Barite-bearing “septarian concretions” (upper Cretaceous – Paleocene) from the Biban area (north Algeria)

Abstract

The "septarian barite concretions" have garnered significant interest worldwide. This study presents the first detailed investigation of these sedimentary structures in Algeria, with a primary focus on elucidating their genesis through petrography, SEM-EDS, major and trace elements (including REEs), and sulfur isotope analysis. The sampled concretions occur in Upper Cretaceous-Paleocene marl layers from the Biban area, primarily in two main sectors (Koudiat Djebassa and Sidi Ziane), situated within the Southern Tellian Atlas in Northern Algeria (Maghrebides). While these barite concretions exhibit diverse morphologies, our analysis focuses specifically on oblate ellipsoidal concretions. The flattened shape of these concretions may stem from compressed echinoid tests or small, oblate septarian cracked concretions representing the flattened cores. Notably, these concretions feature an outer cortex of radial fibrous barite crystals, measuring centimeters in thickness. Petrographic examination coupled with SEM-EDS analysis suggests that concretion growth occurred from porewaters in unconsolidated, soft sediments at shallow burial depths beneath the sediment-water interface, manifesting in three growth episodes: (i) an initial stage characterized by primarily thin lamellar barite crystals intertwining to form rosette-like structures in concretion cores, (ii) a later stage marked by radial-fibrous barite crystal growth in concretion margins, and (iii) the filling of septarian cracks by barite. The presence of terrigenous sediment, particularly abundant in rosette-like barite crystals, indicates growth within highly porous sediment. Conversely, the radial fibrous outer cortex is related to an effective displacive growth mode. The δ34S values of the barite concretions (ranging from 16 ‰ to 24.4 ‰), consistent with seawater values, which document that seawater was the primary sulfur source. Furthermore, REE geochemistry further supports the seawater origin of diagenetic fluids rather than a hydrothermal one. Concretions are suggested to have formed in organic-rich, fine-grained sediments at shallow depth below the sea floor, in sedimentation halts, and as a result of microbial degradation of organic matter. Barium was supplied to porewaters within the methanogenic zone, precipitating as barite when, migrating upwards, reached the boundary with the overlying sulfidic zone.