areal distribution and bulk rock density variations of the welded … · the areal distribution is...

Turkish Journal of Earth Sciences (Turkish J. Earth Sci.), Vol. 13, 2004, pp. 249-267. Copyright ©TÜB‹TAK

249

Areal Distribution and Bulk Rock Density Variationsof the Welded ‹ncesu Ignimbrite, Central Anatolia, Turkey

ULRIKE MUES-SCHUMACHER1, ROLF SCHUMACHER1,LOTHAR GEORG VIERECK-GÖTTE2 & PETRA LEPETIT2

1 Hauptstraße 3, D-71672 Marbach – Germany(e-mail: ulrike.mues-schumacher@ uev.de)

2 Institut für Geowissenschaften, Friedrich-Schiller-Universität, Burgweg 11, D-07749 Jena – Germany(e-mail: viereck@ geo.uni-jena.de )

Abstract: The 2.8 Ma ‹ncesu ignimbrite is one of the most densely welded ignimbrites of the Cappadocian VolcanicProvince. Estimates of areal extent and volume reveal at least 7,760 km2 and 38 km3 of erupted magma. Thesource area can be located beneath the northeastern flanks of the Erciyes Da¤–Koçda¤ stratovolcano. Today, theignimbrite typically forms relatively small plateaus or isolated remnants and caps on hill tops. Field characteristicsof the deposit include a local dark-brown or black basal vitrophyre and horizontally zoned changes in colour whichcorrespond to changes in modal composition. Upwardly increasing amounts of lithic fragments and pumice areobvious in many locations. Depositional structures are scarce and visible only in places of obviously lower bulk rockdensity. The areal distribution is strongly controlled by palaeotopography of older volcanic cones and the Taurusforehills that channelised individual flow portions and partly terminated their runout.

Bulk rock density of the ignimbrite is as high as 2.34 g/cm3 in the basal vitrophyre of the type section in thetown of ‹ncesu. It generally decreases upsection to 1.77 g/cm3 at the top of the outcrop, but superposing densitypeaks are observed in the central and upper parts. Laterally, bulk rock densities vary systematically with increasingdistance from source and in relation to palaeotopography: depositional lobes which are not terminated bytopographic barriers show a consistent decrease in bulk rock density, from proximal dense to distal incipientwelding. Those lobes, which are deposited against obstacles, show an unexpected re-increase in bulk rock densityin front of the barrier.

The amount of crystals and lithic fragments is unimportant in the determination of bulk rock density such thatthe multiple density peaks represent zones of higher degrees of welding. The observed vertical pattern can beexplained by the model of a compound cooling unit made up of several emplacement units which are separatedfrom each other by pauses in the depositional sequence. Welding of the ‹ncesu ignimbrite is thought to haveresulted 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 sectionand the frequent occurrence of the highest densities in the lower parts, respectively. The model also explains highbulk rock densities in front of topographic barriers which may have formed by local overthickening due to partialback flow and, thus, increased load pressure.

Key Words: Cappadocia, ignimbrites, welding, rock density

Kaynaflm›fl ‹ncesu ‹gnimbiritinin Bölgesel Da¤›l›m› ve Kayaç Yo¤unlu¤u De¤iflimleri, Orta Anadolu, Türkiye

Özet: 2.8 My yafl›ndaki ‹ncesu ignimbiriti Kapadokya Volkanik Bölgesinin çok iyi kaynaflm›fl ignimbiritlerindenbirisidir. Bölgesel yay›l›m ve hacim tahminleri en az 7,760 km2 ve 38 km3’lük magman›n püskürdü¤ünü ortayakoymaktad›r. Bu ignimbiritin kayna¤›, Erciyes Da¤–Koçda¤ kompozit volkan›n›n kuzeydo¤u kanad›n›n alt›nda yeralmaktad›r. Günümüzde, ignimbiritler küçük platolar, kal›nt› tepecikler ve tepelerde örtüler oluflturmaktad›r. Yerselkoyu kahverengi veya tabandaki siyah cams› doku ile kaya bileflimine ba¤l› olarak geliflen yatay kuflaklar halindekirenk de¤iflimleri ignimbiritlerin arazi özelliklerini içermektedir. ‹gnimbiritlerin üst seviyelerine do¤ru litik tane vepomza miktar›ndaki art›fllar bir çok yerde aç›kça gözlemlenmifltir. ‹gnimbiritlerin çökelimiyle ilgili yap›lar nadirolarak geliflmifl olup, genelde kayaç yo¤unlu¤unun az oldu¤u alanlarda gözlemlenmifllerdir. ‹gnimbiritlerin bölgeselda¤›l›m›, her bir ak›nt›y› kanalize eden veya ak›nt›lar›n önünü kesen volkanik koniler ile Toros Da¤lar›n›npaleoco¤rafyas› taraf›ndan etkin olarak kontrol edilmifltir.

‹ncesu kasabas›ndaki tip kesitinin taban kesimlerinde ignimbiritlerin kayaç yo¤unlu¤u 2.34 g/cm3 kadaryükselebilmektedir. Yo¤unluk üst kesimlere do¤ru 1.77 g/cm3 ye kadar düflmekte, ancak tip kesitin orta ve üstbölümlerinde yüksek de¤erlere ulaflmaktad›r. Kayaç yo¤unlu¤u yanal olarak kaynaktan uzaklaflt›kça vepaleotopografyaya ba¤l› olarak sistematik bir de¤ifliklik gösterir: topo¤rafik bariyerler tarf›ndan engellenmeyençökelim loblar› kayaç yo¤unlu¤unda sabit bir azalma – proksimal alanlarda daha yo¤un, ›rak kesimlerde ise az

