Hardground development and drowning of a Miocene carbonate ramp (Latium-Abruzzi): from tectonic to paleoclimate

Abstract

Three Upper Miocene hardgrounds have been analysed in this study, outcropping in the Latium-Abruzzi Apennines (Italy). The central Apennine hardgrounds all lie on top of the Latium-Abruzzi carbonate ramp succession and in each case are overlain by hemipelagic Orbulinamarls; these marls are linked to plate flexure-related to drowning and coeval input of terrigenous sediments. The hardground age ranges from Tortonian to Early Messinian. Phosphate precipitation in the investigated hardgrounds was confined to a thin layer (up to 15 cm) close to the sediment water interface. Here oxic to suboxic conditions prevailed, resulting in early-diagenetic iron cycling and subsequent phosphogenesis in oxygenated bottom-waters. Glaucony only occurs in the planktonic-rich marls that overlie and infill the phosphatized hardground level in the Latium-Abruzzi succession. An upwelling flux triggered phosphogenesis, promoting the early lithification of the sea floor on the platforms. After upwelling event neritic carbonate production could not be re-established on the Latium-Abruzzi platform because of the persisting eutrophic conditions and the high rates of tectonic subsidence and terrigenous input linked to Appennine orogenesis. The Latium-Abruzzi phosphorites are coeval with the Tortonian phosphogenic phase reported in the Mediterranean. Despite being a global event, regional and local factors played a major role in the hardground deposition at each site.