Chihuahua Trough Research Articles

Aptian–Albian sequence stratigraphy, biostratigraphy, chemostratigraphy, and chronostratigraphy: Sonoran shelf and Tamaulipas Basin, Mexico

The Late Jurassic extensional Bisbee Basin in northwestern Mexico and southwestern United States lay between the Pacific magmatic arc and subduction zone and the cratonic Comanche shelf. Aptian to late Albian fluvial, deltaic, and shallow marine sediments and patch reefs of the Bisbee Group filled the basin. The upper Barremian(?)–lower Aptian Cerro de Oro Formation and its equivalent Rancho La Colgada Formation are overlain by the Morita, Mural, Cintura, and La Juana formations. The paralic Morita Formation and the marine Mural Limestone grade southeast into the marine Agua Salada and Lampazos formations, which correlate with deep-water facies in the Chihuahua trough and Tamaulipas Basin. Three late Aptian to middle Albian long-term depositional cycles were approximately 2.5 myr in duration.Foraminifers, calcareous algae, bivalves, rudists, corals, dinoflagellates and scarce ammonites compose the Mural Limestone biostratigraphy. First-occurrence datums of Aptian–Albian benthic Foraminifera are integrated with planktonic Foraminifera and rudists and define biostratigraphic zones that correlate with the Comanche shelf. In the Chihuahua trough and Tamaulipas Basin planktonic Foraminifera and nannofossils identify global zones. Stage boundaries on the Sonora shelf are correlated by a comprehensive biostratigraphic database.Carbon isotope chemozones correlate with Tethyan chemostratigraphy. The lower Aptian OAE 1a and uppermost Aptian–lowest Albian OAE1b chemozone are in the Lampazos section. The negative δ13C excursions and fossil evidence correlate with global Jacob, Kilian, Paquier, and Leenhardt levels. Low-oxygen water masses flooded the shallow shelf and even affected coral-rudist buildups.

Impact of Mexican Border rift structural inheritance on Laramide rivers of the Tornillo basin, west Texas (USA): Insights from detrital zircon provenance

Abstract Late Cretaceous to Eocene Laramide basement–involved shortening fragmented the Sevier and Mexican foreland basins. This resulted in a major drainage reorganization in response to the emerging topography of Laramide basement–cored uplifts and Mexican inverted Border rift basins. This study presents new depth-profile detrital zircon U-Pb data (3679 ages from 28 samples) from Upper Cretaceous–Eocene fluvial strata of the Tornillo basin in west Texas to determine sedimentary provenance and reconstruct sediment dispersal through the U.S.-Mexico border region. Detrital zircon U-Pb data are dominated by Hauterivian–Coniacian (130–87 Ma; ~20%) and Coniacian–Ypresian (87–52 Ma; ~30%) ages that represent Cordilleran and Laramide arc magmatism, respectively. Subordinate age groups are Paleoproterozoic–Mesoproterozoic (1900–1300 Ma; ~12%), Ectasian–Tonian (1300–900 Ma; ~8%), Tonian–Pennsylvanian (900–300 Ma, ~10%); Permian–Triassic (300–200 Ma; ~8%), and Jurassic–Early Cretaceous (200–130 Ma; ~11%). Detrital zircon maximum depositional ages provide new constraints on the chronostratigraphic framework of the Tornillo Group, the stratigraphic nature of the Cretaceous-Paleogene boundary, and the stratigraphic level of the Paleocene–Eocene thermal maximum. Depth-profile core-rim age pairs yielded Paleoproterozoic–Mesoproterozoic and Jurassic cores with Cretaceous–Paleogene rims, which represent zircons derived from Laramide magmatic rocks that intruded Yavapai-Mazatzal basement and Cordilleran-Nazas magmatic rocks. Zircon grains with Ectasian–Tonian cores and Paleozoic rims likely represent Appalachian-derived and/or Coahuila terrane zircons recycled from the inverted Mesozoic Bisbee basin and Chihuahua trough. These results demonstrate that fluvial strata in the Tornillo basin were sourced from Laramide and Cordilleran magmatic rocks, Yavapai-Mazatzal basement, and recycled Mexican Border rift sedimentary rocks in the southwest United States and northern Sonora, and these sediments were delivered via a large (>103-km-long), axial-trunk river. Additional recycled detritus from Mexican Border rift sedimentary rocks in the Chihuahua fold belt was delivered via transverse tributaries. This drainage reconstruction indicates that the Tornillo river flowed along an inversion-flank drainage corridor adjacent to topography formed by the inverted Mexican Border rift. Therefore, inherited Mexican Border rift architecture represented a first-order control on sediment routing to the Tornillo basin.