Introduction

Volcanism in Turkey is closely related to closure andsubduction of the Neotethys. Four major volcanicprovinces developed during the Neogene and Quaternary,namely the Western Anatolian, Central Anatolian, EasternAnatolian and Galatean volcanic provinces (Figure 1). Thevolcanic activity prograded from west to east with theoldest rocks in Western Anatolia. The Central AnatolianProvince – extending for about 300 km in a NE–SWdirection within the Anatolides – exposes the mostprominent tephra series that is chiefly exposed betweenthe Tuz Gölü and Sultansazl›¤› depressions. This tephraseries is composed of at least 10 major and two minorignimbrite sheets which were emplaced during the past12 Ma (Figure 2). They are separated from each other byvarious products of other, less voluminous tephraeruptions, products of effusive activity, and continentalepiclastic deposits such as lahars, and fluvial andlacustrine sediments (Pasquarè 1968; Innocenti et al.1975; Pasquarè et al. 1988; Schumacher et al. 1990; LePennec et al. 1991, 1994; Temel 1992; Schumacher &Mues-Schumacher 1997; Toprak et al. 1994; Mues-Schumacher & Schumacher 1996). Although welding oftephra is not common in Cappadocia, a general increasein welding intensity of individual ignimbrites can beobserved in the upper parts of the stratigraphy sequence.

With respect to the welding of tephra, the classicalwork by Smith (1960a & b) defines welding as thatprocess which promotes the union or cohesion of glassyfragments, and further describes the transition fromincipient to complete welding as the progressive loss ofpore space and deformation of shards and pumice.Incipient welding in rhyolitic pyroclastic flow deposits

needs conservation of minimum temperatures of 600 ±25 °C after emplacement (Ragan & Sheridan 1972;Riehle 1973; Sheridan 1979). Recent interest hasconcentrated on the influence of composition,temperature, and depositional thickness on the final bulkrock density (Riehle et al. 1995). Streck & Grunder(1995) described the welding intensity, recrystallisationof glass shards and vapour-phase crystallization in thewelded Rattlesnake Tuff ignimbrite in Oregon, U.S.A.Finally, Sparks et al. (1998) showed the influence of re-dissolution of volatiles into glass shards on the weldingintensities of tephra deposits.

The 2.8 Ma ‹ncesu ignimbrite constitutes theuppermost part of the Cappadocian tephra sequence(Figure 2). It is one of the most widespread and one ofthe most densely welded ignimbrites of central Anatolia.We therefore studied in detail its vertical and lateralvariations in bulk rock density to elucidate the influenceof palaeotopography, particularly the influence oftopographic barriers which partly terminated the runoutof the corresponding pyroclastic flows.

In the following, we maintain the stratigraphic term‹ncesu ignimbrite introduced by Pasquarè (1968) todescribe a welded ignimbrite exposed around the town of‹ncesu. Some contributions, notably by Le Pennec et al.(1994) and Temel et al. (1998), refer to this unit as theValibaba Tepe ignimbrite.

Areal Distribution

We mapped the semi-continuous distribution of the‹ncesu ignimbrite over an area of about 100 x 100 km,which takes in the 115 locations we have studied (Figure

WELDED ‹NCESU IGNIMBRITE, CENTRAL ANATOLIA

250

kaynaflm›fl fleklinde bir de¤iflim – sunarlar. Buna karfl›n, bariyerler taraf›ndan engellenmifl ak›nt›lar özelliklebariyerlerin önününde kayaç yo¤unlu¤unda beklenmedik bir art›fl göstermektedir.

‹gnimbirit istifi içinde gözlemlenen çok say›da yo¤unluk art›fllar›n›n kaynaflman›n belirgin/çok iyi oldu¤udüzeylere denk düflmesi, gerek kristal gerekse litik tanelerin miktar›n›n kayaç yo¤unlu¤unun belirlenmesinde çokda önemli olmad›klar›na iflaret eder. ‹stif içerisinde dikey yönde gözlemlenen yo¤unluk de¤iflimleri birbirlerindenbelli bir zaman aral›¤› ile ayr›lan çok say›daki ak›nt›dan oluflan bir istifin varl›¤› ile aç›klanabilir. ‹ncesu ignimbiritininkaynaflmas›n›n ak›nt›n›n dura¤an hale gelmesini takip eden dönemde s›k›flmaya/yo¤unlaflmaya ba¤l› olarak geliflti¤idüflünülmektedir.

Gaz al›koyma rejimi uçucu gazlar›n kül içeren hamurda erimesini teflvik etmifl olabilir. Buda, tip kesitintaban›ndaki cams› düzeyin oluflmas›na ve istifin alt kesimlerinde yüksek yo¤unluk düzeylerin oluflmas›na nedenolmufl olabilir. Önerilen model, topo¤rafik bariyerlerin önünde gözlemlenen ve olas›l›kla bariyere çarpan ak›nt›n›nk›smende olsa geri dönmesine ba¤l› geliflen yersel kal›nlaflma ve bunun do¤al sonucu olan yüksek yük bas›nca ba¤l›oluflan yüksek kayaç yo¤unluklar›n› da aç›klamaktad›r.

Anahtar Sözcükler: Kapadokya, ignimbirit, kaynaflma, kayaç yo¤unlu¤u

U. MUES-SCHUMACHER et al.

251

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3). The distribution area is transected by the Ecemifl FaultZone, two feeder branches of which form a pull-apartbasin – the Sultansazl›¤› depression – which is filled withyoung sediments. Limitations in field exposure are alsodue to a younger ignimbrite cover and the Erciyes Da¤stratovolcano; thus, we found no outcrops of the ‹ncesuignimbrite in the centre of the study area.

The present-day exposure constitutes an area of 595km2 (Table 1). The reconstructed area originally coveredby the ‹ncesu ignimbrite but later eroded or buried byyounger deposits has been divided into three qualitativecategories: 'certain', 'likely' and 'probable', indicatingincreasing degrees of uncertainty (Table 1). The 'certain'category includes areas connecting present-day outcropsand takes in another 385 km2. 'Likely' takes in 3,150 km2

and is the most probable reconstruction of the individualflow lobes relative to topography, the proximal areastoward the assumed source area beneath Erciyes Da¤ andin the Sultansazl›¤› depression. 'Probable' was assigned tolobes connecting well-correlated but presently isolateddistal remnants and encloses at least 1,370 km2. Hence,the ‹ncesu ignimbrite covered an area of 5,500 km2,which might be the minimum value of this conservativeestimate. Based on these data, a more probablereconstruction of distal lobes reveals an areal extent ofabout 7,700 – 7,800 km2.

