LITHOSTRATIGRAPHY OF THE 250,000 YEAR RECORD OF LACUSTRINE SEDIMENTS FROM THE VALLE-DI-CASTIGLIONE CRATER, ROMA

Sediments drilled by an 88 m borehole through the middle of the drained lake bed (1 km diameter) were studied by macroscopic, grain size, mineralogical and geochemical analyses. The catchment area (60 km2) is composed of leucitic undersaturated volcanic rocks, mainly of the pyroclastic type. The lithostratigraphic sequence can be summarized as consisting of two alternating lithotypes, which allows different palaeoclimatic influences to be drawn: the first consists mainly of tuffaceous silts, generally enriched with organic matter and aceolian quartz, and contains heavy and weathered minerals. It is interpreted as a sediment deposited with a predominantly terrigenous supply by the abundant surface waters flowing into the lake during wet climatic phases. The second lithotype consists of silts with a predominantly carbonate component, rich in mollusc shells but with low contents of organic matter, aeolian quartz, heavy and weathering minerals. It is interpreted as a sediment deposited during relatively dry climatic phases with an abundant calcium bicarbonate supply, which is relatively higher than the terrigenous one. Most of the sequence can be correlated with the palaeoclimatic variations inferred from oxygen isotopic stratigraphy. Dry intervals seem to be related to the winter solstice in the northern hemisphere when the earth is at perihelion.