Abstract The Carboniferous Tesnus Formation in the Marathon basin of west Texas was deposited as a large submarine-fan complex in a tectonically active, migrating foredeep. Primary depositional fabrics in siliciclastic mass-flow deposits of the Tesnus Formation were extensively modified during intense soft-sediment deformation. Fluidization and clastic intrusion were common processes and produced clastic injection structures possessing a remarkable array of shapes and orientations with respect to bedding. The most commonly recognized intrusive bodies are dikes, which were nonsystematically injected into overlying host sediments at angles ranging from just greater than 0–90°. Dike orientations, corrected for tectonic deformation, show no correlation with paleoslope or later structural trends and, based on traditional decompaction methods, are interpreted to have been injected at relatively shallow burial depths (tens to hundreds of meters). Clastic sills are nearly as common as dikes and, based on their generally greater thickness, are interpreted to have been responsible for accommodating a greater proportion of postdepositional sediment remobilization than any other type of intrusion. Other, more unusual clastic injection structures in the Tesnus include concordant and discordant cylindrical clastic pipes; irregularly shaped, wavelike intrusions in which one surface is concordant and the other is discordant; and intrusions that are both concordant and discordant along different parts of the injection body. With clastic dikes and sills as end members, previously undescribed clastic injection structures in the Tesnus Formation define a spectrum of features with geometries transitional between concordance and discordance.

Abstract Sedimentary rocks of Ouachita facies are here defined as rocks lithologically similar and strati-graphically equivalent to sedimentary and low-grade meta-sedimentary rocks exposed in the Ouachita Mountains of Oklahoma and Arkansas. Data obtained from numerous well borings show that rocks of Ouachita facies form a long, folded, geosynclinal belt that passes beneath the Cretaceous overlap in southern Oklahoma, bends around the Central Mineral Region of Texas in a buried structural belt, strikes almost due west, is exposed at the surface again in the Marathon and Solitario uplifts of extreme southwestern Texas, and crosses the border into Mexico. In the outcrop areas, and throughout the length of the Ouachita structural belt, these rocks show strong similarities in lithologic character, fabric, and meta-morphic grade, which may be used to differentiate them from sediments of foreland or Arbuckle facies. In the Ouachita Mountains, rocks of late Mississippian and Pennsylvanian age reach a maximum thickness of at least 22,000 feet and are divided into two major groups. These are (1) strata widely exposed in the central part of the Ouachita Mountains, and (2) strata of frontal zone Ouachita facies exposed only in small slivers in the western part of the Ouachita Mountains in Oklahoma. The former comprise, from oldest to youngest, the Stanley shale, Jackfork sandstone, Johns Valley shale, and Atoka formation. In the frontal zone they comprise, from oldest to youngest, the Caney shale, Springer formation, Wapanucka limestone and Chickachoc chert, and the Atoka formation. The rocks of late Mississippian and Pennsylvanian age in the Marathon uplift in southwest Texas attain a maximum thickness of 12,800 feet and consist of four formations. From oldest to youngest, these are the Tesnus formation, Dimple limestone, Haymond formation, and Gaptank formation. In both outcrop areas and in the buried structural belt, sediments of Ouachita facies were down-warped in an active geosyncline and subsequently deformed and uplifted. Most of the beds were laid down as sediment flows or deposits from turbidity currents in deep water, but the Wapanucka, Chickachoc, and Dimple formations are shallow-water, shelf-type deposits formed mostly in the zone of wave action. The Caney shale and dark siliceous shales in several formations were deposited in deep unaerated basins with part of the sediments derived from ash falls. The boulder beds in the Jackfork, Johns Valley, and Haymond formations apparently were formed by submarine landslip and mudflows. The Caney shale of the frontal Ouachitas is of late Mississippian age and is the stratigraphic equivalent of the Stanley, Jackfork, and lower part of the Johns Valley formations of the central Ouachitas. The upper part of the Johns Valley is of early Pennsylvanian (Morrow) age and is equivalent to the Springer, Wapanucka, and Chickachoc formations of the frontal Ouachitas. The lower part of the Atoka is also of Morrow age, but the upper limit of its age has not been established. The Tesnus formation is of late Mississippian and early Pennsylvanian (lower Morrow or Springer) age. The Dimple limestone is of Morrow age; the Haymond formation is early to middle Pennsylvanian (Morrow and Atoka) in age; and the overlying Gaptank formation is of middle and late Pennsylvanian (Des Moines to Virgil) age. During the last stages of the deformation of the geosynclinal belt, rocks of Ouachita facies were thrust over rocks of foreland or Arbuckle facies for considerable distances. The foreland rocks underlying the thrust sheets are potentially petroliferous; exploration for petroleum production in these buried shelf sediments should be more rewarding than in sediments of Ouachita facies.

