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.