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Late Triassic timescale: Carnian-Norian boundary



From:  Ben Creisler
bcreisler@gmail.com

A new online paper:

S.G. Lucas, L.H. Tanner, H.W. Kozur, R.E. Weems & A.B. Heckert (2012)
The Late Triassic timescale: Age and correlation of the Carnian-Norian boundary.
Earth-Science Reviews (advance online publication)
doi: http://dx.doi.org/10.1016/j.earscirev.2012.04.002
http://www.sciencedirect.com/science/article/pii/S0012825212000517?v=s5


The Late Triassic timescale is poorly constrained due largely to the
dearth of reliable radioisotopic ages that can be related precisely to
biostratigraphy combined with evident contradictions between
biostratigraphic and magnetostratigraphic correlations. These problems
are most apparent with regard to the age and correlation of the
Carnian-Norian boundary (base of the Norian Stage). We review the
available age data pertaining to the Carnian-Norian boundary and
conclude that the “long Norian” in current use by many workers, which
places the Carnian-Norian boundary at ~ 228 Ma, is incorrect. The
evidence supports a Norian stage that is much shorter than proposed by
these workers, so the Carnian-Norian boundary is considerably younger
than this, close to 220 Ma in age. Critical to this conclusion is the
correlation of the Carnian-Norian boundary in nonmarine strata of
Europe and North America, and its integration with existing
radioisotopic ages and magnetostratigraphy. Three biostratigraphic
datasets (palynomorphs, conchostracans and tetrapods) reliably
identify the same position for the Carnian-Norian boundary (within
normal limits of biostratigraphic resolution) in nonmarine strata of
the Chinle Group (American Southwest), Newark Supergroup (eastern
USA-Canada) and the German Keuper. These biostratigraphic datasets
place the Carnian-Norian boundary at the base of the Warford Member of
the lower Passaic Formation in the Newark Basin, and, as was widely
accepted prior to 2002, this correlates the base of the Norian to a
horizon within Newark magnetozone E13n. In recent years a correlation
based solely on magnetostratigraphy has been proposed between the
Pizzo Mondello section in Sicily and the Newark section. This
correlation, which ignores robust biostratigraphic data, places the
Norian base much too low in the Newark Basin section (~ at the base of
the Lockatong Formation), correlative to a horizon near the base of
Newark magnetozone E8. Despite the fact that this correlation is
falsifiable on the basis of the biostratigraphic data, it still became
the primary justification for placing the Carnian-Norian boundary at ~
228 Ma (based on Newark cyclostratigraphy). The “long Norian” created
thereby is unsupported by either biostratigraphic or reliable
radioisotopic data and therefore must be abandoned. While few data can
be presented to support a Carnian-Norian boundary as old as 228 Ma,
existing radioisotopic age data are consistent with a Norian base at ~
220 Ma. Although this date is approximately correct, more reliable and
precise radioisotopic ages still are needed to firmly assign a precise
age to the Carnian-Norian boundary.