The Saharan Platform domain of southern Tunisia has recorded very thick Mesozoic successions that developed during the syn- and post-rifting stages in the Tataouine basin. These strata hosted a huge number of faunal and floral fossils that widely contribute to providing the necessary chronostratigraphic data for the establishment of the southern Tunisian stratigraphic chart. However, the lower Cretaceous deposits cropping out along the Dahar escarpment are worldwide known from the numerous biota sites discovered over the past twenty years which yield macro- and microfossil vertebrate remains   as well as very well preserved fossil plants  . The present work provides new biostratigraphic data based on fossil bivalves and aims to: 1) refine the age of the Lower Cretaceous succession of the Tataouine basin and 2) correlate these lower Cretaceous strata with those of the Chotts basin.
2. Study Area
The Lower Cretaceous deposits are widely exposed in the Saharan Platform domain along the Dahar escarpment which runs for about 300 km in a roughly N-S direction along the northeastern margin of the Tataouine basin (Figure 1(a)). At the type locality, the Late Jurassic (Oxfordian-Kimmeridgian)-Early Cretaceous (Aptian) Merbah el Asfer Group, siliciclastic dominated succession, is divided into three formations which are from the base to top: Bir Miteur, Boulouha, and Douiret  (Figure 1(b)). The deposits of Bir Miteur and Douiret formations encompassed several fossiliferous beds which have yielded faunal and floral assemblages characterizing the Kimmeridgian and the Early Aptian respectively. However, the Boulouha Formation is attributed to different ages including the Barremian-Aptian   and the Hauterivian (Figure 1(b)). Each Formation is bounded by erosional surfaces of regional extent called D1-D4  . The Fossil bivalves assemblage recently discovered from the Boulouha Fm. allowed giving new insights on its age.
The Boulouha Formation consists of approximately 80 meters of interbedded sandstones and greenish clays with frequent intercalation of dolomitic beds characterizing a siliciclastic/carbonate tidal flat setting  (Figure 1(b)). The greenish clay horizons have previously yielded fossil plants  and for the first time well preserved bivalve specimens were collected from a claystone bearing horizon situated at about 10 m above the Kimmeridgian carbonate marker bed (Figure 1(b)). This horizon contains the following bivalve species, Anomia laevigata (Sowerby); Modiolus sp. cf. M. dahuashuensis (Yu et Li); Astarte sp.; and ?Mytilus sp.. This bivalve association indicates a Hauterivian-Barremian age. The temporal distribution of the Anomia laevigata (Sowerby) and Modiolus sp. cf. M. dahuashuensis (Yu et Li) is limited to the Aptian formations of England, northeastern China, Japan and Korea  .
Henceforth, the Hauterivian-Barremian Boulouha Formation of the Saharan domain constitutes coeval strata of the Hauterivian-Barremian Bouhedma Formation of the Chotts domain.
The Hauterivian-Barremian age is for the first time attributed to the Boulouha Formation of the Dahar escarpment, Saharan Platform domain, based on well
Figure 1. (a) Location of the study area; (b) Lithostratigraphy and key fossiliferous beds; (c) Stratigraphic ranges updates of the studied area at the prominent Jeffara Escarpment.
preserved bivalve assemblages. This new direct dating of the lowermost strata of the Boulouha Formation (=Bouhedma Formation of the Chotts basin), just few meters above the D2 which in turn is capping the widespread Kimmeridgian carbonate marker bed, corroborates the hypothesis  suggesting the presence of a major gap (20 Ma) between the Jurassic and the Early Cretaceous in the Sahara Platform domain.
This work is supported by MESRS of Tunisia, and is a contribution to UNESCO-IUGS IGCP 679 project.