Showing posts with label isotopic dating. Show all posts
Showing posts with label isotopic dating. Show all posts
New Isotopic Dates for Strata in the Late Triassic Chinle Formation, Petrified Forest National Park
I'm thrilled that this is finally out, along with the recent Irmis et al. paper. This new paper by Ramazani et all provides high-precision isotopic dates for a variety of stratigraphicl levels of the Chinle Formation in Petrified Forest National Park. Furthermore it provides the first dates for the Mesa Redondo and Owl Rock Members which essentially contrain the entire park section. The Mesa Redondo date is particularly important because it comes close to representing the base of the Chinle Formation in Arizona (see stratigraphic discussion in Irmis et al., 2011). The stratigraphic sequence and scheme used for this study is based upon independent work conducted by Dr. David Fastovsky of the University of Rhode Island and some of his students, as well as work done by a group from Baylor University. Thus it doesn't match the current park stratigraphic nomenclature introduced by Martz and Parker (2010) and Parker and Martz (2011). Still the two are close and if anyone has any questions where these new dates fit into our scheme please contact me.
Some important points made by the paper:
- Chinle deposition in Petrified Forest ranged from about 225 - 208 million years ago or a duration of 17 million years.
- There is not enough stratal thickness in the Sonsela Member to account for the elapsed amount of time, so there must be many hiatuses in this unit.
- Despite the purported presence of numnerous hiatuses in the Sonsela none of them are large-scale, and there is little evidence for a sizable regional unconformity (i.e., the TR-4). [Note that this is also supported by other stratigraphic and biostratigraphic data].
- These authors argue that the basal Chinle date demonstrates overlap between the Chinle and Ischigualasto formations, suggesting that the rise of early dinosaurs was not diachronous (but see Irmis et al., 2011).
Ramezani, J., Hoke, G. D., Fastovsky, D. E., Bowring, S. A., Therrien, F., Dworkin, S. I., Atchley, S. C., and L. C. Nordt. 2011. High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs. Geological Society of America Bulletin. First published online August 19, 2011, doi: 10.1130/B30433.1
Abstract - The Triassic successions of the Colorado Plateau preserve an important record of vertebrate evolution and climate change, but correlations to a global Triassic framework are hampered by a lack of geochronological control. Tuffaceous sandstones and siltstones were collected from the Upper Triassic Chinle Formation exposed in the Petrified Forest National Park, Arizona, USA, within a refined stratigraphic context of 31 detailed measured sections. U-Pb analyses by the isotope dilution–thermal ionization mass spectrometry (ID-TIMS) method constrain maximum depositional ages for nine tuffaceous beds and provide new insights into the depositional history of the Chinle fluvial system. The base of the Blue Mesa Member of the Chinle Formation is placed at ca. 225 Ma, and the top of the Petrified Forest Member is placed at 208 Ma or younger, bracketing an ∼280-m-thick section that spans nearly the entire Norian Stage of the Late Triassic. Estimated sediment accumulation rates throughout the section reflect extensive hiatuses and/or sediment removal by channel erosion. The new geochronology for the Chinle Formation underscores the potential pitfalls of correlation of fluvial units based solely on lithostratigraphic criteria. A mid-Norian age (ca. 219–213 Ma) for the distinctive Sonsela conglomeratic sandstone bed constrains the Adamanian-Revueltian land vertebrate faunachron boundary. Our new data permit a significant time overlap between the lower Chinle sequence and the dinosauromorph-rich Ischigualasto Formation of northwestern Argentina. Near-contemporaneity of the trans-American deposits and their faunal similarities imply that early dinosaur evolution occurred rapidly across the Americas.
