CORALLINE RED ALGAE FROM THE UPPER EOCENE CALCARE DIE NAGO (LAKE GARDA, NORTHERN ITALY)by Davide Bassi (Ferrara)
ABSTRACT - Coralline red algae are highly abundant in the Middle-Upper Eocene Calcare di Nago as fragments, crusts, and rhodoliths. The Calcare di Nago is a carbonate unit of Bartonian-Priabonian age which is well exposed at the north-eastern end of Lake Garda, on the western margin of the Lessini Shelf (Southern Alps). Large foraminifera, corals, bryozoans, echinoderms, and molluscs are also present. The quantitative and qualitative analyses carried out on the Priabonian part of this formation enabled five facies to be distinguished based primarily on coralline red algae and large foraminifera:
1) Algal crust-branch rudstone,
2) Algal/Discocyclina packstone,
3) Coral-algal boundstone,
4) Rhodolith mound wacke/packstone, and
5) Rhodolith pavement.
According to the coralline assemblages, coralline growth-forms, and large foraminiferal associations, the five facies reflect solid and soft substrate types. Some of these facies are dominated by in situ rhodoliths, others by reworked algal debris. In the architecture of an interpreted prograding carbonate ramp, shallow water facies are dominated by coralline algae of the subfamily Mastophoroideae, while deeper water facies are dominated by members of the subfamily Melobesioideae and family Sporolithaceae. There is a significant increase both in size and in constructional voids of the rhodoliths with depth. A concomitant decrease in algal species diversity with depth has also been recognized.
According to the systematics of present-day representatives, the taxonomic investigation led to the identification of fourteen taxa, most of which belong to the nongeniculate corallines (Melobesioideae, Mastophoroideae, and Sporolithaceae). They are well preserved and many show previously undescribed vegetative and reproductive features. These algae comprise two species of Lithothamnion, one of Mesophyllum, three of Spongites, two of Neogoniolithon, two of Lithoporella, one mastophoroid genus indet., and two species of Sporolithon. Geniculates sensu lato, one species of Peyssonneliacean (red alga), and one of Halimedacean (green alga) were also recognized.