The Besano Formation is a

geological 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 expo ...

in 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, ...

of northwestern

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...

and southern Switzerland. This formation, a short but fossiliferous succession of dolomite and

black 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, especiall ...

, is famous for its preservation of

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma ...

marine life including fish and aquatic reptiles. It is exposed in the vicinity of

Monte San Giorgio Monte San Giorgio is a mountain and UNESCO World Heritage Site on the border between Switzerland and Italy. It is part of the Lugano Prealps, overlooking Lake Lugano in the Swiss Canton of Ticino. Monte San Giorgio is a wooded mountain, risin ...

and is among the formations responsible for the area being designated as a UNESCO

World Heritage Site A World Heritage Site is a landmark or area with legal protection by an international convention administered by the United Nations Educational, Scientific and Cultural Organization (UNESCO). World Heritage Sites are designated by UNESCO for ...

. In Switzerland, it is also known as the Grenzbitumenzone.

Geology

General geology

The Grenzbitumenzone is a relatively thin band of dark dolomite and shale, approximately 5 to 16 meters in total thickness. It extends about 10 km from east to west along the northern edge of Monte San Giorgio. In individual outcrops, the Grenzbitumenzone overlies the lower part of San Salvatore Dolomite, a thick and widespread carbonate-rich formation. The later parts of the San Salvatore Formation, exposed north of Monte San Giorgio, are partially isochronous with the Grenzbitumenzone (formed at the same time). At its upper extent, the Grenzbitumenzone grades into the San Giorgio Dolomite, a formation with fewer fossils and a lower concentration of organic matter. The San Giorgio Dolomite itself is succeeded by the fossil-rich

Meride Limestone Meride is a village and former municipality in the district of Mendrisio in the canton of Ticino in Switzerland. On 14 April 2013 the former municipalities of Besazio, Ligornetto and Meride merged into the municipality of Mendrisio.

. As its name indicates, the sediments of the Grenzbitumenzone are

bituminous Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term ...

, rich in

organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...

to the point that they burn readily. Grey laminated (finely-layered) dolomite with about 20% organic matter comprises the majority of the formation. The width of laminae in these dolomite layers vary widely from sub-millimeter to sub-centimeter scales, as a function of mineral or grain size variation. Invertebrate fossils and isolated

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

grains are common in the dolomite, while vertebrates and

radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The elab ...

n molds are preserved less often. Finely-laminated

with up to 40% organic matter makes up a smaller portion. Radiolarians and vertebrate fossils are common in the shale. However, invertebrates are practically absent, and crystals of dolomite and detrital quartz are rare. These major dolomite or shale layers show very little evidence of bioturbation or disturbance.

Pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

is present but uncommon, likely a consequence of low

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

availability. The organic matter can be characterized as type II kerogen, enriched in hopane and

porphyrin Porphyrins ( ) are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent of porphyrin is porphine, a rare chemical ...

compounds, though strongly depleted in

Carbon-13 Carbon-13 (13C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth. Detection by mass spectrometry A mass ...

. These

biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, p ...

s, when combined, indicate that most of the organic material was derived from cyanobacteria. Other sediments and rock types are uncommon in the formation. Thin layers of fine-grained laminated dolomite with a white color extend over a wide area. They have very little organic matter and instead contain shell fragments and peloids. This white dolomite likely represents distal

turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites w ...

deposits, collapsed from nearby carbonate sources. A similar origin is inferred for massive (unbedded) dolomite layers, which have a porous texture and heterogenous grain sizes. There is some evidence of reworking, as thin dolomite layers rarely show wavy layering or are ripped up into clasts by deep currents. The black shale layers occasionally preserve bands of

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...

, derived from radiolarian blooms. Numerous narrow

bentonite Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-m ...

layers ( volcanic tuffs) occur throughout the formation. They are mostly composed of

illite Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina ...

and montmorillonite, with occasional crystals of

sanidine Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal syste ...

. Unlike most Triassic tuffs from the Southern Alps,

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

crystals are completely absent.

Palaeoenvironment

The formation is representative of a small intraplatform basin, a deep and stable marine environment which would have been positioned among shallow-water reefs and

carbonate platform A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually mic ...

s. The carbonate platforms themselves are preserved in thick sequences, such as the San Salvatore Dolomite further north and west, and the Esino Limestone further east. The Grenzbitumenzone basin may be up to 20 km wide, if the Perledo-Varenna Formation east of Lake Como also belongs to the basin. This system of carbonate platforms and basins developed along a western tongue of the

Tethys Ocean The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean that covered most of the Earth during much of the Mesozoic Era and early Cenozoic Era, located between the ancient continents ...

, which transgressed eastwards during the Middle Triassic. Alternations between dolomite and shale in the Grenzbitumenzone are probably a consequence of sea level fluctuations. Raised sea levels would have submerged the carbonate platforms, which may have enhanced dolomite deposition in the basin. Alternatively, it would have connected the basin to other nutrient-rich areas, leading to phytoplankton blooms and thus more shale deposition. Laminations within dolomite layers correspond to fluctuating carbonate levels, possibly linked to runoff from carbonate platforms during storms. The sediments of the Grenzbitumenzone are undisturbed by

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

(seabed-living) organisms, while well-preserved fossils, organic matter, and heavy metal ions are prevalent. This evidence supports the traditional view that the seabed of the basin was completely

anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...

