San Cassiano Formation (

Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Trias ...

Carnian The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followe ...

) is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

located on the Southern

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, ...

(Northeast Italy) in the

Dolomites The Dolomites ( it, Dolomiti ; Ladin: ''Dolomites''; german: Dolomiten ; vec, Dołomiti : fur, Dolomitis), also known as the Dolomite Mountains, Dolomite Alps or Dolomitic Alps, are a mountain range located in northeastern Italy. They form pa ...

. These Triassic dolomites are considered to be a classic example of ancient carbonate platforms. Bosellini, A., Gianolla, P., & Stefani, M. (2003). Geology of the dolomites. Episodes, 26(3)./ref> As the allochthonous elements in the Shale strata ( Cipit boulders) show a good preservation, fossils and

microbialites Microbialite is a benthic sedimentary deposit made of carbonate mud (particle diameter < 5 μm) that is formed with the mediation of microbes. The constituent carbonate mud is a type of The landscape of the Dolomites is dominated by Triassic carbonates, deposited from the

to the

. In the Lower Carnian (Julian) the percentages of carbonates increments due to the diminishing eustatic sea-level change. The last part of the reefal evolution is the development of patch reefs constituted by colonial corals (

Scleractinia Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mo ...

). Large areas of the reefs of the Dolomites suffered karstic erosion due to sea regression in the Anisian. A new transgression in the Carnian permitted new carbonate platforms to develop in swallow basins.[Bosellini, A. & Rossi, D. 1974. Triassic Carbonate Buildups of the Dolomites, Northern Italy. In: Laporte, L.F. (ed). Reefs in time and space: Selected examples from the recent and ancient. Soc Econ. Paleont. Miner. Spec. Publ 18: 209 – 233. Tulsa.]

Stratigraphy

The San Cassiano Formation overlays the Wengen Formation (a volcanic flysch sequence); the limit between this two formations is arbitrary; usually considered as the first carbonate strata in the

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments (clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

sequence.[ Espinoza-Campuzano, C. A. (2013). Análisis geoquímico de microbialitas leiolíticas selectas contenidas en calizas Cipit de la Formación San Casiano (Triásico MedioSuperior, NE de Italia). Thesis, Universidad Nacional Autónoma de México, Distrito Federal, Mexico.] The San Cassiano Formation shows a variable thickness from 300 m to 500 m; it was described by Ogilvie in 1893, and it has two Members.[Urlichs, M. (1974). Zur stratigraphie und ammonitenfauna der cassianer schichten von Cassian olomiten, Italien Schrift. Erdwiss. Komm. Österr. Ak. Wiss., 2, 207–222.] The Inferior Member is constituted by an intercalated sequence of

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especia ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

, volcaniclast (pseudoflysch) and marl; deposited from the Late

Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic) ...

(Mid Triassic) to Early Carnian (Superior Triassic). The Superior Member is a sequence of pseudoflysch, marl, limestone and mudrock (in this strata some Cipit Boulders occur). This sequence was deposited from during the Middle Carnian. The strata are very similar in both members; they are divided by

biostratigraphic Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...

criteria based in the aon zone (Inferior Member) and aonoides and austracum zones (Superior Member). Dürrenstein Dolomite (a massive dolomite unit) overlies the San Cassiano Formation.

Paleoenvironment

In the 20th century the research in San Cassiano Formation was mostly focused in the study of

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

analysis. The paleoenvironments represented in San Cassiano Formation ürsich, F. T., & Wendt, J. (1977). Biostratinomy and palaeoecology of the Cassian Formation (Triassic) of the Southern Alps. Palaeogeography, Palaeoclimatology, Palaeoecology, 22(4), 257–323./ref> are as follows: Reefal environments consisting of patch reefs; shallow marginal basin; carbonate platform; continental slope; and deep basin.