High-resolution U-Pb ages from the Chinle Formation with a Stratigraphic Revision of the Chinle and Support for the Diachronous Rise of Dinosaurs
This is the full paper providing two new radioisotopic ages for Chinle Formation strata in New Mexico. One for the purported base of the Blue Mesa Member (thus a basal Chinle age constraint) from Six Mile Canyon in New Mexico, and the other from the younger Hayden Quarry, which has produced a number of new dinosauromorphs including the type specimens of Dromomeron romeri and Tawa hallae (Irmis et al., 2007; Nesbitt et al., 2009). Although the Blue Mesa date has been widely cited these have previously only appeared in abstract form (Irmis & Mundil, 2008, 2010).
Jeff's and my contribution to this paper is mainly a revised hypothesis of stratigraphic relationships of units in the lower part of the Chinle Formation based on several years of detailed fieldwork in Arizona, Utah, and New Mexico. Our revised correlations demonstrate that the date from Six Mile Canyon is more likely from strata from the uppermost part of the Blue Mesa Member (Chinle Formation) type section in Petrified Forest National Park (PEFO), and not from the base of the unit as previously supposed. Therefore this date is from stratigraphically younger rocks than previously believed. We also argue that the lower part of the Blue Mesa Member type section at PEFO is equivalent to the Bluewater Creek Member in New Mexico and northeastern Arizona, and that the Bluewater Creek is not directly equivalent to the Mesa Redondo Member. Instead we argue that the Mesa Redondo Member is mainly a lateral equivalent of the Shinarump Member. These are novel hypotheses that we have much supporting data for and we hope to have a full manuscript detailing these revised correlations prepared in the near future.
Irmis, R. B., Mundil, R., Martz, J. W., and W. 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
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.
REFERENCES
Irmis, R., Mundil, R., 2008. New age constraints from the Chinle Formation revise global comparisons of Late Triassic vertebrate assemblages. J. Vertebr. Paleontol. 28, 95A(Suppl.).
Irmis, R.B., Mundil, R., 2010. New U–Pb zircon ages from the Chinle Formation (western US) revise understanding of Late Triassic terrestrial vertebrate evolution. Geol. Soc. Am. Abstracts Prog. 42, 393.
Irmis, R.B., Nesbitt, S.J., Padian, K., Smith, N.D., Turner, A.H., Woody, D., Downs, A., 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317, 358-361.
Nesbitt, S.J., Smith, N.D., Irmis, R.B., Turner, A.H., Downs, A., Norell, M.A., 2009. A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs. Science 326, 1530–1533.
Jeff's and my contribution to this paper is mainly a revised hypothesis of stratigraphic relationships of units in the lower part of the Chinle Formation based on several years of detailed fieldwork in Arizona, Utah, and New Mexico. Our revised correlations demonstrate that the date from Six Mile Canyon is more likely from strata from the uppermost part of the Blue Mesa Member (Chinle Formation) type section in Petrified Forest National Park (PEFO), and not from the base of the unit as previously supposed. Therefore this date is from stratigraphically younger rocks than previously believed. We also argue that the lower part of the Blue Mesa Member type section at PEFO is equivalent to the Bluewater Creek Member in New Mexico and northeastern Arizona, and that the Bluewater Creek is not directly equivalent to the Mesa Redondo Member. Instead we argue that the Mesa Redondo Member is mainly a lateral equivalent of the Shinarump Member. These are novel hypotheses that we have much supporting data for and we hope to have a full manuscript detailing these revised correlations prepared in the near future.
Irmis, R. B., Mundil, R., Martz, J. W., and W. 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
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.
REFERENCES
Irmis, R., Mundil, R., 2008. New age constraints from the Chinle Formation revise global comparisons of Late Triassic vertebrate assemblages. J. Vertebr. Paleontol. 28, 95A(Suppl.).
Irmis, R.B., Mundil, R., 2010. New U–Pb zircon ages from the Chinle Formation (western US) revise understanding of Late Triassic terrestrial vertebrate evolution. Geol. Soc. Am. Abstracts Prog. 42, 393.
