Gasconadian Stage

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. 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. It 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. In central Texas the Gasconadian is represented by the Tanyard and Standebach.

Time and duration

More recent estimates as to time and duration place the Gasconatian as beginning anywhere from 488.3 million years ago(ICS) to 500 million years ago (Cosuna) with a duration ranging from 9.7 to 10 million years respectively. Both estimates are more than half the Early Ordovician, ICS's almost 60 percent, Cusuna's 2/3 . The thicknesses of different strata in the El Paso Group and the evolution of cephalopods in the Early Ordovian suggest otherwise, making the Gasconadian probably no more that 30 percent the total four part Canadian.

The Gasconadian in southern New Mexico

Overall the Gasconadian is shown diagramatically in stratigraphic sections from southern New Mexico to be about 20 percent ( one fifth) the total Lower Ordovican. A generalized stratigraphic section of the Lower Ordovian in the Franklin Mountains outside of El Paso Texas indicates that Gasconadian-age rocks are notably less than 20 percent the total thickness while further north in the Caballo Mountains of Southern New Mexico the Gasconadian portion of the El Paso section is about 40 percent.

Cephalopod evolution and the Gasconadian

In cephalopod evolution, the Gasconadian was a time of diversification at the generic level in the Ellesmeroceratidae and towards the end of sequences leading up to the Endocerida and Tarphycerida. The first great diversification of cephalopods took place in the Early Ordovian after the Gasconadian (Tremedocian) and involved not only new orders (Endocerida and Tarphycerida) but new families and general within each. In addition there are forms like the Baltoceratidae and families included in the Cyrtocerinina and the very first of the Orthocerida. These did not appear at once, but over separate spans of time, each probably near equivalent to that of the Gasconadian itself.

Usage

The Gasconadian is a common term in the paleontological literature of the previous century, whether used simply for a span of time or for strata laid down during that time, and is well understood.

References

*(Cosuna); Amos Salvador, Chronostratigraphic and Geochronometric Scales in COSUNA Stratigraphic Correlation Charts of the United States, AAPG Bul. V 69, No.2, Feb 1985, pp 181-189
*(ICS) chronostratigraphic data and correlations in Geowhen website:
*Flower, Rousseau H. The Nautiloid Order Ellesmerocerida (Cephalopoda), New Mexico Bureau of Mines and Mineral Resources Memoir 12, Socorro NM, 1964.
*________, Franklin Mountains Section in New Mexico Geological Society Guide Book, Fourth Field Conference, Southwestern New Mexico, p15.
* Correlation Chart of the Early Paleozoic in New Mexico, Fig 2, p 117, New Mexico Geological Society Guide Book, Sixteenth Field Conference.
*Seager __ Composite Columnar Section of Paleozoic and Mesozoic Rocks in the Caballo Mtns (from Kelly and Silver 1952)