A record of Late Jurassic vertebrates from Texas

ABSTRACT We present the first description of Jurassic vertebrate fossils from Texas. The vertebrate specimens were collected from the Upper Jurassic Malone Formation in the Malone Mountains of western Texas. The specimens are fragmentary and not particularly diagnostic, but probably represent elements of plesiosaurians. One specimen is similar to the caudal vertebra of a pliosaurid plesiosaurian, whereas another may be a partial propodial of a small plesiosaurian. Additional bone fragments are not identifiable at this time. These vertebrates were discovered along with abundant plant and invertebrate fossils. Previous studies of the invertebrate fossils indicate a Kimmeridgian to Tithonian age for the Malone Formation, which is consistent with a single grain age of 151±2 Ma from detrital zircon U–Pb geochronology obtained in this study. The Malone Formation was deposited in shallow marine to marginal marine environments along the northern edge of the Chihuahua trough. It is correlative with the La Casita and La Caja Formations of northern Mexico, where similar marine vertebrates have been reported. The Malone Formation is also correlative with the Morrison Formation to the north.

Albian (Lower Cretaceous) carbon isotope chemozones, Texas Comanche Shelf and Mexican Chihuahua Trough: Implications for OAEs

Organic-carbon-rich black shale beds that are associated with positive carbon isotope excursions and yield pelagic fossils are evidence of short-term changes in ocean water mass and are important chronostratigraphic correlation tools. Five recognized Cretaceous Oceanic Anoxic Events (OAEs) are well documented in many basins on the Texas Comanche Shelf and the northern Mexico Chihuahua Trough: lower Aptian OAE 1a, Aptian-Albian OAE 1b, upper Albian OAE 1c, uppermost Albian OAE 1d, and Cenomanian-Turonian OAE 2. Two of these chemozones have previously been documented in Mexico and Texas, OAE 1b by a positive δ13C excursion of 2‰ and OAE 1d with a positive δ13C excursion of 1.6–1.9‰. Here we document for the first time OAE 1c in gray marine shale of the Boracho Formation in West Texas by a positive δ13C excursion of 1‰; the Boracho records the late Albian flooding of the Western Interior. On the interior shelf in shallow water carbonates the absence of carbon isotopic excursions suggests that dysaerobic waters did not affect the upper part of the water column. Review of biotic assemblages of each Gulf of Mexico Albian OAE suggests that water depths ranged from at least 300 m to just below storm wave base at about 15–30 m. These Albian OAEs were identified in the Mediterranean area and in the deep Atlantic Ocean and are recognized on the North American cratonic blocks, which supports the hypothesis that ocean/climate conditions favoring low-oxygen water masses were widespread although deposition was in local basins.

Paleostress and Geometric Analysis of the Southern Rio Grande Rift in the Northern Chihuahua Trough: Franklin and Potrillos Mountains as examples

Paleostress and Geometric Analysis of the Southern Rio Grande Rift in the Northern Chihuahua Trough: Franklin and Potrillos Mountains as examples

Geochemistry and geochronology of the Sierra de Gomez Limestone-hosted U deposit, Chihuahua: Implications for distribution of Rio Grande rift mineral deposits in northern Mexico

Uranium deposits form in a variety of settings. They are partially controlled by the secular evolution of Earth processes, including deposits in extension-related settings such as the intra-cratonic Rio Grande rift. Plio-Quaternary volcanism, mineral deposits, and hydrothermal spots occur along the Chihuahua Central Graben. The age of the Sierra de Gomez U-deposit is 1.8Ma (based on LA-MC-ICP-MS dating on a uranophane monocrystal), which is contemporaneous with the late mineralization event of the Peña Blanca U-deposit, as well as Rio Grande Rift (RGR)-type deposits in Chihuahua and intraplate volcanism. Studies of fluid inclusions in fluorite and late calcite indicate the presence of hydrocarbons and CH4-rich brine. Homogenization temperatures range from 87 to 112°C, and the mean composition (2.0mol NaCl and 0.3mol CaCl with CH4) is comparable to mineralizing brines in MVT deposits and carbonated hydrocarbon reservoirs. Evolution of C and O stable isotopic values for the calcite cement in the Sierra de Gomez Limestone-hosted U deposit illustrates that two separate calcite precipitation events occurred: (1) travertine filling karst structures in the presence of meteoric water and (2) U mineralization during deep hydrothermal fluid circulation that included interactions with a heat source and basement leaching. In a regional context, a metallogenic model suggests that the Chihuahua Trough area is deep enough to generate fluid migration by hydrothermal and/or compaction processes through RGR extensional faults until a favorable trapping horizon is reached. This causes uranium precipitation because water/rock interaction processes generate a local redox barrier.

