CABALERI nora Graciela
Microbialitic lacustrine successions of two sub-basins from the Triassic Cuyana rift, Argentina
Lugar: Londres; Año: 2015 p. 1 - 43
The Triassic Cerro de las Cabras and Cerro Puntudo Formations represent the sedimentary infilling of the Cuyana rift basin. The basin is composed of two asymmetric half-grabens, situated from north to south and linked by a topographic high. These half-grabens are the Cerro Puntudo sub-basin, in the north containing the Cerro Puntudo Formation and the Potrerillos sub-basin, in the south containing the Cerro de las Cabras Formation. Both formations contain continental deposits and conform to the typical alluvial-fluvial-lacustrine sequences of rift basins. Previous work has provided absolute dates, confirming that the deposits are coeval in age (Anisian). Most significantly, carbonate deposits with abundant microbialites are present in both units, allowing a comparative analysis across the Cuyana Basin. Sedimentologic studies in the Cerro de las CabrasFormation reveal two facies associations that characterize two subenvironments: (1) Sandflat and Mudflat facies association represented by the coarse sandstones, massive intraclastic sandstones, and massive mudstones facies, and (2) Palustrine limestone facies association represented by mottled-nodular limestones and stromatolitic limestones facies. These subenvironments are interpreted to represent an ephemeral playa lake depositional system with common features of subaerial exposure and pedogenesis. In the Cerro de las Cabras Formation evidence for biota in the stromatolitic limestone facies indicates induced precipitation of carbonate. Also the palustrine limestones facies with abundant rhizohaloes and rhizoliths indicate vegetated paleosols. Sedimentologic studies of the Cerro Puntudo Formation reveal six facies associations: (1) Alluvial fan with matrix-supported conglomerates and stratified intraclastic sandstones facies; (2) Sandflat represented by massive sandstones and laminated sandstones facies; (3) Mudflat with laminated mudstones and laminated siltstones facies; (4) Carbonate channels with oncolitic mudstones and oncolitic laminated sandstones facies; (5) Palustrine limestones with mottled limestones, nodular imestones, and stromatolitic limestones facies; and (6) Lacustrine limestones with laminated mudrocks and oncolitic boundstones facies. These subenvironments formed part of an alkaline playa lake system fed mainly by groundwater and with ephemeral surface water supply. Some sedimentary features suggest the presence of an apron discharge area in the sub-basin. All the facies associations except for the Lacustrine limestones show abundant subaerial exposure and pedogenesis. The Cerro Puntudo paleolake contains a microbialite assemblage that reflects a simple aquatic paleoecosystem with a simple trophic network. This is composed of filamentous algae that constitute the primary producers with lithified structure through bio-mediated carbonate precipitation. Also, charophytes have been identified fulfilling the same task. As heterotrophs, ostracodes were found probably feeding from detritus and bacteria as OM decomposers. The sedimentary and paleontologic data obtained allowed the characterization of the two lacustrine depositional systems containing balanced-filled lakes reflecting sedimentation controlled by tectonics (synrift stage) as well as climate (groundwater and surface water input).