Dolomia Principale

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The Dolomia Principale is a Mesozoic formation from the Upper Carnian , Norian and Rhaetian of Italy . It consists mainly of light gray to white dolomite . Its thickness is variable - in the Adige Valley it is only 250 meters, whereas in the Carnic Pre-Alps it is up to 2000 meters. The rock was deposited in the tidal range on a wide, muddy carbonate platform under tropical conditions , with characteristic cycles of supratidal, intertidal and subtidal sediments formed.

etymology

In Italian , the feminine noun dolomia means dolomite and the adjective principale means mainly, outstanding, predominantly, principally. The Dolomia Principale corresponds Eastern Alps the main dolomite .

Initial description

The Dolomia Principale of the Southern Alps was first described scientifically by Richard Lepsius in 1876. The equivalent formation of the Eastern Alpine main dolomite was first defined in the Bavarian Alps in 1857 by Carl Wilhelm von Gümbel .

introduction

The Crozzon di Brenta (right) is made entirely of Dolomia Principale

In the outgoing Carnian ( Tuvalium ), the preceding shelf platforms and basins were displaced by various shallow water environments and terrigenous sediments, evaporites and carbonates from the Raibler layers were deposited . The uneven morphology was leveled and with the beginning of the Norium a carbonate platform was formed in northern Italy , which stretched from eastern Lombardy to Slovenia . The Dolomia Principale was deposited on it under subtropical conditions - an extremely powerful dolomite formation that can even reach up to 3000 meters in Val Camonica . It appears thickly and internally shows an alternation of very fine-grained dolomite micrite and stromatolite dolomite. The generally monotonous-looking formation actually represents a varied sediment complex, which can reveal local interposition of internal, partly turbid basin sediments such as in Val Menaggio , but also of restrictive basin sediments. The latter dolomites are bituminous and appear in the hanging wall , examples can be found near Tremezzo , in Val Brembana and in Val Trompia . Towards the end of the Dolomia Principale, the platform began to differentiate more strongly and larger sinking basins formed, which were framed by synsedimentary faults . Two factors were then decisive in the hanging wall of the norium: (1) a significant drop in sea level, which led to the surface of the platform and the settling of a condensed horizon in the basin. (2) a change from arid to humid climate. The observed changes can best be explained by global cooling, which was responsible for the drop in sea level and the abrupt end of the dolomite sedimentation, and by a shift in the climate belt, which led to a change in precipitation. The end result was the transition to clayey sedimentation on the western edge of Tethys.

Geographical distribution

The roof made of Dolomia Principale in the north face of the western pinnacle

The Dolomia Principale extends in the Southern Alps from western Lombardy (where it occurs only to a very limited extent) to Slovenia ( Julian Alps ). Its main area of ​​distribution is clearly to be found in the Dolomites , examples being the Brenta , the Three Peaks , the Latemar , the Sella , the Schlern and the Puez group . But it can also be found in the Bergamasque Alps and in Friuli . Other occurrences are even known from the Penninic , for example in the Sestri-Voltaggio Zone , in the Sea Alps , in the Valle Maira and in the Cottian Alps .

Outside the Alps , the Dolomia Principale appears in the Apennines of Abruzzo , Basilicata , Campania , Lazio and also Calabria .

Sedimentology

The Monte Prena seen from the Monte Brancastello. The summit structure consists of the Dolomia Principale pushed obliquely to the north (left) over the Jura.

Facies

Overall, the Dolomia Principale was characterized by a very uniform facies in its different areas of distribution. An inner shelf platform facies of great extent, made up of microcrystalline dolomite, clearly predominated . Progressing marginal facies and platform trough facies were much rarer. In the Gran Sasso of the Central Apennine, for example, its cyclically structured inner platform facies reached 600 meters in thickness, sediments from platform-internal basins were subordinate here. In the southern Apennines, the formation experienced a facial differentiation into a biogenic marginal facies, which consisted either of serpulids or crust-forming organisms (in platform furrows) or of corals and sponges (in more open basins). The latter marginal facies also formed in the eastern sector of the Southern Alps.

Cycles

The Dolomia Principale internally in peritidal cycles organized whose individual thickness averages in the meter range, and (the flattening tendency to hanging wall English shallowing upward ) applied to the day. The lower part of the cycle is subtidal and consists of micrite and bioturbate Wackestone / Packstone (where the sediment was ransacked between the bioclasts). The upper section, however, is intra- to supratidal and shows flat to wavy stratosphere layers. In the eastern southern Alps, green marls and fine breccias with black and / or stromatolite clasts can also appear in the upper cycle . In places, subtidal facies can also appear, the fossils of which (for example snails such as worthenia and colonies of megalodonts preserved in their living position ) indicate a higher energy level. Often there are also bioclastic layers created by traction currents (tempestite layers and channel deposits). These can be up to a meter thick, but usually remain in the centimeter range and lead Dasycladaceen , Bivalven and Gasteropods . The presence of reptile tracks and freshwater plants indicates that parts of the platform have temporarily dried out.