Upper Albian OAE 1d event in the Chihuahua Trough, New Mexico, U.S.A.

Oceanic anoxic events are clues to ocean processes and are correlation datums. In North America only OAE 1a and 2 are well documented. Based on a low-resolution sampling program, a multi-proxy geochemical approach constrained by a biostratigraphic framework was utilized to identify OAE 1d in the upper part of the upper Albian Mesilla Valley Formation near El Paso, Texas. Chronostratigraphic and biostratigraphic evidence indicate that the OAE 1d event in the Mesilla Valley section is located in the lower part of the upper Albian–Cenomanian Ovoidinium verrucosum zone, which correlates with the uppermost Albian Parathalmanninella appenninica and Stoliczkaia dispar zones. The chronostratigraphic age of the geochemical event in the Mesilla Valley Formation is uppermost Albian (97.39–97.30 Ma).The classic geochemical signatures for OAEs are enriched total organic carbon (TOC) concentrations and coupled positive δ13C excursions. OAE 1d at this location records TOC values ranging from 0.25 to 0.69 wt.% throughout the Mesilla Valley Formation, where TOC increases during the OAE (21.0–40.0 m) to more than 0.40 wt.%. Interestingly, the organic matter in the Mesilla Valley is dominantly type III, which indicates a pervasive terrigenous source. Although marine organic matter is abundant from the base into the middle of the proposed OAE interval, it is progressively replaced by terrestrial material above the OAE section during progradation. The δ13Corganic values record a positive δ13C shift of +1.6‰ from −26.41 to −24.80‰ across the stratigraphic interval from 21.0 to 40.0 m, which correlates with OAE 1d.Mn and Fe geochemistry suggest the depositional conditions of the Mesilla Valley Formation were dominated by anoxic and possibly Fe-rich bottom waters, specifically during the time period associated with the OAE 1d event. This interpretation is supported by the presence of Fe enrichment recorded by FeTotal/Al and FeHighly Reactive/FeT with the lack of Fepyrite/FeHighly Reactive associated with Mn depletion.

Stratigraphy and age of Upper Jurassic strata in north-central Sonora, Mexico: Southwestern Laurentian record of crustal extension and tectonic transition

Stratigraphy, sedimentology, and geochronology of the Upper Jurassic Cucurpe Formation in north-central Sonora, Mexico, provide new insights into Late Jurassic rifting along the southwestern margin of Laurentia. The Cucurpe Formation is the fill of the Altar-Cucurpe Basin. This basin developed upon attenuated crust of the Triassic–Middle Jurassic continental arc and was part of the Arivechi-Cucurpe seaway; a narrow marine embayment oriented parallel to and located west of the Chihuahua trough. The Cucurpe Formation unconformably overlies Middle Jurassic arc assemblages and represents upward-coarsening marine prodeltaic deposits. New U-Pb zircon geochronology and a Kimmeridgian ammonite ( Idoceras cf. I . densicostatum ) constrain its age to between ca. 158 and 149 Ma. Detrital zircon ages from the unconformably overlying Lower Cretaceous Bisbee Group indicate a maximum depositional age of 139 ± 2 Ma (2σ error), demonstrating a hiatus of at least 10 m.y. between Jurassic and Cretaceous strata. Detrital zircon ages and petrographic data indicate the provenance of Cucurpe Formation and lowermost Bisbee strata. The lower part of the Cucurpe was derived dominantly from Middle Jurassic volcano-sedimentary successions. The upper part of the Cucurpe Formation was largely derived from syneruptive volcanic material equivalent to the Ko Vaya volcanic suite of southern Arizona and northern Sonora. Lowermost Bisbee strata were derived from Middle Jurassic arc rocks and exhumed Caborcan basement, Paleozoic–Lower Jurassic sedimentary cover, and Lower Cretaceous intermediate volcanic rocks. Revised stratigraphy of the Cucurpe-Tuape region indicates that several conglomeratic units, formerly interpreted as Late Jurassic pull-apart basin deposits, are not of Late Jurassic age.

