Geologists take drill to Triassic park Arizona rock core to yield coherent picture of turbulent period. Nature 502, 14–15 (03 October 2013) doi:10.1038/502014a nature/news/geologists-take-drill-to-triassic-park-1.13866 Tourists flock to Petrified Forest National Park in Arizona to marvel at great glittering logs of petrified wood. But geologists hope to flock there this month in search of something less visible and more scientifically significant: a core obtained by drilling half a kilometre into rock more than 200 million years old. Drillers will spend several weeks boring through layers of rock that house the fossils of tiny early dinosaurs and giant crocodile-like phytosaurs, as well as the leaves and pollen of an entire fossilized ecosystem. The goal of the US$970,000 drilling project is to stitch together a complete picture of most of the middle and late Triassic period, a turbulent interval that saw both a mass-extinction event and the emergence of dinosaurs. Geoscientists hope to use the decay of radioactive uranium in layers of volcanic ash in the core to precisely date events between about 205 million and 235 million years ago, just before the supercontinent Pangaea began to break apart. “It’s a unique opportunity to put together a coherent time framework for a critical part of the Triassic,” says John Geissman, a geologist at the University of Texas at Dallas and one of the project’s leaders. “Sure, we have other continental Triassic records, but the Petrified Forest area is pretty darn good when it comes to details.” The Petrified Forest effort has been years in the making. It is a follow-up to a project in which a Triassic core was drilled from New Jersey’s Newark sediment basin between 1990 and 1993 (ref. 1). That project aimed to tease out changes in the amount of sediment that was deposited as Earth went through cyclical shifts in the shape of its orbital path around the Sun. “If we can show that the Newark timescale is correct, we can empirically calibrate the Solar System’s behaviour,” says Paul Olsen, a geologist at the Lamont–Doherty Earth Observatory in Palisades, New York, and a member of the project team. “That’s probably the most exciting aspect for me.” The effort, funded by the US National Science Foundation and the International Continental Scientific Drilling Program, might also help to resolve a simmering dispute. Comparisons of the Newark data with data from Triassic rocks in the Mediterranean have led some researchers to suggest radically revising the period’s history. This reworking would lead to one subdivision — the Norian stage — taking up nearly half of the entire Triassic period, drastically changing dates of key evolutionary events, including the emergence of certain dinosaurs. The idea of a ‘long Norian’ remains fiercely controversial2, and the Petrified Forest core would need to capture a sufficient record to settle the debate. But the rocks have plenty of chronological gaps, owing to weathering or abrupt geological events. Because of surface erosion, for example, the core will not capture the very end of the Triassic around 200 million years ago, when a mass extinction swept across the planet, killing many dinosaurian relatives. The core will instead start in rocks dating to around 205 million years ago, in layers known as the Chinle formation (see ‘Hidden history’). It will travel, with several breaks in time, through the Moenkopi formation and stop in rocks about 235 million years old. The record then skips tens of millions of years into rocks from the Permian period that preceded the Triassic. “We know parts will be missing,” says Geissman. But getting a nearly complete record for much of the Triassic, in such well-studied rock layers, is bound to offer a trove of information. Geologists have explored the Petrified Forest area since the 1850s, most recently for its rich array of Triassic fossils. Since 2004, for instance, several skeletons have been unearthed of an extinct crocodile-like animal called Revueltosaurus, previously known only from its teeth. Early dinosaurs such as the dog-sized Coelophysis also roamed there, and radiometric dating has shown how these dinosaurs were related to those in other parts of the Americas3. Spectacular, fossil-bearing rocks sprawl almost everywhere in the park, says Bill Parker, Petrified Forest’s palaeontologist. The challenge is tying separate discoveries into a coherent, well-dated story. Many surface rocks are weathered so badly that they distort fossil relationships and make radiometric dating all but impossible. “It’s not like the Grand Canyon, where you can just hike down and see all the rocks in their proper order,” says Parker. “A core eliminates all of that problem, when you get one single section all the way down.” Drilling in US national parks is allowed at the discretion of the park superintendent. Petrified Forest is unusual in calling itself a science park and in having Parker on staff as a full-time palaeontologist. The big question for researchers is when drilling can begin. The team had hoped to start on 8 October, but that plan is now in doubt. Petrified Forest National Park, along with the rest of the US government, shut down on 1 October. The park will not reopen until Congress agrees on a plan to keep the government funded. If delayed too long, drilling may have to be rescheduled for next spring. Ultimately, if the project’s science findings are strong, it will pave the way for further studies of the Triassic’s buried history. The team already has its eye on other cores that it could drill. References 1. Olsen, P. E. et al. Geol. Soc. Am. Bull. 108, 40–77 (1996). 2. Lucas, S. G. et al. Earth-Sci. Rev. 114, 1–18 (2012). The Late Triassic timescale: Age and correlation of the Carnian–Norian boundary Earth-Science Reviews Volume 114, Issues 1–2, August 2012, Pages 1–18 sciencedirect/science/article/pii/S0012825212000517 Abstract 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. 3. Irmis, R. B. et al. Earth Planet. Sci. Lett. 309, 258–267 (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 Volume 309, Issues 3–4, 15 September 2011, Pages 258–267 sciencedirect/science/article/pii/S0012821X11004316 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. 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.
Posted on: Fri, 04 Oct 2013 13:51:43 +0000
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