ABSTRACT Upper Cambrian through Middle Ordovician sedimentary strata of the Marathon/Solitario Basin (west Texas), which were folded and thrust-faulted during late Paleozoic Appalachian-Ouachita orogenesis, preserve evidence of the pre-Pangean history of the central southern Laurentian margin. New detrital zircon analyses reported here are from three Marathon Basin/Solitario formations: the upper Cambrian Dagger Flat Sandstone; the Lower Ordovician Marathon Formation, including the Rodrigues Tank Sandstone Member; and the Middle Ordovician Ft. Peña Formation. The far-southwestern outcrops of those Iapetus margin strata are within the Solitario dome (Presidio and Brewster Counties, Texas). Solitario zircon U/Pb geochronological results (laser ablation–inductively coupled plasma–mass spectrometry [LA-ICP-MS], sensitive high-resolution ion microprobe [SHRIMP]) expand the record of Cryogenian rifting as the Cuyania terrane separated from Laurentia. We evaluated these new data along with earlier geochronological and geochemical results from rift-related lava clasts in Lower–Middle Ordovician sedimentary subaqueous debris-flow deposits in the northwestern Marathon Basin. Deepening of the Iapetus seaway near the Laurentian margin (late Cambrian–Middle Ordovician) stimulated headward erosion of drainages, reflected in the systematic north-northwestward shift in zircon provenance from the west Texas Grenvillian and Southern Granite-Rhyolite Provinces to Yavapai-Mazatzal and Cheyenne Belt sources. The Cuyania rifted terrane underwent subduction at the western Gondwanan margin of the Iapetus Ocean in mid-Ordovician time (486 ± 7 Ma to 463 ± 4 Ma), and the resulting volcanism in the Famatina complex (Argentina) was most intense from ca. 472 to 468 Ma. Magmatic zircons from Ft. Peña bentonitic layers have identical U/Pb (488–468 Ma) and biostratigraphic (Darriwilian) ages to those from Famatinian bentonites at Talacasto (470 ± 5 Ma) in the Precordillera of Cuyania. Geologically constrained paleomagnetic reconstructions for 470 Ma depict the proximity of the Famatina arc, the rifted Cuyania terrane, and southern Laurentia at low southern latitudes (equator to ~30°S). These first U/Pb geochronological data from the Marathon/Solitario depocenter of western Iapetus appear to be compatible with such a configuration and can serve as test data for emerging tectonic interpretations.

Abstract In 1937, P. B. King published his classic treatise on the rocks of the Marathon Basin. Today, that work stands as the definitive reference and primary source for the west Texas part of the Ouachita fold-thrust belt. This paper summarizes the present state of knowledge about the basin (emphasizing recent insights), raises more questions than answers, and demonstrates the soundness of King's work to this day.

Abstract Gas and oil fields producing from Devonian reservoirs in the Permian basin of West Texas and New Mexico form a remarkably uniform pattern—fields producing only gas on the south and west, fields with high gas/oil ratios and gas-condensate fields near the center of the region, and fields which produce only oil on the north. The methane content of gases in these fields has a similar regional variation from more than 90 per cent methane on the south and west to less than 30 per cent on the northeast. Analysis of variation of other gas components and crude oil suggests that the methane variation is independent of both other gases and crude oil composition. Relation of methane variation to variation in gas volume suggests that variation in gas distribution is primarily a function of methane distribution. Limited data suggest that carbon dioxide may be anticipated in increasing volumes along the southern margin of the region. Uniformity of the observed pattern indicates that only gas will be found in Silurian-Devonian reservoirs in the Delaware and Val Verde basins. The gas-distribution pattern bears no consistent relation to Silurian-Devonian lithofacies or structural relief. There is simply an increase in the amount of methane toward the Marathon thrust belt. It is proposed that the gas pattern may have been effected by distribution of methane in solution in formation water migrating away from the Marathon thrust belt under the influence of supernormal pressures resulting from overthrusting. Carbon dioxide may have been distributed similarly at a later time and in smaller volumes than methane. Differential distribution of both gases may have been a function of volume of methane or carbon dioxide in solution in a given area and of the volume of gas-bearing water reaching the area.

