Lithic breccias in intermediate volume phonolitic ignimbrites, Tenerife (Canary Islands): constraints on pyroclastic flow depositional processes

Lithic occurrences within Quaternary phonolitic ignimbrites on Tenerife (Canary Islands) fall into three broad groupings: (1) massive ignimbrite with large `floating' lithics; (2) basal lithic-rich bedforms; and (3) matrix- to clast-supported lithic breccias within ignimbrite. Massive ignimbrite divisions commonly contain large (outsized), either solitary 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 several ignimbrites. Most bedforms are asymmetric, with steeper lee-sides, and they are buried by stratified and/or massive ignimbrite. Internally, most bedforms are massive. Some, however, exhibit an upstream-inclined stratification, indicating upcurrent migration of the bedform, which is interpreted to record antidune-like flow regimes. Locally-derived basalt clasts are important constituents of some of these bedforms, which have also been sourced through substrate erosion. Matrix- to clast-supported, pebble- to boulder-sized lithic breccias occur at various heights within massive ignimbrite. Lithic breccias at or near the top of most ignimbrites contain vent-derived lithics, and record a late flux of lithic material from the vent during a sustained pyroclastic flow event. Extensive, boulder-sized breccias at the top of two ignimbrites are interpreted to record the onset of caldera collapse late in the pyroclastic flow-forming eruptions. Some breccias contain boulders that are oriented with long-axes transverse to flow, indicating late-stage tractional transport. Thin-bedded, matrix-supported lithic breccias or lithic trains and stratified ignimbrite are commonly developed adjacent to, or immediately above topographic highs. These facies laterally merge into massive ignimbrite infilling topographic lows. Massive ignimbrite is interpreted to be deposited from a high particle concentration, hindered settling zone. Inverse grading and long-axis parallel to bedding imbrication of lithics within the thin-bedded breccia facies are interpreted to record deposition under modified grain flow conditions. A variety of depositional processes are thus recorded, and the relationship between lithic breccias/ignimbrite facies and topography indicates that there were spatial variations in depositional mechanisms at any one instant. The vertical sequence of bedform–stratified ignimbrite–massive ignimbrite characterises the lower part of several ignimbrites, and is interpreted to record increasing rates of deposition from a sustained current. Multilayered lithic breccias at various heights within massive ignimbrite are a form of clast compositional zonation, and reflect temporal changes in supply, flow competence and capacity.