Geologic structure of the northern margin of the Chihuahua trough: Evidence for controlled deformation during Laramide Orogeny

In this article we studied the northern part of the Laramide foreland of the Chihuahua Trough. The purpose of this work is twofold; fi rst we studied whether the deformation involves or not the basement along crustal faults (thin- or thick- skinned deformation), and second, we studied the nature of the principal shortening directions in the Chihuahua Trough. In this region, style of deformation changes from motion on moderate to low angle thrust and reverse faults within the interior of the basin to basement involved reverse faulting on the adjacent platform. Shortening directions estimated from the geometry of folds and faults and inversion of fault slip data indicate that both basement involved structures and faults within the basin record a similar Laramide deformation style. Map scale relationships indicate that motion on high angle basement involved thrusts post dates low angle thrusting. This is consistent with the two sets of faults forming during a single progressive deformation with in - sequence - thrusting migrating out of the basin onto the platform. We found that the style of deformation in the Chihuahua trough is variable. In places such as the East Potrillo Mountains and Indio Mountains is typical of the thin- skinned style, associated with the Cordilleran thrust belt, while in other places, the thick – skinned deformation present is typical of the Laramide orogeny in the southern Rocky Mountains. The Franklin Mountains record the transition from thick- to thin – skinned deformation. We notice that this difference in the style of deformation is related to the thickness of the Cretaceous section within the Chihuahua trough. On the other hand, the orientation of the shortening direction can be explained based on the geometry of the trough and especially the strike of its eastern margin. Along strike variations in shortening direction and kinematics are controlled by the curved northeast margin of the trough and refl ect stress reorientation along the weak interface between the strong platform and weak basin interior. These processes were wide spread affecting the 300 km long eastern margin of the Chihuahua trough between El Paso and the Big Bend region of west Texas.

Upper Aptian-Lower Albian Mural Formation: stratigraphy, biostratigraphy and depositional cycles on the Sonoran shelf, northern México

The Upper Aptian-Lower Albian Mural Formation of the Bisbee Group in northern Sonora consists of eight members, in ascending order the Fronteras, Rancho Bufalo, Cerro La Ceja, Tuape Shale, Los Coyotes, Cerro La Puerta Shale, Cerro La Espina and Mesa Quemada. These members represent shallow marine facies of a major marine transgressive-regressive event that reached the Bisbee basin, except for the Rancho Bufalo Member which is a tongue of the fluvial Morita Formation that underlies the Mural Formation. The facies, succession and regional correlation of its members indicate that deposition of the Mural Formation occurred in northern Sonora on a broad carbonate shelf, the Sonoran shelf, where depositional environments varied from restricted shelf with local terrigenous deltaic and fluvial influence to open shelf with coral-rudist buildups, to offshore shelf. Facies of the Mural deepen across eastern Sonora into the Chihuahua trough and the formation pinches out northwestward in Sonora. The Sonoran shelf was flooded by three long-term depositional cycles of approximately 2.5 myr duration. The first cycle deposited the Fronteras and Rancho Bufalo members and is older than 115.5 Ma, about 118 Ma. The marginal to shallow marine deposits of the Fronteras Members disconformably overlie fluvial strata of the Morita Formation and are sharply overlain by fluvial deposits of the Rancho Bufalo Member. The second cycle deposited the Cerro La Ceja, Tuape Shale and Los Coyotes members and probably embraces the Aptian/Albian boundary; the Cerro La Ceja Member disconformably overlies the Rancho Bufalo Member and Morita Formation in northeastern Sonora but gradationally overlies the Morita in the sections of north-central Sonora. The third cycle deposited the Cerro la Puerta Shale, Cerro la Espina and Mesa Quemada members and is Early Albian in age. The Mesa Quemada Member is gradationally overlain by fluvial deposits of the Cintura Formation. Biostratigraphy is based on ammonites, bivalves, gastropods, benthic and planktic foraminifers, and colomiellids and two biozones are erected to correlate the measured sections.

Tectonic history of the Chihuahua trough, Mexico and adjacent USA, Part II: Mesozoic and Cenozoic

