The Circum-Laurentian Carbonate Bank, the Western Ouachita-Cuyania Basin, and the Prodigal Llanoria Landmass

Within the southern circum-Laurentian carbonate bank, a 1000-km-long 550-km-wide (621 342 mi) basin, the Ouachita-Cuyania Basin, developed during the Cambrian–Middle Ordovician. The western Ouachita-Cuyania Basin, including the Marathon-Solitario subbasin (west Texas), was flanked by fully correlative platform carbonate successions on the southern margin, as well as on the north. The southern carbonate complex, with its homologous sponge-algal reef organisms, is now preserved in the Cuyania terrane (greater Precordillera of Argentina). The Marathon-Solitario subbasin and Cuyania, with common fundaments of Laurentian Mesoproterozoic (Grenvillian) basement, evolved together, as evidenced by isotopic, lithostratigraphic, biostratigraphic, and chronostratigraphic data, as well as by new high-precision paleomagnetic determinations. Ages, lead-isotopic data, and geochemical data for Cuyania and for west-central Texas crystalline basement rocks, particularly the Llano uplift and the Pecos mafic intrusive complex, are markedly similar. Mesoproterozoic through Eocambrian rocks of the central and western Ouachita embayment furnished detrital zircons to Cambrian siliciclastic deposits in the Precordillera. Faunas of the carbonate platform sequences that developed on both sides (e.g., El Paso and Chica de Zonda) of the basin were particularly well developed around the Laurentian Ouachita margin. The Marathon-Solitario subbasin received sediments from both north and south; much detritus came from the northern shelf, as well as from elevated blocks within the basin, and included decimeter-scale olistoliths bearing a shelf fauna. Erosional gaps on the platform correlate with intervals of coarse carbonate detritus in the basin. Cuyania constituted the vanished early Paleozoic landmass of Llanoria, the long-sought southern source for volcaniclastic, metaigneous, and metasedimentary detritus—most probably the Famatina eruptive complex and western Sierras Pampeanas—in siliciclastic deposits of the Marathon succession. Newly discovered pyroclastics and igneous rocks in the Marathon Formation suggest that coeval volcanic vents may also have developed on the western Ouachita margin during separation of Cuyania from Laurentia. Extensional block faulting, variable carbonate platform and outer-shelf and/or slope sedimentation, and explosive volcanism characterized the western Ouachita-Cuyania Basin from the Cambrian into the Middle Ordovician. Stratigraphy and structures of the basin are consonant with paleomagnetically derived plate reconstructions for that period, which place Cuyania and western Gondwana at low southern latitudes (26S) and adjacent to southern Laurentia (omitting most of Mexico). The western Gondwanan margin trended west, that is, about 90 clockwise from its present orientation, and faced southern Laurentia. The Ouachita-Cuyania Basin was thus positioned to receive sediments from both Gondwana on the south and Laurentia on the north. The biostratigraphic data can readily be accommodated through faunal interchange by means of equator-parallel currents. During the Sandbian, Cuyania moved beyond range of faunal exchange with Laurentia, and tholeiitic basalts with enriched mid-ocean ridge basalt (E-MORB) characteristics were intruded into off-shelf turbidites down the length of the western Precordillera. The attenuated Laurentian slab broke apart with continued oblique dextral (transtensional) separation of Laurentia and Gondwana, and the early Paleozoic Llanoria landmass departed with the southern megacontinent. Geochronologic, paleomagnetic, and biostratigraphic data all attest to accretion of Cuyania to Gondwana well before the onset of Taconic orogenesis in eastern Laurentia.