Field of Science

Using Detrital Zircons to Reconstruct Chinle Paleogeography

Dickinson, W.R., and G.E. Gehrels. 2008. U-Pb Ages of Detrital Zircons in Relation to Paleogeography: Triassic Paleodrainage Networks and Sediment Dispersal Across Southwest Laurentia. Journal of Sedimentary Research 78:745-764.

Abstract - Integration of detrital zircon studies with data on biostratigraphy, isopachs, facies, and paleocurrents can improve reconstructions of paleogeography for ancient fluvial systems. U-Pb ages for 1808 individual detrital zircon grains in 20 samples of Triassic fluvial sandstone on the Colorado Plateau and High Plains, U.S.A., help constrain paleodrainage paths and patterns of sediment dispersal across southwest Laurentia. The dominant paleoflow was from southeast to northwest from sources as distant as the Ouachita orogen, and segments of Mesoamerica beyond it, to the Cordilleran margin in the Great Basin. Detrital zircons in Middle Triassic Moenkopi strata deposited along the distal eastern flank of the Sonoma foreland basin were derived from varied sources to the south and southeast, as were similar populations of detrital zircons in Upper Triassic Chinle strata exposed along the southern margin of the Colorado Plateau. Basal Chinle (-Dockum) strata of the High Plains to the east contain detrital zircon populations dominated by Cambrian grains derived from the nearby Amarillo–Wichita uplift, as do Gartra strata of northeast Utah to which the Cambrian grains were transported across eroded roots of Ancestral Rocky Mountains uplifts along upstream reaches of the Eagle paleoriver of northwest Colorado. The central locus of the Chinle–Dockum fluvial system lay along the tectonic furrow of an elongate backarc basin formed by dynamic subsidence inland from the Cordilleran magmatic arc. A lower Chinle–Dockum trunk paleoriver and the upper Chinle–Dockum Cottonwood paleovalley document longitudinal paleoflow parallel to the axis of the basin. Detrital zircon populations show that headwaters of both paleodrainages tapped the Ouachita orogen, the Permian–Triassic East Mexico arc, and associated rock assemblages of southwest Laurentia and adjacent Mesoamerica. Sediment sources in the Mesozoic Cordilleran arc became increasingly more prominent during Late Triassic time.
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This is part of an ongoing project to reconstruct Triassic paleogeography using detrital zircons, especially the Chinle-Dockum paleoriver systems. The results reinforce the hypothesis of Riggs et al. (1996) that during early Chinle and Dockum deposition the two basins were linked by a single river system. One aspect of this paper that caught my eye is that the authors acknowledge the current confusion regarding ranks among Chinle-Dockum stratigraphic units, and leave this unresolved. Their results hinge on hypothesized unit correlations (supported in part by detrital zircon provenance), namely that the Sonsela, Poleo, and Trujillo sandstones are equivalents as are the Shinarump, Santa Rosa, and Agua Zarca. However, recent paleomag work by Kate Zeigler has suggested that the Sonsela and Poleo are probably not equivalent (Zeigler et al. 2008) and this work needs to be expanded to the other units. It is important to note however, that the "Sonsela" of Dickinson and Gehrels is only equivalent to a medial sandstone bed (Jasper Forest bed) in the expanded Sonsela Member of the Chinle (see Jeff Martz's detailed discussion of the stratigraphy of the Chinle in Petrified Forest National Park), and that other portions of the member may be Poleo equivalent. Nonetheless, the Dickinson and Gehrels paper is an important part of a plethora of new research projects that are clarifying the stratigraphy, biostratigraphy, and paleogeography of the Chinle and Dockum.

As part of this, I'm excited that the stratigraphic and biostratigraphic work being done in Petrified Forest National Park (and mentioned by Jeff), is being supplemented by paleomag work, isotopic dating, provenance work, chemostratigraphic work, and a host of sedimentological and paleosol studies. As a result of the work in the park and regional studies such as this, we are getting very close to finally being able to put together a detailed paleontological and geological history of the park, which will hopefully serve as a standard for the rest of the American southwest.

REFERENCES

Riggs, N. R., Lehman, T. M., Gehrels, G. E., and W. R. Dickinson. 1996. Detrital zircon link between headwaters and terminus of the Upper Triassic Chinle-Dockum paleoriver system. Science 273:97-100.

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.

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