Lithostratigraphy and biostratigraphy of the Chinle Formation in Lisbon Valley, Utah

The stratigraphy of the Chinle Formation in Lisbon Valley, Utah has been somewhat controversial. This paper is the result of several seasons of fieldwork and provides an intense revision of the local stratigraphy as well as discussion of the fossils found in these rocks. Martz et al. provide a large amount of information and are careful to make sure that their work is repeatable as possible. To this end they provide coordinates and labelled photos of their measured sections as well as a list of voucher specimens for all of the fossil taxa. Martz's work continues to raise the bar not only for studies in the Upper Triassic of Utah, but also for stratigraphic and biostratigraphic work in general.



Martz, J. W., Irmis, R. B., and A.R.C. Milner. 2014. Lithostratigraphy and biostratigraphy of the Chinle Formation (Upper Triassic) in southern Lisbon Valley, southeastern Utah; pp. 397-448 in MacLean, J.S., Biek, R.F., and J.E. Huntoon (eds.), Geology of Utah's Far South. Utah Geological Association Publication 43.

Abstract - We present here a detailed study of the lithostratigraphy and preliminary vertebrate biostratigraphy of the Upper Triassic Chinle Formation in Lisbon Valley, southeastern Utah. Triassic salt tectonism resulted in a period of erosion and possibly non-deposition that removed the top of the Lower Permian Cutler Group, the Early-Middle Triassic Moenkopi Formation, and the Late Triassic (earliest Norian) Shinarump Member of the Chinle Formation. Chinle Formation deposition in Lisbon Valley began or resumed sometime during the middle-late Norian or Rhaetian and terminated before the end of the Rhaetian. Chinle Formation sediments are mostly siltstone to fine-grained sandstone dominated by planar cross-bedding and climbing ripple cross-lamination; these sediments generally exhibit poor paleosol development, with interbedded conglomerates dominated by clasts composed of reworked intrabasinal sediments and containing only minor extrabasinal silica. The regional climate during Chinle deposition was becoming increasingly arid, with fluctuating seasonal rainfall. Deposition by the braided and meandering rivers of the lower Kane Springs beds filled paleovalleys incised into the Cutler Group, and was followed by a poorly drained interval of poorly oxygenated swamps and lakes crisscrossed by small streams that produced the middle Kane Springs beds. These conditions transitioned back to the slowly aggrading braided and meandering rivers of the upper Kane Springs beds, probably by Rhaetian time. The Kane Springs paleoenvironment, probably at least partially syndepositional with that of the Owl Rock Member, was inhabited by conifers, freshwater ostracods, bivalves and gastropods, indeterminate phytosaurs, and the aetosaur Typothorax. The shift to the overlying Church Rock Member was gradational and probably involved only subtle shifts in the depositional system, largely related to better-drained sediments. Braided channels with seasonally variable discharge crossed well-drained, rapidly aggrading and well-oxygenated floodplains. Rare paleosols (including entisols, vertisols, and aridisols) indicate seasonal wetting and drying in a generally arid climate. Rivers and lakes were inhabited by the phytosaur taxon Machaeroprosopus (including the derived form “Redondasaurus”), rare metoposaurids, coelacanths, a diverse actinopterygian fish fauna, conchostracans, ostracods, bivalves, and gastropods. The flora included conifer trees, giant horsetails, Cynepteris (a fern), Zamites (a bennettitalean), the small shrub-like conifer Pelourdea, and the enigmatic Sanmiguelia. Terrestrial tetrapods included the aetosaur Typothorax (suggesting that the genus endured into the Rhaetian), paracrocodylomorphs, and small theropods. Eventually, wind-blown eolian deposits entered the region, and late in the Rhaetian, prior to 201.3 Ma, the eolian Wingate erg swamped small braided channels still inhabited by the phytosaur “Redondasaurus,” actinopterygian fishes, and small theropods.

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.

April Fieldwork - 2011 - Petrified Forest National Park Area

In April, despite the high winds, I was fortunate enough to spend a couple of days with the local landowners exploring parcels in the expansion area of Petrified Forest National Park. These sites are a continuation of the area in the park locally known as the "Devils Playground", named by paleontologist Charles L. Camp in the early 1920s.

Incredible clast supported conglomerate in the upper Brown Sandstone (Sonsela Member, Chinle Formation), a rare facies for this unit but one that clearly shows deposition in good sized channels.

Isolated mesa and one of Charles Camp's documented fossil localities. These strata represent belong to the upper part of the Sonsela Member.

In-situ phytosaur snout fragment found by one of the local landowners.