Based on these area/thickness data, we can furtherestimate the aspect ratio, which is the ratio of maximumthickness to runout distance, considered an indicator ofthe energy of an ash flow (Walker 1983). The aspectratio of the ‹ncesu ignimbrite is between 1:5,000 and1:22,000, depending on conservative or optimisticestimates of 4 m / 20 km or 3 m / 66 km, respectively.Both aspect ratios are within the range of extensive, high-energy, low-aspect-ratio ash-flow tuffs.

Volume

To calculate the erupted volume, we use all thickness datawith conservative minimum thicknesses of present-dayexposures combined with the reconstructed distributionareas and associated thicknesses in the differentcategories (Table 1). A total (minimum) volume of 45.4km3 of welded tephra is obtained. The correspondingmagma volume is less because of variations in weldingintensity. Using approximate average densities in eachpartial volume, we obtained a total magma volume


252

non-weldedtephra

pumice falls

welded tephra

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Figure 2. The generalized stratigraphic section of the Cappadociantephra sequence showing the major ignimbrites that areinterbedded with extensive epiclastic deposits. Although theSar›maden ignimbrite is the oldest welded unit of thesequence, welding becomes important only in the upper partof the section with the ‹ncesu ignimbrite which is the mostdensely welded deposit in Cappadocia.

erupted of about 38 km3. This volume is on the order ofthe Crater Lake eruption of Mt. Mazama and the Taupoeruption (Smith 1979; Walker 1983), and less thanestimates made by Le Pennec et al. (1994) and Temel etal. (1998), who supposed an areal extent of 5,200 km2

and a tephra volume of 100 km3 for their 'Valibabaignimbrite'.

Field Characteristics

The ‹ncesu ignimbrite generally forms small plateaus(Figure 4a), a few tens of square kilometers wide, or

occurs as isolated remnants and caps on or near hilltops.Outcrops typically expose 3 – 10 m of a reddish-brownignimbrite; its thickness exceeds 15 or 20 m in only a fewlocations. The outcrops commonly have coarse columnarjointing with individual columns up to about 5 m indiameter (Figure 4b). Distally, particularly in the southand southeast, the ‹ncesu ignimbrite became channelisedin the valleys of the Taurus forehills that terminated therunout. In spite of the palaeotopography and channelisingof flow portions, distinct valley ponds of the ignimbriteare rare. The ignimbrite lacks rheomorphic features asdescribed from the Rattlesnake Tuff of eastern Oregon


253

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Figure 3. Geological sketch map of the study area showing the semi-continuous regional distribution of the ‹ncesu ignimbrite. No exposures werefound in the centre of the area due to faulting, young sedimentary and ignimbritic cover, and the growth of Erciyes Da¤›.

(Streck & Grunder 1995) or the Gribbles Runpalaeovalley tuff of central Colorado (Chapin & Lowell1979).

The ‹ncesu ignimbrite typically lacks basal pumicefallout deposits, in contrast to most of the otherignimbrites of the Cappadocian Volcanic Province. Twolocations, however, location 115 near Büyüksövegenler(Figure 5a) and location 103 near K›z›lhan, show 1.5 – 8cm of a whitish material beneath the ignimbrite that couldbe interpreted as strongly altered and weathered pumicefallout.

The Type Section of ‹ncesu Ignimbrite

The type section of the ‹ncesu ignimbrite, location 4 atthe western margin of the town of ‹ncesu, is composed ofabout 27 – 28 m of densely to incipiently weldedignimbrite (Figure 4c) and subdivided into threeindividual zones that can be identified by differences incolour, concentrations of crystals, pumice, and lithic clastsas well as by density (Mues-Schumacher 1997).

The basal unit is a dark-brown or black vitrophyre,about 1 m in thickness and includes about 25 cm of palebuff-coloured loose material at its base. Thin sectionsshow radial fibrous whiskers of clay minerals and somepreserved Y- and T-shaped glass shards, indicating thatthe loose material is primarily non-welded ash. The basalvitrophyre is overlain by a bright-red to dull brick-red

densely welded central unit. Up section, the colour gradesthrough brownish-violet or dark pinkish grey into palegrey within the upper unit of the ‹ncesu ignimbrite. Thewelding intensity of the ‹ncesu ignimbrite decreases fromthe bottom to the top of the deposit, as is obvious fromflattening ratios and the vesicularity of pumice clasts (seealso Peterson 1979), the degree of which increasesupwardly. Pumice clasts are extremely flattened in thebasal vitrophyre and the lower central unit. Higher up,the decrease in flattening can be documented by MP7-values (average flattening ratios of the seven most-flattened pumices of a stratigraphic level). Throughoutthe uppermost 15 m of the ‹ncesu ignimbrite, MP7

decreases from 8.85 in the pinkish-grey zone to 4.31 atthe top of the outcrop (Figure 6).

The amount of crystals and lithic fragments alsochanges with stratigraphic position, corresponding to thechange in colour. The basal vitrophyre has 2 – 5 volume% of phenocrysts, but the amount abruptly increases to8 – 15 volume % up-section at the transition to thecentral red unit (Lepetit 1999). Lithic fragments aregenerally scarce within the basal vitrophyre, whereas afew small pieces were found in the central unit.Throughout the upper unit, the lithic-fragment contentincreases significantly to about 10 volume %, and the sizeof the fragments increases correspondingly; the ML5-value(average diameter of the five largest lithic fragments of astratigraphic level) increases within the upper units from


254

Table 1. Area and volume estimates of the ‹ncesu ignimbrite. The calculation of the erupted magma volume is referred to the maximum density forthe basal vitrophyre of 2.35 g/cm3.