La Cuenca de Chihuahua es una cuenca pull-apart de movimiento lateral derecho que empezo a formarse hace 159156 Ma (Oxfordiano) durante un periodo de rotacion relativa de la placa de Norteamerica en sentido antihorario. Los mares jurasicos estaban bien establecidos a finales del Oxfordiano y la configuracion de la cuenca tuvo pocos cambios a lo largo del Jurasico Tardio, Neocomiano y Aptiano. Estructuras de una amplia zona de lineamientos preexistentes que se intersectan, de orientacion noroeste y norte, localizados a lo largo del borde suroeste del craton de Norteamerica, proveen la fabrica para el desarrollo de la cuenca pull-apart entre las plataformas Diablo y Aldama. Durante el Titoniano y Neocomiano, la tasa de sedimentacion eventualmente supero a la de subsidencia tectonica y, al iniciarse un evento “regresivo” resultante, ocurrio extensivo deposito de evaporitas (incluyendo halita) en el area oriental de la cuenca de Chihuahua . Hacia el final del Aptiano, durante el deposito de la Formacion Cuchillo y formaciones equivalentes, ceso el fallamiento a lo largo de las margenes de la cuenca de Chihuahua y los mares empezaron a transgredir las areas adyacentes de plataforma. En el Albiano medio, los mares habian avanzado sobre areas previamente emergentes y en la cuenca de Chihuahua inicio el deposito de carbonatos de agua somera, la cual prevalecio con interrupciones menores hasta el Cenomaniano temprano. La Formacion Ojinaga (Cenomaniano temprano - Santoniano?) registra un aporte de clasticos marinos hacia la cuenca de Chihuahua, sincronico con las cunas clasticas del Cretacico Superior en el Mar Cretacico del Interior Occidental (Western Interior Cretaceous Seaway) de los Estados Unidos. La regresion del mar cretacico se refleja en la transicion de estratos marinos a no marinos en la Formacion San Carlos del Santoniano y en la sobreyaciente formacion no marina El Pica

Tectonic history of the Chihuahua trough, Mexico and adjacent USA, Part I: the pre-Mesozoic setting

Las estructuras de una amplia zona de lineamientos de orientacion noroeste, localizada a lo largo del margen suroeste del craton de Norteamerica, han estado activos recurrentemente desde el Proterozoico Medio. Los datos estratigraficos y estructurales indican la influencia de esta zona en la geologia del Paleozoico, Mesozoico, Terciario y Re ciente del noreste de Chihuahua. El Carrizo Mountain Group, con edad estimada en 1,400 a 1,300 Ma, debe haberse depositado en una cuenca (?graben de orientacion noroeste?). Ejemplos aislados de rocas graniticas antiguas (ca. 1,350 - 1,270 Ma) sugieren magmatismo distribuido en una amplia area de Chihuahua, que en parte es coetaneo con el emplazamiento de rocas graniticas hacia el norte. La cuenca (?graben?) del Carrizo Mountain Group puede ser la primera manifestacion del grano estructural de orientacion noroeste que domina en la region. La edad es consistente con la de los movimientos de fallas laterales mas antiguos a lo largo de la zona del Texas Lineament (Stockton Pass, Arizona, ca. 1,370± 70 Ma). Siguiendo al deposito del Carrizo Mountain Group, areas al norte y noroeste de la zona donde se encuentra este grupo fueron afectadas por el inferido De Baca rift a 1,260-1,160 Ma. El rift probablemente se extendio hasta la parte norte de Chihuahua y las rocas metasedimentarias registran incursiones marinas, presumiblemente desde un oceano localizado hacia el sur. Se postula que las rocas metasedimentarias y basalticas De Baca/Swisher representan un rift intracontinental y que existe una relacion genetica entre ellas y el rift Midcontinental. Aproximadamente al mismo tiempo en que ocurria el rifting, un primer evento metamorfico afecto a las rocas del Carrizo Mountain Group. Alrededor de los 1,100 Ma, la region fue sujeta al diastrofismo Grenville, que incluyo emplazamiento extensivo de batolitos, cabalgamientos, metamorfismo retrogrado y el deposito sinorogenico de la Formacion Hazel. Las rocas paleozoicas pre-misisipicas de la region fueron la consecuencia de la depositacion distribuida ampliamente en la plataforma cratonica, a lo largo de un margen relativamente pasivo del craton de Norteamerica. Rocas cambricas y ordovicicas incluyen areniscas basales transgresivas que reflejan un avance general hacia el noreste de los mares sobre Norteamerica. El evento mas importante durante el Paleozoico temprano, fue un cambio en el limite del craton norteamericano durante el Ordovicico. Aproximadamente a los 450 Ma, el terreno Cuyania se separo del craton y eventualmente fue acrecionado a Sudamerica. El fallamiento local del Ordovicico, Silurico y Devonico en Texas y Nuevo Mexico, y las discordancias en la Diablo Platform y en la parte central de Nuevo Mexico, indican eventos de exposicion de la plataforma. Estas caracteristicas pueden reflejar actividad tectonica en grandes areas, incluyendo la parte norte de Chihuahua, pero su origen y significado aun no es claro. Cuatro pulsos tectonicos del Paleozoico tardio, que afectaron al sistema Ouachita-Marathon, se reconocen en el noreste de Chihuahua y zonas adyacentes de Estados Unidos: 1) deformacion en el postpais del Ouachita durante el Misisipico- Pensilvanico; 2) orogenia pensilvanica; 3) plegamiento y cabalgamiento del Pensilvanico-Permico; 4) erosion/ truncacion del Permico y basculamiento subsecuente.

