Southern Coahuila Research Articles

Chihuahuan Desert Vegetation Development during the Past 10,000 Years According to Pollen and Sediment Data at Upper Arroyo, Saltillo, Mexico

Pollen and sediment data from a 10.5 m-deep alluvial exposure and a secondary tributary exposure at Upper Arroyo, a seasonal river, in Saltillo, Mexico, were examined with the aim of reconstructing the vegetation and environmental history during the Holocene as a whole. The role of climate change in Chihuahuan Desert flora development after 8800 BP was assessed, in addition to more local physiographic factors, such as erosion and accumulation, soil development and denudation, and hydrological entrenchment. Climate change appeared to have been a principal agent of vegetation change in the Early and Middle Holocene, with a periodic expansion of desert vegetation. A reduction in the environmental carrying capacities for mesophytic flora according to physiographic factors, such as soil erosion and channel entrenchment, was then identified after 2300 BP, also promoting azonal ecological niches for xerophytic vegetation in southern Coahuila, Mexico, that persist despite modern variations in precipitation.

Cacti in the Mazapil Region, Chihuahuan Desert, Mexico: Diversity, Geographical Patterns and Conservation

The Chihuahuan Desert Region (CDR) is the largest desert area in North America, and the southeastern portions contain the richest areas in Cactaceae in the continent. This research was advocated to understand the diversity of cacti in northern Zacatecas and southern Coahuila, as well as to explore species rarity, environmental relations and conservation alternatives. A geographical square measuring a quarter degree was intensively surveyed through 50, 3-km long transects, distributed in 25, 6-minute sub-squares. The geographical square was selected because of its altitudinal and environmental heterogeneity. Cacti were identified and local rareness observed, 764 specimens were deposited at the National Herbarium of Mexico (MEXU). Specimen and locality information were captured in the Database for Cacti of Central and North America. A general evaluation of the cactus diversity (species richness), levels of endemism and rarity were performed. A total of 45 species were found, just below El Huizache, Tolimán and Mier y Noriega, with 7 new records for Zacatecas, 2 for Coahuila and 3 for both Mexican states. We found 26 endemic species from the CDR, five endemic to the Main and Meridional subregions and 21 endemic to the Main subregion, sensu Hernández. Two species are endemic to the state of Zacatecas and three species are endemic to both states. A new index for rarity (Average rarity, Ar) is presented, obtained as the mean of five variables: level of endemism, species relative frequencies, local rareness, and climatic and edaphic (soil) specialization. Rabinowitz rarity (1981) was calculated for all species and transformed into numerical values to compare it with the obtained Ar values. There are 26 rare species following Rabinowitz and 16 rare species following Ar (≥0.5). Ar values resulted smooth and with a higher explanation power (r2). Just five species resulted with a Rabinowitz rarity of 7 (transformed R of 1) and Ar (≥0.82). A general complementarity analysis was used to address conservation in the studied area, which was enhanced with other four complementarity analyses, including our Ar. Nine sub-squares were found relevant for cacti protection. Soil types and a complete climatic analysis from original data were used to search for environmental relations through Principal Component Analyses. The main preferences for climatic variables and anomalies are highlighted. The Ar index is an adequate tool for measuring species rarity and presents it in a balanced and smooth way, not just fixed Rabinowitz categories, allowing the understanding that some species might be rare in different ways in different areas and environmental conditions, something that seems to occur in cacti. Further research is encouraged on Ar for comparisons and even mapping, as well as for Natural Protected Areas (NPA) design.

Age and depositional environment of the “dinosaur graveyard” at Las Águilas, southern Coahuila, NE Mexico

