An Exceptionally Thick Middle Pleistocene Tephra Layer from Epirus, Greece

QUATERNARY RESEARCH ARTICLE NO.

49, 280–286 (1998)

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An Exceptionally Thick Middle Pleistocene Tephra Layer from Epirus, Greece David M. Pyle and Tjeerd H. van Andel Department of Earth Sciences, Godwin Institute for Quaternary Research, University of Cambridge, Cambridge CB2 3EQ, UK

Panayiotis Paschos Institute for Geology and Mineral Exploration, Prevezza, Greece

and Paul van den Bogaard Geomar FZ, Christian-Albrechts-Universita¨t, Kiel, Germany Received July 24, 1997

A newly recognized 2-m-thick trachytic volcanic ash deposit from northwestern Greece is dated at 374,000 { 7000 yr and correlated with the Middle Pleistocene volcanic activity of central Italy. The deposit represents ash fallout from one of the largest volcanic eruptions in Europe of the past 400,000 yr and should provide an important stratigraphic marker within the poorly dated Middle Pleistocene deposits of Italy and Greece. q 1998 University of Washington.

INTRODUCTION

Large-magnitude volcanic eruptions provide invaluable time markers in the geological record, since they may be dated with high precision and have the potential to leave correlateable deposits in both marine and nonmarine settings. Volcanic ash layers are widespread in late Quaternary abyssal sediments of the central and eastern Mediterranean (Olausson, 1971; Keller et al., 1978; Keller, 1981), where they have attracted much attention in the search for deposits of the Bronze Age Minoan eruption of Santorini (Watkins et al., 1978; Federman and Carey, 1980; Guichard et al., 1993). Analysis of marine cores shows that individual ash layers form distinctive lobes radiating southeastward with the prevailing winds (McCoy, 1981) from the volcanoes of the Aegean Sea and southern Italy (Keller et al., 1978). The best known widespread tephra layer is the 30,000 to 40,000yr-old Y-5 ash (Barberi et al., 1978; Thunell et al., 1979; Cornell et al., 1983), which thins from ca. 20 cm south of Italy to 1 cm near Cyprus and is correlated with the Campanian Ignimbrite eruption (Barberi et al., 1978). Deposits of distal tephra in nonmarine Mediterranean set-

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Epirus principally comprises a sequence of Mesozoic to Eocene limestones with some later detrital sediments. After a major mountain-building phase in the middle Cenozoic, large parts of the landscape were leveled by erosion and limestone dissolution to form a rough karstic plain. Deformation resumed in the latest Cenozoic with the development of half-grabens and the formation of many internally drained basins strung along major fault scarps. These basins, known as poljes, accumulate fine-grained, red residual sediments (terra rossas) derived from limestone solution on adjacent slopes. The poljes form distinct bright red patches in an otherwise gray and grayish-brown landscape. Sediment accumulation continues today in many poljes, but uplift has raised others to levels where they may be dissected by headwater erosion.

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tings are rare (e.g., St. Seymour and Christianis, 1995; Vitaliano et al., 1981) and usually poorly documented in terms of age and origin, except where discoveries have been made from archaeological contexts (e.g., Boekschoten, 1971; Vitaliano and Vitaliano, 1974). Here we report the discovery of an exceptionally thick ash deposit of Middle Pleistocene age in western Epirus, Greece. This ash layer has not been reworked and has a chemical composition that links it to an Italian volcanic source 750 km distant. The deposit must have been produced by one of the largest Mediterranean eruptions of the past 400,000 yr and should form a distinctive marker horizon in Pleistocene deposits of southern Italy, mainland Greece, and the Aegean Sea.

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FIG. 1. Profile and cross section of the Morphi tephra locality (397 11* N, 207 30* E). The tephra horizon is exposed 1.5 km NE of the village of Morphi, Epirus, Greece, where it lies east of the Morphi to Paramithia road, before SpathareıF . The profile shows the relative locations of samples 20A, 7A, and 8 within the tephra bed and Munsell chart colors for selected horizons. (Inset) Location of Pleistocene calderas in central and southern Italy and the locations of marine sediment cores around the shores of Italy.

