Lithofacies palaeogeography and biostratigraphy of the lowermost horizons of the Middle Triassic Hallstatt Limestones (Argolis Peninsula, Greece)

Fotini A. Pomoni , Vassilis Tselepidis

1. Department of Geology and Geoenvironment, University of Athens, Panepistimiopolis 157 84, Athens, Greece

2. Institute of Geology and Mineral Exploration, Olympic Village, Acharnae, P.C. 13677 Athens, Greece＊

Abstract Condensed ammonoid beds of the Hallstatt facies (Anisian-Ladinian) are widespread around the Ancient Theatre of Epidaurus, in the locality Theokafta of the Argolis Peninsula (eastern Peloponnesus). The Hallstatt Formation in Argolis appears, generally, in the form of lensoid bodies of variable sizes, inclination and direction and is always found overlying a formation consisting of keratophyric tuffs. In fact, the contact of the keratophyric tuffs with the overlying limestones, specifically evidenced by an in situ brecciated zone, is stratigraphic and constitutes the base of the Hallstatt Limestones. The contact of the Hallstatt Limestones with the overlying radiolarites is stratigraphic as well.

Key words Middle Triassic, Hallstatt Formation, facies analysis, ammonoid biozonation,condensed pelagic sedimentation, palaeoenvironment, eastern Peloponnesus

1 lntroduction

Ammonoid-bearing pelagic carbonate formations on top of platforms, following a significant stratigraphic break, are very common in Middle Triassic sequences of the southern and eastern Alps (Assereto, 1971; Schlager and Schollnberger, 1974; Eptinget al., 1976; Brandner,1984; Angioliniet al., 1992; Bracket al., 2007; Monnetet al., 2008). These formations are condensed, red, micritic,sometimes nodular, rich in cephalopods, conodonts and molluscs, and are known as “Hallstatt-type limestones”.The Hallstatt horizons are of particular stratigraphic and geologic interest, as far as the relationship of the Hallstatt facies with surrounding formations and the determination of the stratigraphic level that coincides with the beginning of Hallstatt facies deposition.

Hallstatt facies correspond to hiatus beds/concretions characterized by discontinuous sedimentation and erosion.Hiatus concretions are hypothesized to form during early diagenesis by reworking of carbonate sediments, after a break in sedimentation or seafloor erosion (Voigt, 1968;Raiswell, 1987, 1988; Spears, 1989; Wetzel and Allia,2000). A prerequisite for the growth of concretions is that they should remain for a considerable time within the sulfate reduction zone (7000 years; Coleman and Raiswell,1993). The identification of such horizons has a great stratigraphic and sedimentologic value, as they indicate markers of significant interruption of sedimentation that otherwise would not be noticed (Fursich and Baird, 1975;Baird, 1976). Such discontinuity surfaces are commonly not manifested as biostratigraphic gaps (e.g., Wilson,1985). The hiatus beds, and specifically the hiatus concretions are characterized by a multiphase diagenetic history that occurs not very deep below the sediment surface and includes exhumation, boring and/or encrustation, burial and cementation (Savrda and Bottjer, 1988).

In terms of sequence stratigraphy, hiatus concretions and beds are genetically linked to rising or high sea-level(e.g., Van Wagoneret al., 1988). They are thought to form during the initiation of transgressions (e.g., Voigt, 1968;Fursichet al., 1991), as well as during the time of maximum rate of transgression in areas where sediment input is strongly reduced (‘‘condensed section”). Similar deposits, considered to have formed during sea-level highstand,were also reported from drowned carbonate platforms by Kendall and Schlager (1981).

However, this simple sequence-stratigraphic interpretation of hiatus beds and condensed sections, is not always valid, because hiatus beds seem to form more frequently during times of tectonic activity than times of intense sealevel changes (Wetzel and Allia, 2000). Other processes,such as a differential subsidence, may produce sediment starvation or seafloor erosion, providing the necessary conditions for formation of hiatus concretions (e.g., Hesselbo and Palmer, 1992).

The Hallstatt facies occur in the following areas of Greece: in Chios Island (Skythian-Lower Anisian; Bender, 1970; Jacobshagen and Tietze, 1974; Gaetaniet al.,1992; Mertmann and Jacobshagen, 2003), in Hydra Island (Anisian-Carnian; Römermann, 1968; Angiolinietal., 1992), in Argolis Peninsula (Upper Anisian-Norian;Renz, 1906a, 1906b, 1910a, 1910b, 1955; Sakellariou,1938; Bornovas, 1962; Dercourt, 1964), in Crete (Lower Carnian; Creutzburget al., 1966), in Orthrys (Carnian-Ladinian; Mitzopoulos and Renz, 1938), in Attica (Bender, 1962), in Western Peloponnesus (Tsoflias, 1969) and in Naxos (Dürr, 1975).

For estimating the significance of this unique event, in the context of the Tethyan realm, precise dating and correlation is a necessary prerequisite. In the present paper,new constraints on the lithofacies palaeogeography and biostratigraphy of the Middle Triassic ammonoid-bearing pelagic formations (Hallstatt Formation) of the Argolis Peninsula (NE Peloponnesus), are presented.

2 Stratigraphic location and geological setting of the Hallstatt facies in Argolis

Hallstatt Limestones exposed in the Argolis Peninsula have been investigated since the start of the twentieth century. Renz (1906a, 1906b), first confined Hallstatt facies to the Triassic at the Theokafta locality, following the discovery of the ammoniteJoannites diffisusHAUER by Douvillé (1900), attributing to an Upper Anisian-Middle Carnian age.

In the Argolis Peninsula, the Hallstatt Limestones is considered to overlie stratigraphically, volcaniclastic formations consisted of keratophyric tuffs (Renz, 1910a,1910b, 1939, 1955; Bender, 1962; Tselepidiset al., 1989).Bender and Kockel (1963) attributed an Early Triassic age to the keratophyric tuffs, suggesting that the Hallstatt facies began in the upper Anisian. Jacobshagen (1967), Bannert and Bender (1968) and Pelosio (1973) also refer to Hallstatt facies as condensed horizons of Upper Anisian-Carnian age that are normally deposited on the tuffic substrata.

