Chinle Confusion

The word 'nomenclature' (syn: terminology) is defined as “a system of terms used in a particular science” and allows for more precise communication and understanding of ideas. All scientific disciplines have established nomenclature (called ‘jargon’ by those outside of the discipline) and many have rules regarding the establishment of nomenclature. The Chinle Formation (mainly) of Arizona, Utah, and western New Mexico and the Dockum Group (mainly) of eastern New Mexico and western Texas are Late Triassic terrestrial units that are often correlated to each other on the basis of similar fossil content. Both have been widely studied by geologists and paleontologists since the beginning of last century and it is still debated whether the units were deposited within separate basins. Current thinking shows that the two depocenters may have been connected early in their history (Riggs et al., 1996) and separated later (Lehman and Chatterjee, 2005). The Dockum was named in 1890, the Chinle in 1917.

In 1993 Spencer Lucas advocated raising the name Chinle to Group status (and thus all of its constituent members to Formation rank). In doing so he reduced to Dockum in Texas to formational rank and included it within his Chinle Group. His rationale was that both units were the same age (based on fossils) and deposited in the same basin. It is important to note that he included all Late Triassic terrestrial rocks (including the Popo Agie Fm. of Wyoming) in his newly established group and eliminated some other older names (e.g., Dolores Fm. of Colorado). In later papers Lucas and colleagues completely abandoned use of the name Dockum as well. Lucas et al. (1985) argued that the term Dockum was meaningless, having been widely applied and lacks specificity as it used to include strata later determined to be Middle Triassic in age (Anton Chico Formation). Lucas argued that the term Chinle was better established; however, when originally defined, the Chinle contained units that are now considered to be Jurassic in age (Glen Canyon Group). Nonetheless, this is irrelevant as lithostratigraphic units can be time transgressive and do not need to be restricted by chronstratigraphic boundaries. Moreover, revising lithostratigraphic units does not invalidate them.

Furthermore, the term Dockum has long (since the 1890s) been generally understood to be restricted to Upper Triassic strata exposed around the southern High Plains in eastern New Mexico and west Texas. However, the term “Chinle” has been applied in a much more varied fashion, not only to the upper Triassic strata of the Colorado Plateau, but to strata in northern Utah and Colorado (e.g., Eagle Basin) deposited in a separate basin, and to strata within the Dockum Group which is probably only equivalent to parts of the Chinle Formation on the Colorado Plateau. However, by 1993 the scope of the term “Chinle” was well understood. Ironically the most confusing application of the term “Chinle” has been its extension to all Upper Triassic strata in the southwest by Lucas (1993), which has created two very different understandings of the term in the literature. Given that Lucas (1993) argued that the term Dockum should only be restricted to its type area in Texas, his radical extension of the term “Chinle” through the whole western U.S. is puzzling.

Nonetheless, since first being proposed in 1993, the use of Chinle as a Group has been published in hundreds of papers and abstracts by Lucas and colleagues and has also been used by a few other workers as well. There is also a large group of researchers who insist on leaving the Chinle at Formational rank, as well as researchers from Texas who refuse to abandon the term Dockum. Arguments as to why the Chinle should not subsume the Dockum were provided by Lehman (1994) and Dubiel (1994) and most recently addressed by Carpenter (1997) who wrote: “substituting one name for another (Chinle Group for Dockum Group) violates nomenclatural stability. Furthermore, as a stratigraphic unit, the Dockum Group is not defined by time” making the arguments by Lucas and colleagues “meaningless”. He then writes “the term Dockum Group must be retained, and because it has priority, can be used to encompass the Upper Triassic formations of the American southwest”. He also notes that because Upper Triassic formations in Colorado, Wyoming, and Idaho were most likely not deposited in the same basins as the Chinle or Dockum they should be left alone.

Although Carpenter (1997) was just making a point and not really advocating using the term Dockum in Arizona and Utah, this is where the issue of nomenclatural utility comes to play. Admittedly these names were erected at different times by different researchers who may not have been looking at all of the Triassic rocks in the western U.S. as a whole, but this does not negate their utility. When we hear the term Dockum we think Texas, whereas Chinle suggests Arizona., to mix the two only causes confusion. Even more so IMHO combining all of these units under a single name tells us nothing new scientifically, we already knew that they were roughly of the same age. In fact, again IMHO, “Chinle Group” appears to basically be synonymous with “Upper Triassic” and tells us nothing about local lithostratigraphic variation. This is contrary to Lucas’ (1993) stated intent to “simplify” the basic nomenclatural framework. Essentially, you have two groups of workers, one producing a plethora of papers in in-house bulletins and geological society guidebooks, independently utilizing two differing schemes. In summary, not only is one scheme considered to be against the North American Stratigraphic Code, the situation also creates serious confusion especially among outside researchers, as I will document next with a couple cases.

Weishampel et al. (2004) provide a listing of all known dinosaur occurrences along with their stratigraphic information. Under their Triassic section for North America they refer to the Chinle Formation, but also refer to the members within it also as formations (e.g., “Chinle Formation/Petrified Forest Fm”.). This is non-sensical as they are actually referring to the Petrified Forest Member of the Chinle Formation. Furthermore they list “Chinle Formation/Santa Rosa Fm” regardless of the fact that the Santa Rosa has never been considered a member of the Chinle Formation. Again this makes no sense and would only confuse workers with no familiarity with the conflicting schemes. It certainly does appear that Weishampel et al. (2004) were confused.