In the raised polje fill at Morphi (Fig. 1) we found a 2.5-m-thick layer of grayish-white volcanic ash intercalated between a lower sequence of gray clay and a red upper zone. The polje, now raised and tilted ca. 67 east, is being dissected and stripped by a small tributary of the Acheron River. The polje deposits (van Andel, in press) are very-finegrained (70–90% õ10 mm) and contain a small amount of clay minerals in a disordered matrix. The remaining fraction comprises fine quartz silt (ca. 10–40 mm), presumed to be of long-distance aeolian origin since it resembles in grain size and composition dust falls from modern deserts (Pye, 1992). The intercalated tephra bed, which lies in sharp contact with the underlying gray clays (Fig. 1), is weakly bedded, ca. 2.5 m thick, and slightly friable and has a bulk density of ca. 800 kg03. The major constituent is fresh microvesicular

pumice. Rarer tricuspate glass shards with a refractive index õ1.52 are also present. The grain size is predominantly 50 to 250 mm and shows no significant change from bottom to top. Toward the top, a few ill-defined reddish bands suggest that slope-wash of mixed terra rossa and tephra may have been added after deposition of the main mass, but the lower half shows no contamination. The lower half of the deposit is only weakly stratified, while the uppermost meter is parallellaminated on a scale of tens of centimeters. Compared to late Quaternary ash deposits in abyssal sediments of the eastern Mediterranean (Keller et al., 1978; Vergnaud-Grazzini, 1985; McCoy and Cornell, 1990), even the minimum thickness of the Morphi ash deposit is exceptional, since Morphi must be ca. 750 km distant from any plausible source region. Minoan tephra from Santorini are only 150 mm thick at sites 1000 km downwind from the source (Guichard et

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TABLE 1a Summary of Mineralogical Information Phase

Relative abundance within the crystal population

Potassium feldspar Plagioclase feldspar Clinopyroxene Biotite Magnetite Hornblende Apatite

High Ç10% of total feldspar Moderate Moderate Moderate Rare Rare

Composition Or69 – 26Ab28 – 59An3 – 15 ; Median Or51Ab45An4 An58 – 29Ab50 – 59Or2 – 15 Median: Wo46Fs13En37Ac3 mg# 0.75 { 0.03 Median: mg# 0.69 { 0.01 Usp8 – 11Mgt92 – 89

Note. mg#-ionic Mg/(Mg / Fe).

al., 1993). The great thickness at Morphi is comparable to tephra from the large-magnitude Bishop Tuff eruption in California, which is reportedly ca. 60 cm thick 1800 km downwind from the known vent (Izett et al., 1970). Electron microprobe analysis shows the glass to be uniformly trachytic on the basis of its total alkali and silica contents (Table 1). The trachytic glass composition strongly suggests that the tephra was derived from an Italian volcano. Crystals make up less than 1 vol% of the deposit and are dominated by potassium feldspar, with lesser amounts of plagioclase feldspar, a pale-green pleochroic clinopyroxene, and biotite (Table 1; Fig. 2). No leucite has been found in the mineral concentrates. The glass is close in composition to Pleistocene tephras erupted from the Phlegrean Fields, Italy (Fig. 1), which are distinguished from other eastern Mediterranean distal tephra units by their trachytic composi-

tions and elevated total alkali contents with K2O ú Na2O (Keller et al., 1978). The crystal assemblage is also fairly similar to that reported from Pleistocene ash layers from southern Italian sources, although direct comparison is difficult because previous workers have not reported mineral compositions from distal tephra. Age of the Morphi Deposit Feldspar separates were prepared from samples 7A and 20A with heavy liquids. The 20- to 63-mm fraction was analyzed by laser step-heating at the Geomar Tephrochronology Laboratory for the purposes of 40Ar/39Ar dating. Sample 7A-A did not yield a plateau age, but instead a range of steps with apparent ages from 0.37 to 8.7 Myr. The old ages probably represent contributions from xenocrysts of alkali

TABLE 1b Representative Microprobe Glass and Mineral Compositions Sample Wt%

Glass ({1s) 9322-1

Glass 9322-5

Glass 9322-16

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2Oa K2O Total

60.2 { 0.4 0.53 { 0.03 17.8 { 0.2 2.82 { 0.08 0.18 { 0.04 0.47 { 0.10 1.19 { 0.04 4.8 { 0.3 6.6 { 0.2 94.6