Irmis, R.B., Nesbitt, S.J., Padian, K., Smith, N.D., Turner, A.H., Woody, D., Downs, A., 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317, 358-361.
Nesbitt, S.J., Smith, N.D., Irmis, R.B., Turner, A.H., Downs, A., Norell, M.A., 2009. A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs. Science 326, 1530–1533.
New Isotopic Age Constraints for Middle Triassic Rocks in Argentina
Mancuso, A. C., Chemale, F., Barredo, S., Ávila, J. N., Ottone, E. G., and Marsicano, C. In Press. Age constraints for the northernmost outcrops of the Triassic Cuyana Basin, Argentina, Journal of South American Earth Sciences (2010), doi: 10.1016/j.jsames.2010.03.001
Abstract - Age constraints on the Cerro Puntudo Formation at the northernmost exposures of the continental Triassic Cuyana Basin are presented based on palynological data and U-Pb zircon dating. The lacustrine facies of the upper part of the Cerro Puntudo Formation contain a low diverse palynological assemblage that is dominated by spores of ferns and lycopsids with subordinated inaperturated forms of uncertain affinity. The U-Pb SHRIMP zircon age of 243.8 ± 1.9 Ma (Anisian), obtained from juvenile magmatic zircons in a tuff interbedded in the same beds, provides the first chronostratigraphic date of the Cuyana Basin infilling at its northernmost exposures. According to additional dates already known from the southern part of the basin, a new stratigraphic correlation is proposed thus restricting the first tectono-sedimentary sequence (Synrift I depositional event of Kokogian et al.) to the Anisian. This depositional event has already been recorded in the Cacheuta and Rincón Blanco sub-basins but it is for the first time timely constrained across the whole basin.
Abstract - Age constraints on the Cerro Puntudo Formation at the northernmost exposures of the continental Triassic Cuyana Basin are presented based on palynological data and U-Pb zircon dating. The lacustrine facies of the upper part of the Cerro Puntudo Formation contain a low diverse palynological assemblage that is dominated by spores of ferns and lycopsids with subordinated inaperturated forms of uncertain affinity. The U-Pb SHRIMP zircon age of 243.8 ± 1.9 Ma (Anisian), obtained from juvenile magmatic zircons in a tuff interbedded in the same beds, provides the first chronostratigraphic date of the Cuyana Basin infilling at its northernmost exposures. According to additional dates already known from the southern part of the basin, a new stratigraphic correlation is proposed thus restricting the first tectono-sedimentary sequence (Synrift I depositional event of Kokogian et al.) to the Anisian. This depositional event has already been recorded in the Cacheuta and Rincón Blanco sub-basins but it is for the first time timely constrained across the whole basin.
No Carnian aged deposits in the Chinle Formation?
For the last couple of years one of the more intriguing hypotheses regarding the Chinle Formation is the possibility, based on proposed revisions of the Late Triassic time scale (Muttoni et al., 2004; Furin et al., 2006), that the entire unit was Norian-Rhaetian in age, rather than Carnian-Norian. The Carnian-Norian age was based on palynology (e.g., Litwin et al., 1991) as well as vertebrate biostratigraphy (e.g., Lucas and Hunt, 1993). In fact, a faunal and floral turnover near the middle of the Chinle Formation was hypothesized by many workers to possibly represent the Carnian-Norian boundary and was evidence for an end-Carnian terrestrial extinction. First noticed by Camp (1930) and Gregory (1957) this turnover was more fully documented by Long and Ballew (1985) who noted the presence of two distinct faunas differentiated by the phytosaurs “Rutiodon A” and “Rutiodon B”, as well as the aetosaurs Calyptosuchus and Typothorax. Lucas and Hunt (1993) subsequently named these the Adamanian and Revueltian land-vertebrate faunachrons.
