Aptian Paleoclimates and Identification of an OAE1a Equivalent in Shallow Marine Environments of the Southern Tethyan Margin: The Bir Oum Ali Section, Northern Chott Chain, Southern Tunisia
Abstract:Alternations between siliciclastic, carbonate and evaporitic sedimentary systems, as recorded in the Aptian mixed succession of southern Tunisia, reflect profound paleoceanographic, paleoclimatic and tectonic changes in this area of the southern Tethyan margin.
Whereas the early Aptian is interpreted as having been dominated by slightly humid conditions, the onset of a gypsiferous sedimentation near the early to late Aptian boundary is associated with the appearance of palygorskite and sepiolite, which supports the installation of arid conditions in the study area. The evaporitic sedimentation may have also been promoted by the peculiar tectonic setting of the Bir Oum Ali area during the Aptian, where local subsidence may have been tectonically-enhanced in relation to the opening of northern and central Atlantic.
Chemostratigraphic (δ13C) correlation of the Bir Oum Ali succession with other peri-Tethyan regions complements biostratigraphic findings, and indicates that a potential expression of the Oceanic Anoxic Event (OAE) 1a may be preserved in this area of Tunisia. Although the characteristic negative spike in δ13C values located at the base of this event is not recognized in the present study, a subsequent, large positive excursion is of similar amplitude and absolute values to those reported from other peri-tethyan regions. This comparison supports the identification of isotopic segments C4 to C7 of the OAE1a, during which the dominance of macroalgae over other benthic organisms is observed at Bir Oum Ali. The absence of the negative spike may be linked to either non preservation or non deposition: the OAE1a occurred in a global transgressive context, and since the study area was located in the innermost part of the southern Tethyan margin during most of the Aptian, stratigraphic hiatuses may have been longer than in other regions of the Tethys.