SEDIMENT BUDGETS IN CATCHMENT–ALLUVIAL FAN SYSTEMS OF THE NORTHERN TIAN SHAN (CHINA): IMPLICATIONS FOR MASS-BALANCE ESTIMATES, DENUDATION AND SEDIMENTATION RATES IN OROGENIC SYSTEMS
Résumé
Mass balances are often used to calculate sediment fluxes in sedimentary basins and denudation
rates in adjacent topographies on intermediate to long timescales (from a few tens of
thousand to a million years). In this study, we focus on simple Quaternary catchment–alluvial
fan systems in the northern Tian Shan Range and its foreland basin to discuss some ideas
about sediment production, storage, release, and bypass in relatively short (100 km long) sediment
routing systems. Based on a geometrical reconstruction of the fans, we estimated the
volumes of sediments exported out of the range and deposited in its alluvial piedmont during
the Middle-Late Pleistocene and the Holocene. This work shows that the two areas clearly
present evidence of a temporary but significant storage of sediments during the Pleistocene.
These sediments were then excavated and delivered farther into the foreland basin during the
Holocene. The difference between the volumes of materials released from the range and piedmont
areas and the volume stored in the contemporaneous fans downstream indicates that
the latter did not trap the whole sediment load transported by the rivers. The alluvial fans were
bypassed by 27 to 78% of this load toward the downstream alluvial plain. It implies a major
volumetric partitioning of the deposits between the fans and the alluvial plain, with a sedimentation
rate about twenty times higher in the fans than in the plain. However, this volumetric
partitioning might only occur during periods with a very specific hydrological regime such
as the Holocene deglaciation. Eventually, the peculiar sediment storage and release pattern
within the range and piedmont areas during the Pleistocene and Holocene complicates the
calculation of mean paleodenudation rates using either sediment budgets or in situ produced
cosmogenic nuclides.