The Tournaisian is in the
ICS geologic timescale the lowest
stage or oldest
age of the
Mississippian, the oldest
subsystem of the
Carboniferous
The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...
. The Tournaisian age lasted from Ma to
Ma. It is preceded by the
Famennian (the uppermost stage of the
Devonian
The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...
) and is followed by the
Viséan. In global stratigraphy, the Tournaisian contains two substages: the Hastarian (lower Tournaisian) and Ivorian (upper Tournaisian). These two substages were originally designated as European regional stages.
Name and regional alternatives
The Tournaisian was named after the
Belgian city of
Tournai. It was introduced in scientific literature by Belgian geologist
André Hubert Dumont in 1832. Like many Devonian and lower Carboniferous stages, the Tournaisian is a unit from West European regional stratigraphy that is now used in the official international time scale.
The Tournaisian correlates with the regional North American Kinderhookian and lower
Osagean stages and the Chinese Tangbagouan regional stage. In the British Isles, where the Hastarian and Ivorian are difficult to distinguish, the entire Tournaisian was equivalent to the Courceyan regional stage.
Stratigraphy
The base of the Tournaisian (which is also the base of the Carboniferous
system
A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...
) is at the first appearance of the
conodont ''
Siphonodella sulcata'' within the
evolutionary lineage from ''Siphonodella praesulcata'' to ''Siphonodella sulcata''. The first appearance of
ammonite species ''
Gattendorfia subinvoluta'' is just above this and was used as a base for the Carboniferous in the past. The
GSSP for the Tournaisian is near the summit of La Serre hill, in the
Lydiennes Formation of the
commune of
Cabrières, in the
Montagne Noire (southern France). The GSSP is in a section on the southern side of the hill, in an 80 cm deep trench, about 125 m south of the summit, 2.5 km southwest of the village of Cabrières and 2.5 km north of the hamlet of
Fontès.
The top of the Tournaisian (the base of the Viséan) is at the first appearance of the
fusulinid species ''
Eoparastaffella simplex'' (
morphotype 1/morphotype 2).
The Tournaisian contains eight conodont
biozones:
*the zone of ''
Gnathodus
''Gnathodus'' is an extinct conodont genus in the family Idiognathodontidae.
Use in stratigraphy
The Tournaisian, the oldest age of the Mississippian (geology), Mississippian (also known as Lower Carboniferous), contains eight conodont biozon ...
pseudosemiglaber'' and ''
Scaliognathus anchoralis''
*the zone of ''Gnathodus semiglaber'' and ''
Polygnathus communis''
*the zone of ''
Dollymae bouckaerti''
*the zone of ''Gnathodus typicus'' and ''
Siphonodella isosticha''
*the zone of ''Siphonodella quadruplicata'' and ''
Patrognathus andersoni'' (upper zone of ''Patrognathus andersoni'')
*the lower zone of ''Patrognathus andersoni''
*the zone of ''Patrognathus variabilis''
*the zone of ''Patrognathus crassus''
Paleoenvironments
The Tournaisian coincides with
Romer's gap, a period of remarkably few terrestrial fossils, thus constituting a discontinuity between the
Devonian
The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...
and the more modern terrestrial
ecosystem
An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...
s of the Carboniferous.
The middle of the Tournaisian is marked by a southern glaciation event, of a slightly lesser extent than the glaciations which swept over Gondwana in the later Carboniferous and the very end of the Devonian. During the Tournaisian,
South America was located at south polar latitudes and formed the westernmost part of the
supercontinent Gondwana. The southwestern coastline of Gondwana was bustling with distinctive cold-water brachiopod and bivalve faunas.
Coal is less common in the Tournaisian than in the rest of the Carboniferous, and forests and swamps were at low-density despite some trees reaching heights of up to 40 meters (131 feet).
Anabranching channels and
anastomosing rivers (with permanent channels splitting around large vegetated islands) would not develop until the Viséan, and river systems of the Tournaisian were more similar to those of the
Late Devonian.
Flora
The Tournaisian saw a new diversification of arborescent (tree-sized)
lycophytes and giant
sphenophytes (horsetails). They coexisted alongside
ferns and lignophytes (
wood-bearing plants), including early
seed plants such as
lyginopteridalean pteridosperms ("seed ferns").
The Tournaisian was a transitional stage for lignophyte evolution: Devonian
progymnosperm taxa such as ''
Archaeopteris'' had gone extinct, but new types of woody trees such as ''
Pitus'' and ''
Protopitys'' set the stage for even greater morphological diversity. There is still much debate over the proportion of spore-bearing (progymnosperm) to seed-bearing (spermatophyte) woody plants, but both were evidently major parts of Tournaisian ecosystems.
