Gasconadian Stage
Encyclopedia
The Gasconadian Stage is the first stage of the Ordovician
geologic period in North America and of the Lower Ordovician Canadian Epoch
, coming immediately after the Late Cambrian
Trempealeauan
and preceding the middle Canadian Demingian Stage. The Gasconadian is equivalent to the European Tremedocian and roughly to the Skullrockian of the Ibexian series.
The Gasconadian is named for the Gasconade Formation of Missouri, named in turn for the Gasconade River, and is represented by the Tribes Hill Formation in New York state and by the upper Bliss Fm and the Sierrite Ls of the El Paso Group in southern New Mexico
and west Texas
.
The Gasconadian is a common term in the paleontological and stratigraphic literature of the previous century, whether used simply for a span of time or for strata laid down during that time.
The Gasconadian (Tremadocian) is represented in southern New Mexico and west Texas by the upper part of the Bliss Formation and the Sierrite Limestone which is the lowermost formation of the Lower Ordovician (Canadian) El Paso Group. The Bliss is a near shore, deltaic, and beach sandstone deposit; the lower part spans the Upper Cambrian. The Sierrite Limestone is a shallow water, carbonate shelf deposit. The contact between the two is gradational.
Generalized stratigraphic sections from the region indicate Gasconadian age strata take between 20 and 40 percent the Lower Ordovician section; 20 percent in Franklin Mtns outside of El Paso, Texas; 40 percent in the Caballo Mtns further north in southern New Mexico. This suggests the Gasconadian may actually take up well less than half the Early Ordovician rather than the principal amount.
Among these were Clarkoceras
and Ectenolites
which continued from the Late Cambrian, along with the exclusively Ordovician Ellesmeroceras the similar Eremoceras, both cylindrically orthoconic, and the compressed endogastirc Dakeoceras. All have essentially straight, short septal necks and thick concave connecting rings in the siphuncle.
The latter Canadian, following the Gasconadian saw the onset and early diversification of two new orders, the Endocerida and Tarphycerida
, evolution of new families within the Ellerocerida (e.g. Baltoceratidae and Bathmoceratidae), and near the end the first of the Orthocerida, Actinocerida, and Discosorida.
The limited suprageneric cephalopod diversity of the Gasconadian compared to the ordinal diversity that followed also suggests a relatively short duration for the stage.
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
geologic period in North America and of the Lower Ordovician Canadian Epoch
Canadian Epoch
The Canadian is the Lower or Early Ordovician in North America. The term is common in the older literature and has been well understood for more than a century...
, coming immediately after the Late Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
Trempealeauan
Trempealeauan
The Trempealeauan is the upper or latest stage of the Upper or Late Cambrian in North America, spanning about 4 million years from about 492.5 to 488.3 m.y.a., equivalent to the Fengshanian of China...
and preceding the middle Canadian Demingian Stage. The Gasconadian is equivalent to the European Tremedocian and roughly to the Skullrockian of the Ibexian series.
The Gasconadian is named for the Gasconade Formation of Missouri, named in turn for the Gasconade River, and is represented by the Tribes Hill Formation in New York state and by the upper Bliss Fm and the Sierrite Ls of the El Paso Group in southern New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
and west Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
.
The Gasconadian is a common term in the paleontological and stratigraphic literature of the previous century, whether used simply for a span of time or for strata laid down during that time.
Time and Duration
Recent estimates (ICS and Cosuna) place the beginning of Gasconadian, and Ordovician, anywhere from about 488 to 500 million years ago, with a duration ranging from 9.7 to 10 million years respectively putting the end about 464 m.y.a. Both give estimates of over half the Early Ordovician. 60 percent according th ICS, 2/3 according to Cosuna.Gasconadian Strata
- New Mexico -West Texas -The Gasconadian (Tremadocian) is represented in southern New Mexico and west Texas by the upper part of the Bliss Formation and the Sierrite Limestone which is the lowermost formation of the Lower Ordovician (Canadian) El Paso Group. The Bliss is a near shore, deltaic, and beach sandstone deposit; the lower part spans the Upper Cambrian. The Sierrite Limestone is a shallow water, carbonate shelf deposit. The contact between the two is gradational.
Generalized stratigraphic sections from the region indicate Gasconadian age strata take between 20 and 40 percent the Lower Ordovician section; 20 percent in Franklin Mtns outside of El Paso, Texas; 40 percent in the Caballo Mtns further north in southern New Mexico. This suggests the Gasconadian may actually take up well less than half the Early Ordovician rather than the principal amount.
Gasconadian Cephalopods
Cephalopods from the Gasconadian belong exclusively to the nautiloid order Ellesmerocerida, and predominantly to the Ellesmeroceratidae.Among these were Clarkoceras
Clarkoceras
Clarkoceras is a breviconic ellesmerocerid cephalopod, one of only two genera known to have crossed from the Late Cambrian, Trempealeauan, into the Early Ordovician, Gasconadian...
and Ectenolites
Ectenolites
Small, slender, cylindrial members of the Ellesmeroceratidae that resemble small but proportionally narrower Ellesmeroceras. Septa, as typical for ellesmerocerids, are close spaced with shallow lobes on either flank. The body chamber is proportionally long, the shell itself slightly compressed...
which continued from the Late Cambrian, along with the exclusively Ordovician Ellesmeroceras the similar Eremoceras, both cylindrically orthoconic, and the compressed endogastirc Dakeoceras. All have essentially straight, short septal necks and thick concave connecting rings in the siphuncle.
The latter Canadian, following the Gasconadian saw the onset and early diversification of two new orders, the Endocerida and Tarphycerida
Tarphycerida
The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...
, evolution of new families within the Ellerocerida (e.g. Baltoceratidae and Bathmoceratidae), and near the end the first of the Orthocerida, Actinocerida, and Discosorida.
The limited suprageneric cephalopod diversity of the Gasconadian compared to the ordinal diversity that followed also suggests a relatively short duration for the stage.