EXTRAPOLATING NUMERICAL AGES FOR THE MESOZOIC GEOLOGIC TIMESCALE – A STATUS REPORT
The Early Triassic timescale is well constrained by U-Pb zircon ages and cycle-magnetostratigraphy. The Middle Triassic has fewer radio-isotopic constraints, therefore relative durations of ammonoid zones in some intervals are are scaled according to their number of subzones. Even though the Late Triassic contains an extensive cycle-scaled magnetostratigraphy from the Newark lacustrine formations and there are precise radio-isotopic ages for the Triassic-Jurassic boundary, the correlation of the marine-based Norian lacks a consensus among stratigraphers. For example, it is debated if the uppermost stage, the Rhaetian, spans 3 or 8 million years.
A linear decrease in Strontium isotope ratios through the Early Jurassic supported by radio-isotopic ages and cycle-stratigraphy provides durations of the ammonite zones. Middle Jurassic ammonoid zones are scaled by cycle stratigraphy from boreholes and outcrops or correlation to Pacific M-sequence magnetic anomalies. Cycle-scaled magnetostratigraphy indicates a progressive slowing of spreading rates for the M-Sequence from Callovian through Barremian.
Aptian-Albian zones are scaled via cycle stratigraphy that is constrained by radio-isotopic ages. Late Cretaceous (Cenomanian-Campanian) ammonite zones of the Western Interior have numerous Ar-Ar and U-Pb radio-isotopic ages. The magnetic polarity time scale for the latest Cretaceous (Campanian-Maastrichtian) is cycle scaled in ocean drilling cores, thereby providing numerical ages relative to the base-Cenozoic radio-isotopic age of 66.0 Ma.
The main time intervals lacking definitive radio-isotopic ages with unambiguous biostratigraphic calibrations are the Late Triassic and nearly all of the Middle Jurassic through early Cretaceous.