40Ar/39Ar Dating of a Langhian Biotite-Rich Clay Layer in the Pelagic Sequence of the Co©Nero Riviera, Ancona, Italy

Earth and Planetary Science Letters 194 (2001) 111^126 www.elsevier.com/locate/epsl

40Ar/39Ar dating of a Langhian biotite-rich clay layer in the pelagic sequence of the Co©nero Riviera, Ancona, Italy

Dieter Mader a, Alessandro Montanari b,Je¨roªme Gattacceca c, Christian Koeberl a;b;*, Robert Handler d, Rodolfo Coccioni e

a Institut fu«r Geochemie, Universita«t Wien, Althanstrasse 14, 1090 Vienna, Austria b Osservatorio Geologico di Coldigioco, I-62020 Frontale di Apiro, Apiro, Italy c Ecole des Mines de Paris, CGES, 35 rue Saint-Honore¨, F-77305 Fontainebleau, cedex, France d Institut fu«r Geologie und Pala«ontologie, Universita«t Salzburg, Hellbrunnerstrasse 34, 5020 Salzburg, Austria e Istituto di Geologia, Universitaª degli Studi Sogesta, Localita© Crocicchie, 61029 Urbino, Italy Received 16 July 2001; received in revised form 6 September 2001; accepted 4 October 2001

Abstract

A nearly complete and undisturbed Miocene carbonate sequence is present in the easternmost part of the Umbria- Marche basin, Italy, which is ideal for detailed and integrated stratigraphic investigations of the Miocene Epoch. In this study, we were trying to obtain evidence for the presence or absence of distal ejecta from the 15 Ma Ries impact structure in southern Germany, located about 600 km to the north^northwest of the Umbria-Marche basin. The first step is to find coeval strata in the Umbria-Marche sequence. At the La Vedova section, Co©nero Riviera, we dated a volcaniclastic biotite-rich clay layer, the Aldo Level, which is situated within planktonic foraminiferal Zone N8, at 14.9 þ 0.2 Ma, using the 40Ar/39Ar method. Together with detailed geologic and stratigraphic information about the Aldo Level, the resulting age can be used confidentially to calibrate the Langhian stage. Besides providing new constraints on Miocene geochronology, this age can now be used for impact stratigraphic studies. To directly correlate the biotite ages of the La Vedova section with rocks from the Ries impact event, Ries impact glass was also analyzed and found to be coeval. Although unrelated to this impact event, the biotite-rich clay layer should help in the search for evidence of distal ejecta related to the Ries crater. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: Ries Crater; ejecta; Langhian; Ar/Ar; Italy; stratigraphy; volcaniclastics

1. Introduction in age from the Late Triassic to the Pleistocene ([1], and references therein). Three tectonic re- The Umbria-Marche (U-M) region in the gimes occurred during this time. Rifting during northeastern Apennines consists of a continuous the Late Triassic to Early Jurassic between Eu- sequence of marine sedimentary rocks that range rope and Africa led to the formation of new oce- anic basins, such as the Penninic-Ligurian Ocean [2], which surrounded the Adria microcontinent [3]. Several pelagic basins, such as the U-M basin, * Corresponding author. developed in response to this extensional phase, E-mail address: [email protected] (C. Koeberl). which continued until the earliest Cretaceous, on

EPSL 6042 13-12-01 112 D. Mader et al. / Earth and Planetary Science Letters 194 (2001) 111^126 the northern margin of an African promontory. eastward, were ¢lled by thick £ysch deposits By the Early Cretaceous, the irregular horst and (sandstones and marls). graben topography, which developed in this ¢rst In the eastern part of the U-M basin, in con- tectonic phase, was almost completely leveled by trast to the synorogenic siliciclastic sedimentation deep water, pelagic carbonate sediments. In the occurring throughout the rest of the Apennines, Late Cretaceous, after a prolonged time (Aptian pelagic limestones and marls continued to accu- to Turonian) of regional tectonic quiescence and mulate until the Late Miocene. Thus, a nearly pelagic sedimentation, the U-M basin experienced complete and undisturbed carbonate sequence is a renewed extensional tectonic activity, which was present in the easternmost part of the U-M basin, progressively reduced until the Oligocene. This which is extensively exposed on the coastal cli¡s second regional tectonic phase was followed, in of the Co©nero Riviera, just south of the port city the early-mid Miocene, by a reversal of tectonic of Ancona (Fig. 1). For this reason, the Miocene regime, from extensional to compressional, repre- exposures of the Co©nero Riviera are an ideal area senting, in this area of the Apennines, the begin- for detailed and integrated stratigraphic investiga- ning of the Alpine^Himalayan orogenesis. The tions of the Miocene Epoch [1,4]. onset of folding and thrusting was preceded by Several volcaniclastic clay layers, in the form of the formation of NW^SE elongated foredeep ashfall deposits of distant volcanic eruptions, are troughs, which, as the orogenic front advanced interbedded and well preserved in the Upper Pa-

Fig. 1. Location map of the study area (from [4]). See text for explanation.

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foraminiferal Zone N8 [24] (mid-Langhian; see Fig. 3). An age of about 15 Ma was inferred by extrapolating the radiometric ages of two dated clay layers in the Serravalian section of the Co©- nero Riviera [4]. At a small outcrop at L'Annun- ziata near Apiro, a similar (maybe the same) biotite-rich level was identi¢ed in the same biozone, and yielded a preliminary 40Ar/39Ar laser fusion age from nine sanidine grains of about 15 Ma [14]. Besides helping to constrain the Miocene geochronology, this age might have speci¢c implications for impact stratigraphic studies [1], as the Co©nero Riviera is located within 600 km of the Ries impact structure in southern Germany, which has been dated at 15.1 Ma [15^17]. If it is possible to constrain the age of the layers in the La Vedova section (Fig. 3), the search for impact signatures related to the Ries event, such as shocked quartz or geochemical anomalies, or un- usual bioevents within the La Vedova section, will be possible. To obtain a direct radioisotopic correlation between the biotite ages of the La Vedova

Fig. 2. Panoramic view of the La Vedova outcrop (base of photograph approximately 2 m). 24.1 m is the distance from the base of the Vedova section, which is some meters to the right of the base of this outcrop (just below 17 m).

