LATERAL VARIATIONS WITHIN COARSE CO-IGNIMBRITE LITHIC BRECCIAS OF THEKOS PLATEAU TUFF, GREECE

Coarse, co-ignimbrite lithic breccia, Ebx, occurs at the base of ignim brite E, the most voluminous and widespread unit of the Kos Plateau Tu ff (KPT) in Greece, Similar but generally less coarse-grained basal li thic breccias (Dbx) are also associated with the ignimbrites in the un derlying D unit. Ebx shows considerable lateral variations in texture, geometry and contact relationships but is generally less than a few m etres thick and comprises lithic clasts that are centimetres to a few metres in diameter in a matrix ranging from fines bearing (F2: 10 wt.% ) to fines poor (F2: 0.1 wt.%), Lithic clasts are predominantly vent-d erived andesite, although clasts derived locally from the underlying s edimentary formations are also present. There are no proximal exposure s of KPT. There is a highly irregular lower erosional contact at the b ase of ignimbrite E at the closest exposures to the inferred vent, 10- 14 km from the centre of the inferred source, but no Ebx was deposited . From 14 to >20 km from source, Ebx is present over a planar erosiona l contact. At 16 km Ebx is a 3-m-thick, coarse, fines-poor lithic brec cia separated from the overlying fines-bearing, pumiceous ignimbrite b y a sharp contact. This grades downcurrent into a lithic breccia that comprises a mixture of coarse lithic clasts, pumice and ash, or into a thinner one-clast-thick lithic breccia that grades upward into relati vely lithic-poor, pumiceous ignimbrite. Distally, 27 to >36 km from so urce Ebx is a finer one-clast-thick lithic breccia that overlies a non -erosional base. A downcurrent change from strongly erosional to depos itional basal contacts of Ebx dominantly reflects a depletive pyroclas tic density current. Initially, the front of the flow was highly energ etic and scoured tens of metres into the underlying deposits. Once dep osition of the lithic clasts began, local topography influenced the ge ometry and distribution of Ebx, and in some cases Ebx was deposited on ly on topographic crests and slopes on the lee-side of ridges. The KPT ignimbrites also contain discontinuous lithic-rich layers within text urally uniform pumiceous ignimbrite. These intra-ignimbrite lithic bre ccias are finer grained and thinner than the basal lithic breccias and overlie non-erosional basal contacts. The proportion of fine ash with in the KPT lithic breccias is heterogeneous and it; attributed to a co mbination of fluidisation within the leading part of the flow, turbule nce induced locally by interaction with topography, flushing by steam generated by passage of pyroclastic density currents over and depositi on onto wet mud, and to self-fluidisation accompanying the settling of coarse, dense lithic clasts. There are problems in interpreting the K PT lithic breccias as conventional co-ignimbrite lithic breccias. Thes e problems arise in part from the inherent assumption in conventional models that pyroclastic flows are highly concentrated, non-turbulent s ystems that deposit en masse, The KPT coarse basal lithic breccias are more readily interpreted in terms of aggradation from stratified, wan ing pyroclastic density currents and from variations in lithic clast s upply from source.