Erosion of Tortonian phosphatic intervals in upwelling zones: The role of internal waves

Abstract

During the Miocene, the upwelling is assumed to be important in the formation of many Mediterranean phosphate deposits in carbonate platform successions.There are different types of upwelling mechanisms such as equatorial upwellings, ice-edge upwellings and coastal upwellings. The carbonate platforms are mainly affected by wind-driven coastal upwelling systems. During upwelling season, the seawater in coastal areas is strongly density stratified and the permanent pycnocline rises inshore forming an inclined frontal layer. The liaison between upwellings and internal waves is widely documented in the literature. Internal waves result from perturbations of the hydrostatic equilibrium, where balance is sustained between the force of gravity and buoyant restoring force. Many perturbations of the pycnocline can propagate as an internal wave. In a coastal upwelling, a key role for wave propagation is played by permanent pycnocline. The run-up swash currents develop during the break of internal waves and produce erosional processes. In particular, the backwash return flow may induce the erosion of seafloor sediments and transport these sediments downslope as bedload.Erosion of phosphate-rich intervals has been extensively documented. The deposition of gravity-flow sediments is followed by intervals of non-deposition, incipient lithification, phosphogenesis and erosion. In this work, we evaluate the role of internal waves in the formation of the resedimented phosphate-rich intervals characterising the basal portion of the hemipelagic Orbulina marls. These marls are deposited on the low-angle Latium-Abruzzi carbonate ramp and represent deposition in a foreland basin during the initial stages of evolution of the Central Apennine Belt during the early Tortonian age.