LITHIC BRECCIAS IN INTERMEDIATE VOLUME PHONOLITIC IGNIMBRITES, TENERIFE (CANARY-ISLANDS) - CONSTRAINTS ON PYROCLASTIC FLOW DEPOSITIONAL PROCESSES

Lithic occurrences within Quaternary phonolitic ignimbrites on Tenerif e (Canary Islands) fall into three broad groupings: (1) massive ignimb rite with large 'floating' lithics; (2) basal lithic-rich bedforms; an d (3) matrix- to clast-supported lithic breccias within ignimbrite. Ma ssive ignimbrite divisions commonly contain large (outsized), either s olitary or scattered lithics, which are mostly monomictic, and derived locally from substrate erosion by pyroclastic density currents. Low- and high-aspect ratio, Lithic-rich bedforms occur at the base of sever al ignimbrites. Most bedforms are asymmetric, with steeper lee-sides, and they are buried by stratified and/or massive ignimbrite. Internall y, most bedforms are massive. Some, however, exhibit an upstream-incli ned stratification, indicating upcurrent migration of the bedform, whi ch is interpreted to record antidune-like flow regimes. Locally-derive d basalt clasts are important constituents of some of these bedforms, which have also been sourced through substrate erosion. Matrix- to cla st-supported, pebble- to boulder-sized lithic breccias occur at variou s heights within massive ignimbrite. Lithic breccias at or near the to p of most ignimbrites contain vent-derived lithics, and record a late flux of lithic material from the vent during a sustained pyroclastic f low event. Extensive, boulder-sized breccias at the top of two ignimbr ites are interpreted to record the onset of caldera collapse late in t he pyroclastic flow-forming eruptions. Some breccias contain boulders that are oriented with long-axes transverse to flow, indicating late-s tage tractional transport. Thin-bedded, matrix-supported lithic brecci as or lithic trains and stratified ignimbrite are commonly developed a djacent to, or immediately above topographic highs. These facies later ally merge into massive ignimbrite infiIling topographic lows. Massive ignimbrite is interpreted to be deposited from a high particle concen tration, hindered settling zone. Inverse grading and long-axis paralle l to bedding imbrication of lithics within the thin-bedded breccia fac ies are interpreted to record deposition under modified grain flow con ditions. A variety of depositional processes are thus recorded, and th e relationship between lithic breccias/ignimbrite facies and topograph y indicates that there were spatial variations in depositional mechani sms at any one instant. The vertical sequence of bedform-stratified ig nimbrite-massive ignimbrite characterises the lower part of several ig nimbrites, and is interpreted to record increasing rates of deposition from a sustained current. Multilayered Lithic breccias at various hei ghts within massive ignimbrite are a form of clast compositional zonat ion, and reflect temporal changes in supply, flow competence and capac ity. (C) 1998 Elsevier Science B.V. All rights reserved.