The 'persistent red silcrete' horizon and upper Sonsela strata near the Devil's Playground.

Fallen blocks of the Brown Sandstone, this accumulation is named the "Storyteller Doll" by the local landowners, after a popular puebloan indian figure.

Sequence Stratigraphy of the Upper Triassic Succession in Algeria

Bourquin, S., Eschard, R., Hamouche, B.  High-resolution sequence stratigraphy of Upper Triassic succession (Carnian - Rhaetian) of the Zarzaitine outcrops (Algeria): a model of fluvio-lacustrine deposits, African Earth Sciences (2010), doi: 10.1016/j.jafrearsci.2010.04.003

Abstract - The detailed facies analysis of the Zarzaitine outcrops (Illizi Basin in the In Amenas area of Algeria) allows the depositional environment of the Upper Triassic succession to be defined: braided rivers within an arid and humid alluvial plain, low sinuosity rivers within a humid alluvial plain, lake deposits and marginal sabkha. The description of the outcrops helps to define three types of genetic units from a proximal to a distal depositional environment: fluvial, fluvial-lacustrine and lacustrine. The spatial and temporal evolutions of the genetic unit were characterised by five specific stages from dry to humid climate conditions, inducing sediment supply and lake-level variations. During the first two stages (stage 1 and 2) under a dry climate, the lake level was low and sediments mostly by-passed and poorly preserved. During stages 3 and 4, an increase in humidity and rainfall induced a rise in the lake-level and the development of vegetation, as well as a decrease in the sediment supply, although the sediment preservation were then at its maximum. The last stage (stage 5) marked the beginning of a decrease in the humidity, the minimum of sediment supply and the maximum of the lake level. Therefore, the recognition and the description of genetic units within this fluviallacustrine environment help to demonstrate the interaction between climate, sediment supply and lake-level variation, at the scale of these units. Six stratigraphic cycles have been recognised in the vertically stacked genetic units and can be grouped in three megacycles (denoted as I, II and III). The first megacycle, attributed to Carnian to early Norian, corresponds to the base of braided river systems with some ephemeral channels developed in an arid environment where some aeolian deposits were preserved. It evolved trough time to humid conditions favouring the development of extensive floodplain, associated with hydromorphic soils, and perennial lake environments. During the Carnian times and Norian, the Zarzaitine area was not connected to the Berkine basin northward, a Hercynian unconformity palaeorelief forming a drainage divide. According to the directions of the palaeocurrents, the sediment provenances were mostly from the southwest and the north. The connection with the Berkine basin only occurred during the upper part of the megacycle II deposition, characterised by fluvio-lacustrine environments. The maximum flooding of the megacycle I, Norian in age, could be correlated with the early Norian maximum flooding observed on the Saharan platform. The late Norian-Rhaetian second megacycle was mostly a lacustrine environment associated with extensive floodplain, with the development of a hydromorphic palaeosol with root imprints attributed to a warm and humid climate. The maximum flooding episode of this second megacycle could be
attributed to the major marine trangression recorded in Algeria, during the Rhaetian, and
could be correlated with a relative sea-level rise. A dolomitic level, attributed to sabkha
environments, marks the beginning of the retrogradational trend of the third cycle,
attributed to the Rhaetian-Liassic. The palaeocurrent of the upper part of megacycle II
and of the megacycle III were always oriented toward the northeast, attesting that the
relief was located to the southwest.

Revised Stratigraphy of the Sonsela Member (Chinle Formation) in the Southern Part of Petrified Forest National Park, Arizona

One of the immediate problems facing me when I started working at Petrified Forest National Park in 2001 was the inability to often locate myself stratigraphically using existing lithostratigraphic models. This became particularly irksome in the winter of 2001 when I recovered a partial skeleton of the aetosaur Calyptosuchus wellesi from just below a prominent sandstone ledge in the central portion of the park. Calyptosuchus (=Stagonolepis) has been proposed as a biochronological index fossil of the Adamanian land vertebrate faunachron, which at that time was considered to represent the latest Carnian. The existing lithostratigraphic scheme divided the Petrified Forest Member of the Chinle Formation into upper and lower parts using a local sandstone called the Sonsela Sandstone bed. A major part of the problem was that various researchers differed on exactly what sandstone ledges in the park (and there are many) represented the Sonsela. Revisions by Andrew Heckert and Spencer Lucas in the Spring of 2003 (Heckert & Lucas, 2003 [imprint 2002]) and later that year by NAU graduate student Daniel Woody (Woody 2003, 2006) looked to address this problem by expanding the Sonsela Member to include more of these beds and to provide hypotheses of correlation. Unfortunately, attempts to map the park in 2006 at the 1:24000 scale realized that the new proposed correlations looked great on paper, but didn't work at the outcrop or on the map (Raucci et al., 2006). This reinforced doubts I was already having because of difficulty using the new scheme in the field, and simply stating that it represented a complex fluvial system wasn't helping.

