Calcare di Sogno

The Calcare di Sogno ("Sogno Limestone"; also known as the Sogno Formation) is a geological formation in Italy, dated to roughly between 182 and 169 million years ago and covering the Lower Toarcian-Late Bajocian stagess of the Jurassic Period in the Mesozoic Era.^[1] Thalattosuchian remains are known from the formation, as well as fish and other taxa.^[2]

Calcare di Sogno
Stratigraphic range: Lower Toarcian-Late Bajocian ~182–169 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Sub-units	Livello a Pesci
Underlies	Formazione delle Radiolariti
Overlies	Calcare di Domaro, Calcare di Morbio, Unnamed limestones[note 1]
Thickness	Typically 120–140 m (390–460 ft) East and west 70–100 m (230–330 ft)
Lithology
Primary	Marl, marly limestone & abundance of clay Secondary: Alternation of marly limestones and marls, presence of graded Chalcudites
Other	Limestones with nodules of flint & subordinate marls
Location
Coordinates	45.8°N 9.3°E / 45.8; 9.3
Approximate paleocoordinates	33.4°N 18.9°E / 33.4; 18.9
Region	Lecco Province
Country	Italy
Type section
Named for	Sogno
Named by	Gaetani & Poliani
Calcare di Sogno (Italy) Show map of Italy Calcare di Sogno (Lombardy) Show map of Lombardy

Description

During the Early Jurassic, concretely towards the Toarcian, the Lombardy Basin became a relatively deep, fully pelagic area, located between the so-called Lugano High, at the west, and the Trento Plateau to the east, with several troughs and palaeohighs (West to east: Monte Nudo Trough, Lugano High, Generoso Trough, Corni di Canzo High, Albenza Plateau, Monte Cavallo High, Sebino Trough and Botticino High).^[3] The formation is characterized by a disposition of regional deposition equivalent to the German Posidonia Shale, with a benthic setting and deposition trends, mostly populated by marine fauna.^[4] The environment of the formation was related to a marginal marine deposit, with probably epicontinental deposition from near land environments, being connected to the central European seas and the North African currents of the Toarcian.^[5] The formation is linked with the Toarcian Anoxic Event, that is measured in the “Fish Level”, that is also the most fossiliferous section.^[6]

Environment

Two cores, the Colle di Sogno and Gajum are among the best sections that recovered the ecological changes in the Pliensbachian-Toarcian Lombardy Basin.^[7] Carbon-and oxygen-isotope data calibrated against nannofossil biostratigraphy has shown that the palaeobathymetry of the deposits was about 1000 and 1500 m, being the deepest records of the T-OAE in the western Tethyan region.^[8] As the Sogno Formation was deposited mostly in a pelagic setting, influenced by both the European and African bioregions, taxa of several provenances mix in this layer. The Nannofosil assemblage, that ranges from moderate/poor to good decreasing in the Toarcian AOE (drastic decrease in total abundance is observed in the Fish Level), includes the taxa Lotharingius (L. hauffii, L. sigillatus, L. crucicentralis, L. velatus), Discorhabdus ignotus, Diductius constans, Carinolithus (C. poulnabronei, C. superbus), Mitrolithus jansae and Watznaueria sp.1 in the Gajum Core, while the Sogno Core shows abundance of the genera Biscutum, Calyculus, Carinolithus and Crepidolithus, whereas Bussonius, Diductius, Similiscutum, Parhabdolithus and Tubirhabdus are extremely rare.^[9] The overall structure of this microtaxa assemblage trends to suggest a correlation with the biohorizon seen in coeval layers in the Lusitanian Basin, where a common trend is observed in the Western Tethys of north–south migration pathway for several organisms, including calcareous nannoplankton and ammonites.^[9]

A local index genus for environment evolution is Schizosphaerella spp. (specially S. punctulata), showing a lower valve size than in coeval layers on connected basins (Lusitanian and Paris Basins), as local result of the Lower Toarcian Jenkyns Event, indicating changes in ocean acidification and fertility rather than temperature.^[10]

Fossil content

Ichnofossils

Genus	Species	Location	Material	Type	Origin	Notes	Images
Planolites[7]	P. isp.	Colle di Sogno	Cylindrical burrows	Pascichnia	Polychaetes	Burrow-like ichnofossils referred to vermiform deposit-feeders. Sometimes considered a junior synonym of Palaeophycus.[11]	Planolites fossil

