Dachstein Formation

{{Short description|Geologic formation in the southeastern Alps of Europe}} {{Infobox rockunit | name = Dachstein Formation | image = Mitterhorn.JPG | caption = Well bedded Dachstein limestones forming a distinctively stepped profile with clearly defined boundaries on the face of the Mitterhorn (2,506 m) in the Lofer Alps. | type = Formation | age = Norian-Rhaetian<br>~{{fossil range|221|202}} | period = Triassic | prilithology = Limestone | otherlithology = | namedfor = Dachstein Mountains<ref name=ICS/> | namedby = Friedrich Simony | region = Styria, Veszprém, Bessuno, Brescia,<br>South Tyrol | country = Austria, Germany, Hungary, Italy,<br>Slovakia, Slovenia, Switzerland | coordinates = {{coord|47.6|N|14.0|E|display=inline,title}} | paleocoordinates = {{coord|25.4|N|17.6|E|display=inline}} | unitof = | subunits = | underlies = | overlies = | thickness = | extent = | area = | map = {{Location map+ | Alps | relief = 1 | width = 250 | float = center | places = {{Location map~ | Alps | lat_deg = 47.6 | lon_deg = 14.0 | mark = Pink ff0080 pog.svg | marksize = 10 }} {{Location map~ | Alps | lat_deg = 46.4 | lon_deg = 12.3 | mark = Pink ff0080 pog.svg | marksize = 10 }} {{Location map~ | Alps | lat_deg = 45.6 | lon_deg = 10.3 | mark = Pink ff0080 pog.svg | marksize = 10 }} }} | map_caption = }} The '''Dachstein Formation''' or '''Dachstein Limestone''' (German: '''''Dachsteinkalk''''') is a lithostratigraphic term for a geologic formation of Triassic age. It is a carbonate sequence which forms prominent features in the Northern Limestone Alps and also crops out over a number of Tethyan mountain ranges in Austria, Germany, Hungary, Italy, Switzerland, Slovakia and Slovenia.

The Dachstein limestone was first formally described by the Austrian geologist Friedrich Simony in the 19th century, the type locality is the Dachstein Massif in the southern Salzkammergut region of Austria.<ref name=ICS>{{cite journal | title = Field trips in the Eastern and Southern Alps (Austria, Italy): 2nd International Congress on Stratigraphy| journal =Berichte der Geologischen | volume = 111 | editor= Sylvain Richoz |issn = 1017-8880 | url = https://opac.geologie.ac.at/ais312/dokumente/BR0111_001.pdf | access-date = 24 July 2025 }}</ref>

==Age and characteristics== The Triassic age of the Dachstein Formation can be established by the identification of characteristic fossils found within it. Because the Dachstein Limestone is a lithostratigraphic unit the time of deposition (in Ma before present) may be somewhat variable from place to place but in general terms deposition started in the late Carnian and continued through the Norian and into the Rhaetian stages of the Late Triassic period.<ref name=ICS/>

The sequence reaches 1800m in thickness and can be sub-divided into (i) well bedded limestones and (ii) a more massive facies, the massive limestone passes laterally southwards into the well bedded limestone which often includes thick and abundantly fossiliferous reef limestones.<ref name=ERO/>

===The reef limestones=== The Dachstein reefs were connected to lagoonal areas by narrow back-reef belts which now display massive to thick bedded limestones with ooids, oncoids and other coated grains, algae and reef debris.<ref name=ICS/> The patch reefs and detrital limestones had a very rich fauna and flora, more than 50 species have been identified in the reef framework with at least the same number as benthonic reef-dwellers.<ref name=ICS/>

===The bedded limestone=== The bedded limestone is the most widespread facies and this unit is layered on the scale of metres to tens of metres. The thickness of the layers and the clearly defined boundaries often produces a distinctively stepped or banded appearance so the layering can be visible from significant distances in large cliff faces. There is evidence of shallow water cyclic sedimentation,<ref name=ERO>{{cite book | last= Oxburgh | first = E.R. | date= 1968 | title =The Geology of the Eastern Alps | publisher = The Geologists Association | isbn = 978-0-913312-33-9 | url = https://books.google.com/books?id=71XwAAAAMAAJ | access-date = 22 July 2025}}</ref>{{rp|page=17|style=ama}} the details of the layered units and their cyclical nature was first described by Alfred G. Fischer<ref>{{ cite journal | url = https://www.kgs.ku.edu/Publications/Bulletins/169/Fischer/index.html | title = Symposium on cyclic sedimentation: The Lofer Cyclothems of the Alpine Triassic | first = Alfred G. | last = Fischer | editor = D.F. Merriam | date = 1964 | journal = Bulletin of the Kansas Geological Survey | volume = 169 | pages = 107–149 | access-date = 22 July 2025}}</ref> and the cycling beds are commonly referred to as Lofer cyclothems (after the excellent exposures in the Lofer facies of the Lofer Mountains).<ref name=Sch/>