leogene^Lower Neogene deep water carbonate sequence. Their provenance is still unknown, although a Sardinian source was proposed for an Early Miocene volcaniclastic layer in the Bis- ciaro Formation [5,6]. Nevertheless, such ash layers provide an excellent opportunity for cali- Fig. 3. Integrated stratigraphy of the La Vedova section brating, with precise and accurate radioisotopic (from [4]) and correlated stratigraphic position of the Ries dating, the Tertiary time scale, and several de- impact event based on radioisotopic ages from suevite and moldavite [17] glasses. The numerical time scale shown in tailed studies were done on Eocene^Oligocene bi- this ¢gure is derived from the interpolated age of 12.9 Ma otite-rich clay layers in Italy [7^13]. Some biotite- for the Respighi Level, which is located at 101.4 m in the rich levels have also been dated in the Miocene MDC section [4], and the new age of 14.9 Ma for the Aldo formations of the U-M sequence [4]. In the La Level at 24 m in the La Vedova section, as documented in Vedova section at the Co©nero Riviera (Fig. 2), a this work. Biostratigraphic zonations are from: (1) Blow [24]; (2) Iaccarino and Salvatorini [22]; (3) Iaccarino [23]; volcaniclastic biotite-rich layer, named Aldo Lev- (4) Martini [25]; (5) Okada and Bukri [26]; (6) Fornaciari et el, has been determined to be within planktonic al. [27].

EPSL 6042 13-12-01 114 D. Mader et al. / Earth and Planetary Science Letters 194 (2001) 111^126 section and rocks from the Ries impact event, and In contrast to the western areas of the U-M to exclude interlaboratory bias between published basin, where the Schlier Formation displays re- Ries ages and our data for the La Vedova bio- duced thicknesses and is overlain by the Marnoso tites, we also dated glass from Ries crater suevites Arenacea £ysch since the Langhian, the eastern (polymict impact breccia; see, e.g. [18]) and a part of the basin is received by the siliciclastic moldavite, a tektite probably derived from the turbidites not before the late Messinian. The Ries crater (e.g. [19]). Schlier Formation of the Co©nero Riviera is sub- divided into three lithostratigraphic subunits from 1.1. Geological setting and lithostratigraphy base to top: (a) the Massive Member, (b) the Calcareous Member, and (c) the Marly Member. The marly limestones and marls of the Co©nero The Massive Member is a 35 m thick sequence of Riviera, south of the port city of Ancona, repre- seven massive beds of hemipelagic marls [20]. sent a 300 m thick pelagic sequence covering the Above them follows the Calcareous Member, entire Miocene Epoch. The Middle to Upper with a rhythmic sequence of planktonic foramini- Miocene portion of this sequence (Upper Lan- feral marly limestones and strongly bioturbated ghian to the Lower Messinian) is continuously marls. Within the upper part of this member, fre- exposed along the coastal cli¡s of Monte dei Cor- quent thin beds of black shale are interbedded vi, northwest of the Trave Reef (Fig. 1). It con- between marls and marly limestone couplets. tains the upper part of the Schlier Formation The previously mentioned Cavolo marker is lo- (Serravallian and Tortonian), and the Euxinic cated within the middle part of this member. Shale unit at the base of the Gessoso-Sol¢fera The Calcareous Member is overlain by the Marly Formation (Messinian), which represents the in- Member, which consists of marls and several in- ception of the Messinian salinity crisis. The tercalated marly limestones and black shales. A Schlier Formation is a rhythmic sequence of 3 m thick sequence of marly limestones containing hemipelagic and pelagic marls and marly lime- a biotite-rich layer, named the Rossini Level, is stones interbedded with frequent black shales, found in the upper part of this Marly Member. and containing rare, distal volcanic ash layers. The Schlier Formation is topped by the bitumi- The composite sedimentary sequence of the nous shales and marls of the Euxinic Shale unit, Monte dei Corvi was derived by Montanari et which is, in turn, overlain by the evaporitic al. [4] from three sections. From S to N, these gypsum deposits of the Gessoso-Sol¢fera Forma- sections are: (a) the Monte dei Corvi (MDC) tion. along the beach, (b) the combined sections of The La Vedova section (Figs. 2 and 3), which La Sardella and the higher cli¡s of the Monte was investigated for this study, consists of a 40 m dei Corvi (SAR-MCH), and (c) the La Vedova thick continuous sequence of bioturbated marine (VED), which extends along the beach below the marls and marly limestones at the foot of the homonym locality (Fig. 1). The ca. 90 m thick Monte dei Corvi cli¡s (upper Massive Member MCD section contains two biotite-rich volcani- and lower Calcareous Member). A biotite-rich clastic layers, the Resphigi and Ancona levels, volcaniclastic layer, the Aldo Level, occurs within which yielded 40Ar/39Ar ages of 12.9 þ 0.2 Ma this section at 24.1 m (Gattacceca, 1995, unpub- and 11.4 þ 0.2 Ma, respectively [4]. The SAR- lished report, [21]). This level is thought to corre- MCH section, which is exposed along the slope spond to a similar marker horizon near Apiro: of a landslide scarp, also contains a volcaniclastic the Michelangelo Level of the L'Annunziata sec- layer named the Rossini Level. With the exception tion [14]. Unfortunately, the abundant biotites in of a 10 cm thick turbidite called the Cavolo the Michelangelo Level were not suitable for ra- marker, which is located within the mid-Serraval- diometric dating because they are partially altered lian portion of the Schlier, the whole Langhian to to vermiculite. However, 40Ar/39Ar ages of some Messinian sequence of Monte dei Corvi is virtu- isolated sanidine grains of the Michelangelo Level ally devoid of siliciclastic or detrital beds [4]. show an apparent age of about 15 Ma [14].

