Using volcaniclastic rocks to provide depositional ages : what can (and cannot) be inferred from U-Pb/Zircon dating ? Examples from south East Asia terrestrial sections
Résumé
Volcaniclastic sediments are commonly used to provide constraints on the depositional age of
sedimentary rocks by the mean, for example, of U-Pb geochronology on zircon. These time
constraints allow for correlations between distant basins and are critical to assess the
contemporaneity between major events, such as mass extinctions. In this case, the spatial and
temporal patterns of extinction and recovery provide essential information about the mechanisms
leading to the extinctions. However, assessing the temporal patterns of extinction and recovery using
U-Pb/zircon geochronology on volcaniclastic rocks requires that volcanic activity is coeval with
sedimentation. With the exception of “primary” volcaniclastic rocks (e.g., pyroclastic flows), rare in
the sedimentary record, all the other “secondary” volcaniclastic rocks are made of volcaniclasts that
have been reworked to various extents. In ancient deposits, the distinction between primary and
secondary (reworked) volcaniclasts is notoriously difficult, hindering a direct access to the conclusion
that volcanism and sedimentation were contemporaneous. The use of secondary volcaniclastic rocks
for dating purposes thus often results in obtaining maximum depositional ages that may be
significantly older than sedimentation, but the assessment of the contemporaneity between
volcanism and sedimentation is often overlooked, if not simply ignored.
To assess the contemporaneity between volcanism and sedimentation in ancient
volcaniclastic series, we propose a simple methodology that requires a large number of samples
collected along continuous sedimentary piles. This method relies on the comparison of (i) the relative
stratigraphic positions of the investigated samples, (ii) their maximum depositional age and (iii) the
analytical precision of the measured ages. If the maximum depositional ages get systematically
younger together with the position of the samples in the stratigraphic column, the correlation
between absolute ages and stratigraphy is very likely to demonstrate that some volcaniclasts were
produced contemporaneously with sedimentation. The differences in maximum depositional ages
from two successive samples in a pile, and, in some cases, the uncertainties obtained from the U-Pb
ages can then be used as crude estimates of the ‘lag time’, defined as the difference between the age
of the volcaniclast production and the depositional age of its host strata.
Applied to various Permian and Triassic terrestrial sections located in South East Asia, this
methodology allows to valid, reject or evidence the impossibility to demonstrate the
contemporaneity between volcanism and sedimentation. Such assessments contribute to relativize
the strength on which earlier interpretations about synchronism of major events, such as the
Permian-Triassic mass extinction, are based