– stagnant, oxygen-deprived, and lifeless. However, abundant fossils of

nekton Nekton or necton (from the ) refers to the actively swimming aquatic organisms in a body of water. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the passive organisms t ...

ic (swimming) and

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cr ...

ic (free-floating) life indicate that oxygen was more concentrated in seawater closer to the surface. There has been debate over the origin or intensity of this strong stratification in oxygen content. The basin, though relatively deep, was likely too shallow for stratification via deep saline currents or a strong temperature gradient. The traditional view places blame on a concentration of planktonic bacteria at mid-level in the water column, dividing an oxic upper part of the basin from the anoxic lower part. There is little evidence for microbial activity on the seabed during the deposition of the Grenzbitumenzone. Later studies have argued that the seabed could have been dysoxic – with low oxygen levels, though still greater than in anoxic waters. Among the most common fossils belong to '' Daonella'', a bivalve with strong debate over its habitat preferences. Early studies argued that it was pseudoplanktonic (attached to floating objects) or washed in from shallower areas, congruent with an anoxic Grenzbitumenzone seabed. However, ''Daonella'' is now believed to have lived in place at the bottom of the basin, specializing in a dysoxic environment inhospitable to most other benthic animals. Disarticulation and reorientation of Grenzbitumenzone fossils have favored the presence of weak and oxygenated bottom currents. Early evidence for bottom currents was controversial and perhaps based on a misdrawn illustration, but further specimen sampling supports the same general conclusion.

Taphonomy

Grenzbitumenzone fossils are usually well-preserved, but most are compressed between sediment layers. Compaction is much more pronounced in the thin shale layers than the thicker dolomite layers. Soft tissue preservation is rare but not unheard of: it includes calcified shark cartilage, phosphatized coprolites and gut contents, and organic remnants of reptile scales. In a dysoxic environment, preservation may have been facilitated by bacterial mats, adhering and sealing a skeleton onto the substrate. However, direct evidence for bacterial mats is not present. Two ways to quantify a skeleton’s preservation are completeness (the proportion of a skeleton present in a fossil) and articulation (the proportion of a skeleton preserved in life position). The completeness and articulation of '' Serpianosaurus'' fossils are variable, though are fairly high on average. The two factors are most clearly linked in the head, a heavy part of the skeleton which is most likely to sink first, detaching from the rest of a floating body. Even so, headless and relatively incomplete specimens are rare, so bodies are unlikely to fall apart while floating for an extended period. The vertebrae and ribs tend to be disarticulated, though only over a short distance. Peripheral elements such as the toes are also prone to disarticulation. Disarticulation is probably a result of subtle deep-water currents, during a long period of slow decay on the seabed. Heads and tails tend to curve in the same direction, which may be a consequence of current flow. ''

Saurichthys ''Saurichthys'' (from el, σαῦρος , 'lizard' and el, ἰχθῦς 'fish') is an extinct genus of predatory ray-finned fish from the Triassic period. It type genus family Saurichthyidae ( Changhsingian-Middle Jurassic), and the larges ...

'' fossils of the Grenzbitumenzone are often well-preserved, though disarticulated and twisted specimens are more common than in the Cassina beds of the Meride Limestone. This may be due to the low sedimentation rate of the Grenzbitumenzone relative to the Meride Limestone, providing more time for the influence of bottom currents prior to burial. The Cassina beds also have more direct evidence for microbial mats, which could have played a role in stabilizing decaying carcasses. Conversely,

ichthyosaur Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, altho ...

fossils of the Grenzbitumenzone tend to have higher completeness than those found in Early Jurassic formations elsewhere in Europe. There is no specific region of the body with significantly lower completeness, arguing that the fossils were unaffected by preferential scavenging. Fossil completeness may have been enhanced by the relatively small and isolated nature of the Grenzbitumenzone basin, protected from the influence of stronger marine currents.

Paleobiota

The formation is highly fossiliferous and has provided the following fossils:

Reptiles

Bony fish

Cartilaginous fish

Invertebrates

* Ammonoids ('' Celtites'', '' Protrachyceras'', '' Serpianites'', '' Ticinites'', '' Repossia'', '' Flexoptychites'', '' Proarcestes'', '' Stoppaniceras'', '' Parakellnerites'', etc.) *

Belemnoids Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms o ...

('' Breviconoteuthis'', '' Phragmoteuthis'', '' Mojsisovicsteuthis'', etc.) *

Bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biv ...

('' Daonella'', '' Peribositria'', etc.) *

Conodont Conodonts ( Greek ''kōnos'', " cone", + ''odont'', " tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, w ...

s (''

Gondolella ''Gondolella'' is an extinct genus of conodonts in the family Gondolellidae Gondolellidae is an extinct family of conodonts in the order Ozarkodinida. There are three subfamilies: Mullerinae, Neogondolellinae and Novispathodinae. Refe ...

'', etc.) *

Crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...

s ('' Antrimpos'') *

Gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. ...

('' Frederikella'', ''

Worthenia Worthenia is an extinct genus of sea snail found in the fossil record. This genus is primarily found in rocks formed during the Devonian to Triassic periods (416-200 Ma) from the central areas of North America. ''Worthenia'' was named for the pa ...

'', '' Trachynerita'', '' Omphaloptycha'', etc.) *

Nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded speci ...

s (''

Michelinoceras ''Michelinoceras'' is the oldest known genus of the Michelinocerida, more commonly known as the Orthocerida, characterized by long, slender, nearly cylindrical orthocones with a circular cross section, long camerae, very long body chambers, and a ...

'', etc.)

Plants

* Calcareous

algae Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from ...

(''

Diplopora ''Diplopora'' is an extinct genus of marine dasycladacean algae in the family Diploporaceae. Fossil records This genus is known in the fossil records from the Silurian to the Jurassic The Jurassic ( ) is a Geological period, geologic period ...

'') * Voltzialean

conifer Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All ex ...

s ('' Voltzia'')

References

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