The Ouachita-Marathon-Sonora orogen is a 3000-km-long belt of deformed Paleozoic rocks bordering the southern margin of the Laurentian (North American) craton. Extending from Mississippi and Arkansas (Ouachita) southwestward through Texas (Marathon) and westward through Chihuahua and Sonora (Sonora), the orogenic system formed during a late Paleozoic collisional-subductional event. This event resulted in closure of the Rheic ocean and the development of the orogen as the southern edge of the Laurentia plate was subducted beneath a northward-advancing Gondwanan (South American) continental-margin arc. Foredeeps and foreland basins and uplifts were created on the Laurentian plate continentward of the orogen. Regional stratigraphic and structural relations indicate original physical continuity of the Ouachita-Marathon-Sonora orogenic belt along the entire southern margin of Laurentia. In the Neoproterozoic and Early Cambrian, the supercontinent Rodinia rifted along trends later followed by the Ouachita-Marathon-Sonora orogenic belt. During the breakup of Rodinia, the promontories and embayments that developed along the south margin of Laurentia are related to a northeast-striking rift system (oceanward of the continental edge) deformed by northwest-striking synrift transform faults that offset the rift-parallel cratonal margin. Initial deposits of the rifted margin were sediments deposited in tectonic sags and basins, which opened oceanward, and in depressions adjacent to transform faults. Shelf deposition began with Middle Cambrian clastic and carbonate sediments and continued with deposition of mostly shallow-marine carbonate sediments. Offshelf deposition began with Upper Cambrian (oldest rocks recognized in thrust sheets) clastic sediments and continued into the Early Mississippian with deposition of deep-marine clastic and subordinate carbonate sediments in continental-rise and ocean-basin settings. Deformation of the southern margin of Laurentia resulted from its late Paleozoic diachronous oblique collision with the South American part of Gondwana and development of volcanic-plutonic arc and associated fore-arc and back-arc assemblages along the northern margin of Gondwana. Westward younging of foredeep and foreland-basin depocenters and decreasing age of basin-fill sediments are in accord with the westward migratory closure of the Laurentia–Gondwana suture. Cambrian to Lower Mississippian preorogenic sediments were deposited in offshelf settings along the margins of the two continents, and Upper Mississippian to Permian synorogenic sediments were deposited in the deep-water ocean basin between the two converging continents and associated arcs, in troughs within the evolving allochthons, and in foredeeps and foreland basins on the craton margins. Ocean-basin sediments were scraped off the ocean floor and transported as allochthons formed during continental collision, in advance of synchronous Pennsylvanian and Permian foredeeps and foreland basins and uplifts. These allochthons were part of a large accretionary wedge formed above a south-dipping subduction zone and thrust northwestward 50–200 km above Laurentian continental-shelf and foredeep rocks. The late Paleozoic synorogenic foreland basins and uplifts developed cratonward as far north as the Transcontinental arch and Ancestral Rocky Mountains. The timing and sense of movement of these intracratonic structures have complex relationships to the collisional margin. Deformation in all three segments of the Ouachita-Marathon-Sonora orogenic belt began in mid-Mississippian time and ended diachronously in the Late Pennsylvanian in the Ouachita Mountains, Early Permian in the Marathon region, and Late Permian in Sonora. This represents predominantly north to northwest contraction. The westward migratory termination of orogenesis and the related development of foredeeps along the Ouachita-Marathon-Sonora belt are consistent with oblique convergence of Gondwana (Africa and South America) with Laurentia and require some clockwise rotation of South America following initial collision in the Ouachita Mountains region. The Sonora segment of the Ouachita-Marathon-Sonora orogenic system shows that northwestern Gondwana and its associated volcanic-arc terrane once lay south of western Laurentia (south of central Sonora) and was not restricted to areas to the east of Sonora as often shown in Pangaean reconstructions. Data south of the Laurentia continental margin indicate that the Gondwanan crust was extremely variable. Ashfall tuffs in Upper Mississippian through lowermost Pennsylvanian hemipelagites and turbidites in the Ouachita Mountains and Marathon region, and rhyolite flows and ashfall tuffs (bentonites) in Permian flysch in the southern Pedregosa basin of north-central Mexico indicate explosive volcanism along the northern margin of Gondwana. Geochemistry of the upper Paleozoic extrusive Sabine Rhyolite in the flysch sequence of the Sabine uplift area indicates a continental-arc origin. Several inferred remnants of Gondwana crust and volcanic-arc rocks in the southern United States and northwestern Gulf of Mexico are collectively referred to as the Sabine block. Other inferred remnants of Gondwana crust and volcanic-arc rocks in northern Mexico include the Coahuila block in Coahuila and southeast Chihuahua, and the El Fuerte block in Sinaloa, Sonora, and adjacent Chihuahua. Several lines of critical evidence contradict a late Paleozoic or Mesozoic megashear through Sonora. Both the Rodinia rift system and the Ouachita-Marathon-Sonora orogen represent a continuous southern continental margin westward to Baja and Alta California, across the supposed trace of the hypothetical Mojave-Sonora megashear. Although structures are present that offset the orogen and its associated rift system, their sense of offset is right-stepping rather than left-stepping as required by the megashear. Minor displacements by postorogenic faulting together with right-stepping transform faulting cumulatively represent ∼300 km of displacement in contrast to 600–1100 km of left-lateral offset postulated for the megashear. Stratigraphic facies changes around the southwest end of Laurentia do not support offset by such a megashear. Paleozoic biostratigraphic faunal groups persist throughout the length of the orogen to Baja California and are distinct from those in the Cordilleran margin. These, together with later Mesozoic faunal groups, continue across the hypothetical trace of the megashear with no offset. Finally, paleomagnetic data by other workers are not supportive of the megashear. The megashear concept is not compatible with the information presented in this chapter.