The Hallstatt facies is overlain by a radiolaritic formation, although in places there appears to be a lateral transition between these two formations. However, according to other authors, the whole lithostratigraphic series is reversed (Bender, 1962; Bender and Kockel, 1963), whereas Baumgartner (1985) suggested that the contact of the radiolaritic sedimentary rocks with the Hallstatt facies, as well as with the keratophyric tuffs is tectonic, and suggested that the Hallstatt Formation, in association with the keratophyric tuffs, acted as an olistolith that was translocated in-between the radiolarites.

Krystyn and Mariolakos (1975), based on conodont analysis, extended the interval of Hallstatt facies deposition from the lower Anisian to the lower Norian and considered that a submarine slide, in the frame of a synsedimentary tectonism, relocated the Hallstatt Formation in-between the radiolarites. On the other hand, Dürkoopet al.(1986), considered that the stratigraphic contacts of the Hallstatt facies with the underlying keratophyric tuffs, as well as with the overlying radiolarites, was stratigraphic,reaching up to the Upper Norian (Sevatian).

Red, micritic, ammonoid-bearing limestones of the Hallstatt facies are widespread around the Ancient Theatre of Epidaurus, in the Argolis Peninsula (Figs. 1, 2, 3). Detailed sampling of the Hallstatt facies was conducted in one of the most spectacular sequences, which is situated in the broader area of Epidaurus, 600 m NW of the Ancient Theatre of Epidaurus, in the locality of Theokafta.

The Theokafta outcrop is composed of the following lithologic formations, from base to top: the keratophyric tuffs, the limestones of the Hallstatt facies, the radiolarites and the limestones with cherts (Figs. 4, 5).

The formation of the keratophyric tuffs extends throughout the broader area of Argolis, and is in contact with different formations each time (e.g., the Hallstatt, the radiolarites and the limestones with cherts), resulting in different age estimations. For instance, as substrata of the Hallstatt Limestones, the keratophyric tuffs should be considered older than Lower Anisian, whereas as substrata of the formation of limestones with cherts, should be older than Lower Ladinian.

The Hallstatt Formation in Argolis appears, generally, in the form of lensoid bodies of variable sizes, inclination and direction and is found always overlying the keratophyric tuffs, although due to inversion tectonics the keratophyric tuffs appear in places, to overlie the Hallstatt Limestones (Benderet al., 1960; Jacobshagen, 1967;Bannert and Bender, 1968; Pelosio, 1973; Bachmann and Jacobshagen, 1974; Krystyn and Mariolakos, 1975;Tselepidiset al., 1989; Geol. Sheet Ligourio 1:25000 IGME). In fact, the contact of the keratophyric tuffs with the overlying limestones, as evidenced by anin situbrecciated zone, is stratigraphic and constitutes the base of the Hallstatt Limestones. Due to tectonism, the Hallstatt Formation has, in places, undergone fragmentation and significant translocation. The outcrop studied, at the eastern slopes of the Theokafta Hill (Fig. 2), which appears in the form of a lensoid body with thickness reaching 82 m,is the most significant outcrop of the Hallstatt facies in the Argolis Peninsula, not only referring to its dimensions,but also because of its richness in macro- and microfauna,which covers the entire stratigraphic evolution of the Triassic. Additionally, in Theokafta, the relationship of the Hallstatt facies with the underlying (keratophyric tuffs)and the overlying formations (radiolarites) is very clear(Fig. 4).

Fig. 1 Sketch map of the study area showing the relationship of the Hallstatt Formation with the surrounding formations.

Fig. 2 . Limestones of the Hallstatt facies around the Ancient Theater of Epidaurus in the Argolis, Peninsula.

Fig. 3 Red, micritic, ammonoid-bearing limestones of the Hallstatt facies from the locality Theokafta of Epidaurus (Argolis Peninsula). Age: Upper Anisian (Illyrian, biozone Nevadites).

Fig. 4 The Theokafta outcrop is composed of the following lithologic formations, from base to top: keratophyric tuffs, limestones of the Hallstatt facies, radiolarites and limestones with cherts. The contact of the Hallstatt Limestones with the underlying keratophyric tuffs and the overlying radiolarites is stratigraphic.

Fig. 5 An overview of the studied geological section.

Concerning the relationship of the Hallstatt facies with the stratigraphically overlying radiolarites, it has not yet been clarified as to whether the radiolarites were deposited before or after the deposition of the Hallstatt Limestones(Fig. 6). As it was previously noted, the radiolarites are,in places, in direct contact with the keratophyric tuffs,as well (Fig. 7). In the studied sequence, radiolarites are tectonically overlain by the limestones with cherts (Fig.6) and have been dated as Lower Ladinian-Middle Liassic, on the basis of conodonts (Krystyn and Mariolakos,1975). The limestones with cherts are considered to have been deposited in a neighboring area, to that of the Hallstatt Limestones, directly upon the keratophyric tuffs. The Hallstatt Limestones started to be deposited in the Lower Anisian, whereas deposition of the limestones with cherts should have started later, possibly in the Lower Ladinian.In the area of Theokafta, both the “packet” radiolarites/limestones with cherts, as well as, the “packet” keratophyric tuffs/radiolarites, are inversely stratified (Figs. 6,7). Inverse stratification is the result of post Upper Jurassic compressional tectonism, resulting in folding and over-thrusting (Fig. 8).

Fig. 6 The stratigraphic contact of the Hallstatt facies with the overlying radiolarites and the tectonic contact of the radiolarites with the “limestones with cherts”.

Fig. 7 The tectonic contact of the keratophyric tuffs (Skythian) with the Middle-Upper Jurassic radiolarites.

3 Lithostratigraphy

The studied section is situated in a very short distance north of the “Asklipieion” of Ancient Epidaurus, at the eastern slopes of Theokafta. Due to a system of faults,normal to the stratification, fragmentation and significant horizontal translocation of the resulted blocks is observed.

Starting from the lower part of the section, along the contact with the keratophyric tuffs, towards the top, a formation of brownish-reddish limestones is observed, averaging 8 m in thickness (lithologic units A0 to A8). It is characterized by layers enriched in ammonoids and nautiloids, orientated parallel to the stratification, as well as by concentrations of Fe- and Mn-oxides, along specific surfaces (Figs. 9, 10).

Fig. 8 Radiolarites overthrusted on the keratophyric tuffs and limestones with cherts overthrusted on radiolarites, due to inverse stratification.

