Aptian sequences, sea-level, siliciclastic flux and climate change in the Neo-Tethys domain (Tunisia, N. Spain and S. France)

A carbonate platform to basin transect of the Aptian succession has been constructed using outcrop sections in central Tunisia. Constrained by new biostratigraphic information and a new C-isotope curve a high-resolution sequence stratigraphic model is proposed. It is compared to three reference sections along the Northern Tethyan margin in order to evaluate the timing of the Aptian sequences and sedimentation patterns at the Neo-Tethys scale.

The Tunisian sections, which vary in thickness from 130 m in the inner platform to 850 m in the basin margin, display a rich variation in carbonate, siliciclastic and evaporite sedimentary facies. In the inner platform domain three significant stratigraphic hiatuses have been detected, whereas the basin margin section is complete, and comprises an 80 m thick siliciclastic/oolitic lowstand wedge deposited during the early late Aptian sealevel fall. This fall also marks the abrupt influx of continent-derived siliciclastics. Comparison of the Tunisian section to an equally extended basin margin section in northern Spain (Aralar platform), and basinal sections in southern France (Vocontian Basin) and southeastern Spain (Cau section), show a strikingly similar sequence stratigraphic organization at two orders. At the large scale, two supersequences can be correlated, of which the first one spans the early Aptian to the early part of the late Aptian, and is dominated by shales (including the OAE 1a), carbonates and evaporites. The second-order supersequence covers the remaining part of the late Aptian and earliest Albian and is characterized by a lowstand wedge caused by a eustatic sea-level drop of approx. 60-70 m, accompanied by a strong, basinwide, influx of siliciclastics around the Neo-Tethys Ocean. At the medium scale, six third-order sequences can be correlated across the Neo-Tethys domain.

The 2nd order lowstand wedge and siliciclastic influx are time equivalent with the coldest temperatures reported for the late Aptian, and coincide with the establishment of the Equatorial Humid Belt, all supporting a dramatic late Aptian climate change. The onfollowing overall transgression is co-eval with a gradual rise in temperatures, punctuated by short hyperthermal events. These events are locally expressed by the accumulation of organic matter (Jacob, Kilian, Paquier levels) and occur during the transgressions of the 3rd order sequences, interpreted here as the expression of a pulsed melting of the ice caps that caused the initital 2nd order sea level fall.