60.5 0.58 17.8 2.93 0.16 0.28 1.16 4.6 6.3 94.3

60.5 0.54 17.8 2.86 0.19 0.36 1.18 4.7 6.1 94.3

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total

Pyroxene 8c-8

Pyroxene 8c-37

Biotite 8c-6

50.9 1.02 2.29 8.75 0.87 12.9 22.3 1.16 0.04 100.1b

51.1 0.69 2.35 7.86 0.63 13.2 22.9 0.88 bd 99.8c

36.3 6.06 14.1 11.9 0.23 15.3 bd 0.68 9.31 93.9

Note. Analyses quoted are those determined on single points. Errors quoted for the first glass analysis are the standard deviations (1s) from the mean determined by the analysis of 27 separate glass points across three samples. These errors reflect a combination of instrumental reproducibility and sample homogeneity. bd, below detection limit. a Glass analyses include a 5% correction for Na loss during analysis, determined by repeated analysis of alkali glass standards with a 4-nA current and 5-mm beam. b Includes 0.05 wt% Cr2O3 . c Includes 0.14 wt% Cr2O3.

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record (McCoy and Cornell, 1990; Calanchi et al., 1994), although the age constraints remain very poor. Hole 974B has produced a record of 207 tephra-bearing horizons extending through the Pleistocene and Pliocene (Comas et al., 1996). Preliminary work (van den Bogaard et al., in press) on the tephra horizons from hole 974B shows that the band from 300,000–500,000 yr contains three tephra horizons, of which only the oldest (layer 7509; dated by 39Ar/40Ar at Ç500,000 yr) has a mineralogical composition similar to the Morphi ash. The absence of any firmly correlateable horizons awaits the completion of the more-detailed work on these recent ODP cores. It is entirely plausible that a large eruption need not have left an extensive Tyrrhenian Sea record, for both drilling sites are several hundred kilometers from land, and both are on the ‘‘upwind’’ side of the volcanoes of Latium and Campania (Fig. 1). Constraints on the Scale of the Eruption That Produced the Morphi Deposit FIG. 2. Summary of feldspar and clinopyroxene compositions determined by electron microprobe (using a 20-kV accelerating voltage, a 2mm-diameter beam, and 15-nA current). Feldspar compositions are plotted on an orthoclase (Or)–anorthite (An)–albite (Ab) triangle. Compositions of coexisting feldspars suggest an equilibration temperature close to 800– 8507C. Only one feldspar analysis from over 250 analyses of random grains is close to pure albite and is clearly xenocrystic. Pyroxenes, plotted within a section of the enstatite (En)–ferrosilite (Fs)–wollastonite (Wo) compositional triangle are compositionally uniform.

Knowledge of the implied scale of the eruption could help to place additional constraints on the exact source volcano for the tephra deposit. Studies of modern pyroclastic deposits show that volcanoes typically produce widespread, slowly thinning deposits of fine-grained tephra either by direct fallout from a plinian-style eruption column or by fallout from an ash-cloud associated with the emplacement of voluminous pyroclastic flow deposits (e.g., Sparks and Walker, 1977; Woods and Wohletz, 1991). In both cases, the resultant fall deposits thin exponentially away from their source regions (Pyle, 1989). These deposits may have strongly ellip-

feldspar. The youngest heating step of sample 7A-A yielded an estimated maximum tephra age of 370,000 { 40,000 yr. Sample 20A-A (Fig. 3) yielded a plateau age of 374,000 { 7000 yr, which we interpret as the eruption age. These results place the ash deposit in marine oxygen isotope stage 13 (Imbrie et al., 1984), or just above sapropel S11 in Ionian and Levantine Sea piston cores (Cita et al., 1977; Keller et al., 1978). Although many tephra layers have been reported from these cores, few have been seen, and none correlated, below sapropel S8 (ca. 200,000 yr B.P.; Cita et al., 1977; Vergnaud-Grazzini et al., 1977; Keller et al., 1978). An inspection of the records of the dozen holes drilled in the 1970s by the Deep Sea Drilling Project in the eastern Mediterranean (Ryan et al., 1973; Hsu¨ et al., 1978) did not reveal any evidence of correlative tephra layers, although only holes 125, 126, 132, and 376 cored nannofossil zone NN20, which contains oxygen isotope stage 13 (Berggren et al., 1995). Unfortunately, the coring schedules and the quality of the core descriptions render it likely that tephra either were not sampled or were missed by the describers. The prospects of finding a correlative layer in the tephra records of the more-recently drilled sites 650 (ODP leg 107; Karstens et al., 1990) and 974 (ODP leg 161) in the Tyrrhenian Sea should be fairly good, once a radiometrically dated time scale is in place. Site 650 preserves a good tephra