Here we provide a detailed description of the upper Campanian sediment succession at Las Águilas, southern Coahuila, northeastern Mexico, including the first absolute age dating for this interval, paleoenvironmental reconstructions and taphonomic observations on the abundant dinosaur remains at the locality. Stratigraphic investigations of the dinosaur-bearing succession at the Las Águilas vertebrate fossil area near Porvenir de Jalpa reveal a diverse vertebrate assemblage, including dinosaurs, crocodilians and turtles. New findings in adjacent sites include eusuchian crocodylomorphs, four different kinds of turtles, dromaeosaurids, lambeosaurines, pterosaurs and elasmosaurid plesiosaurs. Strontium isotope measurements on fossil oyster shells provide an absolute age of 73±1Ma for the lower part of the Las Águilas section. The locality is thus of late Campanian age. The vertebrate, invertebrate and plant materials as well as the sediment structures observed in a 50m thick predominantly siliciclastic succession of the Cerro del Pueblo Formation suggest deposition in an extensive delta plain environment. The facies succession indicates a short-termed cyclicity of limnic, brackish and shallow marine environments during the late Campanian–early Maastrichtian Cerro del Pueblo Formation with numerous layers containing dinosaur fossil remains. ResumenAquí damos una descripción detallada de la sucesión de sedimentos del Campaniano Superior en Las Águilas, sur de Coahuila, noreste de México, incluyendo la primera datación absoluta para este intervalo, reconstrucciones paleoambientales y observaciones taxonómicas sobre la abundancia de restos de dinosaurios en la localidad. Investigaciones estratigráficas de los estratos con dinosaurios en el área fosilífera de Las Águilas cerca de Porvenir de Jalpa revelan un ensamblaje vertebrado diverso, incluyendo dinosaurios, cocodrilos y tortugas. Nuevos descubrimientos en sitios adyacentes incluyen crocodylomorfos eusuquios, cuatro diferentes tipos de tortugas, dromaeosauridos, lambeosaurinos, pterosaurios, y plesiosaurios elasmosauridos. Mediciones de isótopos de estroncio en conchas de ostiones fósiles proveen una edad absoluta de 73±1 millones de años para la parte inferior de la sección de Las Águilas. La localidad es por lo tanto de edad Campaniana tardía. El material de vertebrados, invertebrados y de plantas así como las estructuras sedimentarias observadas en una sucesión predominantemente siliciclástica de 50m de la Formación Cerro del Pueblo sugieren una depositación en un ambiente de una extensa planicie deltáica. La sucesión de facies indica una ciclicidad de periodo corto de ambientes limnicos, estuarinos y marinos someros durante el Campaniano tardío-Maastrichtiano temprano de la Formación Cerro del Pueblo con numerosos estratos conteniendo fósiles de dinosaurios.Palabras Clave: Las Águilas, sucesión de dinosaurios, Campaniano superior, ambientes deposicionales, Formación Cerro del Pueblo, Coahuila.

Upper Campanian (Upper Cretaceous) cephalopods from the Parras Shale near Saucedas, Coahuila, Mexico

72 specimens of ammonites and the nautilid Eutrephoceras, collected from the upper Cretaceous Parras Shale at Saucedas in southern Coahuila, Mexico, are here assigned to twelve genera and fourteen species. The assemblage represents three to four upper Campanian biozones reaching from the Western Interior lower upper Campanian Exiteloceras jenneyi to the uppermost Campanian Tethyan Nostoceras hyatti biozone. Eutrephoceras irritilasi and Trachyscaphites sp. are endemic taxa, while Baculites taylorensis is restricted to the northern Gulf of Mexico Coast and E. jenneyi, Oxybeloceras crassum and Solenoceras elegans are Western Interior Seaway elements. Didymoceras donezianum is a southern Euramerican species, while Bostrychoceras polyplocum and N. hyatti occur throughout lower and middle latitudes. Diplomoceras cylindraceum and Phyllopachyceras forbesianum are cosmopolitan taxa with their main occurrences in the Maastrichtian; their record at Saucedas is the oldest of these species in North America. A clear paleobiogeographic signal is identified in the upper Campanian ammonite assemblages at Saucedas by a change from restricted towards geographically widespread faunas. This suggests gradual disappearence of faunal barriers which separated the Gulf Coast from the rest of the world.