In 2008 Jeff Martz was hired in a seasonal position at the park. Jeff had just completed his dissertation at Texas Tech University solving similar problems in the Upper Triassic Dockum Group of west Texas. Realizing that this was a perfect opportunity to examine and hopefully solve this problem once and for all I dragged him out to an area of the park known as the Flattops, near the center of one of the more stratigraphically challenging areas of the park. The Flattops area contains three prominent sandstone horizons termed the Flattops Sandstones #2-4 (Billingsley, 1985) which are only found in this area and restricted in their lateral extent. One of the points Jeff had stressed in his dissertation was the need to completely walk out beds in areas with complex stratigraphy in order to ascertain the proper superpositional relationships and correlations of beds. In the past a lot of this had been done using correlated measured sections or seemingly by 'eyeballing it'. I placed us on top of what was unambiguously Flattops Sandstone #2 and we proceeded to walk out numerous beds in the Sonsela Member. At first we assigned alpha names to each bed (e.g., A, B, C) and did not change these to existing bed level names until we could directly correlate to the 'type sections' for those beds. Fairly quickly we were able to work out most of the major correlations and superpositional relationships. We also were able to directly tie in all of our known fossil sites, including the site that had given me so much trouble since 2001.

The results of this work, which differs in significant ways from that of all previous workers, will be published on Friday, February 19th in the open access journal PLoSONE - Martz & Parker 2010. The media embargo ended today (Feb. 18th) and the paper is actually available now.

Figuring out the stratigraphic relationships of the southern portion of the park has allowed us to see support for various hypothesis regarding the depositional systems and biostratigraphy of the park, including the position of the Adamanian-Revueltian boundary (a proposed faunal and possibly floral turnover event; Parker & Martz, 2009). We have also been successful in correlating the northern and southern ends of the park, something that no previous researcher has been able to do conclusively. This allows us to amalgamate biostratigraphic data from both areas, allowing for a more robust biostratigraphic hypothesis. We are currently working on correlating other Chinle Formation outcrops in Arizona back to the park in hopes of developing regional lithostratigraphic and biostratigraphic models. A paper detailing our biostratigraphic hypotheses has been submitted and a paper discussing the north end stratigraphy and another on regional correlations are in the works.

One of the things we have tried to do with the PLoS ONE paper is to make our study completely reproducible. This was a problem we had with many of the past studies, rendering much of their data unusable. For example, stratigraphic measured sections can be highly subjective both in terms of where on the outcrop it was measured as well as how the individual units were broken out. In complex fluvial systems where beds can thicken, thin, or pinch out laterally over very short distances the ability to precisely relocate where a section was done is very important. To this end we have provided (and advocate that all future studies also do this) GPS coordinates as well as photos of all measured outcrops, showing not only the location but how the outcrop was separated into units. We also provide a copy of our geological map. Mapping was crucial to the determining of correlations, and as is stated in the paper, we have doubts about stratigraphic correlations made without the tool of geological mapping. Furthermore, any proposed mistakes in our work can be easily verified or refuted by future workers by using the map. Very important!

This project has been a lot of work, but also a lot of fun and it is refreshing that I can pretty much accurately place myself stratigraphically anywhere in the park. This is crucial for accurate plotting of fossil sites and for determining stratigraphic ranges of organisms. I hope that our colleagues find this work extremely useful for their own studies on the Chinle Formation as well.

Martz, J.W., and W.G. Parker. 2010. Revised lithostratigraphy of the Sonsela Member (Chinle Formation, Upper Triassic) in the southern part of Petrified Forest National Park, Arizona. PLoSONE 5(2)e9329. doi:10.1371/journal.pone.0009329

ABSTRACT

Background: Recent revisions to the Sonsela Member of the Chinle Formation in Petrified Forest National Park have presented a three-part lithostratigraphic model based on unconventional correlations of sandstone beds. As a vertebrate faunal transition is recorded within this stratigraphic interval, these correlations, and the purported existence of a
depositional hiatus (the Tr-4 unconformity) at about the same level, must be carefully re-examined.

