Areal Distribution and Bulk Rock Density Variations of the Welded İncesu Ignimbrite, Central Anatolia, Turkey

Authors: ULRIKE MUES-SCHUMCHER, ROLF SCHUMACHER, LOTHAR GEORG VIERECK-GÖTTE, PETRA LEPETIT

Abstract: The 2.8 Ma İncesu ignimbrite is one of the most densely welded ignimbrites of the Cappadocian Volcanic Province. Estimates of areal extent and volume reveal at least 7,760 km^2 and 38 km^3 of erupted magma. The source area can be located beneath the northeastern flanks of the Erciyes Dağ-Koçdağ stratovolcano. Today, the ignimbrite typically forms relatively small plateaus or isolated remnants and caps on hill tops. Field characteristics of the deposit include a local dark-brown or black basal vitrophyre and horizontally zoned changes in colour which correspond to changes in modal composition. Upwardly increasing amounts of lithic fragments and pumice are obvious in many locations. Depositional structures are scarce and visible only in places of obviously lower bulk rock density. The areal distribution is strongly controlled by palaeotopography of older volcanic cones and the Taurus forehills that channelised individual flow portions and partly terminated their runout. Bulk rock density of the ignimbrite is as high as 2.34 g/cm^3 in the basal vitrophyre of the type section in the town of İncesu. It generally decreases upsection to 1.77 g/cm^3 at the top of the outcrop, but superposing density peaks are observed in the central and upper parts. Laterally, bulk rock densities vary systematically with increasing distance from source and in relation to palaeotopography: depositional lobes which are not terminated by topographic barriers show a consistent decrease in bulk rock density, from proximal dense to distal incipient welding. Those lobes, which are deposited against obstacles, show an unexpected re-increase in bulk rock density in front of the barrier. The amount of crystals and lithic fragments is unimportant in the determination of bulk rock density such that the multiple density peaks represent zones of higher degrees of welding. The observed vertical pattern can be explained by the model of a compound cooling unit made up of several emplacement units which are separated from each other by pauses in the depositional sequence. Welding of the İncesu ignimbrite is thought to have resulted from compaction welding after the flows came to rest. A gas retention regime may have promoted re-dissolution of volatiles into the ashy matrix and thereby the formation of the basal vitrophyre in the type section and the frequent occurrence of the highest densities in the lower parts, respectively. The model also explains high bulk rock densities in front of topographic barriers which may have formed by local overthickening due to partial back flow and, thus, increased load pressure.

Keywords: Cappadocia, ignimbrites, welding, rock density

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