DIACHRONISM OF KEY FOSSIL STRATIGRAPHIC MARKERS IN THE MESOZOIC: CONSEQUENCES FOR THE GEOLOGIC TIME SCALE AND FOR PALEOENVIRONMENTAL CHANGE
Résumé
Diachronism is a well-known issue in stratigraphy. In the case of various macro-and microfossil groups,
several statistical methods such as constrained optimization (CONOP) or unitary associations have been
proposed to remedy such issues and improve the Geologic Time Scale. The purpose of the latter methods is to
examine misfits, eliminate problematic bio-events in favor of the most reliable biostratigraphic markers and to
draw the best statistical or optimal biostratigraphic series. Such methods do not attempt, however, to quantify
diachronism nor to understand its possible causes besides issues linked to preservation, inadequate sampling
or incorrect dating. The integration of numerous stratigraphic techniques (chemostratigraphy,
magnetostratigraphy, high-precision radiometric dating and cyclostratigraphy) allows today a much better
characterization of potential diachronism in some of the key stratigraphic macro-and microfossil marine
groups. Here, we will examine case-studies from the Mesozoic where well-demonstrated examples of
diachronism call for a modification of some of the “standard” marine microfossil zones but also trace
paleoenvironmental change. Rather than to be considered as a pitfall, more emphasis should be put on potential
diachronism as it can improve both stratigraphy and our understanding of ocean currents, climate, continent
configuration and/or sea-level change.