The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from Ma to Megaannum.[Gradstein et al. (2004)] It is preceded by the Famennian (the uppermost stage of the Devonian) 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
Belgium 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.
[Heckel & Clayton (2006)]
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) is at the first appearance of the
conodont Siphonodella sulcata within the
evolution 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.
[Menning et al. (2006); for the old definition, see Paeckelmann & Schindewolf (1937)] 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 was published by Paproth et al. (1991)] 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 :
-
the zone of Gnathodus 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 and the more modern terrestrial
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.
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). Anabranch and Anastomosis 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)
Lycophyte and giant
Sphenophyte (horsetails). They coexisted alongside
Fern and lignophytes (
wood-bearing plants), including early
Spermatophyte such as
Lyginopteridales 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 Biogeography realm known as the Amerosinian realm.
Northern Asia (Kazakhstania 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.
Gondwana plant fossils are uncommon: southernmost Gondwana was covered by dwarf lycophytes, even smaller than those of the Angaran realm.
Invertebrates
Trilobite 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
Biogeography 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)
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