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1.2. Biostratigraphy and paleobathymetry Bukry [26] respectively, is virtually missing. H. walberdorfensis is absent and the end of the ab- The stratigraphical interval between 7 and 41 m sence interval zone of S. heteromorphus (end of was studied for the planktonic foraminiferal, Zone MNN4b, [27]) occurs at 20 m. This feature benthic foraminiferal, and calcareous nannofossil indicates that the studied stratigraphical interval assemblages [4]. (7^41 m) is to be ascribed to the upper part of Zone NN4 and the lower part of Zone NN5 of 1.2.1. Planktonic foraminifera Martini [25] and to the upper part of Zone CN3 All the events useful for the Langhian in the and the lower part of Zone CN4 of Okada and Mediterranean zonal models of Iaccarino and Sal- Bukry [26]. With reference to the zonation of For- vatorini [22] and Iaccarino [23] were recognized, naciari et al. [27], the studied stratigraphical inter- whereas only some events used by Blow [24] to val spans the upper part of Zone MNN5b to the de¢ne his standard model were identi¢ed (Fig. 3). lower part of Zone MNN5a. Praeorbulina glomerosa sicana is already present In summary, planktonic foraminifera and in the lowest samples of the section, which can calcareous nannofossils indicate that the studied be assigned to the N8 Zone of Blow [24], and stratigraphical interval is Langhian in age. the Praeorbulina glomerosa Zone of Iaccarino and Salvatorini [22] and Iaccarino [23]. Praeorbu- lina glomerosa glomerosa ¢rst occurs at 24 m, followed by the ¢rst occurrences (FOs) of Praeorbu- lina glomerosa curva and Praeorbulina glomerosa circularis at 24.1 m and 28.5 m, respectively. All these events occur within the N8 Zone of Blow [24] and the P. glomerosa s.l. Zone of Iaccarino and Salvatorini [22] and Iaccarino [23]. The FO of Orbulina suturalis is found at 30 m, de¢ning the N8/N9 zonal boundary of Blow [24] and the boundary between the P. glomerosa s.l. and the O. suturalis^Globorotalia peripheroronda zones of Iaccarino and Salvatorini [22] and Iaccarino [23].

1.2.2. Calcareous nannofossils The distribution patterns of six stratigraphically important species (Helicosphaera ampliaperta, Helicosphaera walbersdorfensis, Sphenolithus heteromorphus, Calcidiscus premacintyrei, Reticulofe- nestra pseudoumbilicus, and Cyclicargolithus £ori- danus) were chosen for a detailed biostratigraphic zonation of the studied stratigraphical interval at La Vedova (see Fornaciari, Rio and Mozzato in [4]). For the biostratigraphic classi¢cation of this section, we refer to the standard zonations of Martini [25], and Okada and Bukry [26], as well Fig. 4. X-ray di¡raction patterns of biotite £akes from the as the recently established regional Mediterranean Aldo Level oriented parallel to (001). Sample A shows a par- Middle Miocene zonation of Fornaciari et al. [27]. tial vermiculitization of the biotite, whereas sample B is com- posed of pristine biotite. Sample A was collected in the im- H. ampliaperta, which de¢nes the top of zones mediate subsurface of the outcrop whereas sample B was NN4 and CN3 of Martini [25] and Okada and collected 60 cm deep into the outcrop.

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1.2.3. Paleobathymetry were studied by X-ray di¡ractometry and electron Paleobathymetric assessment is based on the microprobe (Gattacceca, 1995, unpublished re- distribution of benthic foraminiferal index forms port). X-ray di¡ractometry of oriented biotite as reported by Van Morkhoven et al. [28]. This £akes shows very slight vermiculitization from a section lacks faunal components typical of Neo- sample collected in the immediate subsurface of gene abyssal depths, and contains few specimens the outcrop (Fig. 4A), whereas biotite from a of forms restricted to neritic-upper bathyal. On sample collected deeper in the outcrop shows no the other hand, this section contains taxa most traces of vermiculite on X-ray di¡raction patterns common at middle-upper bathyal depths (Ano- (Fig. 4B). malinoides £inti, Anomalinoides helicina, Bulimina Chemical compositions of biotite from the Aldo jarvisi, Cibicidoides pachyderma forma bathyalis, Level were determined on 14 individual £akes Eggerella bradyi, Hanzawaia ammophila, Hoeglun- (Table 1) using a Camebax (Cameca0) electron dina elegans, Laticarinina pauperata, Planulina microprobe with wavelength dispersive spectrom- renzi, and Sigmoilospsis schlumbergeri). This pro- eters. An acceleration voltage of 20 kV and a vides a paleobathymetric estimate of 600 m. Ac- sample current of 20 nA was used. Natural min- cordingly, the P/(P+BU100) ratio (foraminiferal erals and simple oxides were used as standards. planktonic/benthic ratio) shows values typical for The biotite £akes were mounted in araldite, pol- a bathyal depositional environment. ished, and coated with carbon. X-ray absorption, atomic number e¡ect and X-ray £uorescence were 1.3. The biotite of the Aldo Level corrected by standard ZAF procedures. The low standard deviations and a limited scatter of ele- The 3^6 cm thick clayey layer of the Aldo Level mental compositions suggest that the biotite £akes contains biotite £akes up to 0.5 mm in diameter, of the Aldo Level are chemically very homogene- which are suitable for radiometric dating by the ous, and that they represent a population of min- 40Ar/39Ar technique. Feldspar crystals, mostly eral grains produced by the same volcanic erup- plagioclase, are also present in this volcaniclastic tion. This precludes the possibility that this material, but we were not able to separate enough volcaniclastic material is made of mixed and re- sanidine or K-rich plagioclase for additional ra- worked material derived from di¡erent sources. dioisotopic dating. Biotites from the Aldo Level In summary, XRD and microprobe analysis of

Table 1 Chemical composition (in wt%) of biotite £akes from the Aldo volcaniclastic Level

a SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O Total 35.37 3.67 12.72 26.22 7.67 0.23 0.39 8.56 95.20 35.24 3.68 12.49 26.09 7.64 0.01 0.36 8.58 94.41 35.18 4.43 12.56 24.62 7.94 0.03 0.43 8.43 93.83 35.46 4.25 12.40 24.41 7.89 0 0.38 8.30 93.36 35.10 4.36 12.70 24.57 7.86 0 0.45 8.33 93.56 35.06 3.67 12.37 24.41 8.50 0.04 0.22 8.72 93.43 34.78 4.27 12.49 25.18 7.35 0 0.36 8.39 93.16 35.45 4.43 12.65 23.98 8.40 0.02 0.44 8.38 93.98 35.23 3.97 12.64 24.48 8.12 0.01 0.38 8.44 93.55 34.99 4.40 12.64 24.23 8.10 0.01 0.39 8.35 93.33 35.18 4.55 12.56 24.09 8.41 0 0 8.58 93.54 34.91 4.71 12.67 24.60 8.24 0.03 0.46 8.55 94.39 35.03 4.37 12.52 24.05 8.28 0.01 0.43 8.33 93.27 36.12 4.58 12.45 23.92 8.28 0.15 0.41 8.21 94.27 Mean value 35.22 4.24 12.56 24.63 8.05 0.04 0.36 8.44 93.80 Standard deviation 0.32 0.35 0.11 0.72 0.34 0.07 0.12 0.14 0.58 a Total iron as FeO.

