CONICET | Buscador de Institutos y Recursos Humanos

A palaeobotanical biozonation and a chronostratigraphic chart for the continental Triassic of Argentina are proposed. The biostratigraphic study was based on the multivariate analysis of 218 taxa distributed in 15 lithostratigraphic units and 134 taxa from 28 palaeocommunities. The biostratigraphic analysis was also supported on the Permian or Jurassic lineage of several species, the acme, the temporal rechange and the evolution of the paleofloras. This information was evaluated together with the stratigraphic location of the fossiliferous horizons, the geochronologic information and the relationship between palaeocommunities and sedimentary facies and environments. Five biozones were defined: 1.- Biozone Dictyophyllum castellanosii, Johnstonia stelzneriana, Saportaea dichotoma (CSD). Represented by a mixed forest. The tree layer with Corystospermales (Zuberia feistmantelii, Z. brownii) and primitive Ginkgoales (Saportaea) associated to Voltziales (Heidiphyllum elongatum) precursors of Coniferales. The shrub layer or the areas free of trees were dominated by ferns (Dipteridaceae, Dictyophyllum castellanosii) and Corystospermales (Johnstonia stelzneriana). This biozone was identified in the Barreal and Ischichuca Formations. 2.- Biozone Yabeiella mareyesiaca, Scytophyllum bonettiae, Protophyllocladoxylon cortaderitaensis (MBC). Mixed forests of Corystospermales (with two morphological patterns: cuneumxyloid and rhexoxyloid) and Coniferophytes precursors of Coniferales. The herbaceous-shrub-like community was dominated by Kurtziana cacheutensis, Yabeiella mareyesiaca and Scytophyllum bonettiae. This biozone is recognized in the Potrerillos, Cortaderita (lower section) and El Alcázar Formations. 3.- Biozone Yabeiella brackebuschiana, Scytophyllum neuburgianum, Rhexoxylon piatnitzkyi (BNP). Mixed bistratified forests in which the Coniferophytes (Voltziaceae and Protopinaceae) constitute the upper tree layer. The cuneumxyloid pattern declines and in the lower tree layer smaller-sized corystospemal trees survive. Monospecific rhexoxyloid tree forests are restricted to riparian environments. In the areas free of trees, Yabeiella brackebuschiana is prominent, and associated with corystospermales (as Johnstonia coriacea) and Peltaspermales (Pachydermophyllum precordillerae and Scytophyllum neuburgianum). This biozone is identified in the Vera, Potrerillos and Ischigualasto Formations, and in the El Tranquilo Group. 4.- Biozone Dicroidium odontopteroides, D. lancifolium (OL). Represented by a herbaceous and shrub-like palaeocommunity characterized by Dicroidium odontopteroides and D. lancifolium in association to other Pteridosperms (Johnstonia and Xylopteris) and with frequent Yabeiella brackebuschiana. It is defined in the Cacheuta Formation and in the upper section of the Cortaderita Formation. 5.- Biozone Dictyophyllum tenuiserratum, Linguifolium arctum, Protocircoporoxylon marianaensis (DLM). Deciduous forests composed of Coniferophytes and Ginkgoales, and mosoxerophytic forests of Corystospermales. The Coniferophytes surpassed in evolutive rivalry to Corystospermales (only the rhexoxyloids survived in highly restricted environments). The tree layer is composed of Coniferales: Voltziaceae (similar to those of older biozones), evolved Protopinaceae (as Protocircoporoxylon marianaensis) and very incidental Cheirolepidiaceae. As shrub layer or in the areas devoid of trees, abundant Dicroidium crassum is associated with Xylopteris argentina, X. elongata, Dicroidium odontopteroides and D. lancifolium, as well as with taxa of Jurassic affinity such as Cladophlebis, Goeppertella, Linguifolium and Scleropteris. This biozone is identified in the Los Colorados and Río Blanco redbeds, and is completely developed in the Paso Flores Formation and in the Tronquimalal Group. The chronostratigraphic proposal is based on the biozone chronology, in the analysis of the main stratigraphic unconformities and in the evolution of the basin infill. The following stages are defined: BARREALIAN. Follows the Huarpe Tectonism. The western Argentinean basins were opened (Ischigualasto/Villa Unión, Marayes, Cuyo -including the Barreal/Rincón Blanco depocenter- and San Rafael) and the synrift sedimentary infill is controlled by strong tectonic subsidence. This stage includes the palaeofloras of the Puesto Viejo Formation (San Rafael) and those of the CSD biozone. Fossil vertebrates belong to the Cynognathus and Massetognathus biozones. The Barrealian ranges from the Early Triassic to the Middle Triassic. CORTADERITIAN. There is a change from synrift to postrift sedimentation in the western Argentinean basins, due to generalised thermal subsidence. Basal Cortaderitian unconformities are recorded in the Ischigualasto-Villa Unión and the northern Mendoza depocenters of the Cuyo basins, though in Barreal-Calingasta the sedimentary record is continuous from the Barrelian stage. In other regions, the Tunuyán Tectonism caused the closure of the San Rafael basin and the opening of the Patagonian (Los Menucos and El Tranquilo) basins, both characterised by volcaniclastic synrift deposits. The fossil plants (included in the MBC, BNP and OL biozones) reached their maximum diversity and abundance, whereas the vertebrates are grouped in the Exaeretodon biozone. This stage extends from the late Middle Triassic to the early Late Triassic. FLORIAN. Transition form the postrift to the transtensional phase in the basins of western Argentina, where the infill is characterised by redbed deposits. In several depocenters there is a basal uncorformity related to the Río de Los Patos Tectonism. These movements produced the closure of the Los Menucos basin and the opening of the Malargüe and Paso Flores-Lapa rifts which are the precursors of the Jurassic-Cretaceous Neuquén Basin. The Florian finishes with the Río Atuel Tectonism with which the closure of the western Argentinean basins (Ischigualasto-Villa Unión, Marayes, Cuyo) occurred. This stage includes the DLM palaeofloristic biozone and the Jachaleria and Riojasaurus reptile biozones. Its age is upper Late Triassic.

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