FIG. 3. Results of laser step heating analyses of feldspar sample 20AA. The sample was irradiated in an Al-foil capsule with a 1-mm Cd liner for 72 h at the GKSS research reactor, Geesthacht, Germany. TCR sanidine and high purity CaF2 and K2SO4 crystals were used as irradiation monitors. The ca. 15-mg sample was incrementally heated by continuous scanning with a defocused laser beam with the power increasing from 50 mW to ú20 W. Analyses, corrected for interferences, are shown with 1s error boxes. The total gas age is 486,000 { 9000 yr, with a plateau age determined for the filled boxes which represent three consecutive heating steps and account for 58% of the 39Ar release of 374,000 { 7000 yr. This is interpreted as the eruption age.

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tical isopach distributions with aspect ratios of up to Ç10 (e.g., Hildreth and Drake, 1992). The Italian volcanic source of the Morphi deposit must have been at least 750 km distant from Morphi. If it is assumed, conservatively, that tephra fallout formed a very strongly elliptical deposit and covered an area of 750 1 75 km to 1 m depth with ash and that this deposit thinned exponentially, then the bulk volume of fallout tephra included within the 1-m isopach must have been at least 70 km3. After taking into account the more widely dispersed, exponentially thinning tephra (Pyle, 1995; Pyle, in press), the minimum bulk tephra volume was at least 200 km3. By analogy with other large-magnitude eruptions, most of this ash was probably supplied from a comparable volume of simultaneously erupted ignimbrite (Sparks and Walker, 1977). This implies that the deposit formed as a result of the eruption of at least 70 km3, and most probably 130–400 km3, of dense trachytic magma. This exceeds the volume of the largest known Pleistocene eruption in the Mediterranean basin, the 80 km3 Campanian Ignimbrite. Although these constraints are clearly approximate, for they are based on a single thickness measurement, they are nevertheless secure: this volume estimate is linearly dependent on the assumed area of the 1-m isopach and more or less linearly dependent on the assumed primary thickness of the deposit at Morphi. Thus, even if the original deposit was only 20 cm thick at Morphi and it had therefore been overthickened by a factor of 10, the minimum erupted volume of dense magma would have been 20 km3; this would still represent the products of a significant eruption. An eruption of the scale required to produce the Morphi deposit would have formed a caldera, or volcano–tectonic depression, at least 10–15 km in diameter (Spera and Crisp, 1981). The only mapped feature in central Italy which is of this scale and contains products spanning the correct age and composition is the Baccano–Sacrofano caldera, which lies adjacent to the present Lake Bracciano, and forms a part of the Pleistocene Sabatini volcanic complex (De Rita et al., 1983, 1996; Fig. 1). The ‘‘upper pyroclastic flow’’ (or Sacrofano yellow tuff) of the Sacrofano caldera (Alvarez, 1973; De Rita et al., 1983) deserves to be investigated further as the possible source of the Morphi deposit. Potassium– argon dating of sanidines from the Sacrofano yellow tuff gives conflicting ages of 370,000 { 70,000 yr (Alvarez et al., 1976) and 288,000 { 6000 yr (unpublished analysis by J. M. Villa, cited in Fornaseri, 1985). Thus, a correlation with the Morphi deposits cannot be ruled out. From the published literature, the composition of the ‘‘upper pyroclastic tuff’’ remains unclear, although other products of the volcano are reported as being phonolitic to trachytic (De Rita et al., 1983). Whereas there is a lack of accessible, published work on the nature and distribution of the pyroclastic deposits of the Sabatini volcano with which to assess