Considered controversial, the hypothesis that most if not all of the Chinle is actually Norian received strong support by the recent announcement of a new 206Pb/238U age of 219.2 ± 0.7 Ma from the base of the Blue Mesa Member in New Mexico (Mundil and Irmis, 2008). This is important for several reasons, firstly, the Chinle Formation has very well documented vertebrate, invertebrate, plant and trace fossil assemblages and is arguably a keystone unit for study of the Late Triassic terrestrial record. Second, it is the basis for the Adamanian land-vertebrate faunachron which has been used to correlate the first appearance of dinosaurs globally (e.g., Heckert and Lucas 1999, 2000). The implications of this will be the focus of Randy Irmis’ presentation at the Society of Vertebrate Paleontology annual meeting next month and therefore will not be discussed further here. What I would like to discuss is the slight possibility that despite these new findings maybe not all of the lower Chinle is now Norian.
In the four corners are of the western United States (Arizona, Utah, Colorado, New Mexico), the Chinle Formation uncomformably overlies the Middle Triassic (Anisian) Moenkopi Formation and consists of seven members which from oldest to youngest are the Shinarump, Bluewater Creek, Blue Mesa, Sonsela, Petrified Forest, Owl Rock, and Rock Point (Note: another unit, the Mesa Redondo Member, is locally situated between the Shinarump and Blue Mesa members but is lithologically distinct from the Bluewater Creek and the relationships between these units are not completely understood). The date provided by Mundil and Irmis (2008) is near the Bluewater Creek/Blue Mesa contact. As the new hypothesized date for the Carnian-Norian boundary is now around 230 ma, this would still leave approximately 10 million years of Norian time below this contact and would presumably include the Shinarump and Bluewater Creek. This is important because instead of just having an uncomformity between the Moenkopi and Chinle that encompasses the Ladinian, this unconformity would now encompass the Ladinian-Carnian as well as a portion of the early Norian. Whereas this might possibly explain the lack of rhynchosaur material (believed to have died out at the end of the Carnian or in the early Norian) from the Chinle Formation, it would also suggest that the Chinle is not temporally equivalent to all of the Dockum Group (eastern New Mexico and Texas) or most of the Newark Supergroup (eastern U.S.) as previously supposed. I’m getting dangerously close to the contents of Randy’s SVP abstract here which is embargoed for the next month, so I will stop this line of thought, but what I want to look at in more detail is the possibility that the Shinarump may not be Norian.
The Shinarump member consists mainly of extrabasinal conglomerates and sandstones that fill paleovalleys carved into the underlying Moenkopi Formation. Once considered its own formation, the Shinarump is now considered to represent the basal member of the Chinle Formation. Unfortunately, the high energy environment that deposited the conglomerates and sands is not conducive to preserving fossils; however in some places mudstone facies do preserve material, most notably near Cameron Arizona. Ash (2005, 2006) has documented this flora and found that it is distinct from the rest of the Chinle Formation in possessing several forms, most notably a seed fern, that more closely resembles archaic forms from the Paleozoic. This suggests that the Shinarump, although still Late Triassic, may be much older than the rest of the Chinle. Unfortunately the vertebrates are not any help. Heckert et al. (2003) documented material that is purportedly from the Shinarump near Cameron, and found that the fauna contains metoposaur and phytosaur material typical of the rest of the Chinle. Furthermore, the flora does also contain forms found in the younger Chinle units including the pollen (Litwin et al., 1991; Ash, 2005). Finally in his excellent dissertation, Jeff Martz discusses in detail that Riggs et al. (1996), in an important but often overlooked paper, used detrital zircons to correlate the Shinarump (and the Santa Rosa Formation of the Dockum) with the marine Auld Lang Syne Group, which is early Norian in age (Martz, 2008). Thus, these authors (Riggs et al., 1996) had suggested a Norian age for the entire Chinle over a decade ago. Despite this, the idea that the Shinarump may still be Carnian is intriguing because it would suggest the presence of a sizeable unconformity (TR-4?) between that unit and the rest of the Chinle Formation.
