ALBERT

Description: The results of lithologic, petrographic, grain-size, and chemical studies of volcaniclastic sediments recovered during Leg 107 of the Ocean Drilling Program show that a variety of volcaniclastic sediments occur in the Tyrrhenian Basin. The abundance of volcanic glass and presence (or lack) of sediment structure is used to classify the sediments into four sediment-deposit types: (1) tephra fall, (2) volcaniclastic turbidite, (3) debris flow, and (4) volcanic sand. The abundance and distribution of these sediment types at Leg 107 sites are related both to proximity to volcanic sources and pathways of sediment transport to the basin floor. Deposits directly related to volcanic events include tephra fall, debris flow, and some volcaniclastic turbidites. The latter are generated from reworking of tephra fall and from pyroclastic gravity flows that entered the sea at the time of (primary), or closely following (epiclastic), eruption. These turbidites occur throughout the basin, are glass-rich, and are most common in the central and southeastern portions of the basin. A large debris flow encountered at Sites 651 and 650 represents a marine-deposited equivalent of the Campanian Ignimbrite, a large pyroclastic deposit in the Phlegrean Fields produced about 38,000 yr ago. This correlation is confirmed by glass chemistry. Volcanic sands and other volcaniclastic turbidites represent, on the other hand, deposits of more extensively reworked pyroclastics. Heterogeneous volcanic glass and mineral population, abundant detrital crystals, and, occasionally, high clay component attest to the secondary (epiclastic) origin of these deposits. Several large volcanic sands that occur immediately above vitric-rich layers may be derived directly from reworking of the vitric-rich layer, although predominantly in the nearshore and/or shelf environment. Glass chemistry shows that volcaniclastic sediments at Leg 107 sites are mainly of local provenance. At westerly Sites 655 and 653 rhyolitic and trachytic glasses have a source in the nearby Pontine Archipelago. In the central part of the basin at Site 651, volcaniclastic sediments are primarily derived from the Campanian volcanic province. At southerly Site 650 provenance is mainly the Eolian Arc. Two major volcaniclastic turbidites at Site 650 contain calc-alkaline rhyolitic glass, documenting large eruptions of rhyolitic magma not before reported in the arc.