Stratigraphy and structure of the Lower Cretaceous of Lampazos, Sonora, (northwest Mexico) and its relationship to the Gulf Coast succession

The stratigraphy and structure of the Lower Cretaceous (Aptian-Albian) rocks exposed in the area of Lampazos, Sonora, are redefined and correlated to well-known stratigraphic successions of Chihuahua and northeast Mexico (Coahuila and Nuevo Leon). Originally, seven stratigraphic units were proposed for the Cretaceous rocks exposed in the Lampazos area, but in this article only five units (El Aliso, Agua Salada, Lampazos, Espinazo del Diablo, and Los Picachos formations) have been used, and we propose herein to discontinue usage of the other two (Nogal and La Mesa formations). This Aptian-Albian succession of east-central Sonora yields facies and fossil content remarkably similar to coeval sequences in the Gulf of Mexico realm (Chihuahua, Coahuila, and Nuevo Leon). These sections contain abundant microfossils (benthic and planktonic foraminifera, colomiellids, nannoconids, and radiolarians) that permit detailed biostratigraphic determinations. A complete Aptian-lower Albian section (base of K-6 through top of K-15 of the biochronologic scheme of Longoria, 1984) was identified. Abrupt lithic changes through the Aptian-Albian section of Lampazos revealed three major paleoceanographic events having an overall regional marine transgression, including an early Aptian (K-6 to K-7) event, a late Aptian (K-10 to K-11) event, and an early Albian (K-13 to K-14) maximum flooding phase. We used physical and biochronologic data to tie the aforementioned events to geochemically constrained Aptian-Albian oceanic anoxic events (OAEs) in Santa Rosa Canyon of Nuevo Leon. The close correlation between the oceanic events in east Sonora (northwest Mexico) and central Nuevo Leon (northeast Mexico) invokes the closer paleogeographic development of the two regions and undoubtedly links the area of Lampazos to a rapid encroachment of the paleo-Gulf of Mexico through north-central Mexico (the Mexican Sea) into northwestern Mexico (Chihuahua trough and Sonoran basin). The early Aptian (biozones K-6 to K-7) transgression has its maximum extension during the early Albian (biozones K-13 to K-14), culminating with the drowning of the carbonate platforms. The Lower Cretaceous rocks exposed in the Lampazos area are complexly folded and faulted. The more conspicuous structures are isoclinal and box-shaped folds and thrust faults, mostly oriented north-south and northwest-southeast, having vergences to both the northeast and southwest. A very conspicuous feature is the opposite vergence of thrust faults, either converging or diverging. Two dextral strike-slip faults (northeast-southwest and east-west) separate the area into three large blocks that have different patterns of folds and thrust faults. Also, it is evident that the area was affected by at least two episodes of normal faulting. Furthermore, the structural style of deformation present in the Lampazos succession is remarkably similar to the style of deformation of coeval rocks in northeastern Mexico, especially to the structure of the Chihuahua tectonic belt. The Lampazos succession is paleogeographically and tectonically related to the Chihuahua tectonic belt, and it is considered to be the westernmost extension of the ancestral Gulf of Mexico. The fact that the Lampazos succession can be lithocorrelated to known sequences in northeastern Chihuahua (Lucero, Ahumada, and Loma Plata formations) and northeast Mexico (Cupido, San Angel, La Pena, and Tamaulipas formations), which are know to be source rocks in exploration wells in Nuevo Leon and Coahuila, makes the Lampazos area of Sonora potentially attractive as an exploration target.

