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Triassic Basin Fill and Development of the Southern Alps (Italy, Switzerland)

Details 3		  
       
       
  Late Carnian
In the early to mid-Tuvalian (upper Dilleri to lower Subbullatus Zone), accommodation space decreased signifcantly, following a strong third-order eustatic sea-level fall and low subsidence rates. An erosional unconformity developed across much of the southern Alpine basin. In the central and eastern Southern Alps it is overlain by the fluvial, lagoonal and shoreface sandstones/ siltstones, evaporites, shales and dolomites of the Raibl/Travenazes Formation. Thicknesses increase from 30 m in the west to more than 100 m in the east. The Raibl/Travenazes Formation is probably diachronous between the eastern and central Southern Alps. Coastal sabkha, open-marine subtidal and carbonate-evaporitic lagoonal deposits of the middle-upper San Giovanni Bianco Formation (maximum thickness 250 m) prevail in the western Southern Alps. The Raibl/Travenazes Formation in the Southern Alps is not the time-equivalent of the Raibl Formation of the Northern Calcareous Alps (Austria, Germany), where it overlies the late Early Carnian (Julian) to early Norian (Lacian) succession. In the Northern Calcareous Alps, the source areas of the Raibl Formation were the Central European and Fennoscandian ranges. In the Southern Alps, grain size trends indicate a source area in the south, below the present-day Po Plain.		  
       
  Basin geometry and fill of the central SA along an east-west trending transect (Zühlke in Feist-Burckardt et al., 2008).
The figure shows the reconstructed basin architecture in the Late Miocene. Data for the Permian, Jurassic and Cretaceous basin fill are partly projected from the northern part of the central SA. Thickness and geometry of post-Cretaceous basin fill is tentative, as no direct data exist for the central SA.
    
       
  Latest Carnian to Rhaetian
In the late Tuvalian (Anatropites Zone), clastic input to the southern Alpine basin ended and the carbonate lagoonal deposition of the Hauptdolomit/Dolomia Principale commenced. In the eastern and central Southern Alps the base of the succession is marked by extraformational breccias and conglomerates, while in the western Southern Alps, marginal marine mudstones, intraformational breccias (Castro Formation, 50-250 m thick; Jadoul et al. 1992) and subtidal “Dark dolomites” (50-250 m thick) mark the base. Metre-scale, subtidal-intertidal-supratidal cycles are typical within the Dolomia Principale/Hauptdolomit which covered much of the western Tethyan Shelf. Thicknesses range between 250 m in the western part of the central Southern Alps, to 1500 m in the eastern Southern Alps and up to 2000 m (including the Castro Formation and “Dark Dolomites”) in the western Southern Alps.

Structurally controlled intraplatform basins developed during the deposition of the middle to upper part of the Dolomia Principale/Hauptdolomit, while small, anoxic basins existed in the southern part of the central Southern Alps. The Norian development in the western Southern Alps differs considerably from that in the eastern and central Southern Alps. In the late early Norian, large intraplatform basins developed, which were filled by carbonate turbidites of the Aralalta Formation (200-1000 m thick). Dolomia Principale/Hauptdolomit deposition persisted until the late Lacian (?Magnus Zone), when the platform experienced local emergence and clay suffocation. Finally, in the early Alaunian, rapid subsidence and drowning occurred (Jadoul et al. 1992). Black shales, marls and subtidal limestones of the Riva Solto Shale Formation and the lower Zu Formation (cumulative thickness 300-1500 m) onlap the top of the Dolomia Principale/Hauptdolomit. In the early Rhaetian, carbonate ramps of the Middle-Upper Zu Formations prograded from structural highs.
 
The Triassic basin fill in the eastern and central Southern Alps is concluded by the Dachstein Formation, the age of which is loosely constrained as latest Norian(?) to Rhaetian or early Hettangian(?). The depositional environment was similar to that of the Dolomia Principale/Hauptdolomit. However, the Dachstein Formation is dolomitized to only a minor extent, or not at all. In the central to eastern Southern Alps, thicknesses increase from 30m to >1000 m from west to east. In the western Southern Alps, the Upper Zu Formation and the Conchodon Dolomite represent the Rhaetian succession with a combined thickness of 180-220 m. Open-marine carbonate ramps with shoals and coral patchreefs characterize the upper Zu Formation. The Conchodon Dolomite shows subtidal to peritidal mudstones and prograding oolitic bars with partial dolomitization. A depositional gap occurred in the Rhaetian on structural highs in the westernmost Southern Alps.

Subsidence rates during the Norian and Rhaetian were very high. Quantitative models have large error intervals because biostratigraphic resolution is limited and absolute ages vary. Based on the current chronobiostratigraphic framework, Norian subsidence rates in the central Southern Alps range between 320 mm/ka (western Dolomites) and 820 mm/ka (eastern Dolomites). However, in the Rhaetian subsidence decreased significantly to 50 mm/ka and 140 mm/ka in these two areas, respectively. In eastern Lombardy, the Norian-Rhaetian succession is up to 4000 m thick.
 
       
  Publications
Feist-Burkhardt, S., Goetz, A.E., Szulc, J., Borkhataria, R., Geluk, M., Haas, J., Hornung, J., Jordan, P., Kempf, O., Michalik, J.,; Nawrocki, J., Reinhardt, L., Ricken, W., Roehling, H.-G., Rüffer, T., Torok, A. and Zühlke, R., 2008, Triassic, in: T. McCann, ed., The Geology of Central Europe: Mesozoic and Cenozoic, Volume 2, Geological Society London, p. 749-821.
  
       
  References
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