GEOLOGICAL HISTORY OF THE MORVAN

STEPHANO-PERMIAN

Autun (Saône-et-Loire) and Blismes-Montreuillon (Nièvre) basins.


See geologic maps 1/80 000 Avallon, Château-Chinon and Autun; 1/50 000 Avallon, Quarré-les-Tombes, Sémur-en-Auxois, Corbigny, Château-Chinon, Autun, Epinac-lesMines and Lucenay-l'Evêque; explanatory notes Avallon, Quarré-les-Tombes, Sémur-en-Auxois, Corbigny, Château-Chinon, Autun and Lucenay-l'Evêque.


Westphalian/Stephanian A

During this period reliefs built by the Sudetic orogenic phase are eroded and sedimentary deposits are lacking. Late hercynian fracturation, which acts during Late Carboniferous and Early Permian in weakness zones of gneissic basement, allows two micas microgranites dikes emplacement along submeridian direction Wivre, Folin, Microgranite 2 micas and granophyres Dron.

The Chavence leucogranite emplaced at the same period. With its alterations and associated mineralisation, it is similar to the granitic intrusions of Montebras and Echassière (Allier) [REF], [REF].

Stéphanian B

Epinac-les-Mines Basin (gg) [REF], [REF], [REF]

During Middle Stephanian (B) a basin opens at intersection of two strike slip faults in relation to the N-S compression. The Autun basin E-W striking, opens on an old structure of the gneissic basement. In this strongly subsident basin, a shallow fresh water lake extends and is progessively filled in with coarse alluvial sediments (sandstones and conglomerates) carried out by rivers running among a hilly landscape. Organic rests coming from swamp vegetation are interstratified in coal layers with clastic sediments. Few basalts outflow on the shore lake (gg). Compression direction rotates progressively from N-S to NW-SE and then E-W. The Stephanian C coal stratas are folded under this stress, as are folded the coeval beds of the Sincey-les-Rouvray basin [REF].

Stephanian

Blismes Basin (gg) [REF]

This basin is interpreted as a caldeira filled with volcanic rocks and sediments from Middle Stephanian to Saxonian. Stephanian deposits are essentially volcanic: rhyolites, ignimbrites and associated tuffs and breccias, as also numerous intrusive rocks: dikes, sills and stocks of microgranite and granophyre. Volcanic rocks largely prevail in the southern and deeper part (400 m infered) of the basin, while in the shallow (250 m inferred depth) northern part, the same volcanic rocks are interbedded with sediments: reworked tuffs, siltites and coal beds. The identified flora remnants found in the upper half of sediments filling the northern part yield a Stephanian B and C age. Like the Epinac basin, the Blismes basin is done under compression strengths whose directions rotate from NNE-SSW to NW-SE between Stephanian A and Stephanian C.

Sincey-lès-Rouvray Basin

In the northern part of Morvan, the Sincey-lès-Rouvray stephanian basin is a narrow syncline with a complexe structure, pinched in the gneissic domain. Stretched over a length of 25 km and no more 700 m wide, it contains conglomerates , arkoses and coal-seams a time exploited. Rhyolitic volcanism is intercalated in the sedimentary series .

Autunian

Autun Basin (gg) [REF], [REF], [REF], [REF]

During Autunian times (295-275 Ma) a N-S trending extension rejuvanetes the old Autun strike-slip fault that acts as a normal fault and induces basin enlargement already opened during Stephanian B. Rivers draining neighboring lands, still feed the fresh water lake, which is progressively filled in along its depression in relation to the active Autun fault. The subsidence is more intense near the Autun fault where rivers deposit coarse clastic sediments (sandstones and conglomerates). These shore sediments containing few coal beds, grade toward basin axis (offsetted toward the southern Autun fault) to an argillaceous and calcareous sedimentation rich in organic matter: the bitumineous schists rich in fish scales.

Autunien

Falls of wind blown volcanic ashes form gores, light colored intercalations among dark sediments. The lake filling up ends during Late Autunian with limy clays containing a thin but prominent bed of boghead, a coal derived from algae. End of Autunian times, a total thickness of 1400 m of sediments is reached in the deepest part of the basin. Contrary to the other French Massif central coal basins, here Saxonian is lacking [REF].

Blismes Basin (gg) [REF]

Stephanian sediments suceeds without break to Autunian deposits that are composed of siltites, sandstones, conglomerates and coal beds in the northern part of the basin. Rhyolites and rhyolitic tuffs testify of a still active volcanism.

Saxonian

Blismes-Montreuillon Basin (gg) [REF]

Between 275 Ma and 260 Ma the Montreuillon ignimbritic sheet largely extends in the western part of basin overlapping the Lormes granitic domain.

The microgranite and granophyre dikes even scarce in Autunian series are lacking in this volcanic event.

The lamprophyres dikes intrusive in the Montreuillon ignimbrites [REF] are the ultimate magmatic activity of the Morvan Paleozoïc .


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