Ichnofossil Assemblages and Paleosols of the Upper Triassic Chinle Formation, South-Eastern Utah

Adding to our understanding of the understudied Chinle Formation fossils of Utah.

Fischer, S. J. and S. T. Hasiotis 2018. Ichnofossil assemblages and palaeosols of the Upper Triassic Chinle Formation, south-eastern Utah (USA): Implications for depositional controls and palaeoclimate. 

Annales Societatis Geologorum Poloniae 88(2): 127–162.

Abstract - The Upper Triassic Chinle Formation in the Stevens Canyon area in south-eastern Utah represents fluvial, palustrine, and lacustrine strata deposited in a continental back-arc basin on the western edge of Pangea. Previous investigations interpreted a megamonsoonal climate with increasing aridity for the Colorado Plateau towards the end of the Triassic. In this study, we systematically integrate ichnological and pedological features of the Chinle Formation into ichnopedofacies to interpret palaeoenvironmental and palaeoclimatic variations in the north-eastern part of the Chinle Basin. Seventeen ichnofossil morphotypes and six palaeosol orders are combined into twelve ichnopedofacies, whose development was controlled by autocyclic and allocyclic processes and hydrology. Ichnopedofacies are used to estimate palaeoprecipitation in conjunction with appropriate modern analogue latitudinal and geographic settings. In the north-east Chinle Basin, annual precipitation was ~1100–1300 mm in the Petrified Forest Member. Precipitation levels were >1300 mm/yr at the base of the lower Owl Rock Member, decreased to ~700–1100 mm/yr, and then to ~400–700 mm/yr. Two drying upward cycles from ~1100 mm/yr to ~700 mm/yr occurred in the middle and upper part of the Owl Rock Member. In the overlying Church Rock Member, precipitation decreased from ~400 mm/yr at the base of the unit to ~25–325 mm/yr at the end of Chinle Formation deposition. Ichnopedofacies indicate monsoonal conditions persisted until the end of the Triassic with decreasing precipitation that resulted from the northward migration of Pangea. Ichnopedofacies in the northeast Chinle Basin indicate both long-term drying of climate and short-term, wet-dry fluctuations.

More On the Early Jurassic Resting Trace Paper

Andrew R. C. Milner, Jerald D. Harris, Martin G. Lockley, James I. Kirkland, Neffra A. Matthews (2009). Bird-Like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace PLoS ONE, 4 (3) DOI: 10.1371/journal.pone.0004591

This recent article has been getting a bit of attention in the blogosphere (e.g., here, here, and even here), although mainly on the functional implications of theropod manus and arm orientation. However, there are a few other tidbits in this article that I find of interest (and which are relevent to the Chinle Formation and the Late Triassic.

First up is this little comment regarding the rank of the Chinle:

"The Moenave Formation overlies the Chinle Formation (Chinle Group of Lucas [24], but in Utah, group status is not recognized for these same strata)."

By the way, I really dislike numerical citations in journal articles as you have to continuously flip to the reference section to follow along. The same goes for putting all of the figure abbreviations for an entire paper into a single list or appendix rather than under each figure. Hate, it, hate it, hate it.

Anyhow, as many of my readers are possible aware, the rank of the Chinle is heavily debated. As I have stated before I have no problem with the Chinle being raised to Group status, but I do have a problem with it subsuming the Dockum Group (which is an older established name and thus has priority) as advocated by Lucas (1993) (Lucas [1993] states that the Dockum should be lost because of inconsistent usage, but this would also be true for the name Chinle). Thus we are kind of stuck with not using Chinle as a Group because of the confusion any future work advocating this (and leaving the Dockum separate) would cause with the necessity to then state continuously who's version of "Chinle Group" you are using.

In their paper Milner et al. (2009) have come right out (in a Jurassic-themed paper) and voiced their disapproval with using Chinle "Group" in Utah. We feel the same way in Arizona and I know my Texas colleagues feel just a strongly. This leaves only some workers in New Mexico using "Group" and two different versions at that. Of course, the North American Stratigraphic Code allows for units to be members, formations, or groups in different areas, however, it can cause some confusion.

Another important aspect of this paper is the useful taxonomic review of quite a few Late Triassic and Early Jurassic footprint taxa, which have been assigned to theropods, especially those that may be synonymous with Eubrontes and those that show manus impressions. Interesting is the hypothesis that based on the associated manus impressions the tracks of Atreipus are not theropodan, but rather those of a non-dinosaurian dinosauriform or possibly an ornithischian. Based on the body fossil record for the Late Triassic the former is most likely as was previously attributed to such by Olsen and Baird (1986) and Nesbitt et al. (2007).

In summary, although this new paper mainly focuses on proposed posture and behavior in theropods it also contains some key information on Triassic and Jurassic ichnofossil taxonomy and stratigraphy. For those interested this paper is available for free here.

REFERENCES

Lucas, S.G. 1993. The Chinle Group: revised stratigraphy and biochronology of Upper Triassic nonmarine strata in the western United States. Museum of Northern Arizona Bulletin 59: 27–50.

Andrew R. C. Milner, Jerald D. Harris, Martin G. Lockley, James I. Kirkland, Neffra A. Matthews (2009). Bird-Like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace PLoS ONE, 4 (3) DOI: 10.1371/journal.pone.0004591

Nesbitt, S. J., Irmis, R. B., and W.G. Parker. 2007. A critical reevaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5:209-243.

Olsen, P. E. and D. Baird. 1986. The ichnogenus Atreipus and its importance for Triassic biostratigraphy. Pp. 61–87 in K. Padian (ed.) The beginning of the age of dinosaurs: faunal change across the Triassic–Jurassic boundary. Cambridge University Press, Cambridge.