EPSL 6042 13-12-01 D. Mader et al. / Earth and Planetary Science Letters 194 (2001) 111^126 117 biotite from the Aldo Level show that this mate- crushed in a mortar before they were cleaned rial is generally homogeneous and unaltered, and and hand-picked, using the same procedure used it can be considered as a reliable geochronometer for the biotites. for radioisotopic age determinations. The individually hand-picked mineral concen- trates were packed in aluminum foil, and encap- sulated in sealed quartz vials. For calculation of 2. Materials and methods the J-values, monitor minerals (B4M) were placed between each 3^4 unknown samples. The sealed Samples were collected from freshly cleaned quartz vials were irradiated in the MTA KFKI areas about 10 to 30 cm below the outcrop sur- reactor (Budapest, Hungary) for 32 h. Correction face of the Aldo Level. In addition, a sample was factors for interfering isotopes have been calcu- taken from the marl immediately above the Aldo lated from 10 analyses of two Ca-glass samples Level (5^10 cm), which ^ due to bioturbation ^ and 22 analyses of two pure K-glass samples, 36 37 34 contains some biotites reworked from the Aldo and are: Ar/ Ar Ca = 2.6025U10 ( þ 2.3733 39 39 37 34 Level. The collected soft material was disinte- U10 ), Ar/ Ar Ca = 6.5014U10 ( þ 7.4329 39 40 39 32 grated in dilute H2O2 (3%) for several hours. U10 ), and Ar/ Ar K = 1.5466U10 The clay and silt fraction was removed using a ( þ 7.4546U1037). Variation in the £ux of neu- polyester cloth with 63 Wm mesh size under run- trons was monitored with the B4M white mica ning water. The remaining material was dried standard [29], for which a 40Ar/39Ar plateau age under an infrared lamp at a temperature of about of 18.6 þ 0.4 Ma has been reported [30]. After 80³C. irradiation, the minerals were unpacked from In order to remove fragments larger than 500 the quartz vials and the aluminum foil, and Wm, the remaining material was dry sieved and hand-picked into 1 mm diameter holes within separated into the fractions of 500^250 Wm, 250^ one-way aluminum sample holders. 160 Wm and 160^63 Wm. The ¢nest fraction was 40Ar/39Ar analyses were carried out at the In- not used. The remaining two fractions were stitute for Geology and Paleontology at the Uni- washed repeatedly in an ultrasonic bath with de- versity of Salzburg using an UHV Ar-extraction ionized water and isopropanol for 10 min each. line equipped with a combined MERCHAN- After drying at 50^70³C, the loose material was TEK1 UV/IR laser ablation facility, and a VG- removed by further dry sieving. Subsequently, the ISOTECH1 NG3600 Mass Spectrometer. biotite was concentrated with a Frantz isodynam- Stepwise heating analyses of samples are per- ic magnetic separator, using a lateral inclination formed using a defocused (V1.5 mm diameter) of 20³ and a longitudinal inclination of 10³. Most 25WCO2-IR laser operating in Tem00 mode at of the biotite was separated by 0.4 A ¢eld wavelengths between 10.57 and 10.63 Wm. The strength, and some residual grains by 0.8 A. The laser operating conditions and movement are con- isolated and concentrated biotite samples were trolled by computer, and the position of the laser again cleaned in an ultrasonic bath as described on the sample is monitored through a double-vac- above. After drying at 50³C, the biotites were uum window on the sample chamber via a video hand-picked under a binocular microscope for ra- camera in the optical axis of the laser beam on the diometric dating. computer screen. Gas clean-up is performed using Glasses were extracted from suevites from the one hot and one cold Zr^Al SAES getter. Gas Aumu«hle and Heerhof quarries at the Ries impact admittance and pumping of the mass spectrome- structure (cf. [18]), and a moldavite from Janov in ter and the Ar-extraction line are computer con- the Czech Republic was also prepared for radio- trolled using pneumatic valves. The NG3600 is a metric dating. These samples were selected be- 18 cm radius 60³ extended geometry instrument, cause they provide a direct age correlation with equipped with a bright Nier-type source operating the Ries impact event, which has been dated at 15 at 4.5 kV. The measurements are performed on an Ma (see below). These glasses were coarsely axial electron multiplier in static mode, and peak-

