Lots of good new data in this paper that are very useful. Unfortunately it is victimized by having to rely on previously published stratigraphic correlations that are generally erroneous and unsupported by radioisotopic dates, which especially affects some of the key conclusions of the paper. I also still get really confused with the "Chinle Group" terminology especially units like the "Petrified Forest Formation" which is not equivalent to the currently recognized Petrified Forest Member of the Chinle Formation. Trying to go back and forth between the two schemes (Chinle Formation vs. Chinle Group) is very confusing and often leads to mistakes unless one is extremely careful. Still I extend my congratulations to Kate Zeigler in getting this body of work out there. The correlations can be fine-tuned later.
Zeigler, K. E., and J. W. Geissman. 2011. Magnetostratigraphy of the Upper Triassic Chinle Group of New Mexico: Implications for regional and global correlations among Upper Triassic sequences. Geosphere 7:802–829; doi:10.1130/GES00628.1
Abstract - A magnetic polarity zonation for the Upper Triassic Chinle Group in the Chama Basin, north-central New Mexico (United States), supplemented by polarity data from eastern and west-central New Mexico (Mesa Redonda and Zuni Mountains, respectively), provides the most complete and continuous magnetic polarity chronology for the Late Triassic of the American Southwest yet available. Most of the Chinle Group sequence is composed of hematitic mudrocks that typically carry a well-defined, well-grouped magnetization (residing in both pigmentary and detrital hematite), with laboratory unblocking temperatures as high as 680 °C. Demagnetization experiments isolate magnetizations of south- or north-seeking declination and shallow inclination, which are interpreted as early acquired, Late Triassic magnetizations. Our proposed polarity correlations, coupled with biostratigraphic observations and recent U-Pb age determinations on detrital zircon–bearing strata in the Chinle Group in western New Mexico, West Texas, and Arizona, indicate that deposition of Chinle strata likely spanned a much shorter time span than previously considered. If this interpretation is correct, the Chinle Group can be correlated with only part of the Newark Supergroup or the Upper Triassic Tethyan sections. On a local scale, lower Chinle strata in the Chama Basin are significantly older than the Bluewater Creek Formation in western New Mexico, and the base of the Poleo Formation represents a disconformity of >13 m.y. duration. Magnetic polarity chronologies from upper Chinle strata in New Mexico and Utah suggest that strata considered to be part of the Rock Point Formation in north-central New Mexico are not time equivalent to type Rock Point strata in Utah or to the Redonda Formation of eastern New Mexico.
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I thought their paper is a good piece of careful, analytical work, with great pains taken to ensure demag. behavoir was well-documented. However, I also think it is yet another 'lost opportunity' at establishing a reversed polarity chronology of Chinle Group strata. As acknowledged, and indeed, demonstrated by the authors and others, the Chama basin strat. record, while important for its faunas which are by and large restricted to a fairly narrow stratigraphic interval (i.e. canjillon, whitaker, hayden quarries), contains significant gaps in its Chinle record. Why these authors (or others) have not work up the much more comprehensive and better, more-widely exposed Tucumcari basin section escapes me. Their Redonda data is a bit confusing as compared to Reeve and Helsey when tied into local Redonda section to section correlations, which by their cyclic nature is easily accomplished, complete with robust marker beds. Nonetheless, some food for thought to chew on.
ReplyDeleteAs for the concept of the Chinle Group, if viewed in terms of lithostratigraphy, it has great application for establishing a regional framework for non marine Upper Triassic strata of the Western Interior. The same applies to the Newark Supergroup for eastern North American rift basin rocks of the broadly similar age. Both concepts have utility, and if nothing else, might serve to remind those that have an interest in, but work outside of those respective realms, that the Chinle proper does not solely exist in the vacum of the PFNP; nor does the Newark Supergroup only in well-drafted, pretty diagrams of 7 km of core from a single sub-region of its distribution. More and more geologist these days seem to lack a fair-grounding in lithostratigraphic concepts and muddle it up by not separating out chronostratigraphic interpretations. That is just a generalized observation not directed at the authors nor anyone in particular. Lithostratigraphy has its limitations and a unit to unit lithologic correlation does not necessarily imply a chronostratigraphic correlation (yes, we all know that, and shake our heads dismissively but do we remember that when working up our respective data sets and writing up results for publication?) By the way, I love your blogs and read them not often enough!
Phil,
ReplyDeleteThanks for the comment and the compliment. It will be interesting to see how all of the current and upcoming Chinle paleomag shakes out. Not only are there discontinuous sections and unconformities, but also a lack of absolute dates to calibrate the sections.
Regarding the use of Chinle Group. I don't really have a problem with the Chinle as a "group" rank per se, but I don;t think that it should subsume the Dockum or the Popo Agie (older names), plus there is a real regional utility in the old nomenclature which is lost if you simply unite everything under the Chinle as simply synomymous with Late Triassic terrestrial of the Western Interior. I really don't find that it adds any clarity or provides any new information. What would be best is to have the Chinle as equal rank as the Dockum (as Groups) and unite them under a supergroup with a unique name.