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Triassic Chinle Formation redated to middle Norian = younger dinosaurs



From: Ben Creisler
bh480@scn.org

A new paper of interest:


Randall B. Irmis, Roland Mundil, Jeffrey W. Martz and William G. Parker
(2011)
High-resolution U-Pb ages from the Upper Triassic Chinle Formation (New
Mexico, USA) support a diachronous rise of dinosaurs. 
Earth and Planetary Science Letters (advance online publication)
doi:10.1016/j.epsl.2011.07.015
http://www.sciencedirect.com/science/article/pii/S0012821X11004316


Abstract
Though the Late Triassic preserves major paleoenvironmental fluctuations
and is key for understanding the evolution of Mesozoic and modern
terrestrial ecosystems, comparisons of Late Triassic non-marine sedimentary
and fossil records are difficult because global correlations lack precise
radioisotopic ages, and have instead been based upon unconstrained
biostratigraphic ranges of palynomorph and vertebrate fossils. The Chinle
Formation in southwestern North America preserves a major Late Triassic
record of paleoenvironmental and biotic change, including significant early
dinosaur fossils. Previous high-resolution radioisotopic age constraints
for the formation are limited to a single U-Pb zircon age from the upper
third of the formation. The extraction of a geologically meaningful age is
challenging from these redeposited units and preference is given to
considering the youngest age of a deposit as a maximum age and closest
approximation of the depositional age. Because calculating a weighted mean
age (or median age) from a group of ages from such deposits is often not
adequate, the precision of our two new CA-TIMS single crystal zircon U-Pb
ages from the Chinle Formation of New Mexico is limited to ca 0.3% (or ±
0.7 Ma) of the youngest crystal age. Our 206Pb/238U age of ~ 218 Ma from
the Blue Mesa Member in Six Mile Canyon, western New Mexico, demonstrates
that strata, palynomorphs, and vertebrate fossils previously considered to
be late Carnian in age are actually middle Norian in age. Our new age of ~
212 Ma from the Hayden Quarry within the Petrified Forest Member at Ghost
Ranch, northern New Mexico, provides the first maximum age for important
vertebrate assemblages from this area that record the rise of dinosaurs,
and demonstrates that basal dinosauromorphs (?dinosaur precursors?)
co-existed with dinosaurs for at least 18 Ma. These new radioisotopic data
allow a new correlation of the Chinle Formation to the Late Triassic
timescale, suggesting that most if not all of the lower Chinle is Norian in
age. This new correlation has global implications as it allows us to make
more precise comparisons with early dinosaur assemblages from the
Ischigualasto Formation of Argentina, indicating that Chinle dinosaur
assemblages are significantly younger than those from South America. The
revised age of the Chinle Formation also demonstrates that dinosaurs were
much rarer in North America at a time when they were abundant in South
America, supporting hypotheses of paleolatitudinal variation during the
rise of dinosaurs.

Highlights:
We report the first high-resolution U?Pb ages from the Upper Triassic
Chinle Formation. We demonstrate that the Chinle Fm is 5-10 My younger than
previously thought. We revise Late Triassic biostratigraphy for
palynomorphs and vertebrates. Earliest dinosaurs in western N. America are
much younger than in S. America. We discuss alternative statistical methods
for ages from complex zircon suites.


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