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Table 2 40Ar/39Ar analytical data from multigrain incremental heating analysis on volcaniclastic biotites from the Co©nero Riviera, Italy Increment 36Ar/39Ara37Ar/39Arb40Ar/39Ar % 39Ar % 40Arc Age þ 1c (Ma) (Ma) Sample AL160B1 1 0.001809 0.012507 0.700766 2.68 23.72 10.0 2.6 2 0.000508 0.006149 0.392362 5.24 61.75 14.9 1.2 3 0.000647 0.009855 0.316918 2.38 39.70 7.3 2.7 4 0.000329 0.005823 0.325780 6.10 70.14 14.0 0.9 5 0.000240 0.006122 0.337297 9.00 78.99 16.5 0.8 6 0.000253 0.004125 0.311585 10.07 75.99 14.6 0.5 7 0.000479 0.004157 0.403550 5.42 64.95 16.2 1.1 8 0.000526 0.002432 0.367171 7.69 57.63 12.9 0.9 9 0.000790 0.005829 0.417089 2.80 44.00 11.1 2.0 10 0.001661 0.004086 0.723323 5.42 32.15 14.3 1.3 11 0.001000 0.006607 0.448418 3.10 34.13 9.1 2.1 12 0.000152 0.001980 0.253995 2.61 82.30 12.7 2.6 13 0.000301 0.000233 0.256603 2.35 65.32 10.0 2.8 14 0.000202 0.032048 0.305122 10.03 80.42 15.3 0.7 15 0.000027 0.014333 0.258850 19.20 96.96 15.5 0.4 16 0.000135 0.001883 0.261160 5.92 84.71 13.5 1.2 Total 0.000412 0.008411 0.351765 100.00 65.43 14.1 0.1 Sample AL160B2 1 0.002247 0.104335 0.992153 1.35 33.08 21.0 3.0 2 0.001910 0.031917 0.602286 1.62 6.30 1.6 1.6 3 0.001178 0.021030 0.449056 2.42 22.45 5.7 1.3 4 0.000870 0.018323 0.410100 3.47 37.34 9.2 0.9 5 0.000520 0.012343 0.343428 5.30 55.30 11.5 0.5 6 0.000548 0.010450 0.359574 5.98 54.99 12.0 0.5 7 0.000826 0.009816 0.443338 8.29 44.91 12.1 0.5 8 0.000638 0.011357 0.371921 4.00 49.29 11.1 0.9 9 0.000325 0.008188 0.314766 7.63 69.47 13.4 0.4 10 0.000502 0.012146 0.339655 4.17 56.32 11.6 0.7 11 0.000085 0.010039 0.263475 6.44 90.44 14.7 0.6 12 0.000219 0.001934 0.281612 4.72 77.01 13.2 0.8 13 0.000102 0.000839 0.263536 6.15 88.57 14.3 0.7 14 0.000065 0.005321 0.262112 10.20 92.67 15.0 0.4 15 0.000044 0.008259 0.255521 18.90 94.97 15.0 0.3 16 0.000116 0.010966 0.264564 3.87 87.10 14.2 1.0 17 0.000063 0.014299 0.263517 2.62 92.95 15.1 1.5 18 0.000199 0.001600 0.268247 2.87 78.13 12.8 1.4 Total 0.000369 0.011326 0.324130 100.00 66.37 13.2 0.1 Integrated (steps 5^18) 91.14 13.6 0.6 Sample AL250B 1 0.002490 0.156526 0.962041 1.28 23.51 14.6 1.6 2 0.000597 0.020276 0.432623 1.26 59.22 15.9 1.4 3 0.000475 0.016747 0.375636 2.13 62.63 14.5 0.8 4 0.000576 0.012559 0.409282 3.58 58.42 14.8 0.6 5 0.000422 0.010393 0.336565 1.85 62.98 13.0 0.9 6 0.000341 0.009875 0.325060 3.74 68.99 13.8 0.6 7 0.000293 0.008430 0.305288 5.63 71.63 13.4 0.4 8 0.000326 0.006819 0.346507 3.48 72.19 15.4 0.6 9 0.000163 0.007069 0.293253 6.93 83.60 15.1 0.3 10 0.000252 0.006041 0.313369 4.13 76.20 14.7 0.4 11 0.000189 0.006261 0.301164 7.28 81.45 15.1 0.3 12 0.000075 0.003022 0.261672 2.89 91.49 14.7 0.6

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Table 2 (continued) Increment 36Ar/39Ara37Ar/39Arb40Ar/39Ar % 39Ar % 40Arc Age þ 1c (Ma) (Ma) 13 0.000049 0.003964 0.266349 4.28 94.51 15.5 0.4 14 0.000074 0.005498 0.261879 2.98 91.60 14.8 0.6 15 0.000023 0.006871 0.269684 5.55 97.49 16.3 0.3 16 0.000067 0.024107 0.263501 21.32 92.44 15.1 0.2 17 0.000099 0.022041 0.263606 11.56 88.87 14.5 0.2 18 0.000106 0.016866 0.264680 3.75 88.22 14.4 0.5 19 0.000014 0.009810 0.251706 6.39 98.41 15.3 0.3 Total 0.000192 0.014537 0.297608 100.00 80.89 14.9 0.2 Integrated (steps 8^19) 80.53 15.1 0.3 Sample LV160B 1 0.000668 0.006489 0.387058 2.71 49.01 11.5 1.4 2 0.000426 0.003592 0.367153 2.79 65.71 14.9 1.3 3 0.000015 0.002596 0.249343 3.34 98.22 15.1 1.1 4 0.000091 0.002046 0.257496 4.41 89.59 14.2 0.8 5 0.000099 0.003258 0.241792 2.61 87.92 13.0 1.4 6 0.000009 0.003158 0.244744 3.99 98.90 14.9 0.9 7 0.000061 0.001751 0.248754 2.31 92.75 14.2 1.5 8 0.000297 0.004455 0.302482 5.21 70.96 13.1 0.7 9 0.001465 0.002331 0.687497 2.89 37.04 15.7 1.8 10 0.000170 0.006565 0.260927 4.75 80.71 12.9 0.7 11 0.000065 0.001029 0.261649 2.84 92.66 14.9 1.0 12 0.000074 0.003839 0.263246 4.97 91.66 14.9 0.7 13 0.000041 0.006505 0.262234 9.97 95.40 15.4 0.4 14 0.000032 0.005344 0.260008 47.21 96.39 15.5 0.2 Total 0.000129 0.004063 0.279685 100.00 86.33 14.9 0.1 Integrated (steps 11^14) 64.99 15.4 0.3 Sample MJ1 1 0.001892 0.409344 0.682688 1.61 18.09 9.2 2.9 2 0.000712 0.428617 0.291997 2.52 27.94 6.5 2.4 3 0.000705 0.421339 0.268906 1.84 22.50 5.1 2.7 4 0.000216 0.416533 0.262412 3.72 75.72 14.1 2.2 5 0.000188 0.416089 0.251249 9.22 77.83 13.9 2.1 6 0.000089 0.425028 0.238233 11.69 88.98 15.1 2.2 7 0.000034 0.412798 0.232209 8.54 95.62 15.7 2.1 8 0.000004 0.397531 0.226732 9.83 99.49 15.8 2.0 9 0.000011 0.383139 0.205675 7.29 98.41 14.2 2.0 10 0.000010 0.431477 0.222493 8.26 98.69 15.6 2.2 11 0.000010 0.403562 0.212333 6.41 98.64 14.8 2.1 12 0.000013 0.416122 0.214675 5.15 98.17 14.9 2.2 13 0.000009 0.400390 0.245464 2.76 98.90 16.9 2.3 14 0.000200 0.392747 0.235344 5.75 74.90 12.6 2.1 15 0.000183 0.433016 0.247284 6.49 78.17 13.9 2.2 16 0.000290 0.408470 0.300687 5.27 71.54 15.2 2.1 17 0.000319 0.428327 0.281306 3.66 66.48 13.4 2.3 Total 0.000154 0.423451 0.246722 100.00 81.55 14.3 2.1 Integrated (steps 4^13) 72.86 15.1 2.1 Sample MJ2 1 0.000384 0.399121 0.284569 2.56 60.11 12.3 2.1 2 0.000236 0.418736 0.256268 3.90 72.74 13.4 2.2 3 0.000257 0.403767 0.257588 3.47 70.53 13.0 2.1 4 0.000196 0.423444 0.238958 2.38 75.75 13.0 2.2 5 0.000070 0.440377 0.243584 2.70 91.57 15.9 2.3 6 0.000176 0.432226 0.252821 2.88 79.40 14.4 2.3