Lithofacies and biostratigraphic research has been focused on the lowermost horizons of the Hallstatt Limestones of Anisian age (average thickness about 1.30 m),where a dense sampling has been performed, followed by detailed facies analysis (lithologic units A0 to A3). That part consists of two characteristic lithological units (Fig.11):

The first lithological unit, comprises the layers A1 and A2. Layer A1, a thin layer averaging 4-5 cm in thickness,is friable mylonitic volcanic material, located between the Hallstatt Limestones and the underlying keratophyric tuffse.g., this layer marks the beginning of carbonate sedimentation. In fact, the uppermost portion of layer A1 is a thin layer, averaging 3-4 cm in thickness, consisting of fragments of macrofauna and reworking, fine-grained, volcanic keratophyric material. Layer A2 represents the initial layer of the Hallstatt facies (with the subdivisions A2/1a, A2/1b,A2/2, A2/3 and A2/4). Layer A2/1a, averaging 8-10 cm in thickness, is characterized by a brecciated appearance, due to the presence of abundant, mainly angular lithoclasts and crystal clasts of variable size. The lithoclasts are volcanic in origin and are derived from the underlying keratophyric tuffs. In most cases they have suffered calcitization. Layer A2/1b, averaging 5 cm in thickness, is a brownish-reddish limestone that developed on an irregular, eroded surface.Layer 2/2 consists of a lower brecciated facies averaging～7 cm in thickness, whereas the upper part consists of a biomicrite rich in ammonoids.

Fig. 9 The studied section in the locality Theokafta. On the right edge of the photo, at the base of the section, the contact with the keratophyric tuffs is shown. The lowest layers of the section are rich in ammonoids and nautiloids.

Fig. 10 The stratigraphic contact of the grayish keratophyric tuffs with the reddish Hallstatt Limestones.

The second lithological unit, 1 m thick, comprises layer A3 (A3/1, A3/2) and corresponds to the characteristic reddish-brownish, nodular limestones of the Hallstatt facies (Fig. 12), with the unique occurrence of macrofauna. These layers are rich in Fe- and Mn-oxides that mark hardground horizons and reach several centimeters in thickness.

Fig. 11 The studied lowest members of the Hallstatt Limestones, averaging in thickness ～1.30 m, that comprise the first (A1, A2) and the second (A3, A4).

The studied lowest part of the section is characterized by condensed sedimentation. Synsedimentary and early burrowing differentiate the primary texture characteristics of the deposited sediments. Omission surfaces and hardgroundssensuBathurst (1975) and Bromley (1978),are very common along certain horizons. The hardgrounds are characterized by intense relief and concentrations of Fe-Mn-oxides. Along such surfaces, ammonoids are orientated parallel to the stratification. Ammonoids have suffered dissolution, mainly affecting the upper part of the shell and seldom both the lower and the upper part. Sessile foraminifera of the genusTolypamminacommonly are observed on hardgrounds and/or lithoclasts. Hardground surfaces are commonly overlain by thin layers composed of clastic material, averaging 1 mm-4 cm in thickness.Along discontinuities, stromatactis-like cavities have been developed, revealing rapid synsedimentary lithification. It should also be noted that no mixing of the fauna has been detected.

4 Facies analysis of the lowermost horizons of the Hallstatt Limestones

The lowermost horizons of the Hallstatt Limestones of Theokafta represent typical hiatus beds/concretionssensuWetzel and Allia (2000), characterized by discontinuous sedimentation and erosion (Fig. 11). They consist of red ammonoid-bearing hemipelagic limestones with calcium carbonate nodules floating in an Fe-oxides enriched matrix with dispersed lensoid/prismatic calcium carbonate crystals.

Starting from the contact with the underlying kera-tophyric tuffs and towards the top, the following nine lithostratigraphic horizons appear (A/A, A1, A2/1a, A2/1b,A2/2, A2/3, A2/4, A3/1, A3/2), and include radiolarian packstones, volcaniclastic facies, packstones/floatstones with ammonoids and lag deposits:

Strat. Unit A/A: The lowermost carbonate horizon which overlies the basal volcanic rock (keratophyric tuffs).

Facies: Radiolarian packstones (Fig. 12A).

Microcomponents: Radiolarians (micritized and calcified), benthic foraminifera, ostracods and molluscs.

Texture: Nodular. Nodules are surrounded by a blackened rim enriched in Fe-oxides. Dissolution surfaces,along the contacts of coalescing nodules, are marked by Fe-oxide microstylolites. (Fig. 12B).

Interpretation: Limestones with autochthonous hemipelagic faunal elements (radiolarians) and allochthonous reefal detritus (foraminifera, ostracods).

Lithologic Unit A: It comprises the layers A1 and A2 and averages 3-4 cm in thickness.

A1

Strat. Unit: Volcaniclastic horizon (averaging 4-5 cm in thickness), consisting of fragments of macrofauna and reworked, fine-grained, volcanic keratophyric material. It is characterized as “mylonite” due to its friable character,which lies between the keratophyric tuffs and the overlying Hallstatt Limestones and marks the start of carbonate sedimentation. It consists of relic crystals of feldspars (plagioclase), quartz and mafic minerals, as well as lithoclasts(lag deposit). All components are derived from the underlying keratophyric tuffs and are gradually assimilated by a ferruginous micritic and/or crystalline matrix consisting of dispersed lensoid/prismatic calcite crystals (Figs. 12C,13A). Calcium carbonate nodules occur in places, and are assimilated as well by the same matrix (Figs. 12D, 13A,13B).

A2

Layer A2 represents the basal layers of the Hallstatt facies (subdivisions from base to top: A2/1a, A2/1b, A2/2,A2/3 and A2/4).

A2/1a

Strat. Unit: Contact between the volcaniclastic facies and the overlying Hallstatt facies, averaging 8-10 cm in thickness. The contact is marked by circumgranular cracking and Fe-oxides rims (Fig. 13C).

Lower part: Volcaniclastic facies characterized by reddish colour and brecciated appearance due to the presence of abundant, mainly angular clasts of variable size of fragments of crystals (Fig. 13D), including plagioclase, quartz,mafic minerals (kerostilb and rarely biotite) and angular to subrounded lithoclasts of variable size, gradually decreasing upward. Plagioclase crystals are medium basic(andesine) and quartz crystals show magmatic erosion.All components have been derived from the underlying volcanic rock (keratophyric tuff) and are cemented by and intensively impregnated by Fe- and Mn-oxides as well as matrix that gradually assimilates them.

