A HISTORY OF TOURAINE THROUGH ITS ROCKS
5 - IN TURONIAN

Simplified stratigraphical scale (the millions of years (MY) correspond to the period's beginning) :

4500 MY 540 MY 250 MY 65 MY
Eras Precambrian Paleozoic (primary era) Mesozoic (secondary era) Cenozoic (tertiary + quaternary eras)
Periods Triassic Jurassic Cretaceous
Epochs lower Cretaceous upper Cretaceous
96 MY 92 MY 88 MY
Stages Cenomanian Turonian Senonian

The marine deposits become calcareous: a clayey chalk...
In continuity with Cenomanian, the sea deposits another type of sediment: a calcareous-clayey mud which is today a soft chalk, rich in clay, greyish but becoming white in the open air. The sedimentation took place in a calm sea, some tens of metres deep. Fossils are mainly represented by inocerams, characteristic bivalves of the secondary era.


Clayey chalk with inocerams - origin: Verneuil-le-Château

Then a micaceous chalk...
After the clayey chalk, the sea deposits a soft, sandy limestone, named also white tuffeau. This rock contains here and there specks of mica (muscovite), hence the term "micaceous chalk". The sedimentation probably took place into a sea a little less deep than for the clayey chalk. Fossils are mainly external or internal moulds of bivalves.


Micaceous chalk (white tuffeau) with external mould of bivalve - origin: unknown (found in Tours into rubble of construction)

And finally the yellow tuffeau...
At the end of the Turonian stage, the sea is less deep, more coastal and the deposits are rich in detrital elements resulting from the erosion of the close continents (Central Massif and Armorican Massif). These deposits are constituted locally by sands but the most frequent facies is represented by the yellow tuffeau which is a sandy limestone rich in fragments of fossils. This limestone appears mostly in the form of beds, about one metre thick, with hard ground in their summit (surface hardened, encrusted by oxides of iron and manganese, revealing a momentary stop of sedimentation, and thus, a temporary withdrawal of the sea). Besides the numerous fragments of fossils (bryozoans, annelids, echinids, bivalves, …), macrofossils can be locally plentiful and are mainly constituted by internal or external moulds of molluscs.

      
Yellow tuffeau with moulds of bivalves ( Cytherea) - Origin: Lussault-sur-Loire      Fragment of internal mould of ammonite in yellow tuffeau- origin: unknown



Characteristic bivalve of the yellow tuffeau: external mould of trigonia - origin: unknown

See photos of an outcrop of yellow tuffeau near Amboise.

The three geological formations of Turonian contain flints, either in the form of beds, or in the form of isolated nodules. The big slabs of flint from upper Turonian are at the origin of the rich prehistory in the region of le Grand-Pressigny.
The whole Turonian is about a hundred of metres thick.

Other fossils are visible on pages dedicated to Turonian.
See also other comments on Turonian.

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