The Dolomite Mountains and the Dolomite Problem

Diedonnè-Silvain-Guy-Tancrede de Gvalet de Dolomieu, born June 23, 1750 in the village of Dolomieu, was a typical naturalist of his time. With 26 years he travelled trough half Europe, he got interested in the mines of the Bretagne and the basaltic plateau in Portugal, and he visited South Italy to study the aftermath of an earthquake in Sicily and observed an eruption of the Aetna.
In 1789 during a voyage to Italy with his student fellow Fleuriau de Bellevue he also travelled trough Tyrol. In the Brenner Pass area and between the cities of Bozen und Trento he noted a rock similar to limestone, but which showed no appreciable effervescences with acids. He published these observations in July 1791 in a letter to the "Journal of Physique".
Nicolas de Saussure, son of the Alpinist/naturalist Horace Benedict de Saussure requested some samples to analyze it. After some tentative denominations like "Tyrolit" or "Saussurit" in 1792 de Saussure published the "Analyse de la Dolomie" in the "Journal of Physique".
Even if the rock itself was not completely unknown, in fact called "Spat" or "Perlspat" by miners, it was not realized until the publication of Dolomieu that the rock was composed of a peculiar Ca-Mg carbonate. The Italian naturalist Giovanni Arduino (1713-1795) published in 1779 his observations about a peculiar limestone, found in the mountains surrounding Verona, but he didn't delve further into the subject and consider the idea of a new mineral.
So the name "Dolomite" became soon established, and in 1794 Richard Kirman introduced the Dolomite as a new mineral; the name from there became used to name the dolostone rocks and finally gave the Dolomites their actual name.

In the 19th century the genesis of both the Dolomite Mountains as the rock forming them became a major problem in geology. One of the most important achievements' was the recognition that the outstanding peaks and mountain groups are remains of ancient carbonate platforms and coral reefs.
In early days of geology less was known about the bottom of the sea and sedimentation occurring in oceans, only in 1842 Darwin formulated a first hypothesis dealing with the formations of tropic reefs.
Influenced by this model, intensive field mapping was carried out, and in 1860 the German geologist Ferdinand von Richthofen (1833-1905) recognised as first the Schlern Mountain as a slope of an ancient reef and other peaks as the remnants of large carbonate platforms.

Fig.2. In Leopold von Buch´s work "Esquisse d´une carte geologique de la parte meridionale du Tyrol" (1822) the author distinguishes carbonatic (light blue) from dolomitic rocks (dark blue).

After geologists could answer how the most spectacular rock walls and peaks in the Dolomites formed, the next urgent questions was if dolomite was a primary product of marine deposition or a secondary product of alteration of common limestone.

An insight to the problem came from the study of a characteristic geological formation in the Dolomites and its depositional environment: The appropriately denominated Hauptdolomit, the "main dolostone" formation, was defined in the Bavarian Alps by VON GUEMBEL 1857, and introduced in the stratigraphic nomenclature of the Alps in 1876 by LEPSIUS.

Fig.3. Example of Hauptdolomite forming steep rock walls, the Sass dla Crusc (Hl. Kreuz Kofel) 2.907m (with locals).

During the Upper Carnian and the Norian stage (216,5 - 203,6Ma) the Tethyan Sea experienced various regression and transgression phases.
The changing sea level resulted in the development of large water covered carbonate platforms or emerged tidal flats, on which a sequence of homogenous, meter thick carbonate muds with rare fossils (subtidal facies) and laminated bacterial mats and dolomite marls (peritidal facies) were deposited.
These deposits are widely distributed in the Eastern Alps, they can be found in the Southalpine unit (here denominated Hauptdolomit/Dolomia Principale Formation), as well as in a very similar development in the entire Austroalpine unit (Hauptdolomit-Gruppe in the Northern Calcareous Alps, Ortles nappe, S-charl nappe etc.) and in the Apennines and Dinarides, and therefore points at an enormous extension of this tidal sea.

During sea level low stand the muddy flats were colonized by algae and a species-poor faunal community, dominated by gastropods (Worthenia confabulate) and bivalves (Megalodus). Dinosaurs roved through the tidal flat, their tracks have been preserved at some locations. In times of emersion only thin mud layers were deposited by storms, which themselves were colonized by algae and bacterial mats, and which dried out repeatedly.

Fig.4. Detail of the Hauptdolomit - Formation showing algae / bacterial mats.

The extreme shallow water conditions continued uninterrupted throughout the whole Norian. The uniform and slow subsidence of the basement led to deposition of an up to 1.000 meters thick succession of homogeneous cycles of the two facies.
The top of the Hauptdolomite, and the end of the platform succession, is characterized by the development of polycyclic paleosols up to 30m thick, reflecting a major eustatic sea-level fall. One of the most intriguing differences of the Southalpine Hauptdolomite to other corresponding formations is the lack of intraplattform basins, with a succession of dark dolo- and limestone's, found for example in Lombardy and Austria. This fact is explained by missing tectonic activity during the Triassic in the area of the future Dolomite-mountains.

With this proposed reconstruction, geologists tried to find an actual and comparable environment to understand the deposition of dolostone: the large carbonate platform of the Bahamas Bank seemed to fit perfectly the prerequisites: a vast area covered with a shallow, tropical sea, with sparse islands and coral reefs surrounded by large tidal flats - there was only on problem: no or only a limited formation of dolomite is today observed in this environment.

The Dolomite Problem was still unsolved.

To be continued…

Bibliography:

BERRA, F.; JADOUL, F. & ANELLI, A. (2010): Environmental control on the end of the Dolomia Principale/Hauptdolomit depositional system in the central Alps: Coupling sea-level and climate changes. Palaeogeography, Palaeoclimatology, Palaeoecology 290: 138-150
BOSELLINI, A.; GIANOLLA, P. & STEFANI, M. (2003): Geology of the Dolomites. Episodes, Vol. 26(3): 181-185
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VonGUEMBEL C.W. (1857): Untersuchungen in den bayerischen Alpen zwischen Isar und Salzach. Jahrb. K. K. Geol. Reichsanst., Jahrg. 7, H. I.: 146- 151, Wien.