Kimmeridgian (Late Jurassic) cold-water idoceratids (Ammonoidea) from southern Coahuila, northeastern Mexico, associated with Boreal bivalves and belemnites

Here we present two early Kimmeridgian faunal assemblages composed of the ammonite Idoceras (Idoceras pinonense n. sp. and I. inflatum Burckhardt, 1906), Boreal belemnites Cylindroteuthis cuspidata Sachs and Nalnjaeva, 1964 and Cylindroteuthis ex. gr. jacutica Sachs and Nalnjaeva, 1964, as well as the Boreal bivalve Buchia concentrica (J. de C. Sowerby, 1827). The assemblages were discovered in inner- to outer shelf sediments of the lower La Casita Formation at Puerto Pinones, southern Coahuila, and suggest that some taxa of Idoceras inhabited cold-water environments.

Experimental Evidence for Reproductive Isolation betweenAstrophytum coahuilense(H. Möller) Kayser andA. Capricornevar.Senile(Fric) Okumura (Cactaceae)

Abstract: Astrophytum coahuilense and Astrophytum capricorne var. senile have partly sympatric distributions in southern Coahuila and adjacent Zacatecas, Mexico. The apparent absence of natural hybrids in areas of sympatry has been noted by several authors, leading to speculation concerning the mechanisms of reproductive isolation. Experimental crosses between the two species revealed prezygotic isolating mechanisms in some cases and postzygotic mechanisms in others. Several different results were obtained from the cross-pollinations: 1) fruit was not produced and the ovules in the ovary of the floral remnant showed no development; 2) fruit contained incompletely developed, mostly non-germinant seeds; 3) apparently normal seed was produced, but many of the hybrid seedlings manifested morphological anomalies and mortality was high (>97%) due to chlorophyll deficiency. A series of experimental crosses between A. coahuilense and several allopatric taxa of the A. capricorne group revealed only postzygotic iso...

Age and depositional conditions of the marine vertebrate concentration Lagerstätte at Gomez Farías, southern Coahuila, Mexico

A 1.5 m thick coquinite discovered in the Upper Jurassic La Casita Formation of the Sierra El Jabalà near Gomez Farías, Coahuila, northeastern Mexico qualifies as a concentration Lagerstâtte owing to its richness in marine vertebrates. Ichthyosaurs, pliosaurs and crocodilians were described to some detail, but other taxa remained unstudied and the precise biostratigraphical age, as well as paleoecological conditions that led to the formation of the fossil deposit, are not known in detail. Here we describe ammonites, aptychi, bivalves and radiolarians, which allow for a stratigraphic assignation of the deposit to the uppermost Kimmeridgian Beckeri Zone. The unit under consideration accumulated in a hemipelagic mud bottom environment during a period of time characterized by low oxygen conditions, while a short term benthic colonization phase near the top of the coquinite corresponds to increased oxygen availability. A combination of upwelling, bottom currents, winnowing, offshore winds, storm events, circulatory nutrient traps, low oxygenated bottom waters, and a transgressional regime with reduced net sedimentation was crucial factors for the subsequent concentration of fossils, as well as for marine phosphate generation and phosphorus migration.

Structure and woody species diversity of the Dasylirion cedrosanum (Nolinaceae) rosette scrub of central and southern Coahuila state, Mexico

The most prominent vegetation type in the state of Coahuila, in northern Mexico, is Chihuahuan Desert Scrub. This plant formation encompasses the little known Dasylirion cedrosanum (sotol) rosette scrub, a community that extends over more than one fourth of Coahuila and whose most distinctive species is subjected to intense extraction from its native communities for a variety of purposes. Based on a highly replicated sampling procedure that included 131 plots located in the state’s central and southern portions, we analyzed vegetation structure, floristics, and species diversity of this plant community. A cluster analysis allowed us to differentiate five variants (associations) of the Dasylirion cedrosanum rosette scrub, which together host a richness of 97 species of vascular plants distributed in 61 genera and 28 families. These communities occurred across an elevational range of 850-2,550 m a.s.l. Dasylirion cedrosanum density varied between 193 and 705 ind./ha, with the highest value occurring in the Quercus intricata - Dasylirion cedrosanum association. Other prominent species in these scrublands were Agave lecheguilla and Euphorbia antisyphilitica. The Agave lecheguilla-Dasylirion cedrosanum association was the most broadly distributed and it hosted the highest richness species, while the largest Shannon diversity index value was recorded for the Dasylirion cedrosanum - Viguiera greggii association. The effects of human utilization of this species on the structure of those plant communities where it occurs are largely unknown; further studies are needed to better understand its dynamics and to be able to suggest more adequate regulations for its commercial harvesting.

