Evidencing syn-sedimentary volcanism in volcaniclastic series using coupled sedimentological and geochronological (U-Pb/zircon) analyses
Résumé
Volcaniclastic sediments, often under-studied, constitute an important part of the global sedimentary record, both
in marine and continental environments. These sediments are of particular importance in order to constrain the age
of sedimentation, particularly in series where interbedded lava flow are absent. Volcaniclastics sediments are also
used in order to constrain the duration of the volcanic activity and to link volcanism with a specific geodynamic
context. To demonstrate that volcanism and sedimentation were contemporaneous in a given basin, it is crucial to
determine to which extent volcaniclasts present in the volcaniclastic sediments have been reworked. However, this
determination is notoriously difficult.
As a case study, we characterized the Triassic volcaniclastic series from the Luang Prabang Basin, Laos,
using coupled sedimentological and geochronological analyses. Sedimentological and petrographical analyses
show a wild range of depositional environments (alluvial fan, braided river and alluvial plain) and evidence for
reworking of the volcaniclastics in each of the corresponding deposits. U-Pb geochronology conducted on zircon
grains extracted from the volcaniclastic samples of known stratigraphic position indicates that the maximum
depositional ages get younger together with the sedimentary succession. This good correlation between absolute
ages and stratigraphy demonstrates that, despite evidences of reworking, the volcaniclasts were produced, at least
to some extent, contemporaneously with sedimentation. Then, in this specific example, the uncertainties obtained
from the U-Pb ages can be used to indicate the ‘reworking time scale’, defined as the difference between the
age of volcaniclast production and the depositional age of its host strata. Short reworking time scales, of ca. 1
Ma, one order of magnitude smaller than the total duration of the sedimentary record reveal that volcanism and
sedimentation were contemporaneous. The use of coupled sedimentological and geochronological analyses may
thus allow to determine the duration of volcanic activity even in the absence of volcanic deposits. Constraining the
reworking time scales is useful to describe volcaniclastics deposits in which the volcaniclasts were reworked