Evolution of the Var valley landscape
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Evolution of the Var valley landscape Philippe AUDRA
description
Evolution of the Var valley landscape. Philippe AUDRA. GLOMAR CHALLENGER 1970 Deep Sea Drilling Project. Evaporites ≈ 5 Ma. N. Spain. Morocco. 3 (5.3 Ma). 2 (T < 0.5 Ma). 1 (5.9 Ma). Deep -basin, shallow -water model [ Hsü , Cita & Ryan, 1973]. 5.3 Ma: The Flood!. - PowerPoint PPT Presentation
Transcript of Evolution of the Var valley landscape
Evolution of the Var valley landscape
Philippe AUDRA
GLOMAR CHALLENGER 1970
Deep Sea Drilling Project
=> « Messinian » evaporites (6 Ma)
Spain
Morocco
N
Evaporites ≈ 5 Ma
5.3 Ma: The Flood!
Deep canyons onshore
EbroRhône Danube
Nile?
Pô
Nile valley (Egypt) / Aswan dam900 km from shore / 200 m bsl.
A virtual canyon!
A virtual canyon!
Canyon’s geometry
Canyon’s fill
100 km long Canyon
Var PaillonRoyaLoup
-1500 m / 15 km(10 %!)
Var mouth=> Submarine deep canyons
Depth 1000 m
History at La Manda site
Before Messinian (6 Ma)=> Wide and low entrenched valley
History at La Manda site
Messinian Crisis (5,96 – 5,32 Ma)=> Deep Canyon entrenchment
History at La Manda site
Messinian Crisis (5,96 – 5,32 Ma)=> Deep Canyon entrenchment + tectonic uplift
History at La Manda site
Messinian Crisis (5,96 – 5,32 Ma)=> collapse, breccia veneer along canyon slope
History at La Manda site
Pliocene High Sea-level Stand (5,32 – 2 Ma)=> Canyon flooding by seawater
History at La Manda site
Pliocene High Sea-level Stand (5,32 – 2 Ma)=> Raise WT => spongy karst (mixing zone)
History at La Manda site
Pliocene High Sea-level Stand (5,32 – 2 Ma)=> 1/ Filling with marine clays
History at La Manda site
Pliocene High Sea-level Stand (5,32 – 2 Ma)=> 2/ Filling with marine delta conglomerates
History at La Manda site
Pliocene High Sea-level Stand (5,32 – 2 Ma)=> 3/ Filling with fluvial conglomerates
History at La Manda site
Quaternary (2 Ma -> today)=> Entrenchment with levels of fluvial terraces
History at La Manda site
Today…
Pliocene sea level = 350 m alt.
Uplift 300 m
Uplift 700 m
West East
Terraces in alluvium
History at La Manda site
Today…
West East
Lateralrecharge
Vertical transferRiver / alluvium
Pic-Nic Carros Village!
Sea level 0 mBridge +60 m
Canyon bottom -300 / -700 m
Flood plain:« Casier »(compartments)
Active channel:« natural » braided channel
Narrowing of active channel
Sediment transport
Stream power = f(v)
Þ balance between erosion / sediment transport:
Þ if v ↓ => deposition (riverbed ↑ = aggradation)
Þ if v ↑ => erosion (riverbed ↓ = incision)
Þ if v steady => transport (equilibrium)
For a given v:
Þ if sediment load ↑ => deposition => riverbed ↑
Þ if sediment load ↓ => erosion => riverbed ↓
Military map [J. BOURCET DE LA SAIGNE, Ingénieur ordinaire du Roy 1764-1769]
Sediment transport
The sediment transport troubleManda Bridge
Weirs construction
Þ ponding (v ↓)
Þ sediments blocked upstream
Þ riverbed ↑
Þ Braided channel
2 m! Upstream segment
The sediment transport trouble
Manda Bridge
Þ coarse sediments
blocked upstream
Þ fine load crosses
Þ eventually deposits
Þ vegetation dev.
Þ Rectilinear channelMiddle segment
The sediment transport troubleÞ No load anymore!
Þ Erosion restart
Þ Riverbed incision
Þ Meandering channel
Lower segment6-8 m!
Var online…http://www.vigicrues.gouv.fr/niv_spc.php?idspc=22
See you tomorrow on the field!Meeting at 8:00
be on time!!!