Cleveland et al. (2007, 2008a, 2008b) have recently published a series of papers documenting Late Triassic paleosols from New Mexico. The earliest paper (Cleveland et al., 2007) explicitly states that they prefer the nomenclature of Lucas (1993), and in the second paper (Cleveland et al., 2008a) this is implied. However, in the third paper (Cleveland et al., 2008b) they drop the Chinle back to Formational rank, presumably to move away from the Lucas nomenclature (although they do not state why), and return all lesser units back to member rank. Unfortunately, it is not this simple. For example, Cleveland et al. (2008b) now list one of their units as the Painted Desert Member of the Chinle Formation. No such member has ever been proposed. This is the Painted Desert Member of the Petrified Forest Formation of the Chinle Group according to Lucas (1993). Thus, Cleveland et als. (2008b) unit is actually the Petrified Forest Member (Chinle Formation). Likewise, upon dropping Chinle Group they do not appear to resurrect the term Dockum for their units in eastern New Mexico. This provides a similar scheme to that of Weishampel et al. (2004) where the Santa Rosa and Redonda Formations are considered part of the Chinle Formation (equal rank?). Clearly Cleveland and colleagues are confused by the changes in nomenclature and are possibly propagating more confusion. By the way, this is not meant to be a criticism of their paleosol work or general conclusions, I’m just pointing out the stratigraphic nomenclature confusion. Obviously I support their switch back to Chinle as a formational name. I have provided the nomenclature from thier 2007 (top center) and 2008b (bottom left) papers below as well as a rough correction of the 2008b figure (bottom right) below.

To wrap this up (it is already much longer than I planned). IMHO the scheme presented by Lucas (1993) has caused more confusion than clarification regarding the stratigraphic nomenclature of the Upper Triassic rocks in the western U.S. Also, others have argued that his scheme is against the North American Stratigraphic Code. I am not adverse to the Chinle Formation being raised to group rank, but I cannot do so at the expense of the name Dockum. Moreover, given how radically the meaning of the term “Chinle” was stretched by Lucas (1993), this might propagate more confusion. I am certain that my colleagues in Texas will not call their rocks “Chinle”, and rightfully so. Likewise, I would never dream of calling the rocks in Arizona “Dockum”, because of the confusion it would introduce. Maybe this calls for a special symposium to straighten out the issue, or maybe we should all just stop using Chinle Group. Many of the southwestern U.S. Triassic workers already have (or never adopted it to begin with) but it is important that this issue is brought to the attention of outside workers to help cease the confusion.

REFERENCES

Carpenter, K. 1997. A giant coelophysoid (Certosauria) theropod from the Upper Triassic of New Mexico, USA. N. Jb. Geol. Palaeont. Abh. 205:189-208.

Cleveland, D.M., Atchley, S.C., and L.C. Nordt. 2007. Continental sequence-stratigraphy of the Late Triassic (Norian-Rhaetian) Chinle strata, northern New Mexico: Allo- and autocyclic origins of paleosol-bearing alluvial successions: Journal of Sedimentary Research 77:909–924.

Cleveland, D.M., Nordt, L.C., and S.C. Atchley. 2008a. Paleosols, trace fossils, and precipitation estimates of the uppermost Triassic strata in northern New Mexico: Palaeogeography, Palaeoclimatology, Palaeoecology 257:421–444.

Cleveland, D.M., Nordt, L.C., Dworkin, S.I., and S.C. Atchley. 2008a. Pedogenic carbonate isotopes as evidence for extreme climatic events preceding the Triassic-Jurassic boundary: Implications for the biotic crisis? GSA Bulletin 120:1408-1415.

Dubiel, R. F. 1994. Triassic deposystems, paleogeography, and paleoclimate of the western interior; pp. 133-168 in Caputo, M.V., Peterson, J.A., and K.J. Franczyk (eds.) Mesozoic systems of the Rocky Mountain Region, USA. RMS-SEPM.

Lehman, T.M. 1994. Save the Dockum Group. West Texas Geological Society Bulletin 34:5–10.

Lehman, T. and S. Chatterjee. 2005. Depositional setting and vertebrate biostratigraphy of the Triassic Dockum Group of Texas. Journal of Earth System Science 114:325-351.

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

Lucas, S.G., Hunt, A.P., and M. Morales. 1985. Stratigraphic nomenclature and correlation of Triassic rocks in east-central New Mexico, a preliminary report. New Mexico Geological Society Guidebook 36:171-184.

Weishampel, D. B., Barrett, P. M., Coria, R. E., Le Loeuff, J., Gomani, E. S., Zhao Z., Xu X., Sahni, A., and C. Noto. 2004. Dinosaur Distribution, pp. 517-606 in Weishampel D. B., Dodson, P., and H. Osmólska, H. (eds.) The Dinosauria. 2nd edition. Univ. California Press, Berkeley.