“CIPIT BOULDERS”: KEYS TO DECIPHERING MID-TRIASSIC CARBONATE PLATFORMS OF THE ITALIAN DOLOMITES

Mid-Triassic carbonate platforms of the Dolomites have long been recognized as classic examples of ancient reefs. In nearly all of them, dolomitization has destroyed most of the original fabric. The exception is the swarms of allochthonous blacks shed from the margins into adjacent shale basins and protected from alteration. These margin-derived limestone ‘Cipit Boulders’ preserve primary fabrics and organic matter and, locally, original mineralogy.

Our Cipit samples show distinctive boundstone fabrics with sponge, coral and bryozoan skeletal components as well as calcified cyanobacteria (Girvanella, Cladogirvanella) and a wide variety of problematic organisms: Shamovella, Archaeolithoporella, Plexoramea, Macrotubus, Baccanella. These small, typically millimetric, encrusting and baffling microorganisms are in turn intimately associated with micritic microbial deposits, botryoidal and fibrous marine crusts, and cements which fill small fenestrae and stromatactis-like cavities. Late blocky ferroan cements occlude residual cavities. Bright epifluorescence of the calcimicrobes and micrites indicates high organic matter concentrations.

Despite the major changes that occurred during the Permian-Triassic Mass Extinction, the mid-Triassic (about 240 Ma) Ladinian reef communities preserved in the Cipit blocks show many similarities with mid-late Permian reefs. This raises questions concerning the long-term evolution of reef biotas and the environmental conditions that determined their development during the Paleozoic-Mesozoic transition. It appears that the major changes in organisms and early-marine seafloor precipitates reflected in these reefs mainly occurred during the mid-late Triassic rather than near the Permian-Triassic boundary, prolonging Paleozoic-style reef-building into the early Mesozoic. Here we examine the possibility that these included global environmental influences that promoted microbial calcification into at least the early Late Triassic.