James W. McKee

Stratigraphy and provenance of strata along the San Marcos fault, central Coahuila, Mexico

Stratigraphic and petrologic studies, supported by geologic mapping north of the San Marcos fault (central Coahuila), indicate that as much as 3,000 m of coarse Upper Jurassic and Lower Cretaceous detritus was derived from south of the fault. Two coarse Jurassic submarine conglomerate units (Las Palomas and Sierra El Granizo beds of the La Casita Formation) reflect separate episodes of displacement. Younger Jurassic eolianite and marine sandstone (Colorado and Tanque Cuatro Palmas beds of the La Casita Formation) record subsequent (Tithonian) fault inactivity. Neocomian rejuvenation of the fault produced alluvial deposits (San Marcos Formation); finer-grained, paralic or marine strata that are distal and suprajacent, and manifest the same faulting, are the La Mula Formation. Interruption of Neocomian displacement is indicated by a tongue of carbonate (Padilla and Barril Viejo Formations) that divides the San Marcos into upper and lower members. The principal source of this detritus was the wildflysch of the late Paleozoic, arcproximal Las Delicias basin and the plutonic rocks that cut it. Rb-Sr ages on clasts and from outcrops of granitic rocks are Triassic and probably reflect late-stage magmatism within the Las Delicias arc. Although this arc supplied most of the detritus to the Las Delicias basin, Devonian ages on some clasts reveal that an unknown source area lay somewhere nearby during the late part of the Paleozoic Era. The San Marcos fault may have served as part of the system of transform faults that connected sea-floor-spreading ridges of the Atlantic with those of the Pacific. If so, then the Jurassic units record periods of Jurassic activity and inactivity of that system. Neocomian displacment may have been caused by subsequent isostatic adjustment.

Jurassic volcanic rocks in northeastern Mexico: A possible remnant of a Cordilleran magmatic arc

Pre-Oxfordian Mesozoic subaerial volcanogenic rocks occur in a band extending northwest from Ciudad Victoria, Tamaulipas, to Santa Maria del Oro, Durango. These strata include Nazas, Rodeo, and Caopas Formations in Durango, Coahuila, and Zacatecas; La Boca Formation and its underlying volcanic basement at Canon de Huizachal, Tamaulipas; and volcanic units below La Joya Formation at Real de Catorce and Charcas, San Luis Potosi. Rocks at these localities have similar lithologies, stratigraphic positions, and paleontologic and isotopic ages. Field mapping in the Caopas-Pico de Teyra area, northern Zacatecas, and ancillary research provide insight into the nature of this suite. At least 3 km of abundant airfall tuff, tuffaceous siltstone, and uncommon ashflow tuff are present near Pico de Teyra; this sequence appears to belong to a more distal facies than the flows, breccias, and laharic conglomerates of the Nazas and Rodeo Formations exposed 25 km to the north. Porphyritic rhyolite (Caopas Formation) occurs within these volcanogenic rocks as a fault-bounded block and is interpreted as a cogenetic, subvolcanic pluton. A relatively undeformed portion of the Caopas has yielded a zircon U-Pb age of 158 ± 4 Ma. Petrographic and limited chemical data from these formations show that calc-alkaline andesite, dacite, and rhyolite are the most common compositions. The volcanogenic rocks in northeastern Mexico are south of the inferred trace of the Mojave-Sonora megashear. Their large volume, their lithologic and chemical characteristics, and their age suggest that these rocks may be a component of the Jurassic arc of western North America that was translated southeastward along the megashear.