Upper part: Start of Hallstatt sedimentation. Red coloured limestones.

Facies: Packstones/floatstones with ammonoids (Fig.13D).

Microcomponents: Radiolarians, benthic foraminifera(Arenovidalina chialingchiangensisHO and Nodosariidae), molluscs, brachiopods, ostracods, echinoderms and gastropods.

Texture: Nodular in places. Lime nodules are surrounded by a matrix enriched in Fe- and Mn-oxides. Bioclasts show evidence of boring and are coated by Fe-oxide rims. Biomolds are filled with geopetal fill (pelmicrite/sparitic cement).

Interpretation: Strongly condensed sediments due to low rate of sedimentation. Limestones with autochthonous hemipelagic bioclasts (ammonoids, radiolarians,filaments) and allochthonous reefal detritus (benthic foraminifera, ostracods, gastropods). Oxygen-rich bottom water.

Age: Ammonoids of the speciesMegaphyllites chiosensisFASTINI-SESTINI are recorded for the first time in the study area. Fastini-Sestini (1981) recorded the above species in Chios and defined it as Aegeian in age (lower Anisian). Assereto (1974) referred to beds withMegaphyllites evolutusWELTER of lower Aegeian age, whereas Bender(1970) considers them as Lower Anisian. In the study area these beds are considered to be lower Anisian, and possibly upper Skythian (Tselepidiset al., 1989).

A2/1b

Strat. Unit: Hallstatt horizon averaging 5 cm in thickness. It consists of a brownish-reddish limestone that has developed on an irregular, eroded surface.

Facies: Red packstones/floatstones with ammonoids(Fig. 14A).

Microcomponents: Gastropods, molluscs, echinoderms, ostracods, radiolarians and benthic foraminifera.

Texture: Bioclasts are dissolved, and biomolds are filled geopetally. Abundant relic crystals of quartz, plagioclase and mafic minerals (kerostilb) and a few lithoclasts;all components are derived from the underlying keratophyric tuff. Discontinuities are marked by Fe-oxides and have a tendency to form immature hardgrounds. Nodules are rare in this facies.

Interpretation: Strongly condensed sediments due to low rate of sedimentation. Limestones with autochthonous hemipelagic bioclasts (ammonoids, radiolarians,filaments) and allochthonous reefal detritus (benthic foraminifera, ostracods, gastropods). Oxygen-rich bottom water.

Age: Fauna includes the speciesLeiophyllites confucii(DIENER) andLeiophyllites suessi(MOJSISOVICS), in association withProcladiscitescf.brancoiMOJSISOVICS,Procladiscitessp.,Beyrichites,PtychitesandDiscoptychites. The association ofL. confuciiand the genusProcladiscites, according to Asseretoet al.(1980), suggests a lower Anisian age (Aegeian). This age has been attributed to similar layers of Chios.

The age of lower Anisian is supported by the presence of the foraminiferaTolypammina gregariaand Duostomminidae, which appear for the first time in this layer at Argolis, in the study section of Theocafta.

An analogous age (Aegeian-Bithinian) is attributed by Krystyn (1975) to layers withL. confuciiandProclidiscites,which are referred as the lowest horizon of the“Hallstatt” facies. Later on, Krystyn (1983) confines the age of this horizon to the Bithynian, in the Biozoneismidicus(upper Bithynian), which is considered as the oldest biozone of the Hallstatt facies.

However, from the lowest layers of the Hallstatt facies in Theocafta, among others, the speciesM. chiosensishave been determined, suggesting that the base of this horizon should have an older age and it be included in the Biozoneosmani(lower Bithynian), and may be even older. In conclusion, in the lowest 15 cm of the section, the Biozonesosmani(A1) andismidicus(A2) appear, followed by the development of a hardground surface.

A2/2

Strat. Unit: Hallstatt horizon

Lower part:

Facies: Packstones/floatstones with ostracods, radiolarians and echinoderm debris. Crystals, in places intensively impregnated by Fe- and Mn-oxides (Fig. 14B).

Fauna: Macrofauna lacking.

Texture: Dispersed relic crystals of feldspars and quartz assigned to a microbrecciated texture. All components float in micritic matrix and in some places they are cemented by sparite.

Interpretation: Lag deposit due to interruption of sedimentation, during which components from a semilithified hardground surface were reworked.

Upper part:

Facies: Packstones/floatstones with ammonoids (Figs.14C, 14D)

Microcomponents: Abundant benthic foraminifera(Dustomminidae and Nodosariidae), echinoderms, gastropods, molluscs and ostracods.

Texture: Bioclasts have been bored, forming a rim rich in Fe-oxides, and ammonoid biomolds have been filled by cloudy fibrous cement overlain by blocky cement and/or by chalcedony (Fig. 15A).

Interpretation: Limestones with autochthonous hemipelagic faunal elements (ammonoids) and allochthonous reefal detritus (foraminifera, ostracods, gastropods).

Age: Macrofauna are not well preserved and are represented mainly by debris of ammonoids,e.g.,Balatonites balatonicusMOJSISOVICS,Flexoptychites acutus(MOJSISOVICS)and generaBulogites,Discoptychites,Monophyllites,Megaphyllites, which characterize the Biozonebalatonicus. In addition, the unit also containsArenovidalina chialingchiangensisHO, Ataxophragmidae, as well as dispersed bioclasts. The presence ofMeadrospira insolita, which does not occur later than the Pelsonian, further suggests a Pelsonian age for that horizon.

According to Assereto (1974), Krystyn (1983) and Vierlyng (1987) the Biozonebalatonicuscomprises the entire Pelsonian, substituting the Biozonebinodosus, which in Europe refers to the upper Pelsonian of Germany.

A2/3

Strat. Unit: Hallstatt horizon.

Facies: Packstones/floatstones, rich in ammonoids(lower 5 cm, in thickness). Towards the top they are in contact with wackestones-packstones with calcified radiolarians occur (Figs. 15B, 15C).

Microcomponents: Molluscs, echinoderms, gastropods, ostracods, filaments, and foraminifera.

A2/4

Strat. Unit: Hallstatt horizon

Lower part:

Facies: Packstones/floatstones with ammonoids.