Dolomitization

The pervasive dolomitization process still raises many questions today, but it is likely to have already occurred during the sediment embedding or in the early diagnostic stage . The following environmental factors played an important role here: a generous availability of magnesium , a warm climate, the large surface area of ​​the internal platform and its low water cover with very warm water temperatures. For example, intra- to supratidal hanging end laminites in the Brenta consist of aphanitic and partly peloid dolomite micrite. TEM studies have shown that the micrite is actually composed of poorly ordered nanocrystals . It can therefore be assumed that the dolomite sludge was formed directly from the solution via a previously unknown process which made use of the sprouting and agglomeration of nanoparticles. It can also be seen that areas of nanocrystalline dolomite are embedded and cemented by calcareous dolomite. Thus, it seems to be emerging that one of the world's largest dolomite bodies emerged from a dolomite sludge by means of an unusual crystallization process, which was later partially affected by continuous dolomitization.

stratigraphy

At the summit of the Puezkofel, the Dolomia Principale runs over layers of the Jura and the Lower Cretaceous

In general, the Dolomia Principale followed the Raibl layers ( Gruppo di Raiblo or Raibliano - represented by the Travenanzes formation and the Formazione di San Giovanni Bianco ) and the Formazione di Castro Sebino in Lombardy. In the Julian Alps, the Dolomia Principale on its eastern edge reached down to the upper limit of the Tropites dilleri ammonite zone and followed the Portella dolomite here . Further inland to the west, it started over the somewhat younger Monticello Formation , also from the Tuvalium. To the east, the Dolomia Principale progressed towards the basin over the Carnitza Formation of the Tuvalium and the subsequent Baca Formation of the Norium.

The Dolomia Principale is overlaid by the Werfen layers ( Scisti Neri and in particular the Argillite di Riva di Solto in Lombardy and the Calcare di Zu on the Tridentine threshold ). The separation of the overlying Dachstein limestone of the Rhaetium and the following Calcari Grigi is often made very difficult by the intensive dolomitization. In the Belluno Basin , the Dolomia Principale even reached up to the Rhaetium / Hettangium border, where it was then replaced by the Soverzene Formation .

Internally, the Dolomia Principale can be divided into three sections. The sedimentation began at the Carnium / Norium boundary and began subtidal, but then changed very quickly to cyclical, peritidal tidal deposits of a narrowed lagoon . Around 200 to 300 meters of dark, layered dolomites with laminated stromatolites were deposited.

Above that lay an extremely powerful peritidal shelf platform sequence that is internally organized cyclically with a shallowing upwards tendency . The area around Lake Iseo , in which platform-internal basins formed, is an exception here . The eastern edge of the platform was near Tarvisio , 100 kilometers east of the Dolomites. The well-preserved edge of the platform manifested itself in the steeply sloping, overarching sediment deposits, rich in serpulids, dasycladaceae (calcareous algae) and microbe mats and characterized by phreatic cementations. On the huge platform there were sometimes dis- to anoxic depressions rich in carbonate and organic matter, e.g. B. in Friuli and in the Carnic Alps in the east, in the area of Belluno in the south and in Lombardy west of the Dolomites.

The following middle and upper Dolomia Principale was traversed by synsedimentary faults, the evolution of which was controlled by differential subsidence and extensive stretching. This should lead to the prelude to the sub-Jurassic breaking up of the Adriatic Plate , a spur of Africa.

As a result, extensive basin, edge and slope facies with an asymmetrical arrangement were created. The inside of the platform appeared completely in places. In the uppermost section, the edges grew stronger and the platform progressed over the internal basin. The end of the development was the drowning of the platform under the terrigenous sedimentation of the Argillite di Riva di Solto. Drowning was made easier by a lack of carbonate production. The reasons for this were an increased concentration of the clay fraction, made easier by climatic changes and increasing subsidence .

Fossils

The fossil content in the Dolomia Principale is quite rich in places, but fossils of stratigraphic value are very rare. Overall, the existing fossils - Dasycladaceae calcareous algae, mussels and snails like Worthenia confabulata - allow the formation to be assigned to the norium. The presence of Clypeina besici also points to Upper Carnium. In the marginal facies of the Lombard Basin, scattered serpulid reefs (English patch reef ) and microbial mounds (English mounds ) formed. However, corals were largely absent (the area around Tarvisio is an exception) and sponges were rare. However, corals and sponges occur in the peripheral facies of the southern Apennines. Dinosaur tracks from the Carnic Pre-Alps are of particular interest .

climate

During the Triassic, the southern Alps were at around 18 ° north latitude and were therefore part of the tropical belt. With the beginning of the Rhaetian it had drifted to 25 ° north. The sedimentation room was at the western end of both the Paleo and Neotethys . Plate tectonic movements had changed its position very little during the Triassic. Thus, climatic fluctuations at that time are mainly due to shifts in width of the monsoon belt .