Tropical and subtropical swamps, in what is now Europe, North America, and China, represent a low-latitude
paleobiogeographical realm known as the Amerosinian realm.
Divaricating (widely branching) trunks of ''
Lepidodendropsis'' lycophytes are by far the most abundant and widespread plant fossils of the Tournaisian, yet there was some minor variation in other flora through time and space. In eastern North America, lyginopterids and probable progymnosperms were also common, as indicated by leaf
form genera such as ''
Adiantites'', ''
Rhodeopteridium'', and ''
Genselia''.
The progymnosperm leaf ''
Triphyllopteris'' may be more common in Europe while the lycophyte ''
Sublepidodendron'' characterizes Tournaisian China. Late Devonian seed plants like ''
Rhacopteris'' also persisted into the Tournaisian tropics. ''
Lepidodendron'', a massive arborescent lycophyte which would dominate coal forests through the rest of the Carboniferous, first appeared near the Tournaisian-Viséan boundary.
Northern Asia (
Kazakhstan
Kazakhstan, officially the Republic of Kazakhstan, is a landlocked country primarily in Central Asia, with a European Kazakhstan, small portion in Eastern Europe. It borders Russia to the Kazakhstan–Russia border, north and west, China to th ...
and
Siberia) was positioned within subtropical or temperate northern latitudes, and developed its own endemic floras, the Angaran realm. The most common plant fossils in this region were shrub-sized lycophytes such as ''
Ursodendron'' and ''
Tomiodendron'', shorter than their arborescent tropical relatives.
Gondwanan plant fossils are uncommon: southernmost Gondwana was covered by dwarf lycophytes, even smaller than those of the Angaran realm.
Subtropical and temperate lycophytes such as ''Lepidodendropsis'', ''
Archaeosigillaria'', and ''
Frenguellia'' could be found in some parts of the supercontinent, such as
Argentina
Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
and
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
.
In the middle Tournaisian glaciation, species-poor frigid
tundra
In physical geography, a tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. There are three regions and associated types of tundra: #Arctic, Arctic, Alpine tundra, Alpine, and #Antarctic ...
developed in western Argentina.
These south polar tundras hosted the oldest known seed plants in Gondwanan territories, which likely spread south across a
land bridge once the
Rheic Ocean closed between
Laurussia and Gondwana.
Tournaisian terrestrial sediments in South America are additionally characterized by the
miospore index fossil ''
Waltzispora lanzonii''. The floral diversity of Tournaisian southern tundra consists almost entirely of relict Devonian genera; this suggests that Late Devonian land plant extinctions in lower latitudes were mostly driven by competition from new tropical species, rather than global environmental pressures.
Invertebrates
Trilobites experienced their final substantial diversification event in the mid-late Tournaisian, briefly regaining a level of diversity not seen since the
Middle Devonian. Almost all new species belonged to the recently evolved family
Phillipsiidae, while the few surviving Devonian-type trilobites declined. Most early Tournaisian trilobites were widespread deep-water species. By the late Tournaisian, they had recolonized shallower environments and divided into three different
biogeographic zones corresponding to North America, Europe, and East Asia.
Notable formations
*
Albert Formation (New Brunswick, Canada)
*
Agua de Lucho Formation (Argentina)
*
Ballagan Formation /
Cementstone Group (Scotland)
*
Herbesskaya Formation (Russia)
*
Horton Bluff Formation (Nova Scotia, Canada)
*
Mansfield Group (Australia)
*
Price Formation (West Virginia / Virginia, United States)
*
Tournai Formation (Belgium)
References
Bibliography
* ; 1832: ''Mémoire sur la constitution géologique de la province de Liège'', Mémoires couronnés par l'Académie Royale des Sciences et Belles-Lettres de Bruxelles 8 (3), VII.
* ; 2004: ''A Geologic Time Scale 2004'',
Cambridge University Press
* ; 2006: ''The Carboniferous system, use of the new official names for the subsystems, series and stages'', Geologica Acta 4(3), pp 403–407
*
* ; 2006: ''Global time scale and regional stratigraphic reference scales of Central and West Europe, East Europe, Tethys, South China, and North America as used in the Devonian–Carboniferous–Permian Correlation Chart 2003 (DCP 2003)'', Palaeogeography, Palaeoclimatology, Palaeoecology 240 (1-2): pp 318–372
* ; 1937: ''Die Devon-Karbon-Grenze'', Comptes Rendus (2) du Cinquième Congrès International de Stratigraphie et Géologie du Carbonifère, Heerlen 1935 (2), pp 703–714
*
External links
Early Carboniferous timescaleat the website of the Norwegian network of offshore records of geology and stratigraphy
www.palaeos.com
{{DEFAULTSORT:Tournaisian
Mississippian geochronology
Geological ages
.
Stratigraphy of Europe
Tournai