Area (km2) Volume (km2) Erupted magma (km2)

Area Cumulative Associated Ignimbrite Cumulative Assumed density ignimbrite / Volume Cumulativearea thickness volume volume density density vitrophyre volume

(g/cm3) erupted

Isopach area of present day exposure

2 – 4 meters 169 169 3 m 0 507 0.507 1.65 0.70 0.355 0.3554 – 7 meters 282 451 5 m 1.410 1.917 1.80 0.77 1.086 1.4417 – 10 meters 49 500 8 m 0.392 2.309 2.00 0.85 0.333 1.774> 10 meters 94 594 10 m 0.940 3.249 2.25 0.96 0.902 2.676

Reconstructed

Certain 376 376 2 – 6 m 1.504 1.504 1.70 0.72 1.083 1.083Likely 3,154 3,530 8 – 12 m 31.540 33.044 2.10 0.89 28.071 29.154Probable 3,636 7,166 app. 2.5 m 9.090 42.134 1.60 0.68 6.181 35.335

Total 7,760 km2 45.383 km2 38.011 km2


255

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257

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basal

1.00 1.50 2.00

rock density [g/cm ]3

P 4.31P 4.31P 4.31777

ML 4.1 cmML 4.1 cmML 4.1 cm555


P 8.17P 8.17P 8.17777

P 8.85P 8.85P 8.85777


25

20

15

10

5

igni

mbi

rte

thic

knes

s [m

]

Figure 6. Bulk rock density variation of the type section. Note the steep basal density gradientand the general upward decrease that is superposed by different peak density valuesin the central and upper parts; P7 – values denote the aspect ratios (maximumlength/maximum width) of the seven largest pumices, ML5 – values denote the meanof the five largest lithic fragments.

ML5 = 13.8 mm in the pinkish-grey part to ML5 = 40.6mm at the top of the outcrop.

In addition, yellowish to greenish-white haloes withmaximum diameters of up to 40 mm can be found in themiddle of the upper unit, ~ 15 to 22 m above the base;these features are either cored by small lithic particles ortheir interiors are hollow. These haloes commonly appearas alteration or reaction rims around hydrothermallyaltered wall-rock fragments that partly causedbreakdown of the surrounding ignimbrite material(Figure 5c & d).

Lateral Variations

Generally, the internal fabric of the ‹ncesu ignimbritevaries from dense welding in the proximal facies toincipient welding and non-welding distally. The basalzone, however, is vitric only at a few isolated locations.The vertical subdivision based on changes in colour can beidentified almost throughout the study area except forthe southern (Firaktin-Hoflca area) and southeasternlobes (Büyüksövegenler area). The reddish colours of thecentral part are mostly absent here; the dark basal zone,locally as thick as 3 m, is almost directly overlain byviolet-pinkish-grey ignimbrite portions which we considerto represent the upper units.

The lithic-fragment content locally varies significantly;local incorporation of loose debris picked up from theground can be observed at several places. An overallhigher lithic-fragment concentration in the basal segmentwas observed in the southern lobe. Part of the increasemay be attributed to local incorporation of debris, but theoverall scatter of particles 1 – 3 cm in size is attributedto primary effects of higher lithic-fragment contents inthese particular ignimbrite horizons.

The distribution of haloes appears to be more or lessarbitrary because no regional distribution pattern orrelation to distinct stratigraphic positions was perceived.The highest amounts were found in location 23, theHimmetdede quarries, within the upper half of thedeposit (Figure 5d). Location 38, Kalcanc›k, shows anirregular scatter of haloes over the entire section and thusrules out the possible relationship to distinct stratigraphiclevels. Similar relations were observed in the southernFiraktin-Hoflca lobe, where locations 88 and 89, inparticular, also have high concentrations.

Primary depositional structures, such as lithic- andpumice-fragment concentration layers characterisingindividual flow units, are absent in levels of dense weldingor were partly obscured by the welding. They wereobserved, however, at several places between Bünyan andK›z›lhan that show an overall lower degree of welding(Figure 5b). A striking example was found in location 97,Köprübafl›, showing two distinct lithic-fragmentconcentration layers in the basal unit. Moreover,concentrations of undeformed, highly vesicular pumiceclasts in location 108 are exposed in the same generalarea.

Degassing pipes, which are well known from thestratigraphically lower, less intense welded K›z›lkayaignimbrite (Schumacher & Mues-Schumacher 1996), arealmost absent in the ‹ncesu ignimbrite, which may alsohave been obscured by welding. The only knownexception occurs in the distal Himmetdede quarries(location 23): elongated vertical pipes – which are oftensimilar to haloes (yellowish-green rims) – wereinterpreted as gas-escape pipes.

Source Area

By analogy with similar erupted magma volumes ofroughly 50 km3, a small ring structure with calderasubsidence between 5 and 10 km in diameter might beexpected. However, no such structure is exposed and,thus, positioning of the source area of the ‹ncesuignimbrite is rather difficult. Standard means, such asthickness distribution of initial pumice-fall deposits,internal depositional structures, or the decrease/increaserelationships and size distribution of lithic clasts, do notwork. Pasquarè (1968) and Le Pennec et al. (1994) quitegenerally supposed the Kayseri region to locate thesource area. Kürkcüo¤lu et al. (1998) centred the ‹ncesuignimbrite beneath Koçda¤, which forms the easternflank of the Erciyes Da¤› stratovolcano. Those authorsplaced the ignimbrite with subsequent caldera subsidencestratigraphically at the end of the Koçda¤ stage and at thebeginning of the neo-Erciyes stage in the evolution ofErciyes Da¤›.

Bulk rock density data, combined with the assumedspreading paths of the pyroclastic flows – which werededuced from field observations and a series oftopographical cross-sections that we combined to a tree-dimensional block diagram – suggest that the source area


258

can be located beneath the eastern part of the ErciyesDa¤›/Koçda¤ stratovolcano in the vicinity of the easternbranch of the Ecemifl Fault Zone (Figure 7).