The sequence as a whole has been interpreted as having formed in a peritidal setting with the cyclic units comprising lagoonal limestones, thin layers of variegated argillaceous material, thin layers of intertidal to supratidal laminated dolomites and dolomitic limestone.<ref name=ICS/> The cylicity has been attributed to periodic fluctuations of sea-level, superimposed on the general subsidence. An amplitude of up to 15 m and a periodicity of 20,000 to 100,000 years has been inferred for each cycle.<ref name=ICS/> It has been suggested that the sea-level changes were orbitally controlled and can be interpreted by Milankovitch processes.<ref name=Sch>{{cite journal | title =The stratification and cyclicity of the Dachstein Limestone in Lofer, Leogang and Steinernes Meer (Northern Calcareous Alps, Austria) | first = W. |last =Schwarzacher | journal = Sedimentary Geology | volume = 181 | date = 2005 | issue = 1–2 |pages= 93–106 | url = https://www.sciencedirect.com/science/article/pii/S0037073805002289 | doi=10.1016/j.sedgeo.2005.07.001 | bibcode = 2005SedG..181...93S | access-date = 24 July 2025 | url-access = subscription }}</ref>

==Palaeogeography and structure== The sequence was deposited on the NW margin of the Alpine Tethys.<ref name=Sch/> The Dachstein reefs fringed the lagoons of the inner Dachstein platform and they generally faced the open oceanic basin to the south.<ref name=TransD>{{ cite journal | journal = Global and Planetary Change | volume= 199 | date= 2021| title= End-Triassic crisis and "unreefing" led to the demise of the Dachstein carbonate platform: A revised model and evidence from the Transdanubian Range, Hungary | first1 = József | last1 = Pálfy | first2 =Zsófia | last2 = Kovács | first3 = Attila | last3 = Demény | first4 = Zsolt | last4 = Vallner | article-number= 103428 |url = https://www.sciencedirect.com/science/article/pii/S0921818121000138 | doi = 10.1016/j.gloplacha.2021.103428 | bibcode= 2021GPC...19903428P | access-date = 24 July 2025| hdl = 10831/82812 | hdl-access = free }}</ref>

The rock units overlying the Dachstein carbonates generally sit uncomformably on the carbonates and at many places the Triassic-Jurassic boundary is missing because of the stratigraphic gap.<ref name=TransD/> Various reasons have been suggested to explain why the deposition of Dachstein carbonates ceased, amongst the mechanisms suggested are (a) a tectonically-driven platform drowning event terminating shallow marine platform sedimentation (the onset of extensional tectonics) (b) a cessation of carbonate production due to emergence caused by a significant sea level fall (c) changes in ocean chemistry caused by the end-Triassic extinction which led to a change in carbonate production and the ecological collapse of reefs.<ref name=TransD/>

In the field the Dachstein limestones commonly show broad open flexures on the scale of hundreds of metres. Detailed lithological and palaeontological examination shows that there is significant repetition of units indicating that the sequence is built of a pile of near-horizontal thrust sheets, although there is very limited disturbance along the contact between the sheets and they are not intensely folded.<ref name=ERO/>

Parts of the Dachstein Platform are now preserved in various tectonic units disrupted during the Alpine orogeny, significant elements occur in the Austroalpine nappes of the Northern Limestone Alps, other parts are in the Transdanubian Mountains of Hungary<ref name=TransD/> and the Julian Alps.<ref>{{ cite journal | last1=Celarc | first1= B. | last2= Kolar-Jurkovsek | first2= T. | date =2008 | title = The Carnian Norian basin- platform system of the Martuljek Mountain Group (Julian Alps, Slovenia): progradation of the Dachstein carbonate platform | journal = Geol. Carpath. | volume = 59 | pages= 211–224 |url =https://www.researchgate.net/publication/282821394 | access-date = 24 July 2025 }}</ref>

== Fossil content == Fossils dated to the Norian and Rhaetian stages of the Late Triassic are particularly well represented<ref name=FWAT>[https://paleobiodb.org/classic/displayStrata?geological_group=&formation=Dachsteinkalk&group_formation_member=Dachsteinkalk Dachsteinkalk] at Fossilworks.org</ref><ref name=FWSK>[https://paleobiodb.org/classic/displayStrata?geological_group=&formation=Dachstein%20limestones&group_formation_member=Dachstein%20limestones Dachstein limestones] at Fossilworks.org</ref> and the Dachstein limestone of the Northern Limestone Alps has become a classical palaeontological study site because of the exceptionally diverse Norian-Rhaetian reef biota.<ref name=ICS/> thumb|A cross section of a ''Megalodon'' bivalve as typically seen in the Dachstein Limestone

