Abstract In the search for unconventional reservoirs, one of the geological formations most studied in the United States is the Eagle Ford (Upper Cretaceous), which extends from the state of Texas in United States to the northeastern portion of the Mexican republic. The Eagle Ford Formation consists of argillaceous limestones and calcareous siltstone deposited in a mixed environment. These lithologies have petrophysical and geochemical characteristics sufficient to be considered as producing gas and/or oil, depending on the content of organic matter and the degree of maturity thermal reached. In the northeast of the State of Coahuila, based on lithology, paleontological content, and TOC, Eagle Ford formation can be divided into three units: Biozone A: Heterohelix sigali and Helvetoglobotruncana helvética , dominated by limestone, in platform environments, thicknesses of 16 to 100m, and TOC content of 0.56% to 1.65%. Biozone B: Whiteinella archaeocretacea , limestone and calcareous shales, interbedded lithologies, deposited Biozone C: Rotalipora , black shales deposited in suboxic basin, thicknesses of 60 to 95m, and TOC content of 1.86% to 5.2. With the thickness distribution of the proposed units, it can be interpreted that the variation in the water depth depends on the topographic relief that prevailed in the Late Cretaceous (Cenomanian–Turonian) in the northeastern portion of Mexico: in a shelf and basin environment, influenced by Maverick Basin. Geochemical data analyzed and the proposed subdivision, indicates that Biozone C drive is the thickest, and has more TOC content in the northeastern part of the Burro-Picachos Basin; the predominant type of kerogen in the area is the type III. Using the Dykstra-Parson method to determine the homogeneity of the distribution of values of each proposed unit, Biozone A has a higher degree of homogeneity than Biozone C. However, based on TOC and degree of homogeneity, Biozone B has a lower degree of exploration.

The Mojave-Sonora megashear, which bounded the Jurassic southwestern margin of the North America plate from 170 to 148 Ma, may be linked northward to Alaska via the previously recognized discontinuity between the Insular and Intermontane terranes and co-genetic regional elements such as transtensional basins, transpressional uplifts, and overlapping correlative magmatic belts. The longer, continental-scale fault thus defined, which is called the Mexico-Alaska megashear, separated the North America plate from a proto-Pacific plate (the Klamath plate) and linked the axis of ocean-floor spreading within the developing Gulf of Mexico with a restraining bend above which mafic rocks were obducted eastward onto Alaskan sialic crust that converged against the Siberian platform. The fault, about 8000 km long, lies among more than a dozen large basins (and numerous smaller ones) many of which formed abruptly at ca. 169 Ma. The basins, commonly containing Middle and Late Jurassic and Cretaceous clastic and volcanic units, distinguish a locally broad belt along the western and southwestern margin of the North America plate. The basin margins commonly coincide with easterly striking normal and northwesterly striking sinistral faults although most have been reactivated during multiple episodes of movement. The pattern of intersecting faults and the rarely preserved record of displacements along them suggest that the basins are structural pull-aparts formed at releasing steps of a sinistral continental margin transform and are therefore transtensional. The width of the zone delineated by the basins is a few hundred km and extends west-northwesterly from the Gulf of Mexico across northern Mexico to southern California where it curves northward probably coincident with the San Andreas fault. Principal basins included within the southern part of the transtensional belt are recorded by strata of the Chihuahua trough, Valle San Marcos and La Mula uplift (Coahuila, Mexico), Batamote and San Antonio basins (Sonora, Mexico), Little Hatchet and East Potrillo Mountains and Chiricahua Mountains basins (New Mexico), Baboquivari Mountains Topawa Group (Arizona), regional Bisbee basin (Arizona, New Mexico, and Sonora, Mexico), Bedford Canyon, McCoy Mountains, Inyo Mountains volcanic complex and Mount Tallac basin (California). The latter probably extend into Nevada as part of the Pine Nut assemblage. At the southern margin of the Sierra Nevada of California, the inferred fault steps west then north, roughly along the Coast Range thrust and into the Klamath Mountains. The Great Valley (California) and Josephine ophiolites (Oregon) record these two major, releasing steps along the Mexico-Alaska megashear. From the northwestern Klamath Mountains, the Mexico-Alaska megashear turns east where Jurassic contractional structures exposed in the Blue Mountains indicate a restraining bend along which transpression is manifest as the Elko orogeny. Near the border with Idaho the fault returns to a northwest strike and crosses Washington, British Columbia, and southern Alaska. Along this segment the fault mainly coincides with the eastern limit of the Alexander-Wrangellia composite terrane. West of the fault trace in Washington, the Ingalls and Fidalgo ophiolites record separate or dismembered, co-genetic, oceanic basins. Correlative sedimentary units include Nooksack, Constitution, and