Zeigler, K. E., Kelley, S., and J. W. Geissman. 2008. Revisions to stratigraphic nomenclature of the Upper Triassic Chinle Group in New Mexico: New insights from geologic mapping, sedimentology, and magnetostratigraphic/ paleomagnetic data. Rocky Mountain Geology 43:121-141. DOI: 10.2113/gsrocky.43.2.121
This is a recent paper with some interesting and possibly key implications for stratigraphic correlations in the Chinle Formation. Historically, stratigraphic correlation for Late Triassic non-marine rocks in western North America has been accomplished mainly through the use of biostratigraphy (e.g., Camp, 1930 [phytosaurs]; Colbert and Gregory, 1957 [vertebrates]; Litwin et al., 1991 [palynomorphs]; Good, 1993 [invertebrates], Lucas, 1993 [vertebrates]); however, these biostratigraphic-based correlations have not been unambiguous and are often contested and revised (e.g., Long and Murry, 1995; Langer, 2005; Rayfield et al., 2005; Hunt et al. 2005; Parker, 2006). More recent studies (e.g., Riggs et al. 1996; Zeigler, 2008; Irmis and Mundil, 2008; Dickinson and Gehrels, 2008) generating isotopic dates as well as magnetostratigraphic data may be more reliable to provide correlations and reconstruct Late Triassic paleogeography. However, much of this work is in preliminary stages and/or unpublished outside of dissertations and/or abstracts. Nonetheless, some important resolutions have been made regarding the Late Triassic of the southwest U. S. such as confirming that the lower portion of the Glen Canyon Group is Triassic in age and not Jurassic (Molina-Garza et al. 2003), providing isotopic ages for the Chinle Formation (Riggs et al., 2003; Irmis and Mundil, 2008); and reconstructing the Chinle-Dockum River systems (Riggs et al. 1996; Dickinson and Gehrels, 2008).
Zeigler et al. (2008) examine the sedimentology and magnetostratigraphy of the Chinle Formation in the Chama Basin of north central New Mexico. This area is well known for its paleontology containing no less than four major vertebrate quarries (Canjilon, Snyder, Coelophysis, and Hayden) and the areas have produced the type materials of several taxa including Coelophysis bauri, Eucoelophysis baldwini, Typothorax coccinarum, and Dromomeron romeri (e.g., Colbert, 1989; Hunt and Lucas, 1993; Long and Murry, 1995; Sullivan and Lucas, 1999; Zeigler et al., 2003; Irmis et al., 2007). Furthermore, this area has produced the vast majority of known dinosauromorph (including dinosauriforms and dinosaurs) material from the Late Triassic (e.g., Colbert, 1989; Irmis et al., 2007). Thus, correlating these deposits to the rest of the Chinle Basin is of extreme importance.
In the Chama Basin, the members of the Chinle from oldest to youngest are the “Mottled Strata”, Agua Zarca Sandstone, Salitral Shale tongue, Poleo Sandstone Lentil, Petrified Forest Member, and the Siltstone Member (Stewart et al. 1972). Lucas (1993) raised the Chinle Formation to group status and correlated the Agua Zarca “Formation” to the Shinarump “Formation” and the siltstone member (erroneous termed the Red Siltstone Member by Zeigler et al. 2008) to the Rock Point “Formation”. This last correlation has been highly contested with other workers (e.g., Dubiel, 1989) suggesting correlation with the Owl Rock Member instead. In later papers, Lucas and colleagues (e. g., Lucas et al., 2003) separated out various members of the Salitral “Formation” and the Petrified Forest “Formation”. It is important to note that the Coelophysis Quarry is situated in the siltstone member; whereas the Snyder, Canjillon, and Hayden Quarries are in the Petrified Forest Member. Based on stratigraphic position and vertebrates Lucas (1993) correlated the siltstone member (his Rock Point) with the Redonda Formation (Dockum Group) in eastern New Mexico and the Poleo Sandstone to the Sonsela Sandstone of northeastern Arizona. Finally, Heckert and Lucas (2003) formally named the “Mottled Strata” the Zuni Mountains Formation.
One of the goals of Zeigler et al. (2008) was to test some of these correlations using geological mapping magnetostratigraphy and provide the following results:
1) The Zuni Mountain Formation is not mappable at the 1:24000 scale and thus is not a valid formation.
2) The various proposed members of the Salitral and Petrified Forest “Formations” cannot be distinguished by mapping and therefore are rejected.
3) The Poleo Sandstone is of almost entirely reverse polarity whereas the Sonsela Sandstone bed is of normal polarity and therefore cannot be correlative. [Note that this is the same bed which Jeff Martz and I now call the Jasper Forest bed of the Sonsela Member].
4) The siltstone member is predominantly of normal polarity whereas the type Rock Point (in Utah) and the Redonda Formation are of reverse polarity. Thus the siltstone member is not assignable to the Rock Point, nor is it correlative with the Redonda.
5) The siltstone member (and thus the Coelophysis Quarry) shares a paleo pole position with the lower part of the Moenave Formation of the Glen Canyon Group, which has alternatively been considered to be Triassic or Jurassic.
These conclusions reveal limitations of relying solely on biostratigraphy to correlate non-marine stratigraphic units. Late Triassic terrestrial fossils do have biostratigraphic utility, however, more work needs to be done (e.g., larger sample sizes) to determine accurate stratigraphic ranges of taxa, and this work needs to be done in concert with detailed palynology, magnetostratigraphic, stratigraphy, and anchored with robust isotopic dates.
One aspect of the Zeigler et al. (2008) paper that confuses me is the rank of the Chinle (and Kate and I have discussed this numerous times). In the beginning of the paper they suggest that because Spencer Lucas did not propose his change in rank for the Chinle in peer-reviewed literature, usage of Chinle Group for all non-marine Upper Triassic strata in the western U. S. is not valid. However, without any real justification or explanation they then ‘formally’ raise the Chinle to group rank only in the Chama Basin. When I mentioned this to a Chinle Formation veteran colleague of mine he exclaimed that this would only make things more confusing and Chinle nomenclature was already confusing enough. I have to agree. I really do not see the utility in raising the Chinle to group rank at all, nevermind in such a small portion of the formation. Now if one uses Chinle Group they will have to state whether this is sensu Lucas (1993) or sensu Zeigler et al. (2008). I am really not sure how any of this will be resolved.
Overall the new paper by Zeigler et al. (2008) provides intriguing hypotheses regarding the Chinle Formation and emphasizes how much work is left to do on that unit.
The photo above is of the Hayden Quarry in the Petrified Forest Member and is from here.
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K. E. Zeigler, S. Kelley, J. W. Geissman (2008). Revisions to stratigraphic nomenclature of the Upper Triassic Chinle Group in New Mexico: New insights from geologic mapping, sedimentology, and magnetostratigraphic/paleomagnetic data Rocky Mountain Geology, 43 (2), 121-141 DOI: 10.2113/gsrocky.43.2.121
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