Not all of the rocks in the Italian Dolomites are dolomite

As explained in my last post (https://landscapes-revealed.net/trekking-through-an-ancient-sea-in-the-italian-dolomites/), the Dolomites were named after a French geologist, whose name was also given to the dolomite mineral. Indeed, the spectacular white peaks of dolomite are what catches the eye when in the Dolomite mountains. But there are other rock types with clues to the geologic history of the region, notably thick sequences of volcanic rock that underlie and overlie the carbonate platforms of dolomite. Underlying the carbonate rock are also terrigenous sedimentary rocks made of sand and mud.

Volcanic rocks

Most of the igneous rocks we saw are younger than the carbonate platforms. Here is some of the evidence we saw. For locations, refer to the map in my last post (link above).

This photo was taken on the trail heading west from the Contrin Valley, looking toward the northwest. The dark rocks at the top of this ridge, and extending westward, are igneous rocks that formed when volcanoes erupted and completely covered the underlying light-colored carbonate rocks. The volcanic rocks have a range of compositions and were probably a result of continental rifting. Also note some red-colored layers and some thin layers in the foreground. These rock types have terrigenous (land-sourced) components and are discussed below.
This photo is from the trail heading upward and westward from Lago di Fedeia, at the upper end of the Fassa Valley. The view is westward toward Rifugio Baita Federola, the top of a chairlift to the Fassa Valley below, and a more western ridge of carbonate rocks. All of the darker-colored rocks in the foreground and in front of the distant carbonate ridge are volcanic. They may appear to be beneath the light-colored rocks, but they are not—they filled in the valleys adjacent to the already-formed isolated carbonate platforms. In fact, the carbonate platforms were completely buried by younger volcanic rocks. Because the volcanic rocks are weaker, they have eroded and left the more-resistant carbonate rocks as the highest peaks.
This photo is of Refugio Tobia del Giagher (where we had lunch!) and another chair lift that descends to the Fassa Valley. That is Piz Boe mountain on the horizon on the right. On the left is a peak made wholly of volcanic rocks. Apparently some of the carbonate and volcanic activity occurred at the same time, so this may be an atoll—a volcanic island with a fringing carbonate platform. Alternatively, this peak could be an erosional remnant of volcanic rocks that completely filled the space between the older carbonate platforms.
On our climb up to Pas de la Ciréle (pass) from San Pelligrino we kept seeing igneous rocks (probably of basaltic composition) without an apparent source. Once we got to the pass, however, the source was revealed. We saw igneous dikes (one is delineated by yellow lines) that had intruded into the carbonate sediments. Because the dikes cut the carbonate, they are clearly younger. A close-up of these rocks is shown below. (NOTE: Magma—liquid rock—can flow into fractures and then cool into solid igneous rock.)

Terrigenous rocks

Terrigenous means sediments that have an on-land origin. Whereas carbonate sediments form in-situ, in the ocean, by carbonate-secreting organisms, terrigenous sediments originate on land, by erosion of pre-existing rocks, and are then carried to the ocean by rivers.

This photo is from Refugio Friedrich August, located near Sassolungo peak. Here we could see a clear transition from dark-colored sediments of sand and mud to overlying light-colored sediments of carbonate. When the carbonate platforms were building up, they were isolated from any source of sediment from land. The layers of sand and mud beneath the platforms tell us that sediments eroded from land were reaching this part of the ocean prior to the development of the platforms. The alternating darker and lighter colors are alternating thin mud and sand layers.
This photo was taken on the trail ascending from San Pelligrino to Pas de la Ciréle. Notice the thin beds that underlie the massive carbonates. In this case, the sediments are marl, which means a mixture of limestone and terrigenous mud. This tells us that carbonates were being deposited in the ocean (but not on a platform) along with mud from erosion of rocks on land. This photo shows a distinct debris cone of modern sediment derived from erosion of the massive carbonate peak above it.

Refugio Contrin and Marmolada

I’ll end this post with a photo of a popular refugio—Refugio Contrin—with the south slope of Marmolada peak in the background. At an elevation of 3,343 m (10,968 ft), Marmolada is the highest peak in the Dolomites. It was completely buried by younger volcanic rocks, but because the volcanic rocks are less resistant to erosion, they have been mostly removed, leaving the carbonate peak behind.

In my last post about the Dolomites, I’ll explain how they got uplifted to their present elevations.

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4 Comments

  1. Jane on October 9, 2022 at 7:34 pm

    I am really enjoying your well-documented excursions!

    • Landscapes Revealed on October 9, 2022 at 10:51 pm

      Thanks Jane! Appreciate your comments!

  2. Kim — a r t i c k l e host on October 9, 2022 at 11:33 pm

    Such stunning photos! So informative. ❤️

    • Landscapes Revealed on October 10, 2022 at 8:17 am

      Thanks Kim!

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