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.