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 The Permian marks a time of substantial climatic and tectonic changes in the late Paleozoic. Gondwanan glaciation collapsed after its earliest Permian acme, aridification affected the equatorial region, and monsoonal conditions commenced and intensified. In western equatorial Pangea, deformation associated with the Ancestral Rocky Mountains continued, while the asynchronous collision between Laurentia and Gondwana produced the Central Pangean Mountains, including the Appalachian-Ouachita-Marathon orogens bordering eastern and southern Laurentia, completing the final stages of Pangean assembly. Permian red beds of the southern midcontinent archive an especially rich record of the Permian of western equatorial Pangea. Depositional patterns and detrital-zircon provenance from Permian strata in Kansas and Oklahoma preserve tectonic and climatic histories in this archive. Although these strata have long been assumed to record marginal-marine (e.g., deltaic, tidal) and fluvial deposition, recent and ongoing detailed facies analyses indicate a predominance of eolian-transported siliciclastic material ultimately trapped in systems that ranged from eolian (loess and eolian sand) to ephemerally wet (e.g., mud flat, wadi) in a vast sink for mud to fine-grained sand. Analyses of U-Pb isotopes of zircons for 22 samples from Lower to Upper Permian strata indicate a significant shift in provenance reflected in a reduction of Yavapai-Mazatzal and Neoproterozoic sources and increases in Grenvillian and Paleozoic sources. Lower Permian (Cisuralian) strata exhibit nearly subequal proportions of Grenvillian, Neoproterozoic, and Yavapai-Mazatzal grains, whereas primarily Grenvillian and secondarily early Paleozoic grains predominate in Guadalupian and Lopingian strata. This shift records diminishment of Ancestral Rocky Mountains (western) sources and growing predominance of sources to the south and southeast. These tectonic changes operated in concert with the growing influence of monsoonal circulation, which strengthened through Permian time. This resulted in a growing predominance of material sourced from uplifts to the south and southeast, but carried to the midcontinent by easterlies, southeasterlies, and westerlies toward the ultimate sink of the southern midcontinent.