Freshwater sponges from Triassic carbonates of Argentina (Cuyana Basin)

BENAVENTE, CECILIA A.; MANCUSO, ADRIANA; CABALERI, NORA G.

Konstanz

Congreso; 5º International Limmnological Congress; 2011

University of Konstanz

Freshwater sponges are scarce in the fossil record globally. There exists a unique report of spiculites from the Permo-Carboniferous, and the rest from the Jurassic period. This gap in the sponge record has been related to the effects of the Permo-Triassic extinction. Thus, the present finding of freshwater sponges from a Triassic succession in Argentina (South America) constitutes the first occurrence known from a Triassic freshwater environment in Gondwanaland, giving a useful indication of paleoenvironmental and paleolimnologic conditions. The sponge remains are found in the upper section of Cerro Puntudo Formation (Anisian, Triassic). The succession consists mainly of microbial carbonates that probably correspond to spring deposits that developed on an alluvial plain near a lake margin. An oncolitic boundstone facies is interpreted as small shallow lake (pond) deposits. This is in contrast with previous reports in which freshwater sponges are reported from the shales of Jurassic alkaline lacustrine sequences and silica cherts. The sponge remains of the Cerro Puntudo area are observed in the oncoidal facies as spicules, mainly megascleres with minor microscleres. The megascleres range in size from 1.6 to 4.2 mm in length, are fusiform, straight or slightly curved, and can present spines. Microscleres measure 0.4 mm long, with spines and a preserved central canal. They are found associated with charophyte algae (gyrogonites) and cyanobacteria (filaments) in a micritic and siliciclastic silt- grain matrix. Gyrogonites are found commonly as the nuclei of oncolites and cyanobacterial filaments are present within cortices, while the spicules are dispersed in the matrix. The finding of Porifera representatives is the first from a freshwater environment of the Triassic period globally, what allows reducing the gap in the sponge register from the Permo-Carboniferous to the Triassic. The presence of sponges in this oncolitic facies gives an interpretation of oxic shallow conditions with clear waters, perhaps associated with groundwater springs feeding into small carbonate-rich Cerro Puntudo paleolakes.