Molluscs

Genus	Species	Stratigraphic position	Material	Notes	Images
Bositra[1][4]	B. buchii	Most of the formation	Shells	A posidoniid ostreoidan. The habitat and mode of life of Bositra has been debated for more than a century. There have been different interpretations, such as a pseudoplanktonic organism,[12] a benthic organism living on the open marine floor, where it was the main inhabitant of the basinal settings, and a hybrid mode, where it has a life cycle with holopelagic reproduction controlled by changes in oxygen levels, and even a chemosymbiotic lifestyle, related to the large crinoid rafts, being the main "Safe conduct" to evade anoxic events. All the opinions along the years led to a large study in 1998, where the size/frequency distribution, the density of growth thanks to the lines related to the shell size and the position of the redox boundary by total organic carbon diagrams has revealed that Bositra probably had a benthic mode of life.[13]	Thousands of specimens in one matrix
Collina[1][4]	C. gemma	Mount Brughetto Mount Cornizzolo	Shells	A Dactylioceratidae ammonite. Present and abundant on the Mediterranean Toarcian realm.
Cornaptychus[1][4]	C. lythensis	Mount Brughetto Mount Cornizzolo	Shells	An indeterminate ammonite. Some of the specimens found are very fragmentary, making its identification complex.[4]
Dactylioceras[1][4]	D. polymorphus	Mount Brughetto Mount Cornizzolo	Shells	Type member Dactylioceratinae family of Ammonites. A common Mediterranean genus, found in deposits across all of Europe.
Harpoceras[1][4]	H. sp.	Mount Brughetto Mount Cornizzolo	Shells	Type genus of the Harpoceratinae ammonite family
Hildaites[1][4]	H. sp.	Mount Brughetto Mount Cornizzolo	Shells	A Hildoceratidae ammonite
Mesodactylites[1][4]	M. sapphicus M. sp.	Mount Brughetto Mount Cornizzolo	Shells	A Nodicoeloceratinae ammonite

Arthropods

Genus	Species	Stratigraphic position	Material	Notes	Images
?Antrimpos[14][15]	?A. sp.	Mount Cornizzolo	1 complete specimen, MSNM i10852	A Penaeidae Decapodan.
Archaeopalinurus[16]	A. cfr. A. levis	Mount Cornizzolo	Various specimens	A Palinuroidean Decapodan.
Coleia[14]	C. cf.banzensis	Mount Cornizzolo	15 specimens, complete and incomplete	An Erymidae Decapodan.
?Etallonia[14][15]	?E. sp.	Mount Cornizzolo	Single Isolated chelae, MSNM i10855	An Axiidae Decapodan.
Gabaleryon[15]	G. garassinoi	Mount Brughetto Mount Cornizzolo	Various specimens	A Coleiidae Decapodan. Was confused with Proeryon hartmanni specimens.
Proeryon[1][14][16]	P. hartmanni	Mount Brughetto Mount Cornizzolo	Various specimens	An Erymidae Decapodan Crustacean, common in Mediterranean formations.
Uncina[17]	U. cf.posidoniae U. alpina	Mount Cornizzolo	Isolated chelae, MSNM i10851, il0863, i10864	An Astacidean Decapodan of the family Uncinidae. A large decapodan, with sizes up to 40 cm.

Fish

Genus	Species	Stratigraphic position	Material	Notes	Images
Leptolepis[2][18]	L. coryphaenoides L. sp.	Monte Cornizzolo	+100 specimens	Type member of the family Leptolepidae inside Leptolepiformes. It is the most abundant fish found in the formation.
Pachycormus[2][18]	P. sp.	Monte Cornizzolo	Several specimens	The main member of the family Pachycormidae inside Pachycormiformes. Large sized fish, able to reach near 1.4 m long.
Pholidophorus[2][18]	P. sp.	Monte Cornizzolo	Several specimens	Type member of the family Pholidophoridae inside Pachycormiformes. A small sized fish, mostly related to marine deposits, associated with various predatory behaviours, including coleoids and Crocodylomorphs.

Crocodyliformes

Genus	Species	Location	Material	Notes	Images
cf.Pelagosaurus[2]	cf. P. sp.	Monte Cornizzolo	Various specimens MSNM V4012, MSNM V4013.	A Thalattosuchian marine crocodylomorph of the family Teleosauridae.The specimens found were of small size, with several characters such as opened neurocentral vertebral sutures and non sutured caudal pleurapophyses, that show a possible juvenile or subadult status.

Flora

Several plant leaves and fragments of wood were not identified.^[18]

Genus	Species	Stratigraphic position	Material	Notes	Images
Ginkgo[14][18]	G. digitata	Mount Brughetto Mount Cornizzolo	Leaves	Affinities with the Ginkgoaceae. Arboreal plants related to the modern Ginkgo species.
Pagiophyllum[14][18]	P. kurri	Mount Brughetto Mount Cornizzolo	Leaves	Affinities with the Cheirolepidiaceae and Araucariaceae. Arbustive to arboreal plants with several leaf morphotypes, probably from nearshore environments.

See also

• List of fossiliferous stratigraphic units in Italy
• Toarcian turnover
• Toarcian formations
  • Marne di Monte Serrone, Italy
  • Podpeč Limestone, Slovenia
  • El Pedregal Formation, Spain
  • Mizur Formation, North Caucasus
  • Sachrang Formation, Austria
  • Posidonia Shale, Lagerstätte in Germany
  • Irlbach Sandstone, Germany
  • Ciechocinek Formation, Germany and Poland
  • Krempachy Marl Formation, Poland and Slovakia
  • Djupadal Formation, Central Skane
  • Lava Formation, Lithuania
  • Azilal Group, North Africa
  • Whitby Mudstone, England
  • Fernie Formation, Alberta and British Columbia
    • Poker Chip Shale
  • Whiteaves Formation, British Columbia
  • Navajo Sandstone, Utah
  • Los Molles Formation, Argentina
  • Mawson Formation, Antarctica
  • Kandreho Formation, Madagascar
  • Kota Formation, India
  • Cattamarra Coal Measures, Australia