Report on 2011 Geological Mapping

project

For the Class of 2005 Mapping Fund

Castellane

Nathan Allen & Emma Woodward

Overview of the mapping area

Castellane in the southern sub-Alpine chains – approximately 100km NW of Nice in

southeast France – is world-renowned for its ammonite fauna (including a stratotype

location) and lies in the ‘Réserve Géologique des Alpes de Haute Provence’.

The mapping area was centred around the villages of Chasteuil and Taloire at the head

of the spectacular Verdon gorge, where the Mesozoic strata record a facies shift from

platform to basin (in both time and space); since deformed during the Pyrenean and

Alpine orogenies.

Project objectives

• Identify and define lithological units of the area

• Interpret the transition between platform and basin in both time and space; and

determine the depositional environments

• Investigate structural relationships and attempt to deduce deformation history

This work was carried out over July and August 2010 (see below).

Total duration of field trip 35 days Duration of fieldwork 28 days Total area mapped 34km2

Transport to and from mapping area On foot Transport around mapping area On foot Hours spent daily in field 06:00 – 16:00 Temperature range 4˚C (early morning) – 38˚C

(midday) Accommodation Camping Gorges Verdon (all

geology within 3 hours walk) Land use Mixture of natural scrub, private

grazing land, and artificial forest.

Geological summary The Mesozoic carbonate sequence of Castellane, southeast France, records a transition

from basinal (“Dauphinois”) pelagic facies in the north to platform (“Provençal”)

neritic facies in the south; although this zone of transition is poorly studied and

understood, despite excellent exposure and plentiful fossils. Studying the lithologies

and field relationships of the units has allowed detailed reconstruction of the

depositional environments, palaeobathymetry, and structural history of the area.

The French subalpine basin, a gulf on the NW margin of the young Tethys Ocean,

began is development in the Late Triassic. Shallow-water facies dominated the

mapping area until a transgression at the End-Tithonian, and a basinal environment

persisted throughout the Cretaceous; at the end of which compression from the

Pyrenean Orogeny uplifted the region.

The Alpine Orogeny compressed the area further, reactivating structures from the

Pyrenean compression and original Tethyan rifting, causing the Mesozoic

sedimentary cover to slip in a classic example of thin-skinned tectonics, forming the

nappes of the present-day 100km Castellane Arc (a Cenozoic fold-and-thrust belt),

which forms a bundle of folds whose apex is located in the Castellane area.

Mesozoic normal faults determine planes of weakness which have been reactivated in

various ways in the Cretaceous and Tertiary tectonic deformations, and this earlier

structure is preserved and its inheritance plays a major role in present-day

configuration.

Looking North, Verdon River View from the North Rim

Final Map: