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.

Abstract Late Albian reefs formed the upper part of the Devils River trend that rimmed the north and west margins of the Maverick Basin in south-central Texas and adjacent parts of Mexico (Fig. 42). The Maverick Basin was open to the south after the Stuart City reef trend was drowned in the late Albian. During the medial Albian, Edwards-equivalent time, the lower part of the Devils River trend joined with the Stuart City trend to restrict the Maverick Basin in Mexico and Texas (Smith, 1970, 1981). The Devils River reefs were unique to the late Albian Comanche Shelf. No comparable reefs developed between the Central Texas Platform on the northeast, where it sloped into the East Texas Embayment (Rose, 1972). West of the Devils River trend in places on the Coahuila Platform, rudist patch reefs are found in the Santa Elena Limestone (Scott and Kidson, 1977). Kimbleia occidentalis (Conrad) occurs in the Santa Elena in the Sierra de la Parra, northeastern Chihuahua (Young, 1984). The type specimen of this species (USNM 9840) was collected near the mouth of the Pecos River, Val Verde, Texas, probably from the Devils River Formation. Stratigraphy The Devils River Formation is an unconformity bounded limestone unit as thick as 173 m (570 ft) in southwest Texas and northeast Mexico (Lozo and Smith, 1964; Smith, 1970; Miller, 1984). At its type section near Leakey, Real County, Texas, the lower Devils River consists of thin- to thick-bedded, miliolid, pebid, rudist, bioclast grainstone, packstone and wackestone; dolomite, evaporite collapse

The Solitario displays geologic features that span virtually the entire regional history of Trans-Pecos Texas since Cambrian time. The visible structure (cover) is the eroded remnant of the roof of a radially symmetric late Eocene (38 Ma) laccolith. Erosion of the laccolith roof has exposed a remarkably complete stratigraphic section. The rock record begins with Upper Cambrian Dagger Flat Sandstone. Deposition of Upper Cambrian sand and shale in a shallow sea gave way during Ordovician to deposition of black shales interbedded with some sand and black chert, reflecting more restricted circulation. About 1 km of sediments, from the craton to the north and northwest, accumulated in the Ouachita Trough during Late Cambrian and Ordovician time. The area was elevated and slightly tilted, but not significantly deformed, by the Llanorian Orogeny during Silurian time. Silurian rocks are missing, and the Lower Devonian-Mississippian Caballos Novaculite rests unconformably on the Upper Ordovician Maravillas Formation. More than 1.4 km of flysch, from a source to the southeast, forms the Mississippian-Pennsylvanian Tesnus Formation. No Paleozoic rocks younger than Early Pennsylvanian (Morrowan Series) have been found. The measured thickness of Paleozoic rocks in the Solitario is approximately 2.6 km and represents a time span of 240 m.y., with a single break of ~30 m.y. during Silurian, one of the longest depositional records known. The Paleozoic rocks presently found in the Solitario are allochthonous and were intensely deformed during the Ouachita Orogeny. The orogeny affected the Solitario area from Middle Pennsylvanian (Desmoinesian) until Early Permian (middle Wolfcampian). Transport of the allochthon during the Ouachita Orogeny was at least tens of kilometers from the southeast. Deformation was primarily by folding, with the development of nappes, S-folds, boudinage structures, and local and regional thrust faults evident in the exposed Paleozoic rocks. After the Ouachita Orogeny, the Solitario area remained positive from Early Permian (middle Wolfcampian) on the structural block known as the Tascotal Uplift that formed the southern margin of the Permian sea. Throughout early Mesozoic, the area remained elevated on the West Texas-Coahuila Platform, and was extensively eroded as part of the Wichita paleoplain. In Early Cretaceous (late Aptian), the area was covered by a shallow sea, and 1.2 km of carbonates were deposited. These rocks are now magnificently exposed in cross section in the shutups that cut the rim of the Solitario dome. The Cretaceous rocks are correlative with carbonate units found to the east and south in the Gulf Coast area. At the end of the Cretaceous (Gulfian), the area was elevated once again as the Laramide Orogeny migrated eastward. Regionally, the Solitario lies on a large structural block that is defined by gravity data as a remnant of the Tascotal Uplift. The block appears to have responded to Laramide compression by uplift and rigid-body rotation without undergoing extensive internal deformation. Deformation associated with the Laramide Orogeny had no discernible effect on the later emplacement of the Solitario laccolith. Within the mapped area, Laramide compression is, at most, presently evident only as sparse stylolites in the Cretaceous rim rocks. Mid-Eocene basal conglomerate of the Devils Graveyard Formation, shed from Laramide folds to the west, is found in Fresno Canyon, and is the only Tertiary rock that predates the formation of the Solitario dome. The oldest reliably dated igneous rock in the Solitario is a 37.5 ± 0.8 Ma rhyolite sill. The sill intruded the base of the Cretaceous section immediately prior to the formation of the Solitario dome. The dome was formed by intrusion of ~100 km 3 of silicic magma that formed the present granite laccolith shortly after emplacement of the rim sill. The structural relief of the dome is 1.6 km, and the roof underwent 400 m of radial extension from the center. A crestal graben formed during doming, and the graben block collapsed less than 1 m.y. after formation of the dome, foundering and rotating down to the south after the roof was deeply eroded. The foundering of the crestal graben block was probably contemporaneous with the emplacement of a granite intrusion on the eastern side of the collapsed block and formation of a small caldera south of the crestal graben block. The series of intrusive and extrusive volcanic rocks found within the dome includes 14 mappable rock types, with a wide range of compositions. The Solitario igneous suite was emplaced over a total time span of 11 m.y.; silicic igneous activity was probably limited to the first 3 m.y. of this time. Younger, more mafic rocks have vents within the Solitario dome, and are thus included within the suite, but appear to be genetically and temporally related to the Bofecillos volcanic center, immediately west of the dome. The oldest units of the central basin-filling Needle Peak Tuff were deposited in late Eocene within 1 m.y. after the dome was formed. The roof of the dome was therefore eroded to virtually its present level by the end of the Eocene. The emplacement of the Needle Peak Tuff is associated, at least in part, with the collapse of a small caldera in the south part of the central basin. Volcaniclastic rocks accumulated in surrounding areas during the Oligocene and early Miocene, particularly those erupted from the Bofecillos volcanic center to the west. Early Oligocene Chisos Formation pinches out against the western flank of the dome. These volcanic units eventually lapped high onto the eroded rim of the dome, but did not spill over into the central basin. From early Miocene until the Quaternary, the area was an elevated plain, with the streams at or near their base level. There is no evidence in the map area for significant erosion or deposition from early Miocene until the Pleistocene, when the Rio Grande began actively downcutting its bed to the south. The base level of all local streams was lowered as a result. The map area is presently being rapidly eroded, and the late Eocene topography has been partially resurrected.