Texture: Condensed sedimentation. Along hardgrounds surfaces, Bioclasts have been dissolved.

Age: According to Pelosio (1973), identified species are predominantly those of the family Ptychitidae (more than 70%):Ptychites stolicskaiMOJSISOVICS,P. canavarii MARTELLI,P. oppeliMOJSISOVICS,Flexoptychites flexuosus(MOJSISOVICS),F. cf.studeri(HAUER),F. cf.gibbus(BENECKE),Discoptychitessp. “Paraceratites”trinodusus(MOJSISOVICS), “P.elegans” (MOJSISOVICS), “Paraceratites” sp.Semiornitessp.,Monophyllites sphaerophyllus(HAUER),Megaphyllites sandalinusMO-JSISOVICS,Gymnites obliquusMOJSISOVICS,G. incultus(BEYRICH),Proarcetes escheri(MOJSISOVICS).

According to Mojsisovics (1882), Arthaber (1914),Jacobshagen (1967), Pelosio (1973) and Assereto (1974),the speciesParaceratitesis confined to this horizon and in association with species from the family Ptychitidae and the speciesSemiornitescoincide with the Biozonetrinodosus(Illyrian).

Microfauna are similar to that of the previous horizon,except for the observed decrease ofA. chialingchiangensisHO and extinction ofM. insolita(HO), which favours an Illyrian age.

A zone of condensed fauna, referred to as thebalatonicus-trinodosuszone, developed in Epidavros above theismidicuszone, and is considered to be Pelsonian-Illyrian in age (Krystyn, 1983).

The above data show that although sedimentation was very condensed, condensation did not reach the level of mixed fauna, and for that reason it is possible to differentiate several biozones.

Upper part: Thickness about 3 cm.

Facies: Comprised of small clasts of volcanic origin(plagioclase, titanomagnetite, lithoclasts) and a few bioclasts. Clasts of volcanic origin are arranged parallel to each other.

Lithologic unit B: Includes layers A3 and A4, which correspond to the characteristic reddish-brownish, nodular limestones of the Hallstatt facies, with the unique occurrence of macrofauna and averages 1 m in thickness (Fig.11). These layers are rich in Fe- and Mn-oxides that mark hardground horizons.

A3

A3/1

Strat. unit: Hallstatt horizon. Homogeneous, massive limestone characterized by brownish-reddish colour, averaging 1 m in thickness.

Facies: Packstones/floatstones with ammonoids (Fig.15).

Microcomponents: Molluscs, gastropods, echinoderms, ostracods, radiolarians, filaments and foraminifera.

Texture: Condensed sediments. Discontinuities are marked by Fe-oxides and correspond to immature hardgrounds (firmgrounds). Glauconite has been observed to fill cavities along hardgrounds. Several microstylolites transect the matrix, resulting in a nodular texture.

Interpretation: Limestones with autochthonous hemipelagic faunal elements (ammonoids) and allochthonous reefal detritus (foraminifera, ostracods, gastropods).

Age: The lower part of this layer is poor in macrofauna,whereas within the upper part a characteristic fauna has been detected includingParakellneritessp.,Ptychitescf.oppeliMOJSISOVICS,Proarcestessp. andProcladiscitessp., which taking into consideration the conodonts microfauna included, as well, corresponds to the BiozoneParakellnerites(Krystyn and Mariolakos, 1975; Krystyn,1983). The BiozoneParakellneritesfollows the Biozonetrinodosus, both corresponding to Illyrian, however, due to the lack of characteristic fauna, in-between these horizons,the boundary was not determined.

A3/2

Strat. Unit: Hallstatt horizon, averaging 25 cm in thickness.

Facies: Packstones/floatstones rich in ammonoids with a parallel orientation, enriched in Fe-oxides, and containing dissolution seams. Ammonoids molds are filled with geopetal fill.

Microcomponents: Echinoderms, molluscs, gastropods and foraminifera (Tolypammina gregariaWENDT,Arenovidalina chialingchiangensisHO, Duostomiidae,Ataxophragmiidae and Nodosariidae).

Texture: Ammonoids with a parallel orientation.

Interpretation: Limestones with autochthonous hemipelagic faunal elements (ammonoids) and allochthonous reefal detritus (foraminifera, gastropods).

Age: Ammonoids includeNevadites humboldtensisSMITH,Proarcestes bramandei(MOJSISOVICS),P. esinensis(MOJSISOVICS),P. subtridenticus(MOJSISOVICS),Sturia semiarataMOJSISOVICS and the generaAnolcites,Protrachyceras, which characterize the Biozonereitzi(Nevadites). Concerning the stratigraphic position of the zonereitzi(Nevadites), Krystyn and Mariolakos (1975)and Brack and Rieber (1986) considered it to coincide with the Illyrian (Anisian), whereas Rieber (1973) and Krystyn(1983), considered it to be Fassanian (Ladinian).

In Theokafta, the BiozoneNevaditesis situated on the BiozoneParakellneritesand coincides with the extinction ofPtychitesandParaceratitesand the first appearance of Trachyceratidae and the genusAnolcites, as well as the genus and speciesSturia semiarataandMonophyllites wengensis. According to these data, a Fassanian (Lower Ladinian) age is attributed to this horizon.

5 Biostratigraphy

Detailed study of the ammonoid fauna from the lowest horizons of the studied sequence in Theokafta (Epidaurus)revealed 35 genera, including five that are reported for the first time and 94 species, of which 23 are referred to as new species. The following stratigraphic units have been distinguished (Fig. 16).

Fig. 12 A-Radiolarian packstone consisting of calcitized radiolarians. Bioclasts include foraminiferas, ostracod and mollusc debris(arrow, Layer A/A), 2.5X; B-Radiolarian packstone commonly exhibit a nodular structure. Dissolution surfaces, along the contacts of coalescing nodules, are marked by Fe-oxide-rich microstylolites (Layer A/A), 2.5X; C-Volcaniclastic facies, consisted of relic crystals of plagioclase, quartz and mafic minerals, as well as lithoclasts of variable sizes. All components have been assimilating by the surrounding micritic matrix, enriched in Fe-oxides. Note a relic crystal of andesine (arrow). Pyrite is disseminated in the surrounding matrix (Layer A1), 2.5X (plane polarized light); D-Calcium carbonate nodules (arrow a), assimilated by matrix enriched in Fe-oxides that includes dispersed lensoid/prismatic calcite crystals (arrow b, Layer A1), 5X.

