Stratigraphy, sedimentary facies and palaeoflora of the Potrerillos Triassic succession, Mendoza, Republic of Argentina.

Abstract

New exposures of the classic Triassic localities of cerro Bayo and cerro Cocodrilo (Potrerillos, northwestern Mendoza province, Argentina) allowed to: (1) describe the stratigraphic architecture of the Río Mendoza, Cerro de Las Cabras, Potrerillos and Cacheuta formations; and (2) to improve the biostratigraphic knowledge of the Triassic succession based on new findings of fossil plants. Two vertical sections were measured in detail, and lithofacies analysis has been used to interpret depositional systems. Alluvial fan deposits are recognized in the Río Mendoza Formation and in the lower part of the Potrerillos Formation. Gravely and sandy bedload fluvial deposits appear towards the top of the Río Mendoza Formation and in several intervals of the Potrerillos Formation, whereas the basal part of the Cerro de Las Cabras Formation and almost all the Potrerillos Formation are characterised by high sinuosity stream and flood plain deposits. Some intervals of the Potrerillos Formation, but specifically the Cacheuta Formation, are dominated by black shale successions deposited from suspension fallout in meromictic lakes. Progradational sandstone and conglomerate intercalations within black shale successions suggest the development of hypopycnal mouth bars and distributary channel deposits. Fossil plants within the studied succession are located in 16 fossiliferous levels, and 23 taxa were identified: Equisetites fertilis, Neocalamites carrerei, Cladophlebis mesozoica, C. mendozaensis, Dicroidium argenteum, D. dubium, D. odontopteroides, Johnstonia coriacea, J. stelzneriana, Xylopteris elongata, Zuberia feistmanteli, Feruglioa samaroides, Pachydermophyllum praecordillerae, Kurtziana cacheutensis, Sphenobaiera argentinae, Baiera africana, B. cuyana, Yabeiella mareyesiaca, Y. spathulata, Y. brackebuschiana, Fraxinopsis andium and Gontriglossa sp. and Rochipteris sp. Based on the relationship between the paleobotanical composition of the fossiliferous levels, sedimentary facies and depositional settings, 16 taphocenoses are defined and described. Finally, two assemblage biozones (Yabeiella mereyesiaca- Scytophyllum bonettiae-Protophyllocladoxylon cortaderitaensis -MBC- and Yabeiella brackebuschiana - Scytophyllum neuburgianum-Rhexoxylon piatnitzkyi -BNP-) are identified in the type locality of the Potrerillos Formation. This biostratigraphic information allows to suggest that the Potrerillos Formation was deposited during the late Middle Triassic - early Late Triassic.