the scale of any of the eruptions of the region, it is clear that magmatism over the past 500,000 yr must have been responsible for significant collapse to form both the Baccano–Sacrofano caldera structure and the Bracciano depression (De Rita et al., 1996). Deposits with mineral 40Ar– 39Ar ages of ca. 370,000– 380,000 yr have been reported from Roccamonfina (Luhr and Giannetti, 1987) and the Latera (or Vulsini) caldera (the 381,000 { 9000 yr ‘‘PF0’’ phonolite; Turbeville, 1992; the 342,000 { 7000 yr ‘‘Orvieto-Bagnoregio ignimbrite’’ and 349,000 { 7000 yr pyroclastic fall deposit P2; Santi, 1990; Nappi et al., 1994). However, both of these centers and others in the region have considerably smaller calderas than would be expected for an eruption of the magnitude required to deposit the Morphi ash. The Alban Hills caldera is of the correct scale and has experienced major eruptions in the correct age range (300,000–400,000 yr), in common with many of the Roman province volcanoes, but the Alban Hill products have a very different (phonotephritic) composition than the Morphi tephra (Trigilo, 1995). An alternative testable hypothesis is that the source for the Morphi tephra is no longer visible, having since been covered by younger deposits. One location where such a vent may be hidden is in the Campanian region, near the source of the younger Campanian tuff. Despite the considerable work on Campi Flegrei and the Campanian tuff over the years (e.g., Rosi et al., 1983; di Girolamo et al., 1984; Scandone et al., 1991; Barberi et al., 1991), there is still disagreement over the existence and location of the collapse caldera associated with the ca. 36,000- to 40,000-yr Campanian ignimbrite eruption. While many workers consider the Phlegrean Fields (Campi Flegrei) to be the caldera associated with this eruption, Scandone et al. (1991) have pointed out the existence of a larger volcano–tectonic depression, the Acerra depression, which they suggested may represent a collapse structure associated with the Campanian ignimbrite. One way to confirm the nature of the correlation would be to analyze the Morphi glass for its trace element composition, because there are distinct geographical variations in the trace element chemistry of the young volcanic products of the Roman province (Beccaluva et al., 1991). At present, however, reliable trace-element data are only patchily available for pyroclastic rocks from the potential source volcanoes (because many of the products of large eruptions form welded or otherwise modified deposits; Barberi et al., 1978), and there are even fewer trace-elemental analyses of glasses from marine cores.

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Implications for the Terrestrial Middle-Pleistocene Volcanic and Sedimentary Record of Italy and Greece The potential for establishing an improved chronology for the middle Pleistocene deposits of central and southern Italy based on the volcanic record was noted many years ago

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(Alvarez, 1973), and yet this record remains underexploited. A rich Pliocene–Pleistocene distal tephra record is preserved within the sedimentary basins across the Appenines and southern Italy (e.g., Spadea, 1986; Ermolli et al., 1995; Alvarez et al., 1996), but there is an absence of any firmly correlated units older than 36,000–40,000 yr. In addition to the marine core record, there are also Pleistocene records from Lake Ioannina, Greece, and from elsewhere in northern Greece (e.g., Wijmstra and Groenhart, 1983). The lack of attention paid to these accessible on-land examples is rather remarkable, given the recent great interest in establishing a marine chronology for the eastern Mediterranean for the past 200,000 yr. The chronology of the Pleistocene sedimentary deposits of Greece is almost as insecure as the volcanic history. For instance, a widely held view (e.g., Bailey et al., 1992, p. 140) assigns a Pliocene age to the redeposited terra rossa beds of the poljes of Epirus and contends that the Middle Palaeolithic industries that these beds contain are not in situ and therefore of no archaeological interest. The dating of the Morphi tephra renders this view untenable, with significant consequences for the middle and late Quaternary history and tectonics of northwestern Greece. CONCLUSIONS

The discovery of a 2-m-thick trachytic tephra fall deposit in northern Greece, with an age of 374,000 { 7000 yr, promises to extend the terrestrial tephrochronological record of Italy and Greece far back into the Middle Pleistocene. Fallout tephra from the exceptional eruption which formed the Morphi deposit presents a much-needed ‘‘golden spike’’ in the Pleistocene sedimentary sequence that should be present across southern Italy, mainland Greece, and the Aegean Sea. The Middle Pleistocene chronology of the eastern Mediterranean lacks distinctive regional time-markers, which the Morphi ash significantly relieves. Tephra from this exceptional eruption should occur in Middle Pleistocene deposits across southern Italy, Greece, and Turkey and in marine sediments across the Aegean, Black, and eastern Mediterranean seas. ACKNOWLEDGMENTS We are indebted to Maurice Haslop for feldspar and glass separations, Dan Karner for discussion, and Etienne Juvigne´ and another referee for comments.

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