The Triassic Timescale 2013

Abstract

The current Triassic chronostratigraphic scale is a hierarchy of three series (Lower, Middle, and Upper) divided into seven stages (Lower = Induan and Olenekian; Middle = Anisian and Ladinian; and Upper = Carnian, Norian, and Rhaetian), further divided into 15 substages (Induan = upper Griesbachian and Dienerian; Olenekian = Smithian and Spathian; Anisian = Aegean, Bithynian, Pelsonian, and Illyrian; Ladinian = Fassanian and Longobardian; Carnian = Julian and Tuvalian; and Norian = Lacian, Alaunian, and Sevatian). Ammonoid and conodont biostratigraphies provide the primary basis for the chronostratigraphy. We argue here for reliance on ammonoid-based biostratigraphy for Triassic chronostratigraphic definitions. We advocate a four-stage Lower Triassic and the elevation of the very long Carnian and Norian stages to series. A sparse but growing database of precise radioisotopic ages supports the following calibrations: base of Triassic ~252 Ma, base Olenekian ~251 Ma, base Anisian ~247 Ma, base Ladinian ~242 Ma, base Norian ~221 Ma, and base Jurassic ~201 Ma. Triassic magnetostratigraphy is a series of multichrons at best, and needs vast improvement to make a serious contribution to the Triassic timescale.