Variations in Bulk Rock Density

Analytical Methods and Influence of ModalComposition and Porosity

More than 1200 bulk rock density measurements ofabout 900 samples were performed to study the verticaland lateral density variation of the ‹ncesu ignimbrite. Thesize of the rock fragments used were 3 – 8 cm and 25 –250 g. We used the method of water immersion afterparaffin coating. All measurements were carried out at

room temperature with densities of water and paraffin of1.0 g/cm3 and 0.86 g/cm3, respectively. The precision ofthe method deduced from multiple measurements of thesame sample is about ± 1.5 % and is thus similar to therange of the density (in)homogeneities of the ignimbriteitself. Two or three hand specimens from the samestratigraphic level of a section show a general densityvariation on the order of ± 1 %, but rare deviations areas great as 4 %.

We further analysed the influence of modalcomposition, lithic-fragment content, and porosity of the‹ncesu ignimbrite on the bulk rock density variation at thetype location (Lepetit 1999; Viereck-Götte et al. 2001).Specific densities of the phenocrysts were deduced from


259

ÜrgüpÜrgüpÜrgüp

YeþilhisarYeþilhisarYeþilhisar

SultansazlýðýSultansazlýðýSultansazlýðýDepressionDepressionDepression

TomarzaTomarzaTomarza

KöprübaþýKöprübaþýKöprübaþý

BünyanBünyanBünyan

KAYSERÝKAYSERÝKAYSERÝ

10 km10 km10 km

DeveliDeveliDeveliDaðýDaðýDaðý

ErciyesErciyesErciyesDaðýDaðýDaðý

???

???

1.501.501.50 1.321.321.321.331.331.33

1.111.111.11 1.661.661.661.651.651.65 1.401.401.40

1.491.491.49

1.301.301.30

1.261.261.261.251.251.25

2.342.342.34

2.052.052.052.172.172.17

2.372.372.37

2.252.252.252.282.282.282.072.072.07

2.082.082.08

1.991.991.99

1.741.741.74

2.272.272.272.282.282.28

2.142.142.14

1.921.921.922.132.132.13

2.312.312.311.401.401.40

2.342.342.342.312.312.31

2.312.312.31

1.941.941.94

2.282.282.28

2.292.292.291.521.521.521.671.671.67

1.271.271.27

2.172.172.17

2.372.372.37

1.811.811.811.661.661.66

1.191.191.19

1.441.441.44

1.591.591.59

1.821.821.82 1.261.261.261.261.261.261.591.591.59

2.052.052.05

1.951.951.95

1.771.771.772.082.082.08

HimmetdedeHimmetdedeHimmetdede

KalcançýkKalcançýkKalcançýk

2.302.302.302.072.072.07

1.931.931.93

1.691.691.69

1.641.641.641.651.651.651.531.531.53

2.032.032.031.951.951.952.342.342.34

ÝncesuÝncesuÝncesu2.892.892.89

KoçdaðKoçdaðKoçdað

BüyüksövegenlerBüyüksövegenlerBüyüksövegenler1.771.771.771.861.861.86

2.132.132.131.581.581.58

1.981.981.98

2.202.202.202.082.082.08

HoþçaHoþçaHoþça2.142.142.142.242.242.24

1.761.761.761.921.921.92

2.062.062.061.521.521.52

1.571.571.571.951.951.95 1.861.861.86

1.751.751.75

FirkatinFirkatinFirkatin

1.461.461.46DeveliDeveliDeveli

1.761.761.761.701.701.70

1.941.941.94 1.641.641.641.431.431.431.291.291.29 1.641.641.64 1.371.371.37

1.661.661.66

1.581.581.58

1.571.571.571.581.581.581.491.491.49

KýzýlhanKýzýlhanKýzýlhan

EEE ccc eee mmm iii þþþFFF aaa uuu lll ttt

ZZZ ooo nnn eee

towns & citiestowns & citiestowns & cities

faultsfaultsfaults

ignimbrite densityignimbrite densityignimbrite density

distal extentdistal extentdistal extent

flow directionsflow directionsflow directions

1.761.761.76

Explanations

35° E35° E35° E 36° E36° E36° E

39° N39° N39° N

38° N38° N38° N

36°36°36°35° E35° E35° E

38° N38° N38° N

39° N39° N39° N

NNN

Figure 7. Sketch map illustrating the reconstructed areal extent, flow paths and source area of the ‹ncesu ignimbrite; bulk rock density values referto the basal unit.


260

heav

yfr

actio

n

ignimbrite thickness (m)25 20 15 10 5 0



2.5

3.0

dens

ity

(g/c

m³)

poro

sity

050

100

po

rosi

ty (

%)

rock

de

ns

ity

(g

/cm

³)1.

52.

02.

53.

0

H

P

HR

porous ignimbrite

measured BRD

rhyolite glassrecrystallized glass

dense ignimbrite

ab

Figu

re 8

. Ve

rtic

al v

aria

tions

of

dens

ity a

nd p

oros

ity o

f th

e ‹n

cesu

igni

mbr

ite a

t th

e ty

pe s

ectio

n in

‹nce

su. (

a)th

e de

nsity

var

iatio

ns o

f th

e sp

ecifi

c he

avy

frac

tion

(cry

stal

s an

d lit

hic

com

pone

nts

in t

heir

mod

al p

ropo

rtio

ns)

of t

he ig

nim

brite

and

the

ver

tical

var

iatio

n of

por

osity

; (b

)re

sults

of

mod

el c

alcu

latio

ns t

o qu

antif

y th

e sp

ecifi

c in

fluen

ce o

f th

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avy

frac

tion

and

the

poro

sity

on

the

mea

sure

d bu

lk r

ock

dens

ity.

Poro

us ig

nim

brite

den

otes

den

sity

of

teph

ra s

ampl

es d

evoi

d of

the

hea

vy f

ract

ion

(gla

ss r

ecry

stal

lised

) an

d de

nse

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mbr

itede

note

ste

phra

sam

ple

devo

id o

f an

y po

rosi

ty.