A commonly observed fossil is the Megalodon (bivalve) which can be as large as 20cm across.<ref>{{cite journal | first1 = Sandor | last1 = Kovacs | first2 = Milan | last2 = Sudar | first3 = Eugen | last3 = Gradinaru | first4 = Hans-Jürgen | last4 = Gawlick | first5 =Stefan | last5 = Karamata | first6 = Janos | last6 = Haas | first7 = Csaba | last7 = Pero | first8 = Maurizio | last8 = Gaetani | first9 = Jan | last9 = Mello | first10 =Marian | last10 = Polak | first11 =Dunja | last11 = Aljinovic | first12 = Bogdan | last12 = Ogorelic | first13 = Tea | last13 = Kolar-Jurkovsek | first14 = Bogdan | last14 = Kolar-Jurkovsek | first15 = Stanko | last15 = Buser| title = Triassic evolution of the tectonostratigraphic units in the Circum-Pannonian region | date = 2011 | journal = Jahrbuch der Geologischen Bundesanstalt | volume =151 | issue = 3 | pages = 199–280| url = https://opac.geologie.ac.at/ais312/dokumente/JB1513_199_A.pdf | issn = 0016-7800 | access-date = 1 August 2025 }}</ref> Among others fossils reported from the formation are: ;Reptiles * ''Mystriosuchus planirostris''<ref name=Buffetaut1993>{{cite journal |last=Buffetaut |first=É |year=1993 |title=Phytosaurs in time and space |journal=Paleontologia Lombardia |series=Nuova serie |volume=2 |pages=39–44}}</ref>

;Invertebrates {{div col|colwidth=29em}} * ''Agathammina austroalpina''<ref name=Gazdzicki1979>{{cite journal |last1=Gazdzicki |first1=A. |first2=H. |last2=Kozur |first3=R. |last3=Mock |year=1979 |title=The Norian-Rhaetian boundary in the light of micropaleontological data |journal=Geologija |volume=22 |pages=71–112| url=https://www.researchgate.net/publication/281441626}}</ref> * ''Cnemidium vallisnerii''<ref name=Frech1912/> * ''Dicerocardium curionii''<ref name=Frech1912/> * ''D. jani''<ref name=Frech1912/> * ''Megalodus hoernesi''<ref name=Frech1912>{{cite journal |last=Frech |first=F |year=1912 |title=Neue Zweischaler und Brachiopoden aus der Bakonyer Trias - New bivalves and brachiopods from the Bakony Trias |journal=Resultate der Wissenschaftlichen Erforschung des Balatonsees, II Band: Paläontologie der Umgebung des Balatonsees |volume=1 |pages=1–138}}</ref> * ''M. laczkoi''<ref name=Frech1912/> * ''Involutina communis''<ref name=Gazdzicki1979/> * ''I. gaschei''<ref name=Gazdzicki1979/> * ''I. tenuis''<ref name=Gazdzicki1979/> * ''Nodosaria ordinata''<ref name=Gazdzicki1979/> * ''Rhaetina gregaria''<ref name=Frech1912/> * ''Semiinvoluta clari''<ref name=Gazdzicki1979/> * ''Triasina oberhauseri''<ref name=Gazdzicki1979/> * ''Trocholina acuta''<ref name=Gazdzicki1979/> * ''T. alpina''<ref name=Gazdzicki1979/> * ''T. permodiscoides''<ref name=Gazdzicki1979/> * ''Alpinophragmium sp.''<ref name=Gazdzicki1979/> * ''Involutina sp.''<ref name=Gazdzicki1979/> * ''Nodosaria sp.''<ref name=Gazdzicki1979/> {{div col end}}

== References == {{reflist|3}}

=== Bibliography === * {{citation |last=Buffetaut |first=É |year=1993 |title=Phytosaurs in time and space |journal=Paleontologia Lombardia |series=Nuova serie |volume=2 |pages=39–44}} * {{citation |last=Vörös |first=A |year=1981 |title=A survey of the Rhaetian (Upper Triassic) Bivalvia from Borzavár (Bakony Mts., Hungary) |journal=Annales Historico-Naturales Musei Nationalis Hungarici |volume=73 |pages=33–54}} * {{citation |last1=Gazdzicki |first1=A. |first2=H. |last2=Kozur |first3=R. |last3=Mock |year=1979 |title=The Norian-Rhaetian boundary in the light of micropaleontological data |journal=Geologija |volume=22 |pages=71–112}} * {{citation |last=Frech |first=F |year=1912 |title=Neue Zweischaler und Brachiopoden aus der Bakonyer Trias - New bivalves and brachiopods from the Bakony Trias |journal=Resultate der Wissenschaftlichen Erforschung des Balatonsees, II Band: Paläontologie der Umgebung des Balatonsees |volume=1 |pages=1–138}}

Category:Geologic formations of Hungary Category:Geologic formations of Slovakia Category:Geologic formations of Slovenia Category:Triassic System of Europe Category:Triassic Austria Category:Triassic Germany Category:Triassic Italy Category:Triassic Switzerland Category:Norian Stage Category:Rhaetian Stage Category:Limestone formations Category:Lagoonal deposits Category:Shallow marine deposits Category:Paleontology in Austria Category:Paleontology in Hungary Category:Paleontology in Italy Category:Paleontology in Slovakia Category:Paleontology in Slovenia Category:Geology of the Alps Category:Northern Limestone Alps Category:Southern Limestone Alps Category:Geologic formations of Austria Category:Geologic formations of Germany Category:Geologic formations of Italy Category:Geologic formations of Switzerland

Dachstein Formation

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