Calcare di Sogno | |
Type: | Geological formation |
Age: | Lower Toarcian-Late Bajocian ~ |
Period: | Toarcian |
Prilithology: | Marl, marly limestone & abundance of clay Secondary: Alternation of marly limestones and marls, presence of graded Chalcudites |
Otherlithology: | Limestones with nodules of flint & subordinate marls |
Namedfor: | Sogno |
Namedby: | Gaetani & Poliani |
Region: | Lecco Province |
Coordinates: | 45.8°N 9.3°W |
Paleocoordinates: | 33.4°N 18.9°W |
Subunits: | Livello a Pesci |
Underlies: | Formazione delle Radiolariti |
Overlies: | Calcare di Domaro, Calcare di Morbio, Unnamed limestones |
Thickness: | Typically 120- East and west 70m-100mm (230feet-300feetm) |
The Calcare di Sogno ("Sogno Limestone"; also known as the Sogno Formation) is a geological formation in Italy, dated to roughly between 182-169 million years ago and covering the Lower Toarcian-Late Bajocian stagess of the Jurassic Period in the Mesozoic Era.[1] Thallatosuchian remains are known from the formation, as well fishes and other taxa.[2]
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 benthonic setting and deposition trends, mostly populated by marine fauna.[4] The environment of the formation was related to a marginal marine deposit, with probably epicontinetal 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]
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 on a Pelagic setting, influenced by both the European and African bioregions, taxa of several provenance mix on this layers. 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 is observed a common trend 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 changues in ocean acidification and fertility rather than temperature.[10]
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Genus | Species | Stratigraphic position | Material | Notes | Images |
---|---|---|---|---|---|
Bositra |
|
| 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 related to 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 the change on Oxygen levels, and even a chemosymbiotic lifestile, 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] | |
Collina | Collina gemma |
| Shells | A Dactylioceratidae ammonite. Present and abundant on the Mediterranean Toarcian realm. | |
Cornaptychus | Cornaptychus lythensis |
| Shells | An indeterminate ammonite. Some of the specimens found are very fragmentary, making its identification complex. | |
Dactylioceras | Dactylioceras polymorphus |
| Shells | Type member Dactylioceratinae family of Ammonites. A common mediterranean genera, found on deposits along all europe. | |
Harpoceras | Harpoceras sp. |
| Shells | Type reprensentative genus of the Harpoceratinae ammonite family | |
Hildaites | Hildaites sp. |
| Shells | A Hildoceratidae ammonite | |
Mesodactylites |
|
| Shells | A Nodicoeloceratinae ammonite | |
Genus | Species | Stratigraphic position | Material | Notes | Images |
---|---|---|---|---|---|
?Antrimpos | ?Antrimpos sp. |
| 1 complete specimen, MSNM i10852 | A Penaeidae Decapodan. | |
Archaeopalinurus | Archaeopalinurus cfr. A. levis |
| Various specimens | A Palinuroidean Decapodan. | |
Coleia | Coleia cf.banzensis |
| 15 specimens, complete and incomplete | An Erymidae Decapodan. | |
?Etallonia | ?Etallonia sp. |
| Single Isolated chelae, MSNM i10855 | An Axiidae Decapodan. | |
Gabaleryon[14] | Gabaleryon garassinoi |
| Various specimens | A Coleiidae Decapodan. Was confussed with Proeryon hartmanni specimens. | |
Proeryon[15] [16] | Proeryon hartmanni |
| Various specimens | An Erymidae Decapodan Crustacean, common on the mediterranean rocks. | |
Uncina[17] | Uncina cf.posidoniaeUncina alpina |
| Isolated chelae, MSNM i10851, il0863, i10864 | An Astacidean Decapodan of the family Uncinidae. A large decapodan, with sizes up to 40 cm. | |
Genus | Species | Stratigraphic position | Material | Notes | Images |
---|---|---|---|---|---|
Leptolepis |
| Monte Cornizzolo | +100 specimens | Type member of the family Leptolepidae inside Leptolepiformes. It is the most abundant fish found on the formation. | |
Pachycormus |
| Monte Cornizzolo | Several Especimens | The main member of the family Pachycormidae inside Pachycormiformes. Large sized fish, able to reach near 1.4 m long. | |
Pholidophorus[18] |
| Monte Cornizzolo | Several Especimens | Type member of the family Pholidophoridae inside Pachycormiformes. A small sized fish, mostly related to marine deposits, associated with various predatory behaviours, including coeloids and Crocodrylomorphs. | |
Several plant leaves and fragments of wood were not identified.[18]
Genus | Species | Stratigraphic position | Material | Notes | Images |
---|---|---|---|---|---|
Ginkgo |
|
| Leaves | Affinities with the Ginkgoaceae. Arboreal plants related to the modern Ginkgo species. | |
Pagiophyllum |
|
| Leaves | Affinities with the Cheirolepidiaceae and Araucariaceae. Arbustive to arboreal plants with several leaf morphotypes, probably from nearshore environments. | |