Phylogeography and subspecies revision of the hispid pocket mouse,Chaetodipus hispidus(Rodentia: Heteromyidae)

The hispid pocket mouse (Chaetodipus hispidus) is one of the most genetically and morphologically divergent species within the heteromyid genus Chaetodipus. Four subspecies of C. hispidus currently are recognized, C. h. hispidus, C. h. paradoxus, C. h. spilotus, and C. h. zacatecae, ranging from North Dakota south through the Great Plains and Texas to central Mexico. We investigated the phylogeographic structure within C. hispidus by examining mitochondrial DNA from both freshly collected and museum specimens from localities distributed throughout the range of the species. We also examined 11 cranial characters in 303 specimens to assess morphological variation within the species. Although morphometric analyses were unable to differentiate the subspecies, phylogenetic analyses of molecular data indicated that the 4 currently recognized subspecies of C. hispidus are not genetically distinct. Instead, our results indicate that there are 4 distinct mitochondrial clades of C. hispidus that do not correspond to the currently recognized subspecies, but whose geographic limits instead coincide with major geographical features in the southern United States and northern Mexico. The Southern Coahuila filter-barrier (Durango and Coahuila), the Deming Plains (New Mexico), and the Balcones Escarpment (Texas) likely have acted as intermittent physical barriers to gene flow among the distinct mitochondrial clades, which we recognize as subspecies within C. hispidus.

Botteri's Sparrow (Peucaea botterii) Occurs in Northern Coahuila, Mexico

Botteri's Sparrow (Peucaea botterii) occurs widely in the shrub-grasslands of southern North America. We report a breeding population of the species in the Sierra de la Encantada of northern Coahuila, Mexico, ∼80 km from the Big Bend area of Texas and >300 km from the nearest previously known breeding range in southern Coahuila and central Chihuahua. We captured three individuals, which show a mostly gray dorsal coloration, suggestive of the texana subspecies, occurring from southern Texas to northern Veracruz. The exact affinity of the northern Coahuila population still needs to be ascertained. The presence of Botteri's Sparrow in northern Coahuila may have been overlooked previously or may be part of a (temporary) range expansion. More work is needed to map the occurrence of Botteri's Sparrow in northcentral Mexico grasslands.

El complejo Agave victoriae-reginae (Agavaceae)

El nombre de Agave victoriae-reginae se ha aplicado a un grupo de plantas endémicas al norte de México fácilmente distinguibles de otras especies de Agave por tener hojas con márgenes córneos sin dientes, bandas blancas sobre ambas caras y flores con tubos cortos en forma de embudo. Su revisión taxonómica revela que lo que se conoce como A. victoriae-reginae es un complejo que incluye tres especies, una de ellas con dos subespecies: 1a) A. victoriae-reginae subsp. victoriae-reginae (del occidente de Nuevo León y extremo oriental de Coahuila; 1b) A. victoriae-reginae subsp. swobodae (sur de Coahuila y noreste de Durango); 2) Agave nickelsiae (microendémica al sureste de Coahuila); y 3) Agave pintilla (la de distribución más occidental del grupo, sureste de Durango). En este trabajo se describe Agave pintilla como entidad nueva, se reinstala A. nickelsiae como nombre correcto de una de las especies y se presentan descripciones enmendadas de A. victoriae-reginae subsp. victoriae-reginae, A. victoriae-reginae subsp. swobodae y A. nickelsiae, así como claves para distinguir entre taxa. No se encontraron híbridos naturales entre los miembros del complejo A. victoriae-reginae pero se registran tres con otras especies: Agave nickelsiae x A. asperrima, A. nickelsiae x A. lechuguilla, y A. pintilla x A. salmiana subsp. crassispina.