5.1 Anisian

The lowest members of the study section are attributed to the Anisian (average thickness about 1.45 m). In this interval the following stratigraphic subdivisions are distinguished, from the oldest to youngest (Fig. 16):

Aegaeian

The Aegaeian is determined by the speciesJaponitessp.,Megaphyllitessp.,Proarcestesn. sp. andPtychites oppeli, which characterizes the BiozoneJaponites/Paracrochordicerasor the Biozone “Aegeiceras”ugraof Chios island, that was previously determined in Theokafta. The uppermost biozone includes the new speciesProarcestes jacobshageni, representing its appearance in Aegaeian.The identification of this stratigraphic subdivision demonstrates an age of the keratophyric tuffs as Lower Anisian(Scythian or even older).

Bithynian

The Bithynian includes the BiozonesHollanditesandProcladiscites/Leiophyllites, which are very rich in fauna content (70% of the total fauna).

Fig. 13 A-Carbonate nodules (arrow a) and relic crystals of plagioclase (arrow b), floating in a ferruginous matrix with dispersed pyrite (Layer A1), 1.6X; B-Carbonate nodule (arrow a), partially assimilated by a “mylonitic-like matrix” enriched in Fe-oxides,consisting of relic crystals of quartz and feldspa (Layer A1), 1.6X; C-Contact of the volcaniclastic facies (arrow), with the overlying Hallstatt facies, deliniated by circumgranular cracking and Fe-oxides rims (Layer A2/1a, lower part), 1.6X; D-Packstones/floatstones with radiolarians, molluscs (arrow a), brachiopods (arrow b), ostracods, echinoderms, gastropods (arrow c), as well as benthic foraminifera represented by Arenovidalina chialingchiangensis HO and Nodosariidae. Meniscus cements commonly developed under bioclasts (Layer A2/1a, upper part), 1.6X.

BiozoneHollandites: The BiozoneHollanditesis determined by the presence of the genusHollandites, which is accompanied by the new generaReflingtites,Proarces-tes,Judicarites,DanubitesandMegaphyllites, as well asSturiacf.mohamediandPtychites oppeli. This biozone occupies the lower part of layer A2/1b and corresponds to the lower part of theosmanibiozone of the Bithynia area in Asia Minor. Instead, the BiozoneProcladiscites/Leiophyllitesis more appropriate the upper part of this layer,which is rich in fauna including the speciesLeiophyllites suessiandLeiophyllites confucii, along with the accompanied faunaProarcestescf.svbtridentinus, being equivalent to the remained upper part of the subzoneosmaniand the subzoneismidicusof Bithynia.

Pelsonian

The Pelsonian comprises the biozonesBalatonitesandzoldianus.

BiozoneBalatonites: The biozoneBalatonitesis characterized by the presence of the genusBalatonitesand the accompanying faunaPtychites oppeli,P.opulentus,Flexoptychites acutus,Schreyerites ragazzoniiandReinflingitessp., which most probably corresponds to the subzonebalatonicus.

Biozonezoldianus: The biozonezoldianusis characterized by the speciesBulogites zoldianusand the same accompanying fauna as in the biozoneBalatonites, as well asParaceratitesaff.trinodosus,Megaphyllites sandalinus,Ptychites seebachi,P.progressus,Flexoptychites flexuosus,F.gibbus,Psilosturiasp.,Epigymnites incultusandDanubitessp. The biozonezoldianusthat has been recognized for first time at Theokafta, and is considered to be equivalent to the homonymous subzone of Vörös (1987,1993) in the Balaton area of Hungary.Illyrian

Fig. 14 A-Packstone/floatstone with ammonoids, associated with gastropods (arrow a), molluscs, echinoderms, ostracods, radiolarians (arrow b) and benthic foraminifera debris (Layer A2/1b), 1.6X; B-Packstones/floatstones characterized by a microbrecciated texture and composed of bioclasts (gastropod, arrow), feldspars and quartz crystals, in places intensively impregnated with Fe- and Mn-oxides. All components float in micritic matrix (Layer A2/2, lower part), 1.6X; C-Packstone/Floatstone with ammonoids, associated with echinoderms (arrow a), gastropods (arrow c), molluscs (arrow b) and ostracods and benthic foraminifera. Note the mollusc attacked by microborers (upper right) (Layer A2/2, upper part), 1.6X; D-Packstone/Floatstone with ammonoids, associated with benthic foraminifera (arrow a), echinoderms, gastropods, molluscs (arrow b) and ostracods (Layer A2/2, upper part), 1.6X.

In the Illyrian, three biozones have been distinguished,namely the Biozonetrinodosus, the BiozoneParakellnerites/Reitziitesand the BiozoneNevadites. Krystyn (1983)suggested that the BiozoneNevadites, recorded in the Hallstatt facies, should be included in the Fassanian (Ladinian), instead of the Illyrian (Anisian), as previously was mentioned. However, theNevaditesbiozone is recently considered Anisian because the Anisian/Ladinian boundary has been officially set higher (Bracket al., 2005).

Biozonetrinodosus: The Biozonetrinodosusis rich in fauna, mainly characteristic genera and species of the Ptychitidae family (80%), represented by the generaPtychites,Discoptychites,Flexoptychitesand for first time in Argolis the genusAristoptychites. Additionally, new genera and species appear in this biozone which is characterized as a stable biozone in the alpine system, including:Sageceras walteri,Norites gondola,Bulogitescf.gosaviensis,Philippites erasmi,Lardaroceras krystyni,Lardarocerassp. cf.Lardarocerassp. ind. BALINI,Lardarocerassp.,Judicarites euryomphalus,J. arietiformis,Proarcestes bramantei,Proarcestesaff.obonii,Flexoptychites angus-to-umbilicatus,Flexoptychites studeri,Ptychites uhligi,P.breunigi,P.stoliczkai,Discoptychites suttneri,D.pauli,D.domatus,D.megalodiscus,Aristoptychitessp.,Paraceratitescf.elegans,Epigymnites obliquus,?Anagymnitessp. In addition the following taxa have been recognized:Danubitessp.,Judicarites meneghinii,Ptychites oppeli,P.opulentus,P.progressus,P.seebachi,Flexoptychites flexuosus,F.acutus,Sturia sansovinii,Psilosturiasp.,Epigymnites incultus,Proarcestes esinensis,P.jacobshageni,Megaphyllites sandalinus,Monophyllites sphaerophyllusandLeiophyllites confucii.

