Paleoecological reconstruction of a Triassic paleolake (Agua de la Zorra Formation, Cuyana Basin, Mendoza, Argentina)

Abstract

The Agua de la Zorra Formation outcrops in the Paramillos de Uspallata Area (NW of Mendoza), andrepresents a deltaic-lacustrine system with episodic incursions of peperitic basalts. The successionincludes, from the base to the top, lacustrine distal facies, with proximal shallow facies towards themiddle section. Paleontological record includes invertebrates (spinicaudatans, insects), vertebrates(mainly in the shallow facies), vertebrates and invertebrates traces, and plant remains (mainlyreferred to Umkomasiales) that reflect the presence of plant paleocommunities corresponding to the typical Dicroidium Flora around the paleolake. The kerogen of the distal facies is mostly terrigenous, with scarce amorphous and palynomorphs (disacates and microalgae), it shows a variable percentage of total organic carbon content (between 0.2 and 4.6), and hydrogen and oxygen indices indicating a type III kerogen. In the shallow facies, kerogen is practically absent, and traces registered in proximal facies appear to reflect a sporadic subaerial exposure of the sediments. The data suggest a lake with very low productivity, which can be understood considering diluted hydrological conditions in an overfilled lake-type basin. This is consistent with the Agua de la Zorra paleolake inferred trophic state as oligotrophic (mesotrophic?). Probably low productivity is also linked to the presence of lava in the water column what can be supported by the spinicaudata mortality events registered suggesting changes in the physicochemical parameters. In an alternative explanation, the character of the kerogen could be evaluated in relation to diagenetic processes.