PLACEMENT OF THE TRIASSIC-JURASSIC BOUNDARY ON THE SOUTHERN COLORADO PLATEAU (USA) WITHIN THE MOENAVE FORMATION

For more than 30 years, placement of the Triassic-Jurassic boundary (TJB) in nonmarine strata was confounded by the inability to correlate accurately to marine sections. An incorrect placement in the Newark Supergroup of eastern North America was based on circular reasoning that assumed the TJB = a mass extinction and then identified the largest biotic turnover in the Newark section (which is just below the oldest CAMP basalts) as the TJB. Correction of this error by actual correlation of the boundary to the marine TJB based on biostratigraphy, moves it up into the Newark extrusive zone. This correction has allowed a correct placement of that boundary within the Moenave Formation on the Colorado Plateau. Biostratigraphic datasets—palynomorphs, conchostracans and vertebrates (e.g., phytosaur remains, chirothere footprints)—converge to confirm that the lower Whitmore Point Member of the Moenave Formation and the laterally equivalent lower Wingate Sandstone have a Triassic age, whereas the upper Whitmore Point is Jurassic. Magnetostratigraphy supports this correlation, but is not definitive, as the magnetic polarity within the Moenave Formation is a normal multichron with 2 or 3 short reversals that cannot be unambiguously correlated to other short reversals within the normal multichron that encompasses the TJB elsewhere. Detrital zircon ages from the Dinosaur Canyon and Whitmore Point members of the Moenave Formation are ~ 201.3 Ma and are statistically indistinguishable from each other and indistinguishable from the accepted age of the TJB. Recently published carbon-isotope stratigraphy for the Whitmore Point Member is weak evidence because lacustrine δ¹³C is arguably biased by local and temporary fluctuations in local floral assemblage, lacustrine productivity and preservation. Moreover, the assumption that the lowest occurrence of the theropod dinosaur footprint Eubrontes identifies the base of the Jurassic is an erroneous concept that was disproved more than one decade ago (and needs to be abandoned). Thus, the TJB on the southern Colorado Plateau definitely occurs within the Moenave Formation, and the best data constrain it to a position in the Whitmore Point Member. The fossil record of the Moenave Formation presents no evidence of a terrestrial mass extinction either at or just before the TJB.