One final note is that the faunal turnover mentioned at the beginning of this post is close to the base of the Sonsela Member based on detailed mapping and revised biostratigraphic work done by Jeff Martz and myself in Petrified Forest National Park. Thus this turnover (which may also correspond with a floral turnover) is in the early-middle Norian and does not represent an end-Carnian event. More on this later.
REFERENCES
Ash, S.R. 2005. A new Upper Triassic flora and associated invertebrate fossils from
the basal beds of the Chinle Formation, near Cameron, Arizona. PaleoBios 25:17–34.
Ash, S.R. 2006. Chilbinia gen. nov., an archaic seed fern in the Late Triassic Chinle Formation of Arizona, USA. Palaeontology 49:237–245.
Camp, C. L. 1930. A study of the phytosaurs with description of new material from western North America. Memoirs of the University of California 10:1-174.
Furin, S., Preto, N., Rigo, M., Roghi., G., Gianolla, P., Crowley, J.L., and S. A. Bowring. 2006. High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs. Geology 34:1009–1012.
Gregory, J.T. 1957. Significance of fossil vertebrates for correlation of Late Triassic continental deposits of North America. Report of the 20th Session of the International Geological Congress 1956, Section II:7-25.
Heckert, A.B., and S.G. Lucas. 1999. Global correlation and chronology of Triassic theropods (Archosauria: Dinosauria). Albertiana 23:22-35.
Heckert, A.B., and S.G. Lucas. 2000 [imprint 1998]. Global correlation and chronology of Triassic theropods. Gaia 15:63-74.
Heckert, A.B., Lucas, S.G., and J. W. Estep. 2003 [imprint 2002]. Lower Chinle Group (Upper Triassic: Upper Carnian) tetrapods from the vicinity of Cameron, Arizona. New Mexico Museum of Natural History and Science Bulletin 21:73-76.
Litwin, R.J., Traverse, A., and S.R. Ash. 1991. Preliminary palynological zonation of the Chinle Formation, southwestern U.S.A., and its correlation to the Newark Supergroup (eastern U.S.A.). Review of Palaeobotany and Palynology 68: 269-287.
Long, R. A. and K. L. Ballew. 1985. Aetosaur dermal armor from the Late Triassic of southwestern North America, with special reference to material from the Chinle Formation of Petrified Forest National Park. Museum of Northern Arizona Bulletin 54:45-68.
Lucas, S.G., and A.P. Hunt. 1993. Tetrapod biochronology of the Chinle Group (Upper Triassic), western United States. New Mexico Museum of Natural History and Science Bulletin 3:327-329.
Martz, J.W. 2008. Lithostratigraphy, chemostratigraphy, and vertebrate biostratigraphy of the Dockum Group (Upper Triassic), of southern Garza County, West Texas. Unpublished PhD dissertation. Texas Tech University, Lubbock, 504p.
Mundil, R., and R. Irmis. 2008. New U-Pb age constraints for terrestrial sediments in the Late Triassic: Implications for faunal evolution and correlations with marine environments. International Union of Geological Sciences (IUGS) meeting abstracts Oslo 2008 (online at: http://www.cprm.gov.br/33IGC/1342538.html).
Muttoni, G., Kent, D.V., Olsen, P.E., Di Stefano, P., Lowrie, W., Bernasconi, S.M., and F. M. Hernandez. 2004 Tethyan magnetostratigraphy from Pizzo Mondello (Sicily) and correlation to the Late Triassic Newark astrochronological polarity time scale: Geological Society of America Bulletin 116:1043–1058.
Riggs, N. R., T. M. Lehman, G. E. Gehrels, and W. R. Dickinson. 1996. Detrital zircon
link between headwaters and terminus of the Upper Triassic Chinle-Dockum
paleoriver system. Science 273:97-100.