Arro

ws

H,

P, a

nd R

des

crib

e th

e re

spec

tive

shift

in

dens

ity d

ue t

o ad

ding

the

hea

vy f

ract

ion

(H),

due

to

recr

ysta

llisa

tion

of g

lass

(R

), a

ndco

nsid

erin

g th

e po

rosi

ty (

P).

their optical properties and chemical compositions:plagioclase 2.68 g/cm3, orthopyroxene 3.55 g/cm3,clinopyroxene 3.40 g/cm3, and titanomagnetite 4.89g/cm3. The densities of five different types of lithiccomponents were between 2.09 and 2.70 g/cm3. Wecombined phenocrysts and lithic fragments in their modalproportions into a heavy fraction and calculated theweighted average densities, which were between 2.57and 2.82 g/cm3 (Figure 8a). The glass shards wereanalysed by EDS resulting giving a rhyolitic compositionof 71.8 – 72.1 wt% SiO2 for which we adopted anaverage density of 2.37 g/cm3. XRD analyses ofrecrystallised glass indicate the formation of quartz,cristobalite, tridymite and sanidine. Due to theirestimated modal proportions an average density of 2.45g/cm3 can be assumed. Porosity was measured byallowing the dried samples to imbibe isopropanol for 16hours; the results were plotted versus height instratigraphy.

Bulk rock density and porosity are inversely related;i.e., the minimum porosity occurs within the basalvitrophyre where the rock density is highest, and aconstant upward increase in porosity is superposed bytwo minima in the upper half of the section (Figure 8a).For further evaluation of the influence of porosity, modalcomposition, and recrystallisation, two model calculationswere performed: (i) We simply subtracted the specificheavy fraction, the influence of which on rock density is+0.04 g/cm3 in the basal vitrophyre and +0.37 g/cm3 inthe upper section. Thus we obtained a hypotheticalporous ignimbrite devoid of heavies (Figure 8b); (ii) Thesecond calculation started at the dense glassy matrixsupposing a pure vitric rock devoid of heavy componentsand porosity. The rock density then corresponds torhyolite glass of 2.37 g/cm3. Its recrystallisation wouldproduce an average rock density of 2.45 g/cm3 and, thus,an increase of +0.08 g/cm3. Adding the specific heavyfraction, a hypothetical dense ignimbrite of 2.39 g/cm3 iscalculated from the basal vitrophyre, and of 2.54 – 2.59g/cm3 higher up; i.e., an increase in rock density between0.09 and 0.17 g/cm3 relative to pure rhyolite glass.

To summarise, the modelling illustrated in Figure 8shows that heavy components and recrystallisation willincrease the density between +0.17 g/cm3 and +0.37g/cm3 relative to pure rhyolite glass, but the influence isnegligible relative to the counteracting influence ofporosity. This observation corresponds to the description

by Smith (1960a & b) who found the transition fromincipient to complete welding as progressive loss of porespace. It further shows that bulk rock density datauncorrected for crystals and lithic fragments are valid todescribe variations in welding intensities.

Vertical Variations in Density

Bulk rock densities vary between 1.15 g/cm3 and 2.34g/cm3 in the type section in the western outskirts of thetown of ‹ncesu (Figure 6). The lowest density of 1.15g/cm3 was obtained from loose material at the very baseof the ignimbrite. Bulk rock density rapidly increases to2.07 g/cm3 40 cm above the base and to a densitymaximum of 2.34 g/cm3 in the basal vitrophyre, which isalso the highest value of the entire section detected 80cm above the base. Up section, the density patterns forthe central and upper units show a general decrease,from 2.34 g/cm3 to 1.77 g/cm3 at the top of the outcrop.This general decrease is superposed by at least threeminor density peaks: the first one in the upper half of thecentral unit and two more in the upper units.

Moreover, the western area of the ‹ncesu ignimbritehas its maximum density in the basal (vitric) unit or at thebase of the reddish central unit whereas, in the southernand southeastern distribution areas, the maximumdensities were found within the upper units.

Lateral Variations in Density

In general, maximum densities between 2.20 and 2.35g/cm3 were observed in (proximal) locations around theErciyes Da¤› stratovolcano, whereas (distal) locationsaway from Erciyes Da¤› have maximum densities of 1.40to 1.70 g/cm3 (Figure 7). These regional densitydistribution patterns and lateral gradients are complicatedby anomalies of unexpected higher or lower bulk rockdensities, particularly between the forehills of the TaurusMountains which have controlled the runout andemplacement of the ignimbrite. Three specific areas arediscussed in detail.

(1) The northwestern Himmetdede-lobe (Figure 7)shows a constant decrease in the maximum densityof the basal unit along the assumed spreading axis,from 2.37 g/cm3 in proximal locations at thewestern margin of the Sultansazl›¤› Depression, to


261

1.50 g/cm3 in distant places around the town ofHimmetdede. This distance is about 27 km and thetopography rises by about 100 m. A decrease inmaximum density is also obvious perpendicular tothe spreading axis; i.e., the density decreases from1.66 g/cm3 in the centre to 1.25 – 1.30 g/cm3 inthe lateral outer parts of the lobe. Regionalthickness distribution, depositional facies as well aspresent-day morphology indicate that the runoutand emplacement of the pyroclastic flows were notaffected by palaeotopography.

Similar observations of a constant decrease in densityhave also been made elsewhere, for example, west of‹ncesu where topography rises from 1090 to 1200 mabove mean sea level (a.m.s.l.) within about 5 km. Bulkrock density decrease from 2.34 g/cm3 in the type sectionto 1.93 g/cm3 atop a small plateau. Moreover, betweenthe villages of Köprübafl› and K›z›lhan in the northeasternlobe, bulk rock density decreases from 1.94 g/cm3 to1.66 – 1.44 g/cm3. Palaeotopography of the area partlydeflected flow portions from northeasterly to moreeasterly directions but did not terminate the runout bybarriers which could not be overtopped.

