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

or ???

See you tomorrow on the field!Meeting at 8:00

be on time!!!