Changing Dolomitization Styles from Norian to Rhaetian in the Southern Tethys Realm
- Bruce Purser,
- Maurice Tucker and
- Donald Zenger
Published Online: 14 APR 2009
Copyright © 1994 The International Association of Sedimentologists
Dolomites: A Volume in Honour of Dolomieu
How to Cite
Iannace, A. and Frisia, S. (1994) Changing Dolomitization Styles from Norian to Rhaetian in the Southern Tethys Realm, in Dolomites: A Volume in Honour of Dolomieu (eds B. Purser, M. Tucker and D. Zenger), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304077.ch6
- Published Online: 14 APR 2009
- Published Print: 25 MAY 1994
Print ISBN: 9780632037872
Online ISBN: 9781444304077
- changing dolomitization styles from Norian to Rhaetian in southern Tethys realm;
- Dolomia Principale/Hauptdolomit - phological characteristics and ‘Law of Persistence of Facies’;
- Brenta dolomites;
- Rhaetian-Liassic succession - diverse lithofacies assemblage;
- palaeographic evolution and dolomitization;
- Upper Triassic-Lower Liassic platform carbonates of peri-Mediterranean region;
- Norian Dolomia Principale - reflecting special, nonactualistic palaeogeographic and geodynamic settings;
- Late Triassic carbonates of peri-Mediterranean Alpine region – great amounts of massive dolomite
A comparative study of Upper Triassic carbonate platform successions carried out in the southern Alps and southern Apennines, together with a review of available literature, suggests that two different dolomitization styles characterize respectively the Norian and Rhaetian over most of the peri-Mediterranean region. The Norian is characterized by the dominance of the Dolomia Principale facies, i.e. completely dolomitized peritidal cyclic successions. The bulk of the dolomitization was completed by the end of the Norian and its characteristics are strictly related to this specific stratigraphic interval. Rhaetian facies, on the contrary, are more diversified, typically record a sudden terrigenous input and generally have a much lower dolomite content. Norian dolomites are commonly mimetically replacive and finely crystalline, especially in the inter/supratidal intervals, alternating with coarser, non-mimetic dolomite in the subtidal intervals. Their δ18O values are typically as high as +3.5 per mil (PDB), especially in the fine-crystalline types. Rhaetian dolomites, on the contrary, are mostly coarse and non-mimetic, with field evidence clearly indicating their origin through large-scale fluid circulations. Earlier diagenetic dolomites are of minor importance and are associated only with the inter-/supratidal facies. Moreover, all these dolomites have δ18O systematically lower than texturally analogous Norian dolomites.
Two different styles of dolomitization thus pertain to these Late Triassic ages. The Rhaetian appears as a period of transition between the production of large bodies of early diagenetic dolomite of the Norian and the lack of such in the succeeding Jurassic/Cretaceous, at least in the Tethyan domain. The Rhaetian transition is probably related to a change from the very particular palaeogeographic/tectonic/climatic setting in which deposition and diagenesis of the Norian carbonates occurred. Norian carbonate shelves developed in the extensive Pangea Gulf, which was bordered landward by wide evaporitic deposits, indicating arid climatic conditions. The Rhaetian transgression, with terrigenous input and a more humid climate, by terminating these conditions, greatly hindered surface dolomitization.
A strictly uniformitarian approach can be used only in explaining Rhaetian dolomitization, whereas the very thick dolomite deposits of the Dolomia Principale should be viewed as the result of processes not operating, at least on the same scale, in modern environments.