(2) Contrasting the examples above, the southerndistribution lobe of Firaktin – Hoflca is limited inareal extent by the volcanic rocks of the DeveliDa¤ in the west and by calcareous rocks of theTaurus forehills in the east and south (Figures 3 &9). The pyroclastic flows entered a narrowchannelising valley south of the village of Hoflcaleading 200 m uphill to the Hoflca Pass (1580 ma.m.s.l.), which was overtopped so that the distalflow came to rest beyond the pass in the south. Toillustrate density variations with increasingdistance from source, data from the basal unitwere plotted on a cross-section that parallels thespreading direction (Figure 9). The ignimbriteplatform around Firaktin and particularly in Hoflcashows a rather high density of 2.14 g/cm3 to 2.24g/cm3 (1386 m a.m.s.l.). A constant uphilldecrease down to 1.76 g/cm3 was observed on thenorthern slope at an elevation of 1513 m, and are-increase in bulk rock density to 1.92 g/cm3 wasfound in location 88 beyond the Hoflca Pass on its

southern slope where the basal unit is thickest.Locations 89 and 109 presently mark the distalend of the lobe in the field. They differ completelyfrom each other: location 109 – at the foot of amountain terminating the runout of thepyroclastic flows – shows a density as high as2.06 g/cm3, whereas location 89 beside thespreading axis of the flow only shows a maximumdensity of 1.52 g/cm3.

Similar features were observed in the southeasternÇulha-Büyüksövegenler lobe (Figure 7). Bulk rockdensities are highest at the distal ends of the flows on thestoss-sides of topographic barriers terminating the flows,whereas a constant decrease in density occurs laterallyalong the barriers.

(3) Another density anomaly occurs within theignimbrite sheet north of the village of Köprübafl›.Locations 98 and 108, NW and NE of Köprübafl›,respectively, expose a thick basal unit ofincipiently welded or loose tephra showing lithic-and pumice-fragment concentration layers. Inrelation to the runout distance, the observed bulkrock densities of 1.19 g/cm3 (location 98) and1.27 g/cm3 (location 108) from the basal unit aremuch too low and are surrounded by deposits ofsignificantly higher densities in the range of 1.66g/cm3 (location 97) and 1.81 g/cm3 (location 30;Figure 10). Correspondingly, preserved glassshards are largely undeformed and pumice clastsare significantly less elongated so that MP5-valuesof 2.82 (location 98) and 2.66 (location 108)contrast with the MP5-value of 5.88 within theupper part of the basal zone at location 116(Figure 10).

To shorten the following discussion, we invoke amodel similar to the non-welding of portions of thestratigraphically lower K›z›lkaya ignimbrite (Schumacher& Mues-Schumacher 1996): formation of well developedlithic- and pumice-fragment concentration layers at thebase of and atop individual flow units indicate dilution ofindividual flow portions to promote this particlesegregation. The dilution, in turn, may result fromenhanced fluidisation and thereby enhanced heat


262


263

exchange between the ash particles and the fluidising gasthat the tephra was cooled quickly down to or even belowthe minimum temperature of welding.

Interpretation and Discussion

Vertical Welding Patterns

Many well known ash-flow sheets, like the RattlesnakeTuff (Streck & Grunder 1995), the upper Bandelier Tuff(Smith & Bailey 1965), or the Bishop Tuff (Ragan &Sheridan 1972) show a single density maximum mostly in

the upper part of the lower half of the deposit. This isgenerally interpreted as a result of temperature andespecially compaction of a simple cooling unit which wastheoretically confirmed by the modelling of Riehle(1973). He included the initial depositional thicknesses,emplacement temperatures, and the depositionalenvironment in his computations.

Such a general model, however, is not adequate toexplain the observed welding pattern of the ‹ncesuignimbrite that shows various highs and lows in weldingintensity. Riehle et al. (1995) presented an improved

606060626262

616161

87a87a87a 878787919191

909090888888

898989109109109

DeveliDeveliDeveliDaðDaðDað

5 km

40� 50� E

N

topographicheight

m

1600

1400

1200

BRD1.86

60thickness

1.75

100

2.12 2.142.24

1.76

110 >120>100

>200

1.92 2.06

300(180)

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109889091a 87locations6160 62

b

a

(i) flow (ii) characteristic (iii) (iv)

35° 30�35° 30�35° 30�

38° 15�38° 15�38° 15�

38° 05�38° 05�38° 05�

Figure 9. (a) Detailed sketch map of flow paths and (b) longitudinal cross-section of the southern Firaktin-Hoflca – lobe that overtopped the HoflcaPass. Figures in the map and along the x-axis of the cross-section denote sample locations. The cross-section further shows the thicknessof the basal unit in cm and the corresponding maximum bulk rock density (BRD) in g/cm. (i) The pyroclastic flow covers even groundaround Firaktin; (ii) topography rising toward the Hoflca Pass caused some compression of the flow so that the rock density is higher atthe foot of the slope; (iii) the flow portion which overtopped the pass probably became diluted and partially turbulent behind the edge ofthe pass that resulted in thickening of the deposit in location 88 and correspondingly in slightly higher rock densities; (iv) the flow wasstopped in front of the topographic barrier of location 109: partial backflow from the upper slopes of the barrier caused overthickeningand thereby an increase in rock density at the distal end of the flow.


264

116

116

116

BRD

1.70

BRD

1.70

BRD

1.70

P 5.8

8P

5.88

P 5.8

8555

BRD

1.66

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1.66

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E

5

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6 5 4 3 2 1 0thickness [m]

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ulk

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otes

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he f

ive

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est

pum

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ts.

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e th

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pid

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in

max

imum

bul

k ro

ck d

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ty o

f th

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sal

unit

with

in l

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model that evaluates compound cooling units and avariety of additional factors controlling the final densitypattern of an ignimbrite, including primary thickness,thermal diffusivity of the ash, and some other boundaryconditions. The authors documented the influence of theglass composition and computed the mutual influence oftwo successive flow units. The model calculations show adensity maximum within the centre of each flow unitseparated from the other by a density low, the intensityof which depends mainly on the pause in deposition.When we apply this model of polymodal densitydistribution to the ‹ncesu ignimbrite, we can infer that the‹ncesu ignimbrite is a compound cooling unit composed ofat least three flows or, better said, emplacement units.