Methodology/Principal Findings: Our investigations demonstrate the neglected necessity of walking out contacts and mapping when constructing lithostratigraphic models, and providing UTM coordinates and labeled photographs for all measured sections. We correct correlation errors within the Sonsela Member, demonstrate that there are multiple Flattops
One sandstones, all of which are higher than the traditional Sonsela sandstone bed, that the Sonsela sandstone bed and Rainbow Forest Bed are equivalent, that the Rainbow Forest Bed is higher than the sandstones at the base of Blue Mesa and Agate Mesa, that strata formerly assigned to the Jim Camp Wash beds occur at two stratigraphic levels, and that there are
multiple persistent silcrete horizons within the Sonsela Member.

Conclusions/Significance: We present a revised five-part model for the Sonsela Member. The units from lowest to highest are: the Camp Butte beds, Lot’s Wife beds, Jasper Forest bed (the Sonsela sandstone)/Rainbow Forest Bed, Jim Camp Wash beds, and Martha’s Butte beds (including the Flattops One sandstones). Although there are numerous degradational/
aggradational cycles within the Chinle Formation, a single unconformable horizon within or at the base of the Sonsela Member that can be traced across the entire western United States (the ‘‘Tr-4 unconformity’’) probably does not exist. The shift from relatively humid and poorly-drained to arid and well-drained climatic conditions began during deposition of the Sonsela Member (low in the Jim Camp Wash beds), well after the Carnian-Norian transition.

REFERENCES

Billingsley, G. H.,1985. General stratigraphy of the Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 54:3-8.

Heckert, A.B., and S.G. Lucas. 2002. Revised Upper Triassic stratigraphy of the Petrified Forest National Park. New Mexico Museum of Natural History and Science Bulletin 21:1-36.

Martz, J.W., and W.G. Parker. 2010. Revised lithostratigraphy of the Sonsela Member (Chinle Formation, Upper Triassic) in the southern part of Petrified Forest National Park, Arizona. PLoSONE 5(2)e9329. doi:10.1371/journal.pone.0009329

Parker, W. G., and J. W. Martz. 2009. Constraining the stratigraphic position of the Late Triassic (Norian) Adamanian-Revueltian faunal transistion in the Chinle Formation of Petrified Forest National Park, Arizona. Journal of Vertebrate Paleontology 29(3):162A.

Woody, D. T., 2003. Revised geological assessment of the Sonsela Member, Chinle Formation, Petrified Forest National Park, Arizona. Unpublished M. S. Thesis, Northern Arizona University, Flagstaff.

Woody, D. T., 2006. Revised stratigraphy of the lower Chinle Formation (Upper Triassic) of Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 62:17-45.

Chinle Formation "Bible" Still Available From the USGS

Many papers have been written on the stratigraphy of the Chinle Formation; however, the 1972 USGS professional paper by John Stewart and colleagues still ranks as the best overview of the formation and is crucial for any detailed study of the unit. This is especially true for the basic straigraphic nomenclature, unit descriptions, and key correlations across the Colorado Plateau region.

Bound paper copies of this important work are available for only $12.50 USD from the USGS store. What a great price for a 690 page document complete with maps, fence diagrams, and other fold out plates.

The photo is of the Upper Triassic Chinle Formation capping dark brown cliffs of Middle Triassic Moenkopi Formation in Capitol Reef National Park and is courtesy of the USGS.

REFERENCE

Stewart, J.H., Poole, F.G. and Wilson, R.F. 1972. Stratigraphy and origin of the Chinle Formation and related Upper Triassic strata in the Colorado Plateau region: U.S. Geological Survey Professional Paper, 690, 336 p.

New Issue of Albertiana Out - Triassic Stratigraphy Newsletter/Journal

From its official webpage, "Albertiana is the newsletter of the Subcommision on Triassic stratigraphy". This subcommision is responsible for establishing a standard, global stratigraphic scale for the Triassic system. Albertiana is published twice a year (usually) and features reports on conferences, news items, special reports, bibliographies, etc.. all dealing with the Triassic. The website includes free access to recent and current issues. The newest issue was just released and you can access it here.

While you are at it check out the subcommision on Triassic stratigraphy webpage here.

Latest Literature - August 2008

The latest issue of Palaeontology contains two papers on Late Triassic fossils. The first by Stein et al. examines the aerodymanics of the British kuehneosaurs (gliding reptiles - reconstruction below) and contains a brief discussion of their taxonomy. To determine gliding angle and speeds, the team constructed several plastic and aluminum models and set them in a wind tunnel. Results suggest that the elongate "winged" Kuehneosuchus was an effenctive glider and that the short "winged" Kuehneosaurus was a parachutist. Also of interest is discussion that the taxonomic status of these two genera is unclear. One possibility is that they represent sexual dimorphs of the same species. According to this hypothesis the long "winged" individuals would be males given that the more elongate "wings" provide a strong display apparatus. Accordingly the "wings" would have had an original function and were secondary adapted for gliding.

