The Kizilkaya ignimbrite — an unusual low-aspect-ratio ignimbrite from Cappadocia, central Turkey

The 4.3-m.y.-old medium-volume low-aspect-ratio Kizilkaya ignimbrite (50–100 km3 DRE) is one of the most widespread in the Cappadocian Volcanic Province covering about 8500–10,600 km2. The ignimbrite rests on a relatively fine-grained fan of Plinian pumice-fall deposit (Md of 1.0–1.80 mm in proximal locations). The eruptive center was located in the Misli plain northeast of Nigde, as deduced from thickness and grain-size variations of the fall deposit, flow direction indicators, welding patterns of the ignimbrite and the distribution of certain types ofxenoliths. The massive ignimbrite, generally about 15 m thick, covers a paleoplain throughout at least two thirds of its areal extent. It comprizes two flow units, identified by local pumice enrichment in the upper part of the lower unit. The ignimbrite is completely welded in many places. In other places, the lower flow unit is non-welded, particularly where the initial pumice-fall deposit was eroded, a fine-grained ground layer was deposited, and undulating or cross-laminations with antidunes were developed. The ground layer was derived from the ignimbrite ground-mass by loss of fines < 250–500 μm. tional characteristics indicate that the ignimbrite was emplaced as high-concentration flows with relatively low velocity and low heat loss during runout. Local development of a ground layer and internal bedding structures indicate local increased turbulence only within individual flow portions due to agitated fluidization from engulfed air. The degree of welding of the lower flow unit was controlled by this turbulence and is not related to thickness variations.