In a first approximation, we can further estimateemplacement temperatures and original depositionalthicknesses from the SiO2 content of the glass shards andthe maximum density of the deposit at the type section.We might expect a deposit of 40 m of uncompactedthickness deposited at ± 720 °C (Riehle et al. 1995, p.327, Figure 8).

In addition, the overall vertical density distribution –with the highest density most commonly near the base ofthe deposit – is consistent with the model of Sparks(1997) and Sparks et al. (1998). They propose thatexsolved gasses trapped in the pore space of fresh tephracan be redissolved back into the glass of shards andpumice under the conditions of an increasing pressureregime of a compacting deposit. To support their model,those authors referred to frequent lack of gas-escapepipes and higher flattening of pumice relative to thedeformation of glass shards in welded ignimbrites –characteristics that are also valid for the ‹ncesuignimbrite.

Lateral Density Variations

The model explanation here has to account chiefly for theprincipal differences between the northwesternHimmetdede lobe and the southern Firaktin-Hoflca lobe.

Pyroclastic flows propagate easily several tens ofkilometres from their source vents and are able tosurmount topographic barriers hundreds of meters high(Bursik & Woods 1996). The experiments of Woods et al.(1998) showed that the sedimentation patternsassociated with ridges of different sizes may be verydifferent. Flows involving partial blocking and formation

of upstream propagating bores in front of a large-scaleridge display enhanced sedimentation upstream from theridge, analogous to valley-ponded and caldera-filldeposits. In contrast, if a flow is able to surmount a ridge,the deposits are relatively unaffected by the ridge.

These experiments provide a possible explanation forthe observed density variation in the southern Firaktin-Hoflca lobe of the ‹ncesu ignimbrite. Pyroclastic flowswere obviously able to surmount the Hoflca Passcompletely, in so far as characteristic valley-pondedthickening of the deposit is lacking on the northern slopeof the Hoflca Pass. Down ridge, the observed thickening(and degree of welding) at location 88 is thought torepresent a deposit of lee-side concentration afterdilution of the pyroclastic flow. The southernmostlocation (109) represents the blocking of the flows infront of the Taurus Mountains. The observed relativelydense welding is interpreted in terms of compression ofthe stopped flow combined with partial backflow fromthe upper slopes. The resultant overthickening of thedeposit is made responsible for the high degree ofwelding according to the models of Riehle et al. (1995)and Sparks et al. (1998).

Summary and Conclusions

The ‹ncesu ignimbrite is identified in the field by itswelding intensity and its zoned changes in colourthroughout its distribution area. Changes in colourcorrespond to variations in modal composition. The arealdistribution of the ignimbrite is strongly controlled bypalaeotopography that channelised individual flowportions and partly terminated their runout, especially inthe Taurus forehills. However, distinct valley ponds arerare.

The vertical pattern of welding intensity shows apolymodal distribution in bulk rock density that is notobvious from field observation. The type section in thetown of ‹ncesu and, similarly, the entire distribution areaexcept for the south – southeastern lobes, has its highestbulk rock density in the lower part of the deposit andshows an upward decrease that is superposed by 1 – 3subordinate density maxima. The lateral variation ofwelding intensity discriminates two different types ofdistribution lobes which can be typified by the NW –Himmetdede lobe and the S – Firaktin-Hoflca lobe. Thenorthwestern lobe is not terminated by topographic


265

barriers at its distal end and shows a constant decrease inbulk rock density, from proximal dense welding to distalincipient/non-welding. The southern lobe overtopped the200 m elevation of the Hoflca Pass and finally came torest in front of the Taurus Mountains. In contrast to theHimmetdede area, the location at the distal end of theFiraktin-Hoflca lobe exhibits a higher bulk rock densitythan locations along the spreading path.

The ‹ncesu ignimbrite was emplaced by massivepyroclastic flows which preserved a temperature abovethe welding threshold. Welding in general occurred ascompaction welding after the flows came to rest, andapparently no secondary flow occurred during thewelding process. The polymodal distribution of bulk rockdensity indicates that the ‹ncesu ignimbrite is multiple,composed of several flow and emplacement units whichare separated from each other by short pauses indeposition and, thus, perhaps also in extrusive activity.Each emplacement unit reached the welding threshold asindicated by the various density peaks which areseparated from each other by lows corresponding tointermittent pauses.

Welding may have been promoted by volatileredissolution such that, under compaction of the depositby its own weight, the bulk rock density is highest in thelower parts. The relatively high bulk rock densities at the

distal ends of topographically terminated flows areexplained by the formation of bores in front of thebarrier and partial backflow from the slopes. These boresand backflows caused overthickening of the deposit andthereby a higher degree of compaction and, consequently,higher bulk rock density.

Acknowledgements

A detailed study of the ‹ncesu ignimbrite was initiated byUlrike Mues-Schumacher some time ago when sherecognised distinct vertical and lateral variations in rockdensity. Petra Lepetit performed the studies on theinfluence of modal composition on the bulk rock densityas part of her master’s thesis supervised by LotharViereck-Götte. He further facilitated XRF-analyses ofsamples from remote locations to identify anddiscriminate the ‹ncesu ignimbrite from overlyingdeposits during an earlier stage of the project. We all aregrateful to our Turkish colleagues, particularly VedatToprak of Middle East Technical University, Ankara, fortheir logistical support and help in exporting the samplesto Germany. The manuscript benefited greatly from athorough review with helpful comments by AnitaGrunder, Abidin Temel and Asuman Türkmeno¤lu whocourteously reviewed the manuscript for this journal.


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Received 22 July 2003; revised typescript accepted 15 March 2004

areal distribution and bulk rock density variations of the welded … · the areal distribution is...

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