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Table 2 (continued) Increment 36Ar/39Ara37Ar/39Arb40Ar/39Ar % 39Ar % 40Arc Age þ 1c (Ma) (Ma) 7 0.000210 0.419917 0.244595 2.82 74.58 13.1 2.2 8 0.000258 0.404854 0.267559 4.76 71.51 13.6 2.1 9 0.000154 0.398629 0.252126 6.69 81.96 14.6 2.0 10 0.000214 0.422911 0.239510 8.19 73.63 12.7 2.2 11 0.000177 0.426289 0.250857 9.84 79.12 14.2 2.2 12 0.000176 0.390496 0.254663 7.96 79.56 14.3 2.0 13 0.000135 0.419823 0.244333 13.06 83.64 14.5 2.1 14 0.000158 0.406224 0.249747 12.03 81.27 14.4 2.1 15 0.000195 0.413819 0.250882 8.85 77.09 13.8 2.1 16 0.000152 0.408635 0.259218 7.91 82.63 15.1 2.1 Total 0.000183 0.398980 0.251500 100.00 78.49 14.0 2.0 Integrated (steps 5^16) 87.69 14.2 2.1 a Measured. b Corrected for post-irradiation decay of 37Ar (35.1 days half-life). c 40 36 40 ( Artot3 AratmU295.5)/ ArtotU100; J-value for all samples: 0.036564 þ 0.000366. jumping of the magnet is controlled by a Hall- of 14.1 þ 0.1 Ma. The three largest increments probe. For each heating increment the beam in- (10.07%, 10.03% and 19.20% of total 39Ar re- tensities of 36Ar, 37Ar, 38Ar, 39Ar and 40Ar are leased) have apparent ages of 14.6 þ 0.5, measured, and the baseline readings on mass 15.3 þ 0.7 and 15.5 þ 0.4 Ma. 35.5 are automatically subtracted. Intensities of A total gas age of 13.2 þ 0.1 Ma is indicated by the beams are extrapolated over 16 measured in- sample aliquot AL160B2. The two largest incre- tensities to the time of gas admittance either by a ments, with 10.20% and 18.90% of total 39Ar re- straight line or a curved ¢t. Intensities are cor- leased, show apparent ages of 15.0 þ 0.4 Ma and rected for system blanks, background, post-irradi- 15.0 þ 0.3 Ma. An integrated age of 13.6 þ 0.6 Ma ation decay of 37Ar, and interfering isotopes. Ages is shown by the steps 5^19. and errors were calculated following suggestions by McDougall and Harrison [31] and decay constants reported by Steiger and Jaeger [32]. Table 3 Summary of 40Ar/39Ar age data of samples from the Co©nero 3. Results Riviera, Italy and the Ries impact structure Samples Total gas age Integrated age The 40Ar/39Ar analytical results are listed in Ma þ 1c Ma þ 1c (Ma) (Ma) Table 2 (summarized in Table 3), and are shown as age spectra in Figs. 5 and 6. Increments with AL160B1 biotites 14.1 þ 0.1 39 AL160B2 biotites 13.2 þ 0.1 13.6 þ 0.6 less than 1% Ar released were eliminated. None AL250B biotites 14.9 þ 0.2 15.1 þ 0.3 of the samples yield a £at age spectrum with a LV160B biotites 14.9 þ 0.1 15.4 þ 0.3 plateau age. MJ1 moldavite 14.3 þ 2.1 15.1 þ 2.1 MJ2 moldavite 14.0 þ 2.0 14.2 þ 2.1 3.1. AL160B AL160B: Aldo Level; two aliquots of biotite multigrain samples of the grain size fraction 160^250 Wm; AL250B: Aldo The two biotite aliquots of the Aldo Level Level; one biotite multigrain sample of the grain size fraction 250^500 Wm; LV160B: biotite multigrain sample from (AL160B1 and AL160B2) with grain sizes ranging the bioturbated marl immediately above the Aldo Level between 250 and 160 Wm, show heterogeneous (grain size fraction 160^250 Wm); MJ: two aliquots of mol- spectra. Sample AL160B1 yielded a total gas age davite (Janov, Czech Republic).

EPSL 6042 13-12-01 D. Mader et al. / Earth and Planetary Science Letters 194 (2001) 111^126 121

Fig. 5. 40Ar/39Ar apparent age plots of biotites from the Aldo Level.

3.2. AL250B 3.3. LV160B

The biotite multigrain sample of the Aldo Level Biotites of the Aldo Level, which were depos- with the grain size range of 500^250 Wm displays ited by bioturbation in the marl some 5^10 cm more homogeneous age spectra. Its total gas age above the Aldo Level, also show a fairly homoge- is 14.9 þ 0.2 Ma. Apparent ages of 15.1 þ 0.2 Ma neous pattern. The total gas age is 14.9 þ 0.1 Ma, and 14.5 þ 0.5 Ma are displayed by the two largest and the steps 11^14 yield an integrated age of increments with 21.32% and 11.56%, respectively, 15.4 þ 0.3 Ma. of the total 39Ar released. An integrated age of 15.1 þ 0.3 Ma is calculated by the steps 8^19.

Fig. 6. 40Ar/39Ar apparent age plots of a moldavite (Czech Republic).

