SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY ANALYSIS OF FLUVIAL AND AEOLIAN SYSTEMS WITHIN TERRESTRIAL AND NEARSHORE ENVIRONMENTS: NOVEL INSIGHT AND CLIMATE IMPLICATION
Résumé
Stratigraphic correlations are often considered as impossible in thick accumulations of fluvial deposits
where arid conditions are often underestimated. In fact, most of the time, aeolian deposits in arid environments
dominated by ephemeral alluvial fan or braided river are poorly preserved and particular attention should be
paid to any sedimentological marker of aridity, such as wind-worn stones (ventifacts), sand-drift surfaces and
aeolian sheets. Only detailed study of alluvial plain facies and ephemeral streams provide indications of
climatic conditions within the sedimentary basin, whereas fluvial system only reflects climatic conditions of
the upstream erosional catchment. Moreover, in such context the deposition of conglomerate of alluvial fan
system could correspond to a peak of aridity without an increase in tectonic activity.
Conversely, the sedimentary record of aeolian deposits, often considered as preserved in arid desert
basin, can occur under semi-arid and even humid climates, and from near the Equator to the Arctic Circle.
Ventifacts are climate-sensitive sedimentary features that provide evidence for long periods without any
vegetation in terrestrial environment under hot or cold climate. Therefore, they can allow recognizing desert
conditions, even where no aeolian dune deposits are preserved. Moreover, the well-preserved widespread
paleoregs seem to mark major stratigraphic discontinuity above a geomorphic surface remained a long time
without vegetation as a result of a very dry climate, hot or cold.
Detailed sedimentological study and the recognition of bounding surfaces of regional extend (sanddrift
surface, palaeosols, flooding surface...) allow for the identification of stratigraphic cycles, permit correlation
within time and space and precise the climate conditions. Moreover, these detailed sedimentological and
stratigraphic analyses highlight the influence of climate or tectonic change on sediment facies and depositional
environments.
Applied to various Permian and Triassic continental sections in Europe and USA and Jurassic–
Cretaceous sections in China, this methodology allows to define fluvial and aeolian interaction processes
through time and space, to discuss the allogenic vs autogenic controls and the preserved sediment record within
terrestrial and nearshore environments. The recognition of sand-drift surface and maximum flooding episode
at the scale of genetic unit, provide a valuable tool for stratigraphic correlation within continental environments
devoid of any biostratigraphic markers. Moreover, such results allow to discuss the climate condition of the
basin.