Structural transect of the southern Chihuahua Fold Belt between Ojinaga and Aldama, Chihuahua, Mexico

A restorable structural transect has been constructed across the southern end of the Chihuahua Fold Belt segment of the Cordilleran foreland fold and thrust belt of northeastern Chihuahua, Mexico, and west Texas. The 160‐km transect begins near Ojinaga and ends near Aldama, Chihuahua. It has been found that the Mesozoic Chihuahua Trough was inverted by Laramide tectonism and regionally shortened approximately 20 km (about 9%) along the transect to form the Chihuahua Fold Belt. Along the transect the belt consists of two allochthons of opposing vergence directions. The décollement of the eastern, northeast vergent allochthon shallows from within the Jurassic evaporites, along the center of the belt, and surfaces in Upper Cretaceous clastic rocks at the eastern thrust front south of Ojinaga, Chihuahua. The eastern allochthon was shortened approximately 6 km and may be divided, from west to east, into main ranges and an eastern frontal zone. Structures of the main ranges are detached in Jurassic evaporites and were greatly influenced by syntectonic flowage, resulting in salt‐cored anticlines and salt withdrawal synclines. To the east the décollement shallows over the Tascotal basement block which formed a buttress to northeastward transport forming a frontal zone containing ramp‐related anticlines and emergent thrusts. The western allochthon is southwest vergent. The décollement cuts up section from the center of the belt, where Precambrian basement is detached in the Plomosas Uplift, to Lower Cretaceous carbonates of the western frontal zone. The western allochthon was shortened approximately 20 km at the Paleozoic level and approximately 11 km at the Mesozoic level. Local basement involvement in the Plomosas Uplift may result from strike‐slip deformation along a northwest trending basement fault.

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

Bisbee Basin and Its Bearing on Late Mesozoic Paleogeographic and Paleotectonic Relations Between Cordilleran and Caribbean Regions: ABSTRACT

The Bisbee Group and its correlatives in southern Arizona, the New Mexico panhandle, and adjacent parts of Mexico are composed dominantly of Lower Cretaceous nonmarine, marginal marine, and shallow marine deposits, but apparently also include upper Upper Jurassic and lower Upper Cretaceous strata. Farther west, in southwestern Arizona and southeastern California, lithologically similar nonmarine strata of the McCoy Mountains Formation and its correlatives occupy the same general stratigraphic position as the Bisbee Group, but are poorly dated and may be older. The rifted Bisbee basin was a northwestern extension of the Chihuahua Trough, a late Mesozoic arm of the Gulf of Mexico depression. Basal zones of the Bisbee Group were deposited as alluvial fans marginal to active fault blocks during the rift phase of basin development, and are intercalated locally with lavas and ignimbrites. Subsequent thermotectonic subsidence of thinned crust beneath the Bisbee basin allowed intertonguing fluvial, lacustrine, deltaic, strandline, and marine shelf facies to invade the basin and bury the foundered fault-block topography. The upper Lower Cretaceous (Aptian-Albian) Mural Limestone was deposited during the phase of maximum transgression. Differing sandstone petrofacies of quartzose, arkosic, and volcaniclastic character reflect derivation of clastic detritus from varied sources bordering the Bisbee basin.more » The Bisbee basin and Chihuahua Trough developed in relation to Jurassic opening of the Gulf of Mexico, and were associated with changing plate configurations and motions throughout the Mesoamerican region.« less

Regional Appraisal of Hydrocarbon Potential of Trans-Pecos Texas: Methodology and Conclusions: ABSTRACT

Evaluation of large areas requires a different approach from that used to develop prospects. Regional cross-section networks generally ignore minor correlation problems. We examined and related all potentially significant parameters, and used reduced scales to subdue nonessentials and to cover a large area. Organization and planning must permit free interchange of ideas and close cooperation among those working on the project. Regional structural and stratigraphic analysis of Trans-Pecos Texas strongly suggests that widespread trap-destruction by faulting and erosion led to hydrocarbon leakage and induction of fresh water into prospective zones, which reduced the likelihood of accumulation and preservation of economic reserves. At least 4 major periods of tectonism, pervasive fresh water, mineralization, basic igneous intrusives and extrusives, high heat-flow regime, increasing percentage of carbon dioxide southwestward from the Delaware basin, and contemporaneous vertical movement suggest that the opportunity for major reserves is minimal. A review of 1,000 mi of CDP seismic data from 4 areas disclosed 65 structural leads or traps, of which 45 had significant tests. All these tests were failures; many gave indications of sizable amounts of fresh water. None of the areas have been completely condemned. However, only relatively small reserves can be anticipated for nearshore to continental Jurassic and Cretaceous sections in the Chihuahua Trough, Pennsylvanian and Permian reefs formed on a shelf environment or on the flanks of major uplifts, or in areas such as the Marfa basin. End_of_Article - Last_Page 249------------

