scale. Future advances will come mainly through increasing the amount and quality of data as well as improving biocorrelations among the Ordovician sequences of Mexico.

Abstract The Minas Viejas Formation consists of carbonates and sulfates that are the first evidence of marine incursion into northeastern Mexico during the Late Jurassic (Oxfordian). In the area southwest of Galeana, Nuevo Leon, this evaporite sequence is intensively deformed, but a consistent stratigraphic succession and separation of two members is recognizable. In addition to the Las Minas Member that was defined by Götte (1988), we introduce the La Primavera Member. Our data suggest that only one largely evaporitic succession exists in the region and that the terms Minas Viejas and Olvido are synonyms for the same stratigraphic unit. Lateral and vertical changes of facies in the Minas Viejas Formation are the result of syndepositional normal faulting and relate to the onset of sea-floor spreading in the Gulf of Mexico. Alkaline volcanic rocks occur in the La Primavera Member of the Minas Viejas Formation. This Oxfordian volcanism is hitherto undescribed in the area and links the tectonostratigraphic evolution of northeastern Mexico to early sea-floor spreading in the Gulf of Mexico. In addition, barite deposits in the Galeana area likely are related to this Late Jurassic volcanism. Barite mineralization is restricted mainly to stratigraphic levels older than the alkaline volcanism in the Minas Viejas Formation and is not the result of magmatism of Tertiary age. Apparently, carbonatite magmatism that provided the source for barium by hydrothermal activity was associated with Late Jurassic volcanism.