Evolutionary Relationships of Pocket Gophers (Cratogeomys castanopsSpecies Group) of the Mexican Altiplano

The Southern Coahuila Filter-Barrier (SCFB) effectively subdivides the mammalian fauna of the Mesa del Norte, the northern and most extensive section of the Mexican Altiplano. Pocket gophers of the genus Cratogeomys north and south of this filter-barrier have been informally recognized as 2 distinct species, C. castanops and C. goldmani, respectively. Support for species recognition derives from early morphological comparisons and recent chromosomal and ectoparasite studies. Contradictory conclusions based on the only comprehensive morphometric study have prevented formal recognition of C. goldmani. A morphometric reevaluation based on ratio-transformed data reveals that the previous analysis was unduly biased by size, an ecophenotypically plastic character. When this factor is removed, morphometric variation is fully concordant with chromosomal diploid number and sequence data of mitochondrial and nuclear DNA. We provide synonymies and descriptions for C. goldmani apart from C. castanops, and revise the number of subspecies from a total of 26 to 2 subspecies in each species. The SCFB is most effective in its central portion (Desierto Mayran), and least effective in its western portion (Rio Nazas), which should be geographically broadened to include the neighboring Rio Aguanaval.

VARIATION IN HOME RANGE OF THE SIDE-BLOTCHED LIZARD,UTA STEJNEGERI, IN COAHUILA, MEXICO

We studied seasonal variation in size and fidelity of home range in a population of eastern side-blotched lizards, Uta stejnegeri, in southern Coahuila, Mexico. Field work was carried out during the active seasons of 2002–2004. Home ranges during breeding and non-breeding seasons were larger for males than females and were reduced in both sexes during non-breeding months (autumn). There was greater intersexual overlap of home range during summer and less in autumn. Intrasexually, males had more overlap in home range than females (particularly during summer), which could be related to their large home ranges during that season. Fidelity to home range from one season to the next in a given year was similar in males and females. Although males had greater length and mass than females, neither measure correlated with size of home range. Density of adult lizards was negatively correlated with size of home range during the reproductive period. During non-reproductive months, density of all age classes combined was negatively correlated with size of home range. Our results indicate that home range sizes of U. stejnegeri vary considerably over the year and diverse factors are involved.

Allocation of Populations of Whiptail Lizards toseptemvittatusCope, 1892 (GenusCnemidophorus) in Chihuahua, México, and thescalarisProblem

Abstract We used 11 samples comprising 224 specimens collected between 1966 and 1999 in Chihuahua, Mexico, and Texas to clarify aspects of color pattern and meristic variation in the notoriously difficult gonochoristic Cnemidophorus gularis-scalaris-septemvittatus complex which is characterized by enlarged mesoptychial scales and enlarged and platelike postantebrachial scales. We allocated populations represented in all SEPT-CMX samples except SEPT-CMX5 from northeastern Chihuahua to Cnemidophorus gularis septemvittatus Cope, 1892, type locality Marfa, Presidio County, Texas, represented by sample SEPT-TUS1. This allocation adds 28 newly discovered sites in Chihuahua to the single valid record previously known for the taxon in that state. Our study revealed that three taxonomically distinct forms, one in Trans-Pecos Texas east of Alamito Creek, Presidio County, and another in southern Coahuila, Mexico (both beyond the scope of this study), were included in septemvittatus in a 1962 taxonomic revision. Clar...

Mechanisms for High-Frequency Cyclicity in the Upper Jurassic Limestone of Northeastern Mexico

The 520 m of Upper Jurassic Zuloaga Limestone exposed in the Sierra de Bunuelos in southern Coahuila comprise 118 cycles of peritidal carbonate rock deposited on a gently dipping ramp. Field studies with Fischer plots and time-series analysis suggest that a Milankovitchian glacioeustasy mechanism is inadequate to describe the Zuloaga cycles. Autocyclic progradation may have been the major influence on depositional cyclicity. Depositional cycles in the Zuloaga Formation typically are a few meters thick and asymmetric with subtidal wackestone and packstone grading upward into subtidal grainstone or into intertidal stromatolites. Width of the carbonate ramp is estimated to have been about 150 km. Sedimentation rates for these peritidal carbonate environments apparently exceeded subsidence rates inasmuch as most of the carbonate platform remained near sea level during Zuloaga deposition. The area was tectonically quiescent during the late Jurassic. Autocyclic shoreline progradation is a feasible mechanism for producing the high-frequency cycles, as suggested by (1) poor correlation with predicted Milankovitch periodicity shown by time-series analysis, (2) little evidence of subaerial exposure, (3) development of complete peritidal cycles, (4) general progradational sequences within each third-order unit, and (5) absence of polar glaciation during Late Jurassic.