The second paper by Heckert et al. is a description of a new sphenodontid from the Upper Triassic Ghost Ranch Coelophysis Quarry in northcentral New Mexico. Sphenodontid (represented by the extant tuatara - photo to left) fossils from the Chinle Formation are rare and usually incomplete. This specimen is important because of its more complete nature (both dentaries, possible maxilla, and impressions of a portion of the palate, all toothbearing). The new taxon is named Whitakersaurus bermani in honor of the late George Whitaker who originally discovered the quarry in 1947, and Dr. David Berman of the Carnegie Museum who conducted the last major excavations of the quarry in the 1980s. The Ghost Ranch quarry contains an extensive assemblage of Late Triassic vertebrates including 100s of well preserved specimens of the theropod Coelophysis bauri (Colbert, 1989).

Despite the extensive discussion in this paper by Heckert et al. regarding the stratigraphic position of this spectactular fossil assemblage it should be noted that this is still hotly debated and the subject of current research. The quarry is in reddish and green mottled siltstones (Schwartz and Gillette, 1994) which Colbert (1989) assigned to the Petrified Forest Member (Chinle Formation). Interestingly Colbert considered the quarry to be lower stratigraphically than an occurrence of Coelophysis from Petrified Forest National Park that was described by Padian (1986). Contrary to this all other workers have considered the quarry to be high stratigraphically in either the "siltstone" member (e.g., Stewart et al., 1972), the Owl Rock Member (e.g., Dubiel, 1989), or the Rock Point Member (e.g., Hunt and Lucas, 1993) (Column to right from Parker, 2005). Stewart et al. (1972) were unsure of the correlation of the siltstone member with either the Rock Point or the Owl Rock Members. Dubiel (1989) argued that the siltstone member represents a lateral facies of the Owl Rock Member, which is absent in north central New Mexico. Lucas and colleagues have countered this in numerous papers since 1992, arguing that the strata are equivalent to the Rock Point Member both lithologically and bio-stratigraphically. In fact, they have been unwavering in this hypothesis with the section in Heckert et al. (2008) being almost verbatim to that of Hunt and Lucas (1993).

However, this may need to change. A detailed paleomagnetism study of these strata, which is part of a recently completed PhD dissertation by Kate Zeigler, argues that the siltstone member and Rock Point Member possess differing paleomagnetic polarity signals (Zeigler, 2008). Even more striking, they also have different paleo-pole positions. Even more significant is that the siltstone member has a different paleo-pole from the Chinle Formation as a whole and one that is identical to that of the stratigraphically higher Moenave Formation (once believed to be Jurassic, but now probably Upper Triassic). This would not only be a significant deviation from previous hypotheses stratigraphically but also biostratigraphically. The Coelophysis Quarry fauna is dominated by the theropod Coelophysis bauri, but also contains phytosaurs, poposaurs, "rauisuchians", "sphenosuchians", and an enigmatic archosauriform which has tentatively been referred to Vancleavea campi, all of which are common in the stratigraphically lower members of the Chinle Formation. This would significantly extend the stratigraphic ranges of all of these taxa and demonstrate the survival of the majority of pseudosuchian lineages right up to the Triassic/Jurassic boundary. What is needed now is some independent verification of this hypothesis by isotopic dates.

REFERENCES

Colbert, E.H. 1989. The Triassic dinosaur Coelophysis. Museum of Northern Arizona Bulletin 57:1-160.

Dubiel, R.F. 1989. Depositional and paleoclimatic setting of the Upper Triassic Chinle Formation, Colorado Plateau; pp. 171-187 in Lucas, S.G., and A.P. Hunt (eds.), Dawn of the Age of Dinosaurs in the American Southwest. New Mexico Museum of Natural History, Albuquerque.

Heckert, A.B., Lucas, S.G., Rinehart, L.F., and A.P. Hunt. 2008. A new genus and species of sphenodontian from the Ghost Ranch Coelophysis Quarry (Upper Triassic: Apachean), Rock Point Formation, New Mexico, USA. Palaeontology 51:827-845.

Hunt, A.P., and S.G. Lucas. 1993. Stratigraphy and vertebrate paleontology of the Chinle Group (Upper Triassic), Chama Basin, north-central New Mexico. New Mexico Museum of Natural History and Science Bulletin 2:61-69.

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