Sea bottom current activity recorded on the southern margin of the Vocontian basin (southeastern France) during the lower Aptian. Evidence for a climatic signal

摘要

Numerous current marks are recorded on an Early Aptian unconformity surface observed over a large area of the Vocontian basin and its southern margin (southeastern France, Subalpine ranges). They occur in the transition zone between that basin and the adjacent Provence platform and correspond to various erosion marks, oriented biogenic remains (sponge spicules, belemnite rostra) and trace fossil directions (Rhizocorallium). Three local current orientations occur : 1) a westward orientation in the neritic zone, probably resulting from the trade winds that prevailed in the Tethyan domain; 2) a northward orientation in the deeper hemipelagic and pelagic zones probably resulting from a deviation of the westward currents by the Coriolis force; 3) a southward orientation in the eastern and western margins of the studied area, possibly induced by upwellings resulting from offshore northward deviated currents. These upwellings were characterized by cold water enriched with silica, phosphorous and organic matter. Some characteristics of the Early Aptian unconformity surface result from these different currents : 1) a specific type of erosion linked to mechanical and/or chemical features, 2) an erosion intensity displayed by the importance of the pre-Aptian stratigraphic gaps, 3) an erosion duration illustrated by the importance of the post-Aptian gaps. The spatial distribution of these features was controlled by the water-depth, depending locally on the occurrence of hemipelagic submarine reliefs of tectonic origin. Increases in current activity in the Early Aptian, linked to an acceleration of the atmospheric circulation, and highlighed by erosion and phosphatisation had previously been recorded for many other Eurasian and North Atlantic sites. This large spatial distribution suggests a global cause such as a temporary increase in the latitudinal thermal gradient and seasonal contrasts. According to the literature, these events may be related to the Early Aptian cooling, described by several authors in both high and low latitudes, which led to a remarkable pause in the middle Cretaceous warming.