Calcified algae association from Cerro Puntudo paleolake (Anisian, early Middle Triassic), San Juan, Argentina

La Plata

Congreso; X Congreso Argentino de Paleontología y Bioestratigrafía y VII Congreso Latinoamericano de Paleontología; 2010

Calcified algae are essential in paleoecological studies of carbonate sediments to unravel the environmental conditions that prevailed in a particular setting. A microbialitic limestone of the upper section of Cerro Puntudo Formation was analyzed, emphasizing the role of calcified algae, in order to establish the paleoecological conditions that predominated during their formation. It corresponds to a carbonate lacustrine sequence of the Cerro Puntudo depocenter (Cuyana Basin). The mesostructure of the limestone consists of oncoids that vary in size between 0.5 and 8 cm. The microstructure of the oncoids displays a concentric laminated cortex. Between the laminae there are algal filaments preserved as tubules between 10 and 20 µm long with micritized walls and radial disposition. The algal tubules are straight or softly curved and are found in small aggregates or as tangled masses. The characters of the filaments are considered diagnostic of the morphogroup of tubiform calcified algae (affinity with recent Cyanophytes). The oncoid nucleus may present Charophyte thalli and associated gyrogonites that are rarely articulated. Thalli measure between 0.2 and 0.6 mm long in transversal section. Gyrogonites measure approximately 400 µm (ecuatorial) and 600 µm (polar axis) with a conical apex and base. Charophyte fragments possibly belong to the Porocharaceae family, unique in the Triassic globally. The microfacies is defined as an algal wackestone. The microbialite consists of microbial mats that grow over Charophyte thalli and reproductive structures in the margin of the paleolake (littoral zone) revealing a possible paleoecological association relevant to the paleoenvironmental reconstruction of Cerro Puntudo paleolake. The microbial mats cover Charophyte thalli while the latter were still in situ. This is supported by the association of vegetative and reproductive structures, even rarely articulated, and without signs of transport. Until now, this is the first report of this kind of association for a lacustrine Triassic environment.