Microfacies and depositional environments from the new proposed Upper Cretaceous of Bourzal Formation (Ziban Mounts, Biskra, Eastern Saharan Atlas, Algeria)

Abstract

The newly introduced Upper Cretaceous (Cenomanian–Turonian) Bourzal Formation, outcropped in the Ziban Mounts of the eastern Algerian Saharan Atlas near Biskra, presents a diverse sedimentary sequence approximately 235 m thick. This formation is characterized by alternating layers of carbonate, marl, and gypsum, reflecting dynamic depositional conditions during its formation, The Bourzal Formation is divided into two distinct informal units. The lower unit A, measuring 125 m, consists of thin to thick-bedded limestone interspersed with marl and gypsum beds. The upper unit B, spanning 110 m, is marked by alternating layers of fossiliferous marl, limestone, and dolostone. Detailed field surveys and microfacies analysis led to the identification of eight microfacies types within the formation. These microfacies correspond to three primary depositional environments: tidal flat, lagoon, and shoal. This variety in microfacies and environments indicates significant changes in depositional depth and energy conditions during the formation’s development. The overall depositional setting of the Bourzal Formation is interpreted as a homoclinal ramp, specifically the inner ramp zone. This interpretation is based on the observed facies associations and their vertical relationships. Relatively shallow water depths characterize the inner ramp setting and are typically influenced by wave and tidal processes, the alternation between carbonate-dominated and evaporite-bearing layers suggests fluctuations in sea level or climate conditions during deposition.