Topographic controls on production and deposition of tidal cool-water carbonates, Uzès Basin, SE France.
Résumé
We document an example of a cool-water carbonate deposit interpreted as a flood-tidal delta formed during sea-level fall in the Uzès basin (French Miocene foreland basin). In the Miocene it was a peripheral basin connected to the open sea by a narrow, 15-km-long seaway. The studied Uzès Formation is a 50-m-thick tidal bioclastic carbonate body, formed at the junction between the Uzès basin and the seaway. It is composed dominantly of rhodalgal packstones that are interpreted to have been produced inside the basin. However, most of the deposit was built out by bidirectional hydraulic dunes, suggesting that accumulation of carbonate grains was not controlled by production but rather by tidal currents. The deposit is thickest and coarsest grained, and exhibits the larger dunes at the inlet of the seaway, where currents are expected to have been fastest. The deposit is thinner, finer-grained, and more bioturbated away from the inlet toward the basin; that is interpreted as a result of a decrease of current velocity due to flow expansion. The main deposit exhibits a general upward increase in thickness and grain size of cross-beds, together with a decrease of faunal diversity (from bivalve-rich to rhodalgal facies), consistent with progressive tidal flow acceleration in the seaway resulting from sea-level fall. Lime mud is pervasive in the matrix of even the highest-energy facies. This suggests that one cannot draw any average correlation between permeability and architecture of the deposit. Also, this could favor the observed early marine diagenesis of the deposit and give it a high preservation potential in the stratigraphic record.