Lummi Formations and the Newby Group, within the Methow basin. In British Columbia, the Relay Mountain Group of the Tyaughton basin, and Cayoosh, Brew, Nechako, Eskay, and Hotnarko strata record accumulation from Bajocian through Oxfordian within a northwestward-trending zone. From southern Alaska and northwestward correlative extension is recorded in basins by sections at Gravina, Dezadeash-Nutzotin, Wrangell Mountains, Matanuska Valley (southern Talkeetna Mountains), Tuxedni (Cook Inlet), and the southern Kahiltna domain. The pull-apart basins began to form abruptly after the Siskiyou orogeny that interrupted late Early to Middle Jurassic subduction-related magmatism. Convergence had begun at least by the Toarcian as an oceanic proto-Pacific plate subducted eastward beneath the margin of western North America. As subduction waned following collision, sinistral faulting was initiated abruptly and almost synchronously within the former magmatic belt as well as in adjacent oceanic and continental crust to the west and east, respectively. Where transtension resulted in deep rifts, oceanic crust formed and/or volcanic eruptions took place. Sediment was accumulating in the larger basins, in places above newly formed crust, as early as Callovian (ca. 165 Ma). The belt of pull-apart basins roughly parallels the somewhat older magmatic mid-Jurassic belt. However, in places the principal lateral faults obliquely transect the belt of arc rocks resulting in overlap (southern British Columbia; northwestern Mexico) or offset (northern Mexico) of the arc rocks of at least several hundreds of kilometers. The trace of the principal fault corresponds with fault segments, most of which have been extensively reactivated, including the following: Mojave-Sonora megashear, Melones-Bear Mountain, Wolf Creek, Bear Wallows–South Fork, Siskiyou and Soap Creek Ridge faults, Ross Lake fault zone, as well as Harrison Lake, Bridge River suture, Lillooet Lake, and Owl Creek faults. Northward within the Coast Range shear zone, pendants of continental margin assemblages are interpreted to mark the southwest wall of the inferred fault. Where the inferred trace approaches the coast, it corresponds with the megalineament along the southwest edge of the Coast Range batholithic complex. The Kitkatla and Sumdum thrust faults, which lie within the zone between the Wrangellia-Alexander-Peninsular Ranges composite terrane and Stikinia, probably formed initially as Late Jurassic strike-slip faults. The Denali fault and more northerly extensions including Talkeetna, and Chilchitna faults, which bound the northeastern margin of Wrangellia, coincide with the inferred trace of the older left-lateral fault that regionally separates the Intermontane terrane from the Wrangellia-Alexander-Peninsular Ranges composite terrane. During the Nevadan orogeny (ca. 153 ± 2 Ma), strong contraction, independent of the sinistral fault movement, overprinted the Mexico-Alaska megashear fault zone and induced subduction leading to a pulse of magmatism.

The San Marcos fault is a regional structure in northeast México with a minimum length of 300 km, which separates the Coahuila block from the Coahuila fold belt; the fault dips north-northeast and its trend is west-northwest. The San Marcos fault is a basement structure that has been reactivated multiple times, and along its trace there is stratigraphic and structural evidence of intermittent activity since at least the Late Jurassic to the Pliocene-Quaternary. The structural evidence analyzed in this work suggests that the San Marcos fault accommodated mainly north-northeast crustal extension in pre-Tithonian and Neocomian pulses of activity. This extension may have contributed to development and growth of the Sabinas basin to the north. We found no evidence to support previous proposals of large lateral offset across the fault in Late Jurassic time, but we document a small component of right-lateral slip. At least four reactivation events have been recognized along the San Marcos fault. The first, in Neocomian time, was normal and triggered deposition of the San Marcos Formation. The second reactivation of the San Marcos fault involved reverse slip during Paleogene time, and it must include minor movements along secondary faults associated with the San Marcos fault. Interpretation of the reactivation event of the San Marcos fault as a reverse fault is based on (1) the occurrence of drape folds and minor tectonic transport to the south-southwest along the main trace of the fault; (2) the occurrence of a nearly perpendicular fold axis of different generation in the southwest sector of the Sabinas basin; (3) uplift of progressively older rocks toward the northeast within the San Marcos Valley; and (4) the existence of near perpendicular directions of tectonic transport determined for different structures within the San Marcos Valley (e.g., faults in the western sector of the valley record tectonic transport to the west and faults in the southwest sector of the valley record tectonic transport to the south-southwest). Secondary faults associated with the San Marcos fault vary in orientation from nearly east-west to nearly north-south, and are best represented by the El Caballo and El Almagre faults exposed in western Coahuila and southeastern Chihuahua. Reactivation of the San Marcos fault as