PALEOBIOLOGICAL AND SEDIMENTOLOGICAL EVIDENCE OF PLEISTOCENE-HOLOCENEHIATUSES AND IRONSTONE FORMATION AT THE PONTIAN ISLANDS SHELFBREAK (ITALY)

Iron-rich hardgrounds were grabbed near the shelfbreak NE Ventotene an d SW Ponza, at water depths of 119 and 141 m, respectively. These 1 to 3 cm-thick ironstone layers formed at about the Pleistocene/Holocene boundary during rapid sea-level rise, which began about 16,000 years a go and within about 8000 years brought the bottom of the shelf from a water depth of less than 30-100 m or more. A rapid vertical transition from circalittoral deposits characterized by a coastal detritic commu nity with calcareous algae to ironstones rich in iron-manganese pisoid s or glaucony and containing planktonic foraminifera, siliceous sponge spicules and echinoderm remains, indicates sediment starvation during rapid drowning of the shelf. The crisis of the sedimentary system and the local ironstone formation at sites of favourable morphology were triggered by major paleogeographic changes during final deglaciation a t the close of the Pleistocene, leading to modified circulation patter ns and chemistry of sea-water, reduced carbonate productivity below th e photic zone, and strong activity of northward-directed Tyrrhenian cu rrents intruding onto the outer shelf. Abundance of organic matter tra nsported from the drowned volcanic islands and diminished oxygenation, due to both increasing water-depth and sluggish vertical circulation, were responsible for local reducing micro-environments, as testified by the formation of pyrite, leaching of carbonate bioclasts, and growt h of microbial mats. Offshore sedimentation did not resume until the l ast 2000-4000 years, when the deeper-water and finer-grained Ponza gla uconitic ironstone was covered by a thin veneer of silty clay deposite d on the uppermost slope, beneath the mudline. The shallower and coars er-grained Ventotene winnowed pisoidal hardground, instead, is probabl y still forming today under the action of strong northward directed ma rine currents sweeping the shelfbreak free of sediment.