Revised Biostratigraphy-based Correlations for Determining the Position of the TR/J Boundary on the Colorado Plateau

This is an online published corrected proof utilizing paleomagnetic and biostratigraphic data to attempt to precisely locate the Triassic/Jurassic boundary in terrestrial strata of the Colorado Plateau in the Western U.S.  It provides corrections based on conchostracan biostratigraphy for previous proposed correlations to other global magnetostrat sections presented in a paper just published by many of the same authors this last October. Unfortunately, because of a lack of isotopic dates to calibrate the magnetostrat sections and the limitations of biostratigraphy for such correlations (e.g., Irmis et al., 2010; Olsen et al., in press), it is not yet clear how reliable these corrections are.

One interesting conclusion of this paper is that pseudosuchian extinctions and the first appearance of large theropod dinosaurs (represented by the ichnotaxon Eubrontes) may have occurred within the Late Triassic and not at or even near the Triassic-Jurassic boundary. Of course this observation is simply a result of moving the TR/J boundary upwards in terrestrial strata.

Lucas, S.G., Tanner, L.H., Donohoo-Hurley, L.L., Geissman, J.W., Kozur, H.W., Heckert, A.B., Weems, R.E.in press. Position of the Triassic-Jurassic boundary and timing of the end-Triassic extinctions on land: Data from the Moenave Formation on the southern Colorado Plateau, USA, Palaeogeography, Palaeoclimatology, Palaeoecology (2011). doi: 10.1016/j.palaeo.2011.01.009

Abstract - Strata of the Moenave Formation on and adjacent to the southern Colorado Plateau in Utah–Arizona, U.S.A., represent one of the best known and most stratigraphically continuous, complete and fossiliferous terrestrial sections across the Triassic–Jurassic boundary. We present a synthesis of new biostratigraphic and magnetostratigraphic data collected from across the Moenave Formation outcrop belt, which extends from the St. George area in southwestern Utah to the Tuba City area in northern Arizona. These data include palynomorphs, conchostracans and vertebrate fossils (including footprints) and a composite polarity record based on four overlapping magnetostratigraphic sections. Placement of the Triassic–Jurassic boundary in strata of the Moenave Formation has long been imprecise and debatable, but these new data (especially the conchostracans) allow us to place the Triassic–Jurassic boundary relatively precisely in the middle part of the Whitmore Point Member of the Moenave Formation, stratigraphically well above the highest occurrence of crurotarsan body fossils or footprints. Correlation to marine sections based on this placement indicates that major terrestrial vertebrate extinctions preceded marine extinctions across the Triassic–Jurassic boundary and therefore were likely unrelated to the Central Atlantic Magmatic Province (CAMP) volcanism.

REFERENCES

Donohoo-Hurley, L. L., Geissman, J. W., and S. G. Lucas. 2010. Magnetostratigraphy of the uppermost Triassic and lowermost Jurassic Moenave Formation, western United States: Correlation with strata in the United Kingdom, Morocco, Turkey, Italy, and eastern United States. Geological Society of America Bulletin 122: 2005-2019; doi: 10.1130/B30136.1

Irmis, R. B., Martz, J. W., Parker, W. G., and S. J. Nesbitt. 2010. Re-evaluating the correlation between Late Triassic terrestrial vertebrate biostratigraphy and the GSSP-defined marine stages. Albertiana 38:40-52.

Olsen, P. E., Kent, D. V., and J. H. Whiteside. In Press. Implications of the Newark Supergroup-based astrochronology and geomagnetic polarity time scale (Newark-APTS) for the tempo and mode of the early diversification of the Dinosauria. Environmental and Earth Science Transactions of the Royal Society of Edinburgh.