Paleotectonic Controls on Deposition of Upper Upper Jurassic La Casita Formation, East-Central Chihuahua, Mexico: ABSTRACT

Surface mapping of the basal Mesozoic La Casita Formation (upper Upper Jurassic) in east-central Chihuahua, Mexico, indicates initial Mesozoic sedimentation occurred in a segmented, interconnected subbasin of the Chihuahua trough. La Casite Formation (1200 m thick) is a tectonostratigraphic unit resting with angular unconformity on the Lower Permian Plomosas Formation. It consists primarily of siliciclastic material with sporadic interbedded limestones. The dominant lithofacies, approximately 1000 m thick, consists of turbiditic sandstone units (10-20 m) alternating with thicker, monotonous shale sequences. In the mapped area (approximately 30 km/sup 2/), flute cast measurements indicate flows from both the northeast (N20/degree/E) and southwest (S58/degree/W). Turbiditic sandstone units appear to pinch out and/or interfinger as they extend from the north and south into the central portion of the area. The initial opening of the Chihuahua trough is often associated with Late Jurassic block faulting, related to development of the ancestral Gulf of Mexico. Synrift depositional sequences of a similar age have been described in southern Coahuila, northern Zacatecas, and western Chiapas, Mexico. The subbasin (graben ) examined here may be ascribed a paleoposition near the western edge of the early Chihuahua trough. The western boundary of the early trough may have comprised a seriesmore » of these subbasins, forming a cuspate or serrated coastline. Late Jurassic ammonites recovered from this and other localities along the length of the Chihuahua trough suggest that the subbasins were interconnected by means of an eastern continuous seaway.« less

Confertdiolide, a novel sesquiterpenoid lactone from Parthenium

Four sesquiterpene lactones were isolated from several populations of Parthenium confertum var. lyratum (Gray) collected in Sierra de Arteaga, southern Coahuila. Three of the compounds, hysterin, tetraneurin E and hymenin were previously isolated from other Parthenium species. The fourth constituent is the new cyclopropanoid sesquiterpene dilactone confertdiolide which represents a new structural type. The photolytic conversion of hymenin into confertdiolide confirmed its structure.

Geology of Parras Basin and Adjacent Areas of Northeastern Mexico

The Parras basin, in southern Coahuila and western Nuevo Leon, Mexico, contains 4,500-6,000 meters of Upper Cretaceous and lower Tertiary terrigenous sedimentary rocks. Approximately 1,500-2,100 meters of Lower Cretaceous carbonate rocks and 2,000-3,000 meters of Triassic and (or) Jurassic evaporites, carbonates, and terrigenous rocks flank parts of the basin and underlie large areas within the basin. The Triassic and (or) Jurassic sedimentary rocks exhibit complex facies relations. Lower Cretaceous carbonate rocks are remarkably uniform in large areas of northeastern Mexico. Most of the Upper Cretaceous and lower Tertiary sediments were deposited in a boot-shaped, shallow, subsiding basin between the present-day Sierra Madre Oriental and Coahuila platform. The Upper Cretaceous-lower Tertiary Difunta Group displays intertonguing relations between two distinct lithic types: red, non-marine to brackish-water sandstone and shale alternate with gray to black, brackish-water to marine, calcareous sandstone and shale. Regionally the red strata pinch out or change facies toward the north and east in the basin where marine deposition was continuous from Late Cretaceous to early Tertiary time. Some redbeds change facies down depositional dip into gray marine strata. Red strata also have been discovered in the Upper Cretaceous Parras Shale, which normally is gray to black, calcareous shale, 1,200-1,500 meters thick. During Paleocene or Eocene time, the sediments of the Parras basin were deformed contemporaneously with the adjacent Sierra Madre Oriental structural belt. Deformational intensity in the Upper Jurassic and Lower Cretaceous carbonate rocks of the nearby Sierra Madre appears to be related to the distribution and thickness of the Minas Viejas (Triassic and [or] Jurassic) evaporites. The type and degree of deformation within the Parras basin are not uniform as indicated by three factors. (1) Overturned folds and imbricate thrusts, which probably do not extend below the Parras Shale, characterize the constricted western part of the basin; (2) broad, elongate, open folds in the southeastern part extend downward to folds in Lower Cretaceous strata; and (3) broad, open, domal folds in the n rtheast are related to Lower Cretaceous uplifts on the surface and at depth.