a reverse fault occurred late, relative to an earlier stage of detachment (locally duplicating the stratigraphic sequence) in localities over the Coahuila platform and in the Sabinas basin itself. The relative importance and scale of these detachment folds need to be explored in further detail. The third reactivation event was normal with a left-lateral component (late Miocene–early Pliocene), and the fourth and last event is dominantly normal (Pliocene-Quaternary). These last two reactivation events along the San Marcos fault were recognized along the segment of the fault buried by volcanic products of the Camargo volcanic field in southeast Chihuahua State. These late events might also be present along the San Marcos fault in Coahuila; the lack of Cenozoic sequences atop the fault trace makes their recognition difficult. La falla San Marcos es un lineamiento estructural regional con más de 300 km de largo, rumbo WNW y que se inclina hacia el NNE, separando el bloque de Coahuila del Cinturón Plegado de Coahuila en el noreste de México. La falla San Marcos es una estructura de basamento multirreactivada que, en superficie, muestra evidencias estratigráficas y estructurales que documentan su actividad intermitente por lo menos desde el Jurásico Tardío hasta el Plioceno-Cuaternario. Las evidencias estructurales más antiguas reconocidas en este trabajo documentan actividad de la falla San Marcos durante tiempos pre-Titoniano y Neocomiano, sugiriendo que la falla San Marcos acomodó principalmente extensión de la corteza en dirección NNE. Esta extensión contribuyó al crecimiento de la cuenca de Sabinas; con lo anterior, se pone en duda la existencia de grandes desplazamientos laterales a través de la falla San Marcos por lo menos para estos tiempos. Se han reconocido al menos cuatro eventos de reactivación de la falla San Marcos. El primero fue con componente normal en el Neocomiano y causó el depósito de la Formación San Marcos. El segundo evento de reactivación fue inverso en el Paleógeno y debió incluir a fallas menores asociadas a la falla San Marcos. Se interpreta que el segundo evento de reactivación está representado por (1) la ocurrencia de plegamiento tipo drape y transporte tectónico menor hacia el sursuroeste sobre la traza principal de la falla San Marcos, (2) la ocurrencia de relaciones perpendiculares entre los ejes de pliegues en la parte suroeste de la cuenca de Sabinas, (3) el levan-tamiento de rocas más antiguas progresivamente hacia el noreste dentro del Valle San Marcos y, (4) la existencia de direcciones perpendiculares de transporte tectónico determinadas para diferentes estructuras en el Valle San Marcos (e.g., fallas en el sector oeste del Valle San Marcos registran transporte hacia el oeste y fallas en el sector suroeste registran transporte hacia el sursuroeste). Las fallas menores asociadas a la falla San Marcos presentan orientaciones desde E-W hasta cercanamente N-S como las fallas El Caballo y El Almagre expuestas al oeste de Coahuila y sureste de Chihuahua. Este evento de reactivación inverso de la falla San Marcos es tardío con respecto a una fase anterior de despegues (duplicación de la secuencia por fallas) en localidades de la plataforma de Coahuila y la cuenca de Sabinas. La importancia y escala de los despegues debe ser explorado con mayor detalle en futuros trabajos. La tercera reactivación es normal con componente lateral izquierda (Mioceno tardío-Plioceno temprano) y, la cuarta y última, predominantemente normal (Plioceno–Cuaternario). Estas reactivaciones fueron reconocidas sobre la traza de la falla San Marcos sepultada por productos del Campo Volcánico de Camargo, al sureste de Chihuahua. Los dos últimos eventos parecen estar presentes sobre los segmentos de la falla San Marcos en Coahuila; sin embargo, aquí no afectan a rocas jóvenes por lo que no es posible establecer sus edades.

The low-temperature epigenetic and stratabound Pb-Zn-Cu-Ba-F-Sr–bearing ore deposits enclosed within sedimentary columns historically have been major sources of metals. Exploration companies still find these deposits to be a profitable exploration target due to their simple mineralogy as well as the large tonnage that can present, always considering the mineral districts as a whole. In northeastern México, several nonmagmatic, low-temperature Pb-Zn-F-Ba deposits have been systematically considered as magmatic-related (skarns, high-temperature replacement deposits, epithermal deposits, etc.). Recently, these deposits have been restudied and placed within a scenario of deep fluid circulation of basinal brines through the Mesozoic sedimentary series, enriched in Ba, F, and metals during fluid flow and water-rock interactions. These fluids gave rise to a series of strata-bound epigenetic ore deposits scattered throughout the whole Mesozoic carbonate platform and can be shown to be unrelated to any period of magmatism. There is no intense alteration to the host rocks. Commonly there is a close association with organic matter, either liquid hydrocarbons or bitumen; they have a very simple mineralogy of barite, celestine, fluorite, sphalerite, galena, and have low formation temperatures (90–105 °C) combined with variable salinities. These characteristics make these deposits similar to the Mississippi Valley–type deposits, possibly most similar to the Alpine-Appalachian subtype.