Mountain pass through 230-million-year-old coral reefs. Detailed rock analysis shows transition from shallow reef to deep ocean, recording drowning during a million-year rainstorm.
A Triassic Atoll in the Alps
The road over Giau Pass cuts through the fossilized remains of a massive tropical reef that existed here around 235 million years ago, during the Late Triassic epoch. At that time, this region was a shallow sea on the western edge of the Tethys Ocean. The peaks visible from the pass, such as Ra Gusela, are the remnants of a large carbonate platform, similar to a modern atoll. This structure was not built by the corals we know today, but by a different ecosystem of calcareous sponges, bivalves, gastropods, and early forms of stony corals (Scleractinia).
The primary rock forming these ancient reefs is the Cassian Dolomite. It represents the shallow, sunlit waters of the platform where carbonate-producing organisms thrived. In the surrounding basins, a different rock layer, the San Cassiano Formation, was deposited. This formation, composed of darker marls and shales, contains an exceptionally preserved and diverse fossil fauna. The sediments that make up the San Cassiano Formation came from the erosion of nearby volcanic islands and carbonates washed off the platform. The incredible preservation means that fossils found here, including ammonoids, bivalves, and gastropods, are among the best-known of the entire Mesozoic Era.
The Carnian Pluvial Episode
The transition from the light-colored reef of the Cassian Dolomite to the dark, muddy basin sediments of the San Cassiano Formation records a dramatic global climate event. Known as the Carnian Pluvial Episode (CPE), this was a 1- to 2-million-year period of intense global warming and rainfall that occurred between 234 and 232 million years ago. The cause was a series of massive volcanic eruptions in the Wrangellia Large Igneous Province, now located in western North America. These eruptions released enormous volumes of CO2 into the atmosphere, triggering a 4–8°C rise in global temperatures and disrupting the planet's water cycle.
This "million-year rainstorm" affected the Giau Pass reefs. The once-clear tropical sea became choked with sediment washed from the land by the constant downpours. The reef-building organisms could not survive in the murky, deepening water, and the carbonate platform effectively "drowned." The shutdown of carbonate production is visible in rock layers worldwide. The CPE caused a mass extinction, wiping out many marine and terrestrial species, it also created new ecological opportunities. The recovery period after the event saw the first major diversification of dinosaurs, the spread of modern conifer trees, and the appearance of the first reefs built by modern-style scleractinian corals.
💡Fun Facts
The Dolomites are named for the rock dolomite, which in turn was named for the 18th-century French geologist Déodat de Dolomieu who first described it.
Some gastropod fossils from the San Cassiano Formation are so well-preserved they still show their original color patterns.
The Carnian Pluvial Episode triggered the first widespread formation of amber in the fossil record, likely due to the proliferation of resin-producing conifer trees.
Near the pass, you can still see parts of the "Giau Barrier," a two-kilometer-long wall built in 1753 to mark the border between the Republic of Venice and the Austrian Empire.
The pass is a public road (SP638) and is generally accessible 24/7. However, it can be closed during winter months due to heavy snowfall. It also experiences temporary closures for cycling events.
Admission
Free
Accessibility
The road over the pass is paved. Views of the geology are possible from the road, parking areas, and adjacent hiking trails. The surrounding terrain is mountainous and requires appropriate footwear.