Already during the Upper Travenanzes Formation (Raibler Strata) the conditions were clearly arid with powerful evaporites and pronounced caliche soils. The dolomite layers of the Dolomia Principale also point to drought during much of the norium. Evaporative intervals are absent in the Dolomites, but are present in the subsoil of the central and southern Apennines ( Burano formation ).

tectonics

In the Middle and Upper Norium, the southern Alps were affected by syndepositional tectonics in places. This created smaller basins on the platform, surrounded by faults . The elongation was responsible for the drowning of these basins with the accumulation of a significant sediment load. This explains the aforementioned differences in thickness between carbonate high altitudes and platform-internal troughs. The sediments in the basin consisted primarily of turbiditic limestone and fault breccia from high shoulders.

For the alpine compressive tectonics two phases could be identified in the Southern Alps: the first phase took place in the period from the Upper Cretaceous to the Paleogene , the second from the Lower Miocene . The main stress of the first phase was oriented east-west and produced west vergent thrusts. The main stress of the second phase ran north-south with south vergent thrusts. So-called ramp-flat structures based on the tectonic competence difference of the formations are characteristic. The flats (horizontal sliding planes) usually run in very incompetent locations such as B. the Raibler layers. The ramps (inclined ramps) penetrate competent material (such as the Dolomia Principale) with a typical angle of 25 to 30 °. Very nice examples of this tectonic style are the Gigpel passes (Italian sovrascorrimenti di vetta ) on Puezkofel or Piz Boè , on which Dolomia Principale glided over other units. These are tectonic cliffs that have been separated from their suspending trunk cover by erosion - and no cover remains of gravitational sliding.

Age

The Montasch rests on Dolomia Principale, but the summit area is crowned by Dachstein limestone

The Dolomia Principale is diachronous and was deposited over a period of around 20 million between 228 and 208 million years ago. It generally starts at the carnium / norium border, but can reach down to the highest carnium. Its hanging border is usually on the border with the rhaetium, but can also include the entire rhaetium.

See also

literature

Thrusting over the summit on Piz Boè (3152 m). The base of the Sella mountain consists of Dolomia Principale. The summit structure is based on rhaetian Dachstein limestone and a thin layer of Ammonitico Rosso (left), which in turn is crossed by Dolomia Principale from the east (right).
  • Alfonso Bosellini and LA Hardie: Facies e cicli della Dolomia Principale delle Alpi Venete . In: Memorie della Società Geologica Italiana . v. 30, 1988, pp. 245-266 .
  • Flavio Jadoul, Fabrizio Berra and Silvia Frisia: Stratigraphic and paleogeographic evolution of a carbonate platform in an extensional tectonic regime: the example of the Dolomia Principale in Lombardy (Italy) . In: Riv. It. Paleont. Strat. 98 n.1 , 1992, p. 29-44 .