Fig. 15 A-Ammonoid biomold filled geopetally with internal sediment (arrow) and cloudy fibrous cement overlaid by blocky cement and/or chalcedony (Layer A2/2), 1.6X (Normal Nicols); B-Packstone/Floatstone rich in ammonoids, overlain by a radiolarian packstone consisting of calcified radiolarians. Note the irregular relief of the discontinuity surface. The lower part is intensively impregnated by Fe-oxides (Layer A2/3), 1.6X; C-Discontinuity corresponding to the contact of packstone/floatstone that is rich in ammonoids and radiolarian packstone with calcified radiolarians (upper part). The contact is marked by pyrite rims and irregular relief. The ammonoid shell in the centre has been intensively bored (Layer A2/3), 1.6X; D-Packstone/Floatstone with ammonoids, associated with molluscs,gastropods, echinoderms, ostracods, radiolarians, filaments and foraminifera. Discontinuities are marked by Fe-oxides and correspond to immature hardgrounds (Layer A3/1), 1.6X.

Fig. 16 The principal stratigraphic subdivisions and the respective biozones of Anisian and Ladinian (Tselepidis, 2007).

BiozoneParakellnerites/Reitziites: The BiozoneParakellnerites/Reitziitesis unified, due to the short extent of appearance of the characteristic taxa from both biozones.In the layer that is situated in an height of 1.25 m within this biozone, the characteristic genusParakellneritesoccurs, represented by different species but mainly byParakellneritescf.zoniaensis; the accompanying fauna is represented by the new speciesParaceratitescf.subnodosus,Nevaditessp. 1,Proarcestes extralabiatus,Sturia semiarata, while the occurrence ofFlexoptychites flexuosuscontinues. At 1.35 m above the base, the genusReitziitesappears for first time (Brack and Rieber, 1993). Additionally, the new speciesReitziitesn. sp. is detected in the uppermost layers of this biozone, just above the appearance of the genusParakellneritesand the accompanying faunaJoannites joannis-austriae,Sturia forojulensis,Epigymnites incultus,Procladiscites crassus andMonophyllites wengensis. The genusReitziitesis assumed to have developed after the genusParakellnerites. The short distance between the two stratigraphic layers (1.25 m to 1.35 m),where representatives of the two genera occur, complicates their stratigraphic subdivision in the section at Theokafta.

BiozoneNevadites: The BiozoneNevaditesis attributed to the Anisian and specifically constitutes the uppermost biozone of the Anisian before the Ladinian. It consists of three subzones (crassus,serpianensis,chiesense), but in the same study is unified and is referred asNevadites(sensu lato). In the study section, the BiozoneNevaditesis recognized in layer A5/1, which is extremely rich in fossils, represented by the speciesNevaditescf.crassiornatus,Nevaditessp.,Sageceras haidingeri,Halilucitessp.,Chieseiceras chiesense,Anolcites julium,Proarcestes esinensis,P.escheri,P.extralabiatus,Joannites klipsteini,J.kossmati,J. joannis-austriae,Procladiscites crassus,Flexoptychites angusto-umbilicatus,Sturia semiarata,S.forojulensis,Epigymnites icultus,E.obliquus,E.ecki,E.bosnensis,Japonites raphaelis-zojae,Monophyllites argolicus,M.wengensis, andMojsvarites agenor.

5.2 The Anisian/Ladinian boundary

The subdivision of the Hallstatt-type limestones and the determination of the Anisian/Ladinian boundary is determined from data based on the fauna that is present at the Theokafta section/Epidavros and after considering all the scientific data from respective areas of the Tethys.

Since the genusNevaditesis attributed to the Anisian and belongs to Ceratitidae or the Aplococeratidae, we suggest that the Anisian/Ladinian boundary should be delineated by the presence of fauna of the family Paraceratidae. According to our observations, the Anisian/Ladinian boundary coincides with the first appearance of representatives of Trachyceratidae, especially of the genusEoprotrachyceras(Subcomission on Triassic Stratigraphy, IUGS)

5.3 Ladinian

In the study section, the Ladinian starts at a height of 1.45 m above the base of the Hallstatt Limestones. A distinctive feature of the Anisian/Ladinian boundary is the formation of layers rich in Fe- and Mn-oxides that commonly infill joints vertical to the stratification. The boundary coincides with the base of the layer rich in ammonoids,layer A5/1, which is principally characterized by the genusEoprotrachyceras.

5.4 Fassanian

The Fassanian comprises the lowermost Ladinian. In terms of the study section, only the Biozonecurionii, at the lower part of Ladinian, is considered.

Biozonecurionii: The Biozonecurioniiis characterized by representatives of the family Trachyceratidae,mainly of the genusEoprotrachycerasand the speciesChieseiceras chiesense,Eoprotrachycerascf.margaritosum,Protrachycerassp.,P.gredleri,Anolcitesaff.julium,Megaphyllites jarbas,etc.

6 Diagenesis

The studied Hallstatt facies corresponds to hiatus beds/concretions characterized by discontinuous sedimentation and erosion. Synsedimentary and early burrowing processes differentiated the primary textural characteristics of the deposited sediments. Multiphase diagenesis occurred not very deep below the sediment surface and includes,boring, encrustation, burial and cementation. Similar multiphase diagenetic events have been described by Savrda and Bottjer (1988).

Detailed sedimentological study enabled reconstruction of diagenetic events. Some layers or nodules underwent early lithification, preventing rearrangement during burial. In the uncemented beds, the effects of subsequent mechanical and chemical compaction are clearly recognizable. Grain-supported sediments developed fitted fabrics,whereas in mud-supported sediments dissolution seams were generated.

Cement types, all of which are now calcite, include micrite, fibrous and blocky cement. Micrite cement is homogeneous and/or peloidal and occurs between the components of the volcaniclastic substrata (quartz, plagioclase and mafic minerals crystals; Fig. 12C). Fibrous cement fills voids resulting from dissolution of carbonate hells when exposed to pore fluids. The length of the crystals increase towards the center. The fibrous cement is cloudy and includes fluid inclusions and pyrite grains. Micrite and fibrous cement are usually associated with pyrite. Clear crystals of blocky cement fill the remaining pore space and voids (Fig. 15A). The fact that all cement types contain pyrite, except for the late blocky cement, indicates carbonate precipitation within the sulphate reduction zone.

