GEOLOGICAL HISTORY OF THE MORVAN

Middle to Late Devonian

Succession of the sedimentary and volcanic deposits in the Bourbon-Lancy region

(Saône & Loire)


See geologic maps 1/80 000 Charolles and Autun; 1/50 000 Dompierre-sur-Besbre and Bourbon-Lancy; explanatory notes Dompierre-sur-Besbre and Bourbon-Lancy.


The oldest deposits - volcanic rocks and limestones - of the Morvan belt crop out in this Bourbon-Lancy region. The basement on which they are supposed overly remains unknown. It is inferred that it is the medio-variscan gneissic basement which extends in several belts in the northeastern part of the Massif Central.

The Chizeuil volcanic massif is composed of various lavas flows (spilites and keratophyres or basalt to rhyolite) to which is associated a massive sulfide deposit (mainly pyrite) exploited from 1880 to 1960.

The fossiliferous faunas and floras found in limestones, tuffs and siltites give relatively accurate datations for the sediments deposited from Frasnian to Early Visean, with in addition a radiometric age of 378 Ma obtained for soda-rhyolites of Chizeuil [REF]. These datas allow a reconstitution of the succession of the sedimentary and volcanic events in this région [REF] and by analogy to the entire Morvan belt [REF].


Middle Devonian: Emsian to Givetian

(1) The gneissic basement is eroded (gg).

(2) First deposits of Emsian sedimentary rocks (gg) are also eroded.

(3) Volcanic activity begin ca 380 Ma : extrusion of sub-marine acidic lavas (gg) or finely porphyritic pink-grayish soda-rhyolites constitute a single lava flow in the Chizeuil massif to the South (2 km by 12 km) and two lavas flows separated by a thin tuffitic bed in the Cressy-sur-Somme massif to the North (1 km by 5 km) .

devonian

Late Devonian: Frasnian

In the Chizeuil massif during temporary pause of volcanic outpours, a sub-marine hydrothermalism deposits sulfide ores at hot springs outlets.

:Frasnian

(4) Massive sulfide bodies (gg) and veins of mainly pyrite emplace in soda-rhyolites host rocks (3) which are transformed by the hydrothermal fluids in (5) sericitic "quartzites" (gg) and aluminous "quartzites" , [REF], [REF], [REF], [REF].

(6) Mineralized barite bed (gg) overlying the sulfides deposits and laterally siltites (gg) .

(7) Apart from the volcanic massif, limestones deposits (gg) derive from degradation of coral reefs [REF] (8) (gg) and crinoïds areas. Fossils and microfossils indicate Frasnian age [REF], [REF], [REF], to which some fossils of older age (Emsian) are associated [REF]. A silteous sedimentation follows limestone and dolomite deposits. These first famennian sediments contain few limestone intercalations .

Late Devonian: Famennian

(9) Revival of the eruptive activity: sub-marine flows of basalt (gg) and andesite , along large volumes of pyroclastic deposits: volcanic breccia and mainly basaltic tuffs containing locally lava fragments dragged from the underlying soda-rhyolites .

Famennian

(10) Apart from the volcanic massif, basic pyroclastic material is interbedded with siltites (gg) , , . This sedimentation results from reworked deposits by turbidity currents very probably induced by the instability of the volcanic environment. In this shallow-sea volcanic eruptions could be aerial [REF]. Along the edge of granite intrusions (i.e. Luzy batholith) and locally even inside the volcano-sedimentary belt, siltites are contact metamorphosed .

(11) Interbedded limestone lenses (gg), of late Famennian age [REF], are locally degraded, the limestone clasts being enclosed in coarse grained tuffs of basaltic composition [REF]. Dolerite sills emplaced between andesite and basalt flows.

(12) This first volcanic event ends with dacites extrusions: lava domes and associated breccias (gg)

(13) A fairly large tuff-lava sheet extends between Chizeuil and Saint-Aubin-sur-Loire areas (gg).

Famennian

Late Devonian: Late Famennian

(14) In Givallois area, a basalt-andesite sub-marine flow (gg) crops out below the first sediments containing Lower Carboniferous fossils.

(16) At the same time and under far repercussion of Bretonian orogenic phase, erosion of neighboring emerged lands (gg) produces coarse clatics deposits (15), typical of Early Carboniferous (gg).

Famennian


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