Individual evidence

  1. ↑ F. Berra, L. and M. Delfrati pontoon: Dolomia Principale . In: MB Cita et al. (Ed.): Carta Geologica d'Italia 1: 50,000. Catalogo delle formazioni - Unità tradizionali (1) . series III, 7. Quaderni del Servizio Geologico d'Italia, 2007, p. 63-72 .
  2. ^ GB Carulli et al.: Geologia delle Prealpi Carniche . In: Pubblicazioni del Museo Friulano di Storia Naturale . tape 44 . Udine 2000, p. 1-47 .
  3. ↑ a b c Alfonso Bosellini and LA Hardie: Facies e cicli della Dolomia Principale delle Alpi Venete . In: Memorie della Società Geologica Italiana . v. 30, 1988, pp. 245-266 .
  4. ^ R. Lepsius: Classification of the Alpine Triassic and its relationship to the Ausseralpinen . In: N. Jahrb. Min. Geol. Paleont. Stuttgart 1876, p. 742-744 .
  5. ^ Carl Wilhelm von Gümbel: Investigations in the Bavarian Alps between Isar and Salzach . In: Jahrb. KK Reichsanst. Year 7 Vienna 1857, p. 146-151 .
  6. ^ R. Assereto and P. Casati: Revisione della stratigrafia permo-triassica della Val Camonica meridionale (Lombardia) . In: Riv. It. Paleont. Strat. From 71 to 4. Milano 1965, p. 999-1097 .
  7. ↑ Fabrizio Berra, Flavio Jadoul and Andrea Anelli: Environmental control on the end of the Dolomia Principale / Hauptdolomit depositional system in the central Alps: Coupling sea-level and climate changes . In: Palaeogeography, Palaeoclimatology, Palaeoecology . tape 290 , 2010, pp. 138-150 .
  8. ↑ F. Barattolo and A. Bigozzi: Dasycladaceans and depositional environments of the Upper Triassic-Liassic carbonate platform of the Grand Sasso (Central Apennines, Italy) . In: Facies . tape 35 . Erlangen 1996, p. 163-208 .
  9. ↑ FM Dalla Vecchia and P. Mietto: Impronte di rettili terrestri nella Dolomia Principale (Triassico Superiore) delle Prealpi Carniche (Pordenone, Friuli) . In: Atti Tic. Sc. Terra, ser. spec. tape 7 . Pavia 1998, p. 87-107 .
  10. ↑ Silvia Frisia: Mechanisms of complete dolomitization in a carbonate shelf: comparison between the Norian Dolomia Principale (Italy) and the Holocene of Abu Dhabi Sabkha . In: B. Purser, M. Tucker and D. Zenger - A volume in honor of Dolomieu (eds.): Spec. Publ. Int. Ass. Sedim. tape 21 , 2004, p. 55-74 .
  11. ↑ A. Iannace and A. Frisi: Changes in patterns between dolomitization Norian and Rhaetian in the Southern Tethys realm: clues to the dolomitization of the Dolomia Principale . In: B. Purser, M. Tucker and D. Zenger: "Dolomites, a volume in honor of Dolomieu" (Ed.): Intern. Assoc. Sedimentol., Spec. Pub. tape 21 . Oxford 1994, p. 75-89 .
  12. ↑ Patrick Meister and Silvia Frisia: Dolomite formation by nanocrystal aggregation in the Dolomia Principale of the Brenta Dolomites (Northern Italy) . In: Riv. It. Paleontol. Strat. Band 125 (1) , 2019, pp. 183-1196 .
  13. ↑ F. Jadoul, F. Berra, S. Frisia, T. Ricchiuto and P. Ronchi: Stratigraphy, paleogeography and genetic model of late Carnian carbonate breccias (Castro Formation, Lombardy, Italy) . In: Riv. It. Paleont. Strat. V. 97 n.3-4. Milano 1991, p. 355-392 .
  14. ^ D. Masetti and G. Bianchin: Geologia del Gruppo della Schiara (Dolomiti Bellunesi) . In: Mem. Sc. Geol. Band 39 . Padua 1987, p. 187-212 .
  15. ^ V. De Zanche, P. Gianolla and G. Roghi: Carnian stratigraphy in the Raibl / Cave del Predil area (Julian Alps, Italy) . In: Eclogae geologae Helvetiae . v. 93, 2000, pp. 331-347 .
  16. ^ A. Tommasi: Revisione della fauna a moluschi della Dolomia Principale di Lombardia . In: Paleont. Ital. tape 9 . Pisa 1903, p. 95-124 .
  17. ↑ Marco Marzola and Fabio Marco Dalla Vecchia: New dinosaur tracks from the Dolomia Principale (Upper Triassic) of the Carnic Prealps (Friuli-Venezia Giulia, NE Italy) . In: Bollettino della Società Paleontologica Italiana . tape 53 (1) . Modena 2014, p. 1-18 .
  18. ^ W. Kiessling, E. Flügel and J. Golonka: Patterns of Phanerozoic carboante platform sedimentation . In: Lethaia . tape 36 (3) , 2003, pp. 195-225 .
  19. ↑ J. Marcoux and: Late Norian (215 to 212 Ma) . Ed .: J. Dercourt, LE Ricou and B. Vrielynck, Atlas of Tethys Palaeoenvironmental Maps. Gauthier-Villars, 1883, p. 35-53 .
  20. ↑ B. Martinis and M. Pieri: Alcune notizie sulla formazione evaporitica del Triassico Superiore nell'Italia centrale e meridionale . In: Memorie Soc. Geol. Italiana . tape 4 (1) , 1964, pp. 649-678 .
  21. ^ A. Cozzi: Facies patterns of a tectonically-controlled Upper Triassic platform slope carbonate depositional system (Carnian Prealps, Northeastern Italy) . In: Facies . tape 47 , 2002, p. 151-178 .
  22. ^ F. Jadoul: Stratigra fi a e paleogeogra fi a del Norico nelle Prealpi Bergamasche occidentali . In: Riv. It. Paleont. Strat . tape 91 , 1985, pp. 479-512 .
  23. ↑ Carlo Doglioni: The over thrusts in the Dolomites: ramp-flat system . In: Eclogae geologae Helvetiae . Vol. 78 No. 2, 1985, pp. 335-350 .