Considered controversial, the hypothesis that most if not all of the Chinle is actually Norian received strong support by the recent announcement of a new 206Pb/238U age of 219.2 ± 0.7 Ma from the base of the Blue Mesa Member in New Mexico (Mundil and Irmis, 2008). This is important for several reasons, firstly, the Chinle Formation has very well documented vertebrate, invertebrate, plant and trace fossil assemblages and is arguably a keystone unit for study of the Late Triassic terrestrial record. Second, it is the basis for the Adamanian land-vertebrate faunachron which has been used to correlate the first appearance of dinosaurs globally (e.g., Heckert and Lucas 1999, 2000). The implications of this will be the focus of Randy Irmis’ presentation at the Society of Vertebrate Paleontology annual meeting next month and therefore will not be discussed further here. What I would like to discuss is the slight possibility that despite these new findings maybe not all of the lower Chinle is now Norian.
In the four corners are of the western United States (Arizona, Utah, Colorado, New Mexico), the Chinle Formation uncomformably overlies the Middle Triassic (Anisian) Moenkopi Formation and consists of seven members which from oldest to youngest are the Shinarump, Bluewater Creek, Blue Mesa, Sonsela, Petrified Forest, Owl Rock, and Rock Point (Note: another unit, the Mesa Redondo Member, is locally situated between the Shinarump and Blue Mesa members but is lithologically distinct from the Bluewater Creek and the relationships between these units are not completely understood). The date provided by Mundil and Irmis (2008) is near the Bluewater Creek/Blue Mesa contact. As the new hypothesized date for the Carnian-Norian boundary is now around 230 ma, this would still leave approximately 10 million years of Norian time below this contact and would presumably include the Shinarump and Bluewater Creek. This is important because instead of just having an uncomformity between the Moenkopi and Chinle that encompasses the Ladinian, this unconformity would now encompass the Ladinian-Carnian as well as a portion of the early Norian. Whereas this might possibly explain the lack of rhynchosaur material (believed to have died out at the end of the Carnian or in the early Norian) from the Chinle Formation, it would also suggest that the Chinle is not temporally equivalent to all of the Dockum Group (eastern New Mexico and Texas) or most of the Newark Supergroup (eastern U.S.) as previously supposed. I’m getting dangerously close to the contents of Randy’s SVP abstract here which is embargoed for the next month, so I will stop this line of thought, but what I want to look at in more detail is the possibility that the Shinarump may not be Norian.
The Shinarump member consists mainly of extrabasinal conglomerates and sandstones that fill paleovalleys carved into the underlying Moenkopi Formation. Once considered its own formation, the Shinarump is now considered to represent the basal member of the Chinle Formation. Unfortunately, the high energy environment that deposited the conglomerates and sands is not conducive to preserving fossils; however in some places mudstone facies do preserve material, most notably near Cameron Arizona. Ash (2005, 2006) has documented this flora and found that it is distinct from the rest of the Chinle Formation in possessing several forms, most notably a seed fern, that more closely resembles archaic forms from the Paleozoic. This suggests that the Shinarump, although still Late Triassic, may be much older than the rest of the Chinle. Unfortunately the vertebrates are not any help. Heckert et al. (2003) documented material that is purportedly from the Shinarump near Cameron, and found that the fauna contains metoposaur and phytosaur material typical of the rest of the Chinle. Furthermore, the flora does also contain forms found in the younger Chinle units including the pollen (Litwin et al., 1991; Ash, 2005). Finally in his excellent dissertation, Jeff Martz discusses in detail that Riggs et al. (1996), in an important but often overlooked paper, used detrital zircons to correlate the Shinarump (and the Santa Rosa Formation of the Dockum) with the marine Auld Lang Syne Group, which is early Norian in age (Martz, 2008). Thus, these authors (Riggs et al., 1996) had suggested a Norian age for the entire Chinle over a decade ago. Despite this, the idea that the Shinarump may still be Carnian is intriguing because it would suggest the presence of a sizeable unconformity (TR-4?) between that unit and the rest of the Chinle Formation.
