Ordovician dating

Posted by / 24-Jan-2015 12:54

Ordovician dating

The most recent publication dealing with the chronostratigraphic framework of the Trenton Group and its lateral equivalents for the New York State region was published in 2002 by Brett and Baird.

In their figure, as shown below, there are no specific absolute age restraints placed on the strata from the type section of the Trenton limestone.

Much of the relative geologic time scale had been constructed prior to the 20th century, but much progress has been made in the last half-century toward defining the absolute ages of the rocks on which the relative time scale is based.

The modern Geologic Time Scale as shown above is a compendium of both relative and absolute age dating and represents the most up-to-date assessment of Earth's history.

This site will follow the North American Classification system, as Trenton Falls is one of the primary type sections for this dating system.

These two separate classification systems are presently in use for the establishment of chronostratigraphic age, one called the North American Stage Classification and the other the British or European Stage Classification system.

These include: a) the basal Trenton Unconformity which is accentuated (and of longer duration) in the central Mohawk Valley in the neighborhood of the Canajoharie Arch; b) the upper Trenton Lower Utica disconformity centered in the Western Mohawk Valley region in the vicinity of Middleville, New York; and c) the top Schenectady disconformity centered in the region to the east of Little Falls New York; 2).

The large-scale westward shift in facies belts through time beginning near the end of the graptolite chronozone; and 3) the relative dominance of carbonate facies (in blue) in the Western Mohawk Valley in the Mohawkian and the dominance of shale and flysch facies (in black and stippled gray) in the Eastern Mohawk Valley.

Historically, the identification of European global series and stage boundaries has been difficult in North American carbonate-dominated rocks, such as the Trenton limestone.

This is due, in part, to the lack of the key diagnostic graptolite and conondont species used in the British system.

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The Mohawkian Series is further subdivided into the Ashbyan, Turinian, Rocklandian (Ro. above), and Shermanian stages, so the total duration of time represented within the Trenton limestone (Rocklandian, Kirkfieldian and Shermanian stages) is significantly less.

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