Supercritical-flow Deposits and Their Distribution in a Submarine Channel System, Middle Eocene, Ainsa Basin, Spanish Pyrenees

Abstract

Abstract Studies of supercritical-flow deposits (SFDs) and their spatial distribution in ancient deep-water systems should provide an additional tool to improve the understanding of the flow dynamics during deposition and the architecture of sandbodies. Outcrop recognition of SFDs in ancient deep-marine environments remains poorly documented, although their study dates back to the 1970s. This paper focusses on the criteria for recognizing SFDs and their distribution in three selected depositional environments from an ancient mid-lower slope to a proximal-basin floor setting in the middle Eocene Ainsa Basin, Spanish Pyrenees. From field observations, six facies associations interpreted as related to supercritical flow are defined. These facies associations are grouped in two categories. The first group includes facies associations related to erosional coarse-grained supercritical-flow bedforms related to meter and centimeter-scale scours and backfilling structures interpreted as large-scale cyclic steps or small-scale cyclic steps, respectively. Erosional coarse-grained supercritical bedforms are observed mainly in relatively high-gradient slopes and relatively confined settings. The second group of facies associations are related to depositional fine-grained supercritical-flow bedforms associated with upflow-dipping sandstone lenses, upflow-stacked wavy bedforms, upflow-stacked sigmoidal bedforms, and plane beds, interpreted as unstable and stable antidunes and upper-flow-regime plane beds. Depositional fine-grained supercritical-flow bedforms are observed mainly in relatively unconfined settings such as lower-slope, break-of-slope and proximal basin-floor environments. Two main SFD trends were observed in the Ainsa Basin in: (i) an axial-lateral direction, showing a decrease in SFDs from channel axis to channel margin, and (ii) a longitudinal proximal–distal direction, showing an increase in SFDs from the Gerbe System (mid-slope environment), to the Banastón System (proximal basin-floor environment), to the Ainsa System (lower-slope environment). From this study, two parameters are recognized as likely playing an important role on whether a flow is under supercritical or subcritical conditions: (i) confinement of the sandbodies, and (ii) slope gradient.