GEOLOGICAL HISTORY OF THE MORVAN

Tournaisian to Middle Visean

Succession of the sedimentary and volcanic deposits in the Bourbon-Lancy (Saône-et-Loire) - Mont Beuvray (Nièvre) - Lucenay-l'Evêque (Saône-et-Loire) regions.


See geologic maps 1/80 000 Charolles and Autun; 1/50 000 Dompierre-sur-Besbre, Bourbon-Lancy, Fours, Autun and Lucenay-l'Evêque; explanatory notes Dompierre-sur-Besbre, Bourbon-Lancy, Fours, Autun and Lucenay-l'Evêque.


Early Carboniferous: Early Tournaisian

(15) The first dated Tournaisian sediments (gg) [REF], [REF] even some of Strunian age [REF], [REF] (uppermost Famennian/lowermost Carboniferous) are characterized by alternating beds of sandstones, siltites and conglomerates with pebbles of quartz, quartzites and black lydiennes .

tournaisian

(16) This sedimentation change outlines an erosion revival of neighboring continental lands (gg), probably a repercussion of the orogenic movements (Bretonian phase) which act during this time in the Brévenne region, north of Monts du Lyonnais. It is a deltaic regime, close to the rivers mouths, which carry rests of terrestrial vegetation [REF].

Early Carboniferous: Early Tournaisian

(17) Deltaic sedimentation is composed of alternating sandstones, conglomerates and siltites (gg).

(18) A sporadic volcanic activity continues with a time and regional distribution. According to the various localities in the Morvan belt, few basalt and andesite flows (gg) are associated with volcanic breccias and tuffs .

Tournaisian

(19) Limestone lenses (gg) intercalated in clastic sequences, are characteristically azoic (lacking of fossiliferous rests) and recrystallised in more or less white marble [REF]: for example the marble lenses of Saint-Léon, Arbre Rond, Champ-Robert , le Puits and Argentolle . Thinnely-bedded sediments, pyriteous and tuffaceous siltstones are encountered close to or near the marbles, sometimes associated with coarse-grained tuffs . Pyrite is easily altered, the rock is reddened by Fe-oxides. The sulfuric acid formed during the oxidation reaction leaches the rocks, leaving bleached outcrops.

Locally, bearing limestone conglomerates indicate emersion and erosion of Devonian series [REF].

Early Carboniferous: Middle Tournaisian

(20) From south Mont Beuvray to Lucenay-l'Evêque region, large volcanic deposits (gg) are intercalated in clastic sediments, mainly composed of dacite and rhyolite outpours and extrusions with volcanic breccias .

Tournaisian

(21) Frequent ignimbritic characters of lavas indicate a sub-aerial emplacement .

(22) Progressive emersion of volcanoes allows their erosion and the advent of pebbles and cobbles of volcanic rocks in conglomeratic beds .

Early Carboniferous: Early to Middle Visean

Contrary to the Forez region where Lower to Middle Visean limestones are spread out [REF], the Morvan region offers only two small outcrops at La Chaise and near Cussy-en-Morvan , relics of a possible larger extension (gg).

Tournaisian

The unstable volcanic surroundings are probably responsible of limestone beds degradation. In these two localities, calcareous rocks contain fossils rests [REF], with a special mention for the Cussy old quarry which discloses an abundant microfauna of algas, Bryozoaires and Foraminifers of Early to Middle Visean bottom age [REF]. Conditions of sedimentation shown by the various limestone facies are of shallow sea sediments disturbed by waves and of areas of quiet waters where clastic debris arising from nearby volcanoes mix with limestone deposits.


Late Devonian HOMEHOME Sub-alkaline granites