Sandstones and red-beds in the Lower Cretaceous Sidi Aich Formation, Chotts basin (Southern Tunisia): Facies, architecture and depositional environments

The Lower Cretaceous deposits of North Africa represent a terrestrial to shallow marine sandstone-dominated succession often referred to as the “Continental Intercalaire” (CI) and Nubian Sandstone (NS) informal groups. The upper Barremian Sidi Aich Formation of the CI is distinguished throughout the Chotts basin of southern Tunisia by sandstone-dominated sequences within conglomerate, calcareous siltstone, and paleosol red-bed occurrences. This paper aims to highlight: ( i ) the facies distribution, internal architecture and depositional environment of sandbodies, and ( ii ) the origin, spatial distribution and architecture of red beds that underlined and topped sandbodies. Strata at the Zimlet El Beida anticlinal structure (ZBAS), Chotts basin, offer a good opportunity for detailed field observations and bed-by-bed logging of three representative lithostratigraphical sections. Detailed sedimentological investigation allows the recognition of several lithofacies grouped into three facies associations indicating tidal flat, tidal channel and tidal bars depositional settings characterizing a transgressive tide-dominated estuarine system that developed during an episode of relative sea‐level fluctuations at the Lower Cretaceous. The high-resolution petrographical, mineralogical and geochemical analyses of red beds allow differentiation of three distinguished lithofacies sets including (i) intraformational conglomerates, (ii) calcareous siltstones and (iii) paleosol. Major red beds comprise detrital grains inherited from the host or parent sediment and authigenic minerals (dolomite, calcite, and hematite) precipitated during the diagenetic processes. Grain components have been partially or completely cemented and/ or replaced by terrestrial phreatic dolomite. The negative signatures of δ 13 C and δ 18 O of the dolomite analyses indicate that dolomitization processes have been influenced by fluctuations of the groundwater table or climate change from semi-arid to semi-humid conditions. This study, therefore, may provide useful data for the better understanding of the internal architecture of the red beds associated to sandbodies which represented the main and significant potential reservoir within the Lower Cretaceous CI sandstone-dominated groundwater aquifers of the Chotts basin.