Sourced in the M. Bianco, the highest mountain of the Alps, the Dora Baltea River cuts across the arc of the Western Alps, drains all major Alpine tectonic units from the Helvetic domain to the South alpine domain, and finally joins the Po River. Petrographic composition of clastic detritus carried by the Dora Baltea River and its tributaries faithfully mirrors the complex geology of the Western Alps. High-resolution modal analysis of Dora Baltea sands allowed us to distinguish seven end-member metamorphoclastic provenances. River sands from the M. Bianco External Massif are dominated by quartz, feldspars, and hornblende. Detritus from basement and cover rocks of the Brianconnais zone includes low-grade metasedimentary lithic fragments, epidote, chloritoid, and garnet. Sands from the Internal Penninic Massifs include quartz and subordinate feldspars, garnet, and epidote. Abundant metabasite and serpentine-schist lithic grains, along with epidote, actinolite, and few pyroxenes, characterize detritus from the calcschists and meta-ophiolites of the oceanic Piemonte zone. Sands from the Austroalpine zone include quartz, abundant metamorphic lithic fragments, garnet, and locally significant sillimanite (Dent Blanche kinzigites). The relative contribution of each end member to the Dora Baltea sediment flux was assessed with forward modelling of end-member mixing. Our estimates, calculated from the entire petrographic and mineralogical data set, indicate that the main sources of bed load in the Dora Baltea mountain basin are the granitoid to gneissic rocks exposed in the M. Bianco Massif and in the Austroalpine zone. In order to make allowance for the contribution of a particular bedrock constituent in a compound source area to the derivative sand, the original concept of sand generation index (SGI) was modified to assess the relative contribution of the seven end-member sources of detritus. The highest total-basin SGI indices are calculated for the M. Bianco Massif (SGI=16.4) and Austroalpine Domain (SGI=1.4). The M. Bianco Massif, which represents only 3% of the whole catchment, has the highest capacity of sand-sized bed-load production, owing to its granitoid lithology and extreme relief This actualistic provenance study demonstrates that sediment composition can be markedly influenced by source rocks that occupy only a minor portion of the drainage area. (C) 2004 Elsevier B.V. All rights reserved.
Vezzoli, G., Garzanti, E., Monguzzi, S. (2004). Erosion in the Western Alps (Dora Baltea basin) - 1. Quantifying sediment provenance. SEDIMENTARY GEOLOGY, 171(1-4), 227-246 [10.1016/j.sedgeo.2004.05.017].
Erosion in the Western Alps (Dora Baltea basin) - 1. Quantifying sediment provenance
VEZZOLI, GIOVANNI;GARZANTI, EDUARDO;
2004
Abstract
Sourced in the M. Bianco, the highest mountain of the Alps, the Dora Baltea River cuts across the arc of the Western Alps, drains all major Alpine tectonic units from the Helvetic domain to the South alpine domain, and finally joins the Po River. Petrographic composition of clastic detritus carried by the Dora Baltea River and its tributaries faithfully mirrors the complex geology of the Western Alps. High-resolution modal analysis of Dora Baltea sands allowed us to distinguish seven end-member metamorphoclastic provenances. River sands from the M. Bianco External Massif are dominated by quartz, feldspars, and hornblende. Detritus from basement and cover rocks of the Brianconnais zone includes low-grade metasedimentary lithic fragments, epidote, chloritoid, and garnet. Sands from the Internal Penninic Massifs include quartz and subordinate feldspars, garnet, and epidote. Abundant metabasite and serpentine-schist lithic grains, along with epidote, actinolite, and few pyroxenes, characterize detritus from the calcschists and meta-ophiolites of the oceanic Piemonte zone. Sands from the Austroalpine zone include quartz, abundant metamorphic lithic fragments, garnet, and locally significant sillimanite (Dent Blanche kinzigites). The relative contribution of each end member to the Dora Baltea sediment flux was assessed with forward modelling of end-member mixing. Our estimates, calculated from the entire petrographic and mineralogical data set, indicate that the main sources of bed load in the Dora Baltea mountain basin are the granitoid to gneissic rocks exposed in the M. Bianco Massif and in the Austroalpine zone. In order to make allowance for the contribution of a particular bedrock constituent in a compound source area to the derivative sand, the original concept of sand generation index (SGI) was modified to assess the relative contribution of the seven end-member sources of detritus. The highest total-basin SGI indices are calculated for the M. Bianco Massif (SGI=16.4) and Austroalpine Domain (SGI=1.4). The M. Bianco Massif, which represents only 3% of the whole catchment, has the highest capacity of sand-sized bed-load production, owing to its granitoid lithology and extreme relief This actualistic provenance study demonstrates that sediment composition can be markedly influenced by source rocks that occupy only a minor portion of the drainage area. (C) 2004 Elsevier B.V. All rights reserved.
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