Reconstruction Of The Blue Mesa Member Depositional System

For the general public, most of whom do not have a geology background, it is very hard to understand how drastically what they can see before them may differ from what used to be there. When visitors look out over the Painted Desert from Kachina Point or one of the other overlooks in the north end of the Petrified Forest National Park and are told that the strata before them were formed in an ancient river system, a common response is "oh, yeah, there it is right there!" They are referring to modern day Lithodendron Wash. They do not understand that the modern topography is something carved out by modern drainages, and that an absolutely huge body of sediment was laid down in very different environments and subsequently carved out by post-Mesozoic erosion.

A few months ago, I posted on a reconstruction of the Upper Triassic Chinle Formation depositional system in Petrified Forest National Park, which was paid for years ago by the park service. Such reconstructions are extremely useful for educating the public, as they illustrate the relationship between sedimentary strata and the ancient environments that actually produced them by helping the public understand that they are looking at an irregular cross-section through a series of ancient depositional systems.

However, as I had discussed in my prior blog, this diagram has some inaccuracies. As a result, I decided to do a series of revised block diagrams for the park service, showing the different depositional environments which laid down the Chinle Formation strata, and then one or two others showing this same stratigraphic block diagram carved into modern-day topography.

This project is probably going to take forever to complete, as I have have other illustrations to produce for the park (as well as a lot of other paleontological and geological duties), but I have the first block diagram done. This one illustrates the depositional system of the Blue Mesa Member, the lowest unit of the Chinle Formation in the park which is well-exposed within the traditional park boundaries. Specifically, it shows the depositional system associated with the Newspaper Rock Bed, a prominent sandstone unit in the middle of the member. This reconstruction is based mostly on Tim Demko's work in his 1995 dissertation, and publications by himself and other discussing the Blue Mesa Member depositional system (Demko, 1995b; Demko et al., 1998). The block is a half-kilometer on a side (click on it to see a bigger version).

From Paleo Errata

The Newspaper Rock Bed was deposited by a meandering river system, which displays well-preserved lateral accretion bedding (these are beautifully illustrated by some nice exhumed scroll bars which are visible from the main park road driving south, just before dropping down into the Teepees), while the associated mudstones of the Blue Mesa Member represent overbank and lacustrine deposits. The drab coloration, and relative scarcity of pedogenic carbonate nodules compared to what is seen higher in the Sonsela and Petrified Forest Members, indicate relatively poorly-drained soils and possible wetland conditions. This is supported by the abundance of metoposaurs (large temnospondyl amphibians), in the Blue Mesa Member; these become relatively scarce higher in the section.

Demko's work on plant taphonomy is also very cool, as it allowed him to plausibly reconstruct the distribution of different types of plants around the channel, floodplain, and lacustrine settings. Conifers (which seem to have been a little smaller then those forming the spectacular agatized forests in the Sonsela Member) are mostly found in the channel deposits, and may have been mostly confined to riparian zones along the banks of the river. Cycadeoids (bennettitaleans), ferns, and seed ferns, with less common cycads, ginkos, and conifers, were the main plants growing on the floodplains. Giant horsetails (sphenopsids), as well as smaller horsetails and club mosses, mostly grew along along channels and around the margins of lakes.

The biggest problem with the block diagram is that it is difficult to make out plants and animals at that scale (the little green dashes around the lakes are giant lycopods three meters tall or more, and there is actually an 8 meter long Leptosuchus-grade phytosaur sitting on the point bar. Trust me). At some point, I may do blow ups of different parts of the diagram giving a close-up of the fauna and flora.

REFERENCES

Demko, T.M. 1995a. Taphonomy of fossil plants in the Upper Triassic Chinle Formation. Unpublished PhD dissertation, University of Arizona, Tuscon, 274 pp.

Demko, T.M. 1995b. Taphonomy of fossil plants in Petrifed Forest National Park, Arizona. Pp. 37-52 in D. Boaz (ed.) Fossils of Arizona, Proceedings of the Mesa Southwest Paleontological Society and Mesa Southwest Museum, Mesa AZ.

Demko, T.M., Dubiel, R.F., and Parrish, J.T. 1998. Plant taphonomy in incised valleys: Implications for interpreting paleoclimate from fossil plants. Geology, vol. 26, no. 12, pp. 1119-1122.