Interpretation
The causses of the Minervois are notched by narrow canyons, dug by rivers with very irregular flow. The village of Minerve is thus perched on a calcareous rocky outcrop, at the Cesse and Briant gorges junction, in which erosion indented the calcareous plateau to the paleozoic base (substratum).
Transgression of the Eocene sea ended a 500My stratigraphic hiatus.
The canyons cliffs are of massive limestone, made up of Foraminifera tests accumulation, clearly visible on rock breaks. These organisms (mainly Alveolinacae, but also Miliolacae and Orbitolinacae) are characteristics of shallow marine environment. The limestone of Minerve corresponds to the early Eocene (55 to 50 My) epoch.
Eocene limestone, with a 5°S slope, rests directly on cambrian formations (approximately 550My), with 40-45°N a slope (Hercynian fold). Clearly visible in the Briant gorges, this angular unconformity corresponds to a stratigraphic hiatus of almost 500 My! The Eocene South to North thick decreasing limestone layers testifies to a marine invasion, during which a shallow sea moved over a paleozoic eroded and slightly South facing slope.
A rapidly subsiding sea then deposited a regressive sedimentary sequence.
The Causse landscape is structured by the 3 main levels of the Eocene sedimentary sequence:
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At its base, 50 to 80m thick marine limestone with Alveolinecae forms the cornice which overhangs the Briant gorges. Some sandy insertions divide this massive formation into several distinct layers and form as many narrow bonds in the cornice.
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This limestone is surmounted by lagoon-marine marls rich in oyster fossils, several meters thick. Used in vine cultivation, they constitute the only agricultural component of the landscape.
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The garrigue, which covers the hills on the plateau, grew over pale micritic lacustrine limestone with fresh water gastropod (Limnaeaidae, Planorbidae). Lignitic marl insertions were formerly exploited here.
South of Minerve, the sequence continues with other more recent continental formations (e4 to e6): sandstones, marls and conglomerates…
Thus, approximately 50 My ago, the sea, which invaded the Minervois from the Atlantic, remained a few million years only . After subsiding, continental formations took place:
Looking at global tectonic context, Eocene transgression seems to be a consequence of North Atlantic rifting: the opening of the gulf of Gascony is the origin of the formation of a northern Pyrenean marine gutter whose Eastern edge corresponds to the coastal sea in the Minervois. This rifting then induces the Iberian microplaque rotation to the North West. The consequences are the emerging northern Pyrenean zone, the Pyrenean folding and the sea subsiding in the Minervois. |
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