Las Delicias basin: A record of late Paleozoic arc volcanism in northeastern Mexico

The Las Delicias basin in northeastern Mexico is defined by a thick sequence of mid(?)-Pennsylvanian through Permian marine strata, which forms most of the basement rocks of the Coahuila island, a terrane that stood above sea level during the Late Jurassic and Early Cretaceous. Most of the strata accumulated as mass-gravity deposits derived from an active volcanic arc. Principal components of these deposits are (1) andesitic and dacitic debris, (2) pelagic sediment that underwent postdepositional movement, and (3) limestone debris from the basin margin. Facies patterns and macroscopic soft-sediment structural relations indicate a source to the south; volcanic rocks at Canon Rosillo may be part of the arc. The Las Delicias area is probably not part of the Ouachita-Marathon system; stratigraphic relations and structural setting suggest that the Coahuila island may be an allochthonous stratigraphic terrane.

History of recurrent activity along a major fault in northeastern Mexico

Mesozoic and Tertiary faulting within a 280-km-long, west-northwest–trending zone in Coahuila, Mexico, is inferred from stratigraphic and structural evidence. The fault forms the northern boundary of the Late Jurassic–Early Cretaceous Coahuila Island. Uplift on the south during the Jurassic and Early Cretaceous is indicated by a large detrital lithosome that formed on the northern side of the fault. Clast size within the Early Cretaceous part diminishes northward where the unit is divided into upper and lower parts by a limestone tongue. These two Early Cretaceous detrital units plus two Jurassic conglomerates at Valle San Marcos suggest episodic, pre-Tertiary movement. Deformation of 242 ± 2-m.y.-old granites at Valle San Marcos also reflects several periods of pre-Tertiary movement. Uplift on the north along the same fault zone during the Tertiary (Laramide) raised Permian and Jurassic rocks into juxtaposition with Cretaceous limestones. Extensive left-lateral movement is neither proven nor precluded. Nevertheless, because of the size, orientation, general location, and times of movement of the fault, we suggest that it may either be part of the megashear postulated by several authors for northern Mexico, or a splay of that megashear.

A northwest trending, Jurassic Fold Nappe, northernmost Zacatecas, Mexico

The Caopas, Rodeo, and Nazas formations exposed in the San Julian uplift of northern Zacatecas are distinguished principally on the basis of style and intensity of deformation; they are parts of the same early Mesozoic(?) volcanogenic suite. This suite was the source for overlying volcaniclastic conglomerate and sandstone (La Joya Formation) that appears transitional into succeeding Late Jurassic (Oxfordian?) Zuloaga Limestone. Deformation that was contemporaneous with the deposition of the lower part of the Zuloaga produced an asymmetrical northwest trending fold nappe that was driven southwestward. The massive quartz-porphyry core of the structure (Caopas Formation) moved somewhat independently of the encasing, more ductile rocks (Rodeo and Nazas formations). Phyllonite developed in the nose of the nappe; the upright limb was attenuated and ruptured. In zones where deformation due to the nappe-forming process was weaker, a gently dipping foliation, possibly related to an older episode of recumbent folding, is preserved. Orientation, age, and location of the nappe suggest that it is a manifestation of transpressional stress developed along the Mojave-Sonora megashear as left-lateral movement carried the Jurassic arc toward the southeast.

The Mesozoic La Mula Island, Coahuila, Mexico

Granitic rocks at Potrero de La Mula and Sierra del Fuste, Coahuila, Mexico, are the only known exposures of a Late Triassic pluton that, by the Late Jurassic, stood as a topographically high area until it was inundated by the Cretaceous sea. The principal igneous rock at Potrero de La Mula is a two-feldspar, biotite-hornblende, I-type granite that is cut by comagmatic dikes of six ages and by several shear zones. A Rb-Sr age of 213 ± 14 m.y. and an initial ratio of 0.705 ± .0004 were obtained from nine whole-rock samples. A shallow depth of emplacement is inferred from textural and mineralogical evidence. Mineralogy, chemistry, and isotopes suggest emplacement above a subduction zone, which is consistent with the hypothesis that a late Paleozoic-earliest Mesozoic convergent plate boundary existed in northern Mexico. After being unroofed, the pluton contributed sediment to the adjoining Sabinas Gulf until the mid-Neocomian. By the Hauterivian, the pluton supported, a deeply weathered terrain of moderate relief flanked on its northeastern side by lagoonal environments (Oballos Member, Padilla Formation). Farther south, uplift by faulting along the northern flank of the Coahuila Island produced a clastic wedge, the finer-grained paralic components of which (La Mula Formation) prograded northeastward and eventually covered La Mula Island. Subsequent deposits at Potrero de La Mula, beginning with gypsum, reflect gradually deepening water and include the La Virgen, Cupido, La Pena, and Tamaulipas Formations.