Initiation and Reactivation of Proterozoic Aulacogen, Northern Mexico: ABSTRACT

Geochemical and petrologic affinities of late Proterozoic (~ 1Ga) bimodal igneous rocks of the Franklin Mountains, west Texas, suggest a rift origin. Scattered occurrences of similar rocks southward into the state of Chihuahua, Mexico, indicate a southerly trend for the feature. The feature is bounded by stable blocks: the stable craton of west Texas to the east and northeast, and the Sierra del Nido block to the west and southwest. Separation of the Sierra del Nido block from the craton occurred about 1 Ga. Gravity gradients mark the boundaries of the blocks, and a northwest-trending Bouger gravity high may mark the axis of the aulacogen. The aulacogen and the Sierra del Nido block are truncated to the south by the Mesozoic Mojave-Sonora discontinuity. The aulacogen was reactivated, at least in part, in the late Paleozoic as the Pedregosa basin and in the Mesozoic as the Chihuahua trough. These reactivations were apparently not full-fledged rifting events, but did result in basin development. The Sierra del Nido block was a paleographic high throughout the Paleozoic, and the Aldama platform developed on this block during the Cretaceous. The most recent reactivation of the aulacogen is as the southern extension of the Rio Grande rift, as evidenced by trends of high heat flow, recent mafic magmatism, and regional extensional faulting. End_of_Article - Last_Page 258------------

Geophysical Review of Trans-Pecos Area of West Texas: ABSTRACT

The Trans-Pecos has intrigued and baffled the oil industry, and all exploratory efforts so far have remained fruitless. Our geophysical findings along with other geologic information allow us to analyze the overall hydrocarbon potential for this area. Gravity and magnetic data were helpful in regional mapping but were unreliable for localized information owing to numerous extrusive and intrusive rocks. Seismic mapping shows many undrilled structures. However, the success ratio for the structures already drilled is disappointing (e.g., on the Diablo platform, out of 22 structural leads, 11 have been drilled and all were dry, and in the Marfa basin 17 out of 41 leads were drilled without success). Results were similar in Salt-Flat graben. Many of these wells had good hydrocarbon shows and almost all yielded fresh water. Tectonically the area has undergone several periods of orogeny, the result of the latest being numerous Basin and Range faults. The area is still seismically active and shows appreciable geodetic movement. It is suggested that the traps were destroyed with subsequent leakage of hydrocarbon and repeated induction of fresh water. Trap destruction is apparently beyond the scope of seismic detection. The Chihuahua trough (United States), in spite of many discouraging facts, such as high heat flow, thermal waters, etc, shows some promise because seismic data reveal large thrust anticlines in the lower Paleozoic rocks (approximately 15,000 ft) that are yet to be adequately tested. Other smaller undamaged stratigraphic traps (reefs, truncations, pinch-outs, etc) are possibly present and could be targets for future exploration. End_of_Article - Last_Page 233------------

Paleoenvironments of Middle Member of Quitman Formation, Hudspeth County, Texas: ABSTRACT

The middle Quitman Formation is composed of 95-195 m (310-640 ft) of interbedded shale, sandstone, and limestone in the southern Quitman Mountains and Hueco Bolson, Hudspeth County, Texas. The lower and upper members consist of interbedded sandstone/limestone and thin to thick-bedded carbonate strata, respectively. The Quitman Formation was deposited along the eastern slope of the Chihuahua trough during the late Aptian and early Albian. The formation is interpreted as the oldest transgressive phase of 4 transgressive-regressive cycles that occur throughout the Lower Cretaceous within the Chihuahua trough. Eight major facies representing nearshore to offshore environments of the middle Quitman Formation are listed below. They are: (1) nodular, sandy, peloidal to skeletal wackestone (lagoonal, basinal); (2) nodular, sandy, peloidal to skeletal packstone (lagoonal, basinal); (3) laminated to cross-bedded sandy ooid to skeletal packstone and grainstone (upper shoreface, storm); (4) laminated to cross-bedded sandstone/siltstone (upper shoreface, storm); (5) massive sandstone/siltstone (lower shoreface, lagoonal); (6) Exogyra biostromes (offshore-lower shoreface transition); (7) sandy, bioclastic, marly silt-shale (offshore-lower shoreface transition); (8) mudstone (offshore, lagoonal). Vertical and lateral relations of strata within the middle member have shown several transgressive and regressive cycles. The more applicable terms of progradation and aggradation describe the lateral and vertical buildup of terrigenous and carbonate clastic rocks within the upper half of the middle member in the southern Quitman Mountains. These sequences are not present at the section west of the southern Quitmans. End_of_Article - Last_Page 476------------