THE GEOLOGY OF THE YELLOW TRACHYTIC TUFF, ROCCAMONFINA-VOLCANO, ITALY

The 227 ka Yellow Trachytic Tuff (YTT) of the Roccamonfina volcano is a multiunit ash-, pumice-, scoria- and lithic-ignimbrite with a proxim al sandwave surge deposit. The YTT has an estimated volume of 0.42 km( 3). It erupted in the northern, subsided sector of the volcano from Gl i Stagli caldera, and was channelled down ravines northward between th e limestone range of M. Cesima and M. Camino that bounds the depressio n. Up to 5 YTT units occur close to the outer part of the northern rim of Gli Stagli. The basal four units are separated by lithic-rich mark er layers which are inferred to result from gravity segregation follow ed by shearing. The first three units are consolidated by chabazite ce mentation, the fourth one is not consolidated. The uppermost unit is a ltered. One or two units characterize the YTT deposits in medial to di stal zones, Here, the unconsolidated unit underlies the consolidated o ne. Absence of markers precludes correlation with proximal stratigraph y. The YTT is poorly sorted and, except the surge deposit and the alte red facies which are very fine-grained, has moderate median diameter t ypical of pyroclastic flows, Matrix, pumice, and scoria clasts are poo rly vesicular. Matrix shards are equant, blocky-shaped, hydrated, and range from non-vesicular to vesicular. These features suggest that mag ma-water interaction played a role in the YTT eruption process, with s ome magmatic fragmentation. The complex near-Gli Stagli-rim YTT sequen ce could record the arrival of successive flows from the source vent, or also form by interaction of one or two flows with the caldera rim, In both cases, the absence of basal Plinian deposits in YTT units sugg ests that the eruptions were low pyroclastic fountains. The YTT distri bution was controlled by interaction with the northern rim of Gli Stag li caldera and with the limestone range that bounds the northern depre ssion. The near-rim stratigraphy shows the complete record of the erup tion, whereas the medial to distal sequences provide only the initial pyroclastic flow possibly with the final flow spilling over the calder a rim. The proximal surge episode probably resulted from higher veloci ty of a later pyroclastic flow due to steeper slope of the volcano in that locality.