From build-up to pebbles: brief history of a crustose coralline build-up of the Upper Eocene of Northwestern Italy

Fossil crustose coralline framework are known from the Eocene onward, with occurrences in middle Eocene of Spain, the Upper Eocene of Northern Italy, the Miocene of Malta and Poland and the Plio-Pleistocene of the Mediterranean. The studied example presents a coralline algae framework from the Upper Eocene of Northwestern Italy. Although the carbonate platform where the build-up developed is lost, its remains are preserved. Reworked skeletal grains, transported off shore by debris-flow, are preserved in the Ternate Formation, a deep-water sub-marine fan. In the Upper Oligocene conglomerates of the Gonfolite Lombarda Group, biogenic carbonate pebbles of Upper Eocene age are preserved. Textural characteristics, skeletal assemblages, coralline-algae flora and benthic foraminifers composing the pebbles were studied and compared to those of Ternate Fm. The same rhodalgal skeletal-assemblage, the same species of coralline algae and the same association of benthic foraminifers were found in both set of samples, suggesting their common origin. While Ternate Fm. materials were undoubtedly reworked during transport, the limestone pebbles are pristine fragments of the original carbonate platform which was uplifted and eroded between Upper Eocene and Upper Oligocene. They actually have no internal textural features suggesting reworking (common orientation of the grains, high degree of sorting, presence of rip-up clasts, fragmentation of delicate skeletal elements). The remnant of the crustose coralline framework was preserved in these pebbles. Neogoniolithon sp. is the most common crust-forming species, this alga has the ability to grow directly over fine grained mobile substrate and thus has a pivotal role in framework formation. Although the preserved framework was only observed in limestone pebbles, fragments of Neogoniolithon sp. crusts were commonly observed in all the samples of Ternate Fm., testifying the importance of this framework builder in the carbonate factory. Sporolithon aschersonii and the encrusting foraminifer Acervulina linearis, together with encrusting bryozoans also contributed to the building of the framework. Compared to other European examples of the same age the studied skeletal association lacks fragments of hermatypic corals. The exclusion of corals may have been caused by the general instability of the environment. Ternate Fm. sub-marine fan was supplied by periodic debris flow, probably started by river floods. These events were able to sweep away the platform with enough energy to carry large boulders and to rip away fragments of the substrate several meters in length. Lumps of sediment, rich in hydrocarbons and pyrite, have been observed in the Ternate Fm., suggesting that bottom waters were occasionally and locally depleted in oxygen. The lack of oxygen was probably caused by recurrent high riverine discharge of organic matter and nutrients. The combined stressful effects of bottom instability and riverine discharge probably excluded corals from the association. The integrated study of the abundant but reworked materials of the Ternate Fm. and of the pristine, but rare, limestone pebbles of the Gonfolite Group allowed the reconstruction of a otherwise lost Eocene carbonate factory.