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3.4. Janov moldavite (MJ) dova biotites are relatively young compared to the age from the coarser fraction. This may be due to For comparison with rocks related to the Ries some alteration, as indicated by electron micro- impact event, and to obtain an independent age probe studies of biotites extracted at an earlier calibration point, two aliquots of the Janov mol- date from the same level (Gattacceca, unpublished davite were dated. MJ2 shows more homogeneous report, 1995). The discordant pattern of the spec- spectra than MJ1, but has younger apparent ages. tra of the biotite samples could indicate (a) some MJ1 has a total gas age of 14.3 þ 2.1 Ma and an Ar loss through recoil of 39Ar from the small integrated age of 15.1 þ 2.1 Ma (steps 4^13). The grains during irradiation, (b) enrichment of 39Ar total gas age of MJ2 is 14.0 þ 2.0 Ma, the inte- in K-poor alteration parts of the minerals due to grated age of the steps 5^16 is 14.2 þ 2.1 Ma, recoil from K-richer biotite parts during neutron which is nearly the same. The large errors are activation, or (c) loss of 40Ar as possible reasons most probably due to high 40Ca contents, which for younger apparent age increments (e.g. [33^ lead to high 37Ar, and insu¤cient argon released 37]). This gain of 39Ar in alteration phases only from the small sample used. a¡ects the age spectra, rather than the total gas age, since in this case 39Ar is only redistributed 3.5. Ries suevites [34]. Sample AL160B, however, displays a total gas age younger than the biotite samples Both suevite glasses from the Aumu«hle and AL250B and LV160B, which indicates loss of ar- Heerhof quarries (not shown) have no geologi- gon due to di¡usion. As the small biotite grains cally meaningful spectra. The suevite sample may have undergone Ar loss by recoil e¡ects, from the Heerhof quarry with very heterogeneous their values provide less reliable data for the age spectra, has total gas ages of 12^14 Ma. The sue- of the volcaniclastic Aldo Level. The largest incre- vite glass of the Aumu«hle quarry also shows ex- ments of these two sample aliquots (15.5 þ 0.4 tremely heterogeneous spectra and yields total gas Ma, 15.0 þ 0.3 Ma), as the most probable indica- ages about 14 Ma. Similar to the moldavite sam- tors of the crystallization age of unaltered biotite ple, the error of the apparent ages is about 2 Ma. phases, however, indicate (within errors) an ap- However, the ages of the suevite glasses are use- parent age similar to the more `concordant' spec- less for any interpretation and geologically not tra of the sample aliquots AL250B and LV160B. reliable, as the output of the radiogenic 40Ar (at- According to Roberts et al. [37], extremely high mospheric Ar corrected) in both samples was too and low incremental ages of altered biotites are small (in the range of only 4^37%). caused by the interaction of Ar recoiling and the variable Ar release from K-rich and K-poor biotite phases, and can be overcome by the applica- 4. Discussion tion of a UV laser microprobe. Thus, when using biotites in chronostratigraphic studies, even if The deviant increments with variable ages they appear unaltered, the radiometric data may probably re£ect analytical limitations rather than be more geologically reliable if it is done with a a disturbed argon system. This may be especially UV laser microprobe. true for the moldavite sample. This pattern, to- The spectra of AL250B and LV160B show no gether with the larger errors, may be caused by indication of argon loss by marginal alteration of insu¤cient Ar output due to the small increase in the biotites. Regarding the slight variable incre- the laser output energy for each step of about 0.2^ mental ages, it should be noted that multigrain 0.3%, which were used during the analyses. Thus, samples were analyzed. Slight variations in chem- the released gas amount is, in some cases, only istry and grain size or grain thickness might cause slightly above the background. varying gas output due to di¡erential laser energy The total gas ages of the two aliquots of the absorption between the individual biotites during small grain size fraction (AL160B) of the La Ve- the incremental heating steps. Even the simple

EPSL 6042 13-12-01 D. Mader et al. / Earth and Planetary Science Letters 194 (2001) 111^126 123 fact of several individual micas laying one concluded that the age of the Ries crater is upon the other in a multigrain sample during 15.1 þ 0.1 Ma. stepwise heating by using a defocused laser The fractions of the newly dated moldavite beam, might disturb the homogeneous gas extrac- sample (especially aliquot MJ1) show, within the tion, due to di¡ering reactions of the individual errors, the same age as that obtained by Gentner mineral grains to the energy of the laser beam et al. [15,16] and Staudacher et al. [17] for the [38]. Ries event. The large errors of about 2 Ma are The total gas ages of AL250B and LV160B probably due to the high 40Ca content, as indi- (14.9 þ 0.2 Ma and 14.9 þ 0.1 Ma respectively), cated by the high, but homogeneous, 37Ar/39Ar are interpreted as the most likely apparent age ratio, and due to the small amount of released of the Aldo Level. Sample AL250B represents a argon. coarser grain size fraction, whereas LV160B seems The ages of the suevite glasses are not shown, to be less altered than AL160B. because they are geologically not reliable. Three This radiometric age obtained for the Aldo of the suevite glass samples show discordant spec- Level in the La Vedova section at the Co©nero tra, with highly variable incremental ages that Riviera con¢rms the estimated age of 15 Ma of cannot be geologically interpreted. Some steps this volcaniclastic clayey layer, which had previ- with `old' ages from about 16^24 Ma might indi- ously been derived by stratigraphical extrapola- cate inclusions that were not totally outgased dur- tion [4]. This radiometric age contributes new in- ing the impact event. One suevite glass aliquot formation to the absolute time scale of the from the Aumu«hle quarry, however, displays a Langhian. Both the ¢rst occurrences (FOs) of fairly concordant spectrum and a total gas age the planktonic foraminifera P. glomerosa glome- of 14.6 þ 1.5 Ma, which would ¢t the 15 Ma age rosa and P. glomerosa curva in the La Vedova of the Ries event. However, as described above, section, which are at 24 m and 24.1 m, respec- the insu¤cient portion of radiogenic 40Ar in these tively (Coccioni in [4]), can be related to an abso- suevite glasses does not allow an unambiguous lute date. As mentioned above in the biostratig- interpretation of the data. Denitri¢cation of the raphy section, the FO of O. suturalis de¢nes the glass might have led to the loss of argon and, boundary of the foraminiferal zones N8 and N9 therefore, decreased the ages of several steps and of Blow [24] and the boundary between the P. of the total gas age in general. The problem of glomerosa s.l. and the O. suturalis^G. periphero- insu¤cient radiogenic 40Ar could be prevented ronda zones of Iaccarino and Salvatorini [22] with larger samples. and Iaccarino [23]. A minimum age of 14 Ma Considering the mean sedimentation rate in the for the Orbulina datum event is suggested by area and taking the errors on the ages into ac- 40Ar/39Ar analysis of biotites and sanidines from count, it is conceivable that the Ries ejecta ^ if a tu¡ above the FO of O. suturalis in the Kara- present at all ^ are within close stratigraphic prox- suyama sequence in Japan [39]. By calculating an imity of the Aldo Level. Our preliminary major average sedimentation rate of 38.7 m/Ma between and trace elemental analyses on about 100 sam- the radiometrically dated Respighi and Aldo lev- ples several meters above and beneath the Aldo els, and interpolating with the FO of O. suturalis Level, however, have not indicated any geochem- at 30 m (Coccioni in [4]), an age of about 14.75 ical signatures pointing to Ries ejecta, which Ma can be derived, which may con¢rm the gen- would make it easier to search for petrographic erally assumed estimation at 15 Ma for this zonal impact signatures such as shocked quartz or mi- boundary (e.g. [40,41]). crospherules. The absence of a distinct geochem- The age of the Aldo Level is, within the errors, ical signal is not unexpected, because of the ab- the same as the age of the Ries crater event. Stau- sence of an unambiguous meteoritic component dacher et al. [17] reported Ar^Ar dates of even in proximal Ries ejecta. This led to the sug- 15.0 þ 0.5 Ma for a suevite breccia and gestion that the Ries impactor might have been a 15.21 þ 0.21 Ma for a moldavite. These authors siderophile element-poor achondrite, e.g. [18,42,