Facies

The facies identified in San Cassiano Formation: *1. Volcaniclastic sandstones with cross bedding and clastic turbidites (pseudoflysch) *2. (a) Patch reefs with 10 m to 20 m in length and 10 m of thickness, generally with significant diagenetic alteration :(b) Sequences of intercalated sediments influenced by volcanic activity and near carbonate platforms. Fossiliferour limestones, fossiliferous marl alternated with dismicrits, algal mats, oolit banks, fine to coarse grain size sandstones with cross bedding :(c) Cyclic tidalites, massive dolomites, laminated algal mats, reworked ooliths *3. (a) Dolomitic massive carbonates presenting evidence of karstification (solution cavities, residual sediments and limonite crusts) :(b) Stratified marine debris *4. (Muds or carbonate sediments interrupted by volcanic or carbonate turbidites) :(a) Cipit Boulders consisting mostly of algal biolites, corals, pellets and

micrite Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to four μm formed by the recrystallization of lime mud. Flügel, Erik, ''Microfacies of Carbonate Rocks: Analysis, Interpretation and Application,'' Springer ...

s (microbialites) :(b) Clay sediments with stratified

oolite Oolite or oölite (''egg stone'') is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Ancient Greek word for egg (ᾠόν). Strictly, oolites consist of ooids of diameter 0.25� ...

oncolite Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but, instead of forming columns, they form approximately spherical structures. ...

tuffite Tuffite is a tuff containing both pyroclastic and detrital materials, but predominantly pyroclasts. According to IUGS definition tuffite contains 75 to 25% volcanic (epiclastic) material. There are several classifications that define tuffite. ...

s, turbidites and calcarenites with abundant gastropod and bivalve shells :(c) Tuffites up to 300 m in thickness with sorted strata of

calcirudite Calcirudite is a type of limestone that is composed predominantly, more than 50 percent, of carbonate grains that are larger in size than sand (2 mm in diameter). The grains can consist of either fragments of fossils, fragments of older limesto ...

calcarenite Calcarenite is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) sand-size (0.0625 to 2 mm in diameter), carbonate grains. The grains consist of sand-size grains of either corals, shells, o ...

s and Cipit Boulders. Most of the faunal elements are allochthonous :(d)

Mudrock Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too sma ...

and limestone with fine bedding, intercalated with fine grain turbidite and sporadic calcarenite turbidites These facies are interpreted as the next deposit environments: *1. Volcanic islands and formations *2. Reefal environment near to cost line: :(a) patch reefs :(b) A sequence of variable sediments intercalates with patch reefs :(c) Back reef area of the carbonate platform *3. Carbonate platform *4. Basin: :(a) Cipit Boulders :(b) Marginal shallow basin :(c) continental slope (300 m) :(d) deep basin[Rech, H. (1998). Geobiologie der sogenannten "Cipit‐Kalke â der beckenfazies der Cassianer‐Schcihten, S‘St. Kassian, Dolomiten. In Diplomarbeit am Institut fuÌˆr Geologie und Paläontologie der Georg‐August, (pp. 136pp.+). Institut fuÌˆr Geologie und Paläontologie, Göttingen: Universität Göttingen.]

Biota

The Patch reefs from Valle di Rimbianco present a diverse fauna of fossilized calcitic sponges (Porifera), corals (Cnidaria), bivalves and gastropods (Mollusca), Brachiopoda and

Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea ...

. Large parts of the basin are not fossiliferous. In the deep basin and continental slope facies the fauna consists only of ammonites and pseudoplanktonic bivalves, beside of allochthonous elements eroded from the carbonate platform (Cipit Boulders).

Geochemistry

The facies that present less diagenetic alteration are the Cipit Boulders. The microbialites contained in these elements show a very good preservation and therefore are very useful as geochemical proxies to determine paleoenvironmental conditions of the carbonate platform. The carbonate platform itself suffered intense karstification and dolomitization, because of these alterations the fossils in this facies cannot be used in geochemical analysis.

References

{{clr Geologic formations of Italy Triassic System of Europe Paleontology in Italy Dolomites Lagerstätten