The conclusion that Lisowice/Lipie ÅlÄskie is Norian rather than Rhaetian seems good to me, but a few other things deserve comment:
â No, Dmitry Bogdanov's illustrations are not in the public domain. They must be attributed to their author. I checked
â Like apparently many paleontologists, the authors seem unaware that radiometric ages from "detrital zircons", i.e. zircon crystals found in sedimentary rock other than the most pristine ignimbrites, are maximum ages. The rock can't have been deposited before the youngest zircon crystal in the sample formed, but it can have been deposited later, after that crystal eroded out of volcanic rock and was washed in; there's just no good way to guess how much later. This seems like an evident explanation for why the youngest zircon from a sample lower in the Placerias quarry is younger than the oldest zircon from a sample higher up; both dates can be completely correct, without there being any reason to dismiss the date from the lower sample as the authors do on p. 5. Routinely, the oldest zircon crystals in such samples are a billion years older than the youngest ones; zircon is extremely hard, so the crystals can and do weather out of some volcanic rock, be transported over long distances and reworked several times before we find them. The youngest zircon crystals known from the Triassic sediments of the Karoo Basin are all Permian (McKay et al. 2015); there simply wasn't any Triassic volcanism in the area that could have produced Triassic zircons.
â Speaking of the Placerias
quarry and other sites with similar tetrapod faunas, the authors insist that those are all Carnian rather than Norian. Then in fig. 4 and 5B they merrily place the Carnian-Norian boundary at 22
Ma ago, without any argument, any citation of a source or any indication of uncertainty. The International Stratigraphic Chart
instead places that boundary at "~ 227
" Ma ago (and has been doing so for a while). That's at least half of the "disagreement" right there! For "late Carnian" simply read "early Norian" throughout the paper. :-|
â Bringing Cuvier into the argument seems completely unnecessary to me. The attempt to use his work to construct an argument for doing biostratigraphy preferentially with large tetrapods strikes me as just embarrassing, even without considering how bad that argument is on its own merits; fortunately it, too, is irrelevant to the conclusions.
â Metoposaurus krasiejowensis is not the same as M. diagnosticus. It was originally named as a subspecies (!) of the latter (hence Bogdanov's misspelled filename), but finally raised to species rank in the description of M. algarvensis. In short, its utility for biostratigraphy may be a tad less than implied in fig. 4A. And why is Cyclotosaurus intermedius absent from that figure?
Matthew P. McKay, Amy L. Weislogel, Andrea Fildani, Rufus L. Brunt, David M. Hodgson & Stephen S. Flint (2015)
U-PB zircon tuff geochronology from the Karoo Basin, South Africa: implications of zircon recycling on stratigraphic age controls
International Geology Review 57(4): 393â410
Along the >650 km long southern margin of the Karoo Basin in South Africa, we traversed four evenly spaced stratigraphic transects and collected 22 samples of volcanic, air-fall tuffs thought to be distal deposits derived from the PermianâTriassic Southern Gondwanan volcanic arc. We present 469 new U-Pb zircon ages determined by sensitive high-resolution ion microprobe reverse geometry (SHRIMP-RG) at the StanfordâUSGS Microanalytical Center in order to constrain the maximum depositional ages for the southern Karoo Basin strata. Weighted means of these youngest coherent zircon populations were selected to maximize the number of analyses while minimizing the mean square weighted deviation (MSWD) to increase the robustness and decrease the influence of Pb-loss and inheritance in determining the maximum depositional age. Maximum depositional ages for the marine Ecca Group range from 250 to 274 Ma, whereas in the conformably overlying terrestrial Beaufort Group maximum depositional ages ranged from 257 to 452 Ma. Across the southern Karoo Basin, the Ecca Group tuffs produce maximum depositional ages that young upward; however, the Beaufort Group tuffs yield maximum depositional ages that are geochronologically out of sequence. Furthermore, maximum depositional ages of the Beaufort Group tuffs are consistently older than ash ages within the underlying marine strata. Our results are supported by previously published U-Pb tuff zircon geochronology in the Karoo Basin and demonstrate that the presence of out-of-sequence, older tuff ages are repeatable in Beaufort Group tuffs along the southern margin of the basin. We propose that tuffs in the Karoo Basin are correlative with tuffs in southern South America, and that the age spectra of these tuffs were influenced by magmatic crustal recycling. We use these data to highlight the complexity of U-Pb zircon datasets from tuffs, address the use of U-Pb zircon ages to provide absolute age controls, and discuss the implications of these new age controls on the Permian-Triassic Karoo strata.