Stratigraphy and Structure of Parras Basin and Adjacent Areas of Northeastern Mexico: ABSTRACT

The Parras basin, in southern Coahuila and western Nuevo Leon, contains 15,000-20,000 feet of Upper Cretaceous and lower Tertiary terrigenous clastic sediments. From 5,000-7,000 feet of Lower Cretaceous carbonate rocks and 6,000-10,000 feet of Triassic and (or) Jurassic evaporites, carbonate, and terrigenous clastic rocks flank parts of the basin and underlie large areas within the basin. The Triassic and (or) Jurassic sedimentary rocks exhibit complex facies relations. Lower Cretaceous carbonate rocks are remarkably uniform over large areas of northeastern Mexico. Most of the Upper Cretaceous and lower Tertiary calcareous-arenaceous-argillaceous sediments were deposited in a boot-shaped, shallow, subsiding basin between the present-day Sierra Madre Oriental and the Coahu la Platform. The Upper Cretaceous-lower Tertiary Difunta Group displays intertonguing relations between two distinct lithic types; red, non-marine, arenaceous-argillaceous End_Page 639------------------------------ strata alternate with gray to black, brackish, calcareous-arenaceous-argillaceous strata. The red formations pinch out or change facies toward the north and east in the basin. Marine deposition was continuous from Late Cretaceous to early Tertiary in the eastern part of the basin. A few redbeds grade down depositional dip into gray, marine strata. Red strata have been discovered in the Upper Cretaceous Parras Shale, which normally is a gray to black, calcareous shale, 4,000-5,000 feet thick. During Paleocene or Eocene time, the sediments of the Parras basin were deformed contemporaneously with the adjacent Sierra Madre Oriental. Deformational intensity in the Lower Cretaceous carbonate rocks of the Sierra Madre appears related to the distribution and thickness of the Minas Viejas (Jurassic?) evaporites. The type and degree of deformation in the Upper Cretaceous Parras basin is not uniform as indicated by the following: (1) overturned folds and imbricate thrusts, which probably do not extend below the Parras Shale, characterize the constricted western part of the basin; (2) broad, elongate, open folds in the southeastern part extend downward to folds in Lower Cretaceous strata; and (3) broad, open, domal folds in the northeast are related to Lower Cretaceous uplifts on the surface and at depth. End_of_Article - Last_Page 640------------

Formational Divisions of Difunta Group, Parras Basin, Coahuila and Nuevo Leon, Mexico

The Difunta Group of the Parras Basin, southern Coahuila and western Nuevo Leon, Mexico, consists of two major intertonguing lithic facies with a composite thickness of approximately 4,000 meters. Three gray, brown, red, and green, partly variegated, fine- to coarse-grained calcareous sandstone, siltstone, and mudstone units are thickest on the south in the Saltillo-General Cepeda area; they wedge out north and east in the vicinity of the Saltillo-Paredon-Hipolito railroad. Four units of interbedded, black, gray, brown, and green calcareous shales and siltstones and gray to brown sandstones and shales occur between the red units. East of the Saltillo-Paredon-Hipolito railroad they compose an as yet undifferentiated sequence. The various red and gray-brown units have been mapped westward and northward from the vicinity of Saltillo for distances of approximately 100 and 50 miles, respectively, in a region of relatively complex structure. They are, therefore, mappable lithic units and are thus defined as formations in this paper, being named in descending order: Rancho Nuevo, Las Encinas (redbeds), Cerro Grande, Las lmagenes (redbeds), Canon del Tule, Cerro Huerta (redbeds), and Cerro del Pueblo Formations. Type localities for the various formations are located along or adjacent to major roads leading south-southwest and north-northwest from Saltillo.