Facies analysis of gravity-flow deposits from the Upper Cretaceous of the Alborz Mountains (northern Iran)

Abstract

Field studies of sedimentary successions complemented by petrographic analyses allow distinctions among sub-environments, even in the case of subtle differences in geometry, structure, and composition. In three apparently uniform sections of the coarse-grained siliciclastic K2b unit of Upper Cretaceous age exposed in the southern central Alborz Mountains (northern Iran), careful observations of texture, fabric, composition, and bed thickness have allowed the identification of eight petrofacies, twelve low-rank facies associations, and six high-rank facies associations. The coarse grain size of the K2b unit, the radial paleocurrent pattern indicated by grain orientation, along with the presence of hummocky and swaley cross-stratification, glaucony, Skolithos ichnofacies, and Hedbergellidae planktonic foraminifera suggest a fan-delta prograding onto a shallow shelf. Conglomerate layers, erosion surfaces, plane beds, and oriented grains point to diverse depositional mechanisms, including cohesive debris flows, hyperconcentrated flows, and turbidity currents. Sediment-gravity flows in the K2b unit, generated along the southern margin of the Proto-south Caspian basin, testify to both relative sea-level fall and tectonic uplift in mountainous source areas.