Abstract H-l is the northernmost and longest, about 2000 km, of the three Mexican corridors. The terrane nomenclature used here is based on the distribution of ancient tribes of northern Mexico, and we hope that our names will be employed in future terrane studies in Mexico. From west to east, this transect contains the platform of the southern tip of Baja California Peninsula, the Gulf of California at the latitude of Mazatlan, and mainland Mexico, from the Siena Madre Occidental, through the City of Durango, the Sierra Madre Oriental, and ending in the state of Nuevo Leon, north of Monterrey City. Along this corridor, seven basement terranes have been identified: two of oceanic affinity and five of continental character. Parts of the terrane assemblage of the mainland are overlapped by two sequences in this corridor, one composed of rocks of the Sierra Madre Occidental Magmatic Province, and the other, of the Mesozoic Eastern Sedimentary Province of the Siena Madre Oriental.

Abstract DNAG Transect H-1. Part of GSA’s DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.

ABSTRACT This study documented the stratigraphy and provenance of the El Salto Formation in southern Baja California, Mexico, which represents an early Oligocene–early Miocene forearc basin developed during the subduction of the Farallon plate, in the immediate vicinity of La Reforma caldera, central part of Baja California Sur, Mexico. In the study area, El Salto Formation consists of three stratigraphic members. The lower member is characterized by intercalations of sandstones and conglomeratic sandstones that exhibit eolian large-scale cross-stratification. U-Pb detrital zircon geochronology implies maximum depositional ages of ca. 33–31 Ma. The middle member is characterized by successions of conglomeratic sandstones and sandstones with eolian and tidal large-scale cross-stratification. The member also contains ignimbrites, tuff, and andesite deposits, and its maximum deposition age is ca. 30–28 Ma. The ignimbrite collected at the top of this member has a crystallization age of ca. 28 Ma. The upper member is characterized by conglomerates, sandstones, and shales, with maximum depositional ages ranging from 28 to 23 Ma. Petrographically, sandstones of the El Salto Formation are composed of three petrofacies. Petrofacies A is rich in quartz with a greater contribution of felsitic volcanic lithic grains (Q 55 F 21 L 24 ; recycled orogenic provenance). Petrofacies B is rich in lathwork and microlitic volcanic lithic fragments with minor contributions of quartz and feldspar (Q 39 F 12 L 42 ; recycled orogenic and dissected arc), while petrofacies C is rich in microlitic volcanic fragments and lathwork with subordinate quartz and feldspar (Q 21 F 25 L 54 ; transitional arc setting). U-Pb ages of >600 zircon grains from nine samples contained three populations: (1) 35–23 Ma (early and late Oligocene; 22% of all grains), (2) 120–60 Ma (Cretaceous; 32%), and (3) 170–140 Ma (Middle Jurassic–Early Cretaceous; 46%). Detrital zircon grains with ages of ca. 40–20 Ma showed rare earth element patterns and trace-element ratios similar to those formed in a continental arc. Volcanic rocks sampled in this work contained chemical signatures, including Nb, Pb, and Rb anomalies, that indicate their magmas were created in a subduction zone. In addition, high concentrations of heavy rare earth elements (La/Yb = 14–19) suggest that the magmas contain a component of partial melting of the mantle wedge and crust, probably as a result of asthenospheric upwelling. These features support a model in which the El Salto Formation was developed due to the rollback of the Farallon plate in the period 50–25 Ma.

Abstract: In an attempt to summarize the characteristics and importance of shallow marine phosphogenetic settings we describe three examples from fossil and recent environments. These are the lowermost Cambrian Zhongyicun Formation phosphorites from China; the Tertiary Salada Formation phosphorites from Baja California, Mexico; and recent atoll phosphorites from French Polynesia. Common features of these settings, which range from clastic-dominated to pure carbonate, are a lagoonal or inner shelf to nearshore environment with somewhat restricted water circulation and the influence of an increased supply of nutrients for higher bioproductivity. Organic matter is considered as the major source of phosphorus, but in settings with clastic influence other sources like continental input are feasible. The study of modem atoll phosphorites enabled the introduction of a new phosphogenetic model involving cyanobaclerial mats, which provide an environment favorable for phosphogenesis and serve as nucleation sites for apatite crystals. This model is also valid for the Cambrian phosphatized microbial mats and stromatolites. However, the Salada Formation phosphorites lack discernible microbial mat structures. Here the generation of a phosphogenetic environment may be due to fine-grained sediment layers, which prevent free access of oxygenated water into the sediment. Although only one of the above shallow marine phosphogenetic settings leads to the formation of economic phosphorites, we argue that more time for phosphate accumulation and concentration may have produced such deposits in the other localities as well.

Abstract The La Colorada district in northwest Zacatecas state in Mexico is located near the transition between the Mesa Central and Sierra Madre Occidental physiographic provinces. The district is underlain by a sequence of Cretaceous, deep-water, marine rocks comprised of the Fresnillo, Cuesta del Cura, and Indidura Formations. A dacite flow-dome complex of lower Tertiary age unconformably overlies the sediments and a middle Tertiary sequence of rhyolite tuffs overlies the dacites. Four phases of deformation are seen in the district including tight folding and local thrust faulting of the sedimentary sequence which occurred during the Laramide orogeny, lower Tertiary east-west- and north-northeast-trending normal faulting, a district-scale doming, and postmineral normal faulting with reactivation of earlier faults. Silver-lead-zinc mineralization in the district consists of several sets of predominantly east-west-trending epithermal veins, small vein-related mantos, a deep zone of replacement, breccia, stockwork, and skarn mineralization, and numerous intrusive breccia pipes. Vein-related alteration consists of argilliza-tion above ore and pyrite-illite-sericite-adularia at deeper levels. In sediment-hosted replacement, breccia and stockwork ores within the deep zone, alteration consists of weak silicification and local marble development with abundant calcite veinlets. Skarn exhibits calc-silicate alteration that has been retrograded to a calcite-quartz-chlorite assemblage and the breccia pipes have argillic alteration in the rock-flour matrix. Economic minerals in the various ore types include galena, sphalerite, silver sulfosalts, the tetra-hedrite-tennantite series, native silver, and chalcopyrite. Average ore grades in the minable portions of the veins are 600 g/t Ag with 3.5 percent combined Pb-Zn. The deep zone is characterized by variable but lower grade silver with 6 percent Pb-Zn. The breccia pipes average 90 g/t Ag with 3.0 percent Pb-Zn, and the skarn bodies discovered to date have subore-grade mineralization. All mineralization types are considered to be part of the same lower Tertiary episode of hydrothermal activity and an intrusive body responsible for driving the system is postulated to exist at depth. The vertical transition between the epithermal vein environment and the deeper replacement, stockwork, and skarn environment occurs about 350 m below the current erosion surface and it coincides roughly with the dacite-sediment unconformity in the hanging wall of the No Conocida group of veins. The center of the hydrothermal system is probably located under the eastern part of the district as indicated by the distribution of deep skarn mineralization and the higher temperature breccia pipes. Current resources in the vein and replacement environments stand at 4.33 Mt. The geologic resource in the breccia pipes is 14.3 Mt. Exploration continues at depth along the No Conocida, Recompensa, and Amolillo-Palomas structures, and in the breccia pipes. The development schedule calls for the construction of a minimum 850 tpd underground mine and flotation mill with production beginning in early 2000.