One final note is that the faunal turnover mentioned at the beginning of this post is close to the base of the Sonsela Member based on detailed mapping and revised biostratigraphic work done by Jeff Martz and myself in Petrified Forest National Park. Thus this turnover (which may also correspond with a floral turnover) is in the early-middle Norian and does not represent an end-Carnian event. More on this later.
REFERENCES
Ash, S.R. 2005. A new Upper Triassic flora and associated invertebrate fossils from
the basal beds of the Chinle Formation, near Cameron, Arizona. PaleoBios 25:17–34.
Ash, S.R. 2006. Chilbinia gen. nov., an archaic seed fern in the Late Triassic Chinle Formation of Arizona, USA. Palaeontology 49:237–245.
Camp, C. L. 1930. A study of the phytosaurs with description of new material from western North America. Memoirs of the University of California 10:1-174.
Furin, S., Preto, N., Rigo, M., Roghi., G., Gianolla, P., Crowley, J.L., and S. A. Bowring. 2006. High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs. Geology 34:1009–1012.
Gregory, J.T. 1957. Significance of fossil vertebrates for correlation of Late Triassic continental deposits of North America. Report of the 20th Session of the International Geological Congress 1956, Section II:7-25.
Heckert, A.B., and S.G. Lucas. 1999. Global correlation and chronology of Triassic theropods (Archosauria: Dinosauria). Albertiana 23:22-35.
Heckert, A.B., and S.G. Lucas. 2000 [imprint 1998]. Global correlation and chronology of Triassic theropods. Gaia 15:63-74.
Heckert, A.B., Lucas, S.G., and J. W. Estep. 2003 [imprint 2002]. Lower Chinle Group (Upper Triassic: Upper Carnian) tetrapods from the vicinity of Cameron, Arizona. New Mexico Museum of Natural History and Science Bulletin 21:73-76.
Litwin, R.J., Traverse, A., and S.R. Ash. 1991. Preliminary palynological zonation of the Chinle Formation, southwestern U.S.A., and its correlation to the Newark Supergroup (eastern U.S.A.). Review of Palaeobotany and Palynology 68: 269-287.
Long, R. A. and K. L. Ballew. 1985. Aetosaur dermal armor from the Late Triassic of southwestern North America, with special reference to material from the Chinle Formation of Petrified Forest National Park. Museum of Northern Arizona Bulletin 54:45-68.
Lucas, S.G., and A.P. Hunt. 1993. Tetrapod biochronology of the Chinle Group (Upper Triassic), western United States. New Mexico Museum of Natural History and Science Bulletin 3:327-329.
Martz, J.W. 2008. Lithostratigraphy, chemostratigraphy, and vertebrate biostratigraphy of the Dockum Group (Upper Triassic), of southern Garza County, West Texas. Unpublished PhD dissertation. Texas Tech University, Lubbock, 504p.
Mundil, R., and R. Irmis. 2008. New U-Pb age constraints for terrestrial sediments in the Late Triassic: Implications for faunal evolution and correlations with marine environments. International Union of Geological Sciences (IUGS) meeting abstracts Oslo 2008 (online at: http://www.cprm.gov.br/33IGC/1342538.html).
Muttoni, G., Kent, D.V., Olsen, P.E., Di Stefano, P., Lowrie, W., Bernasconi, S.M., and F. M. Hernandez. 2004 Tethyan magnetostratigraphy from Pizzo Mondello (Sicily) and correlation to the Late Triassic Newark astrochronological polarity time scale: Geological Society of America Bulletin 116:1043–1058.
Riggs, N. R., T. M. Lehman, G. E. Gehrels, and W. R. Dickinson. 1996. Detrital zircon
link between headwaters and terminus of the Upper Triassic Chinle-Dockum
paleoriver system. Science 273:97-100.
Subscribe to:
Posts (Atom)