Paper No. 36
Presentation Time: 9:00 AM-6:00 PM
A TRIASSIC TIMESCALE 2009
The Triassic chronostratigraphic scale is a hierarchy of three series (Lower, Middle, Upper) divided into seven stages (Lower =Induan, Olenekian; Middle = Anisian, Ladinian and Upper = Carnian, Norian, Rhaetian) further divided into 15 substages (Induan = upper Griesbachian, Dienerian; Olenekian = Smithian, Spathian, Anisian = Aegean, Bithynian, Pelsonian, Illyrian, Ladinian = Fassanian, Longobardian, Carnian = Julian, Tuvalian, Norian = Lacian, Alaunian, Sevatian). Ammonoid and conodont biostratigraphies provide the primary basis for the chronostratigraphy. A sparse but growing database of precise radioisotopic ages support these calibrations: base of Triassic ~ 252 Ma, base Olenekian ~ 251 Ma, base Anisian, ~ 247 Ma, base Ladinian ~ 241 Ma, base Jurassic ~ 201 Ma. A U/Pb age of ~ 231 Ma from the Italian Pignola 2 section is lower Tuvalian, and U/Pb ages on detrital zircons from the nonmarine Chinle Group of the western USA of ~ 219 Ma are in strata of late Carnian (Tuvalian) age based on the biostratigraphy of palynomorphs, conchostracans and tetrapods. These data support placement of the Norian base at ~ 217 Ma, and indicate the Tuvalian is almost 15 million years long and that the Carnian and Norian are the longest Triassic stages. Magnetostratigraphic data establish normal polarity for all of the Triassic stage bases except Anisian and Ladinian. An integrated biostratigraphic correlation web for the marine Triassic consists of ammonoids, flat clams, radiolarians and conodonts, whereas a similar web exists for the nonmarine Triassic using palynomorphs, conchostracans and tetrapods. Critical to cross correlation of the two webs is the Triassic section in the Germanic basin, where a confident correlation of nonmarine biostratigraphy to Triassic stage boundaries has been achieved. The major paths forward in development of the Triassic timescale are: finish formal definition of all Triassic stage boundaries, formally define the 15 Triassic substages, improve the integration of the Triassic biostratigraphic webs and develop new radioisotopic and magnetostratigraphic data, particularly for the Late Triassic.