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.
Showing posts with label paleomagnetism. Show all posts
Showing posts with label paleomagnetism. Show all posts
Two New Papers on the Triassic/Jurassic Boundary
Kent, D. V., and E. Irving. 2010. Influence of inclination error in sedimentary rocks on the Triassic and Jurassic apparent pole wander path for North America and implications for Cordilleran tectonics. Journal Of Geophysical Research 115: B10103, doi:10.1029/2009JB007205
Abstract - Because of paleomagnetic inclination error (I error) in sedimentary rocks, we argue that previous estimates of Triassic and Jurassic paleolatitudes of the North American craton have generally been too low, the record being derived mostly from sedimentary rocks. Using results from all major cratons, we construct a new composite apparent pole wander (APW) path for Triassic through Paleogene based on 69 paleopoles ranging in age from 243 to 43 Ma. The poles are from igneous rocks and certain sedimentary formations corrected for I error brought into North American coordinates using plate tectonic reconstructions. Key features of the new APW path are a 25° northward progression from 230 to 190 Ma to high latitudes (off northernmost Siberia) where the pole lingers until 160 Ma, a jump to the Aleutians followed by a hook in western Alaska by ∼145 Ma that leads to the 130–60 Ma stillstand, after which the pole moves to its present position. As an example of the application of this new path we use paleomagnetic results to determine that southern Wrangellia and Stikinia (W/S), the two most westerly terranes in the Canadian Cordillera, lay 630 to 1650 km farther south than at present relative to the craton during the Late Triassic and Early Jurassic. This is consistent with an exotic Tethyan origin as paleontological and mantle geochemical evidences imply. During the Late Triassic through Early Cretaceous, W/S moved northward more slowly than the craton, implying oblique sinistral net convergence over this 130 Myr interval. This was followed by dextral shear in latest Cretaceous through Eocene.
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
Abstract - A composite magnetostratigraphy based on the magnetic polarity data from four sections of the uppermost Triassic and lowermost Jurassic Moenave Formation, Utah and Arizona, USA, can be correlated to the marine successions at Saint Audrie’s Bay (UK), Oyuklu, Turkey, and the Southern Alps, Italy, and to the nonmarine sections in Morocco, northern Africa, and the Newark Basin, eastern North America, all deposited across the Triassic–Jurassic boundary. Our proposed correlation provides a stratigraphic framework to tie Triassic–Jurassic sedimentation in the American Southwest to the marine UK, Turkey, and Italy sections, and to the Pangea rift history, including extrusive igneous rocks, preserved in Morocco and in the Newark Basin. The Moenave polarity record is characterized by mostly normal polarity, as is consistent with other polarity records across the Triassic–Jurassic boundary, and is interrupted by at least two well-defined reverse-polarity magnetozones. On the basis of available paleontologic information, we interpret the oldest well defined, reverse-polarity magnetozone, M2r of the Moenave Formation, to correlate with SA5n.2r or SA5n.3r of the Saint Audrie’s Bay record, H– of the Oyuklu record, BIT5n.1r of the Italcementi Quarry record, the oldest reverse magnetozone in sedimentary rocks in Morocco, and with reverse magnetozone E23r of the Newark Basin. The youngest reverse magnetozone of the Moenave Formation, M3r, is correlated to the latest Triassic magnetozones SA5n.5r of the Saint Audrie’s Bay record, J– of the Oyuklu record, and with the interval of reverse polarity in the “intermediate unit” of the Morocco record. Magnetostratigraphic correlations and marine biostratigraphic information support placement of the Triassic–Jurassic boundary in the middle to upper Whitmore Point Member of the Moenave Formation, the Lias Group of the Saint Audrie’s Bay section, the chert-rich limestone of the Oyuklu section, above the Zu Limestone in Italy, and in the central Atlantic magmatic province extrusive zone in the Morocco and the Newark records.
Abstract - Because of paleomagnetic inclination error (I error) in sedimentary rocks, we argue that previous estimates of Triassic and Jurassic paleolatitudes of the North American craton have generally been too low, the record being derived mostly from sedimentary rocks. Using results from all major cratons, we construct a new composite apparent pole wander (APW) path for Triassic through Paleogene based on 69 paleopoles ranging in age from 243 to 43 Ma. The poles are from igneous rocks and certain sedimentary formations corrected for I error brought into North American coordinates using plate tectonic reconstructions. Key features of the new APW path are a 25° northward progression from 230 to 190 Ma to high latitudes (off northernmost Siberia) where the pole lingers until 160 Ma, a jump to the Aleutians followed by a hook in western Alaska by ∼145 Ma that leads to the 130–60 Ma stillstand, after which the pole moves to its present position. As an example of the application of this new path we use paleomagnetic results to determine that southern Wrangellia and Stikinia (W/S), the two most westerly terranes in the Canadian Cordillera, lay 630 to 1650 km farther south than at present relative to the craton during the Late Triassic and Early Jurassic. This is consistent with an exotic Tethyan origin as paleontological and mantle geochemical evidences imply. During the Late Triassic through Early Cretaceous, W/S moved northward more slowly than the craton, implying oblique sinistral net convergence over this 130 Myr interval. This was followed by dextral shear in latest Cretaceous through Eocene.
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
Abstract - A composite magnetostratigraphy based on the magnetic polarity data from four sections of the uppermost Triassic and lowermost Jurassic Moenave Formation, Utah and Arizona, USA, can be correlated to the marine successions at Saint Audrie’s Bay (UK), Oyuklu, Turkey, and the Southern Alps, Italy, and to the nonmarine sections in Morocco, northern Africa, and the Newark Basin, eastern North America, all deposited across the Triassic–Jurassic boundary. Our proposed correlation provides a stratigraphic framework to tie Triassic–Jurassic sedimentation in the American Southwest to the marine UK, Turkey, and Italy sections, and to the Pangea rift history, including extrusive igneous rocks, preserved in Morocco and in the Newark Basin. The Moenave polarity record is characterized by mostly normal polarity, as is consistent with other polarity records across the Triassic–Jurassic boundary, and is interrupted by at least two well-defined reverse-polarity magnetozones. On the basis of available paleontologic information, we interpret the oldest well defined, reverse-polarity magnetozone, M2r of the Moenave Formation, to correlate with SA5n.2r or SA5n.3r of the Saint Audrie’s Bay record, H– of the Oyuklu record, BIT5n.1r of the Italcementi Quarry record, the oldest reverse magnetozone in sedimentary rocks in Morocco, and with reverse magnetozone E23r of the Newark Basin. The youngest reverse magnetozone of the Moenave Formation, M3r, is correlated to the latest Triassic magnetozones SA5n.5r of the Saint Audrie’s Bay record, J– of the Oyuklu record, and with the interval of reverse polarity in the “intermediate unit” of the Morocco record. Magnetostratigraphic correlations and marine biostratigraphic information support placement of the Triassic–Jurassic boundary in the middle to upper Whitmore Point Member of the Moenave Formation, the Lias Group of the Saint Audrie’s Bay section, the chert-rich limestone of the Oyuklu section, above the Zu Limestone in Italy, and in the central Atlantic magmatic province extrusive zone in the Morocco and the Newark records.
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