ABSTRACT Upper Triassic and Lower to Middle Jurassic strata in the Plomosas uplift of central Chihuahua accumulated in backarc and rift settings, respectively. The succession, as much as ~3250 m thick, consists of four stratigraphic units. The Cerro El Carrizalillo Formation (Carnian–Norian), a volcanic-lithic shallow-marine succession deposited in the (newly named) El Carrizalillo backarc basin, is characterized by predominantly Triassic detrital zircon ages. The overlying Plomosas Formation consists of three members: (1) the Cerro de Enmedio Member (Hettangian–Toarcian), a succession of conglomerate, siltstone, and shallow-marine carbonate strata deposited during the onset of extension in Chihuahua; (2) the Cerro Nevado Ignimbrite Member (176 ± 1 Ma; late Toarcian), a widespread ash-flow tuff; and (3) La Sofía Member (Aalenian–Callovian?), consisting of alluvial-fan conglomerate, fluvial sandstone, tidal sandstone and siltstone, and delta-plain red beds characterized by rapid facies changes, lithic compositions, and diverse Proterozoic, Paleozoic, and Triassic detrital zircon ages characteristic of a rift-basin setting. The extensional basin in which the Cerro de Enmedio and La Sofía members accumulated is termed the Plomosas basin. Improved age control provided by U-Pb maximum depositional ages from detrital zircon and U-Pb zircon analyses of the ignimbrite indicates that the Cerro El Carrizalillo Formation is partly correlative with the Chinle Formation of the Colorado Plateau, and the Plomosas Formation is equivalent to eolianites of the Glen Canyon and San Rafael Groups of the Colorado Plateau. Detrital zircon ages and sandstone textures are consistent with both proximal and distal sediment sources along the Laurentia-Gondwana suture and adjoining Grenville basement of Laurentia, including sources in northern Mexico and the composite Appalachian orogen. Although the depositional setting of the Cerro El Carrizalillo Formation was not connected to fluvial systems of the Chinle Formation, subsequent eolian transport of voluminous sediment to the overlying Cerro de Enmedio and La Sofía members from the Colorado Plateau ergs is suggested by the composition and texture of some sandstone, thick siltstone accumulations, and detrital zircon characteristics that broadly resemble those of the Colorado Plateau eolianites. Thick siltstone in the upper part of La Sofía Member is interpreted as deflated fine-grained sediment that was transported downwind from a time-equivalent erg to accumulate in shallow-marine and coastal-plain settings of the Plomosas basin.

Abstract The Upper Triassic Chinle Formation of the Colorado Plateau in the western United States was deposited as a continental fluvial-floodplain-lacustrine-eolian depositional system near the west coast of Pangea just north of the paleoequator. The deposystem evolved in response to regional tectonics and long-term climate change. Clastic and volcanic sediment was supplied by uplifted highlands and arc magmatism on the margins of the basin. Climate changed from a Pangean megamonsoon regime in the early Late Triassic to an arid climate setting at the close of the Triassic. Chinle fluvial discharge and sediment load varied through time in response to climate, producing degradational and aggradational cycles. Degradation eroded paleovalleys, and aggradation filled them with both meandering and braided fluvial systems, as well as varying amounts of floodplain mudstones. Paleosols developed on paleovalley fills, interfluves, and floodplain siltstones and mudstones, all of which combine to define several scales of degradational and aggradational cycles. An upward stratigraphic trend in Chinle paleosols from Oxisols and Gleysols, to Alfisols (Argillisols) and Vertisols, culminating in Inceptisols (Prototsols) and Aridisols (Calcisols), supports previous interpretations that climate evolved from monsoonal or humid conditions at the beginning of Chinle deposition to progressively more arid climate conditions at the end of deposition.