Comparison of Two Enigmatic Contacts: Palliser-Exshaw, Devonian, Southwestern Canada, and Cupido-La Pena (Cretaceous), Northeastern Mexico: ABSTRACT

In both the Palliser-Exshaw and Cupido-La Pena sequences, uncommonly sharp contacts separate carbonate bank deposits from overlying dark shales. Three environments discernible in each sequence may be attributed to gradual deepening of water during detrital influx. Figure At Potrero de la Mula, Coahuila, the uppermost Cupido consists of poorly sorted, oolith lime grainstones (environment 1). Abundant filled scolecoid burrows 1 to 2 mm in diameter extend 8 cm down into the Cupido from the iron-stained upper surface on which occur gastropods, pelecypods, and unabraded, hemispherical scleractinian colonies (environment 2). Dark shales of the La Pena Formation (environment 3) rest on this surface. Environment 1 was an active shoal with a shifting substratum which may have been stabilized as a result of deepening water (environment 2) permitting occupancy by corals and small burrowers. Bypassing prevented sedimentary accumulation except for End_Page 604------------------------------ shells and burrow fillings. Shales of the La Pena Formation accumulated as still deeper water (environment 3) caused bypassing to cease. At the type section of the Exshaw Formation on Jura Creek, Alberta, beds of the uppermost Palliser Formation accumulated on a shallow-marine carbonate bank (environment 1). These are overlain abruptly by a 2 to 7 cm sandy bed bearing collophane, bone fragments, and abundant pyrite. This bed may have accumulated in turbid, deeper water (environment 2) during bypassing of finer grained sediment. Further deepening of water resulted in decreased currents, and the black shale of the Exshaw Formation accumulated (environment 3). End_of_Article - Last_Page 605------------

Cretaceous Transgression of Coahuila Peninsula, Potrero de la Mula and Sierra del Fuste, Coahuila, Mexico: ABSTRACT

End_Page 495------------------------------Exposures of the Late Jurassic to Early Cretaceous Coahuila peninsula have been reported at three localities northwest of Torreon and at Potrero de la Mula in central Coahuila. Exposures also occur at Sierra del Fuste, about 25 km northwest of the La Mula outcrops. At Potrero de la Mula, granites to granodiorites containing xenoliths are cut by dikes of six ages. Deep pre-Cretaceous weathering, thin transgressive arkose, and overlap by the Padilla Formation confirm these as basement rocks. In early Neocomian time the basement was a source of detritus for the basinward San Marcos Formation. Subsequently, seas partly covered the La Mula area, depositing the lagoonal facies (Oballos Member) of the Padilla Formation, which thins to a featheredge against the higher parts of the basement. A 1 to 2-m arkose, which seems restricted to paleotopographic lows, is present at the base of the Padilla. Overlying the Padilla, marine shales and a progradational sequence of fluvial and marginal-marine sandstones compose the La Mula Formation. Upper La Mula shales grade upward into sabkha deposits of the lower La Virgen Formation. Normal-marine shelf conditions existed at Potrero at several different times, causing carbonate tongues to be deposited in the La Virgen Formation and ultimatel forming the Cupido Formation. Possible subaerial exposure of the Cupido preceded deposition of the La Pena shales and calcareous mudstones, which grade upward into calcareous mudstones of the Aurora Formation. End_of_Article - Last_Page 496------------