EPSL 6042 13-12-01 124 D. Mader et al. / Earth and Planetary Science Letters 194 (2001) 111^126

43]. Thus, the only opportunity to detect ejecta structive reviews by P. Claeys (Brussels) and E. from the Ries event will be to search for a layer Diaz (Madrid).[AC] containing shocked minerals. The Co©nero Riviera area is at a distance of about 25 crater diameters References from the Ries crater, which is in the distal ejecta region (as proximal ejecta are deposited within [1] A. Montanari, C. Koeberl, Impact Stratigraphy - The ¢ve crater radii [1]). A 10 cm thick layer of Ries Italian Record, Lecture Notes in Earth Sciences vol. 93, ejecta was found in Switzerland, at a distance of Springer, Berlin, 2000, 364 pp. about 200 km from the crater (e.g. [44]), so it [2] J. Dercourt, L.E. Ricou, B. Vrielynck (Eds.), Atlas Tethys reasonable to suspect that distal ejecta might Palaeoenvironmental Maps, Gauthier Villars, Paris, 1993, 307 pp. also be present in the U-M basin. The present [3] J.E.T. Channel, B. D'Argenio, F. Horvath, Adria, the work provides the chronological background for African promontory, in Mesozoic Mediterranean paleo- a detailed search for signatures of the Ries impact geography, Earth Sci. Rev. 15 (1979) 213^292. event at La Vedova. [4] A. Montanari, B. Beaudoin, L.S. Chan, R. Coccioni, A. Deino, D.J. DePaolo, L. Emmanuel, E. Fornaciari, M. Kruge, S. Lundblad, C. Mozzato, E. Portier, M. Renard, D. Rio, P. Sandroni, A. Stankiewicz, Integrated stratigra- 5. Conclusions phy of the Middle to Upper Miocene pelagic sequence of the Co©nero Riviera (Marche Region, Italy), in: A. Mon- The new 40Ar/39Ar age of the Aldo Level con- tanari, G.S. Odin, R. Coccioni (Eds.), Miocene Stratigra- ¢rms the stratigraphically extrapolated age esti- phy: An Integrated Approach, Elsevier, Amsterdam, 1997, pp. 409^450. mate of this clayey layer [4], and contributes [5] A. Assorgia, L.S. Chan, A. Deino, C. Garbarino, A. new information to the absolute time scale of Montanari, R. Rizzo, S. Tocco, Volcanigenic and paleo- the Langhian. A further connection can be drawn magnetic studies on the Cenozoic calc-alkalic eruptive se- to impact stratigraphic studies in the pelagic sequence of Monte Furru (Bosa, mid-western Sardinia), in: quences of the Co©nero Riviera [1]. As the age of R. Coccioni, A. Montanari, G.S. Odin (Eds.), Miocene Stratigraphy of Italy and Adjacent Regions, G. Geol. the Aldo Level coincides (within error) with that vol. 56, 1994, pp. 17^29. of the Ries crater event, a detailed search for pos- [6] A. Montanari, S. Carey, R. Coccioni, A. Deino, Early sible impact signatures is now feasible (and in Miocene tephra in the Apennine pelagic sequence: An progress) directly above and below the Aldo Lev- inferred Sardinian provenance and implications for west- el. As the Middle and Upper Miocene in the Al- ern Mediterranean tectonics, Tectonics 13 (1994) 1120^ 1134. pine^Apennine range is mainly represented by [7] A. Montanari, R. Drake, D.M. Bice, W. Alvarez, G.H. synorogenic reworked siliciclastic deposits, the pe- Curtis, B.D. Turrin, D.J. DePaolo, Radiometric time scale lagic carbonate sequence at the Co©nero Riviera for the upper Eocene and Oligocene based on K/Ar and allows future investigations on the regional e¡ects Rb/Sr dating of volcanic biotites from the pelagic se- of the medium sized Ries crater event. quence of Gubbio, Italy, Geology 13 (1985) 596^599. [8] G.S. Odin, Niveaux a© biotite del Apennines autour de la limite Eoce©ne- Oligoce©ne, Bull. Liaison Inf. Int. Un. Geol. Sci. vol. 5, Subcommission on Geochronology, Paris, Acknowledgements 1985, pp. 17^24. [9] P. Mattias, M. Mariottini, G. De Casa, I minerali silica- This study has been supported by the Austrian tici e gli altri minerali compresi nella sequenza eocenica- oligocenica della Valle della Contessa presso Gubbio (Ap- FWF, project Y58-GEO (to C.K.), the Austrian^ pennino Centrale), Miner. Petrogr. Acta 30 (1987) 113^ Italian Scienti¢c and Technical Exchange Pro- 139. gram (Oë AD), project no. 14 (to C.K. and [10] A. Montanari, Geochemical characterization of volcanic A.M.), and the University of Vienna (Internation- biotites from the Upper Eocene-Upper Miocene pelagic al Relations O¤ce) (to D.M.). D.M. would like to sequence of the northeastern Apennines, in: I. Premoli Silva, R. Coccioni, A. Montanari (Eds.), The Eocene-Oli- thank G. Friedl, H. Genser and D. Schneider gocene Boundary in the Marche-Umbria Basin (Italy), (Salzburg) for their help with the mass spectro- IUGS Spec. Publ., F.lli Aniballi Publishers, Ancona, metric analyses. We appreciate critical and con- 1988, pp. 209^227.

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