7 Palaeoenvironment of deposition

The deposition of the red nodular ammonoid-bearing pelagic Hallstatt Limestones of the Argolis Peninsula,upon the carbonate-free volcaniclastic substrata, as hiatus beds/concretions, is considered to be due to anaerobic oxidation of organic matter by microbes, which provided excess alkalinity and induced carbonate precipitation. For the development of such a rich fauna of cephalopods in the Hallstatt Limestones, sunlight is necessary and likely happened on differentially-subsided and intermittently elevated “deep swells” (Hornung and Brandner, 2005; Hornunget al., 2007). After drowning, the sea mounts were covered by pelagic carbonate deposits. Further slight rotation of the blocks along listric faults may have led to additional differential subsidence of the blocks.

In terms of sequence stratigraphy, the studied Hallstatt hiatus concretions and beds are considered genetically linked to rising or high sea-level (e.g., Van Wagoneretal., 1988), formed at the initiation of transgressions (e.g.,Voigt, 1968; Fürsichet al., 1991), as well as during the time of maximum rate of transgression, in areas where the sediment input is strongly reduced (“condensed section”).

During the Lower Triassic, which corresponded to a nonglacial time interval, the shelf occupied a large area of shallow-water deposition, which could often have been affected by storms, resulting in winnowing of the concretions and the formation of hiatus beds. On the other hand,third-order sea-level changes may have played a significant role in the formation of the hiatus beds.

According to the available data from the present research, concerning the deposition, the development and the location of the Hallstatt-type limestones in the Hellenides and generally in the Alpine orogenetic system, it is considered that the depositional area was either united or that there was a direct and broad communication between different palaeogeographic areas, which is confirmed by the distribution of ammonoids and lithology. Taking into consideration the present location of the Hallstatt Formation, in the context of the Hellenides, such an area should be located between the sub-Pelagonian (western part of the Pelagonian zone) and Pindos geotectonic zones, which during the Triassic corresponded to a platform slope and a deep ocean, respectively. According to Bracket al.(2007),in Triassic successions of the Eastern Alps (Dinarides,Hellenides), the widespread Middle Triassic Han Bulog Limestones (ammonoid-bearing pelagic limestones) may have formed partly in similar slope environments.

Apart from the Hallstatt-type limestones, the limestones with cherts are considered to be deposited in the same environment, corresponding to the sub-Pelagonian zone, due to their direct relationship with the Hallstatt-type limestones and their development over the same underlying formation (e.g., the keratophyric tuffs) and also because of their content in cephalopod fauna (siliceous phase) during the Triassic (especially during the Carnian). The fact that during this interval, the formation of limestones with cherts was extended over an area that occupied the external margins of the sub-Pelagonian zone towards the Pindos zone, confirms the aforementioned statements.

8 Conclusions

1) The Middle Triassic Hallstatt facies of the Argolis Peninsula consist of hiatus beds/concretions characterized by discontinuous sedimentation and erosion.

2) The contact of the Hallstatt facies with the underlying volcanic rock, as well as with the overlying radiolarites, is stratigraphic.

3) Nine lithostratigraphic horizons have been distinguished, including radiolarian packstones, volcaniclastic facies, packstones/floatstones with ammonoids and lag deposits.

4) Nine distinct ammonoid biozones from the Anisian to Ladinian have been defined by Tselepidis (2007), documenting deposition on Hallstatt deep swells with a low depositional rate for nearly 5 million years (using the timescale of Gradsteinet al., 2004). The biozones:Japonites/Paracrochordiceras,Hollandites,Procladiscites/Leiophyllites,Balatonites,zoldianus, trinidosus,Reitziites/ParakellneritesandNevaditesare included in the Anisian and thecurioniiin the Fassanian (lower Ladinian). Although sedimentation was very condensed, it did not reach the level of mixing fauna.

5) Sedimentation rate was very low and facies show evidence of early lithification near the sediment/water interface. Omission surfaces, firmgrounds and mineralized hardgrounds were widespread.

6) Synsedimentary and early burrowing processes differentiate the primary texture characteristics of the deposited sediments. Multiphase diagenesis occurred not very deep below the sediment surface and includes boring and/or encrustation, burial and cementation.

7) Cement types include micrite, botryoidal, fibrous and blocky calcium carbonate cement. The fact that all cement types contain pyrite, except for the late blocky cement, indicates carbonate precipitation within the sulphate reduction zone.

8) The deposition of the Hallstatt Limestones, upon the carbonate-free volcaniclastic substrata, is considered to be due to anaerobic oxidation of organic matter by microbes,which provided excess alkalinity, inducing carbonate precipitation.

9) In terms of sequence stratigraphy, the Hallstatt Limestones, as hiatus beds and concretions, are genetically linked to a rising or high sea-level. They are considered to be formed at the initiation, as well as during the time of maximum rate of transgression, in areas where the sediment input is strongly reduced (“condensed section”).

10) Hallstatt facies were deposited on differentially subsided and intermittently elevated “deep swells”. Further slight rotation of blocks, along listric faults, may led to additional differential subsidence of the blocks.

11) Shelf bathymetry and third-order sea-level changes may have played a significant role in the formation of the Hallstatt beds.

12) The depositional area of the Hallstatt-type lime-stones in the Hellenides and generally in the Alpine orogenetic system, was either united or there was a direct and broad communication between different palaeogeographic areas, and is confirmed by the distribution of ammonoids and lithology.

13) Hallstatt Formation deposition occurred between the sub-Pelagonian (western part of the Pelagonian zone)and Pindos geotectonic zones, which during the Triassic corresponded to a platform slope and deep ocean, respectively.

14) The Hallstatt facies of the Argolis Peninsula is considered part of the widespread Middle Triassic Han Bulog Limestones (ammonoid-bearing pelagic limestones) of the Eastern Alps (Dinarides, Hellenides), which may have been formed partly in platform slope environments, similar to those of the Southern Alps.

Acknowledgements

The authors are highly indebted to Professor Feng Zenzhao (the Editor-in-Chief of JOP) and an anonymous reviewer for their constructive suggestions and proposals.