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.

The Caballos Novaculite is composed predominantly (90 percent) of bedded chert; the widespread distribution of two novaculite (milky white chert) lithosome marker units permits the formation to be divided into five members. From the base up, the members are listed with maximum thicknesses and rock type as follows: lower chert, 15 feet, light brown and off-white chert with shale partings; lower novaculite, 150 feet; lower chert and shale, 200 feet, chert of many different colors (chiefly green) and shale partings, and several beds of calcarenite; upper novaculite, 400 feet; and upper chert and shale, 400 feet, chert of many different colors and shale partings. Minor amounts of red and green shale, pebble conglomerate, sandstone and calcarenite are in the chert and shale units. The Caballos Novaculite in the Marathon Basin is a lens-shaped unit from 100 to 700 feet thick, based on 26 measured sections. The Caballos is conformable with the underlying Maravillas Formation of Late Ordovician age and with the overlying Tesnus Formation of Mississippian to Early Pennsylvanian age; and, therefore, probably includes rocks of Silurian, Devonian, and possibly Mississippian age. The novaculite members may be time- stratigraphic units in the center of the Marathon Basin. Chert formed by the diagenetic alteration of opaline skeletal particles, chiefly sponge spicules and Radiolaria. Quartz in the form of subsequent grains of microquartz (<35 μ) and lesser megaquartz is the only silica phase present. Color varieties of chert differ in fabric of microquartz and content of pigmenting impurities; green chert contains abundant illite; tan, brown, and black chert contain iron and manganese oxides and organic matter; red chert contains hematite; blue chert contains apatite. Mottled chert beds formed by submarine slumping. Novaculite (snow-white chert) owes its lack of color to the absence of detrital impurities and chemical pigments, and its milkiness to the dispersion of light by minute water inclusions (Folk, 1965) and reflecting faces of microgranular quartz crystals. Novaculite has a distinct microscopic fabric; subspherical specks up to 200 μ in diameter composed of microquartz grains up to 10 μ long are scattered in a matrix of slightly larger uneven-grained microquartz grains, 5 to 25 μ long; the specks are probably relict Radiolaria. Skeletal ghosts of spicules visible in ordinary light range from 0 to 80 percent of novaculite beds, but average 10 percent. Radiolaria are more abundant than spicules in colored chert beds. The Caballos was deposited in a deep-marine trough adjacent to peneplaned land masses; rate of accumulation (assuming 50 percent compaction) was from 0.1 to 0.5 mm/1000 years. Terrigenous clay was absent during accumulation of proto-novaculite, but was supplied occasionally during accumulation of colored chert and shale beds. Green and red (originally brown?) colors of clay shale reflect original differences in bottom conditions during deposition. Quartz silt in chert includes wind-blown and storm-deposited grains; sandstone, calcarenite, and conglomerate beds were deposited by rare turbidity currents. Depositional fabrics of sediments were modified by lithification processes, burrowing infauna, escaping gases, and submarine slumping. Chertification occurred largely by solution of opal and precipitation of crystalline silica (cristobalite or low-quartz) from pore solutions relatively soon after deposition.