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

摘要

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. Lithofacies and biostratigraphic research has 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. The lowermost horizons of the Hallstatt Limestones of Theokafta represent typical hiatus beds/concretions sensuWetzel and Allia (2000), characterized by discontinuous sedimentation and erosion. They consist of red ammonoid-bearing hemipelagic limestones with calcium carbonate nodules floating in an enriched Fe-oxides matrix with dispersed lensoid/prismatic calcium carbonate crystals. This part of the section is characterized by condensed sedimentation, due to significant lowering of the rate of sedimentation and includes omission surfaces, firmgrounds and hardgrounds along certain horizons. Nine lithostratigraphic units have been distinguished in the lowermost horizons of the Hallstatt Limestones, including radiolarian packstones, volcaniclastic facies, packstones/floatstones with ammonoids and lag deposits. Tselepidis (2007) defined nine distinct ammonoid biozones from the Anisian to Ladinian, documenting deposition of the Hallstatt facies during a low depositional rate over nearly 5 million years (using the timescale of Gradstein et al., 2004). The biozones: Japonites/Paracrochordiceras, Hollandites, Procladiscites/Leiophyllites, zoldianus, trinidosus, Reitziites/Parakellnerites and the Nevadites (Anisian) and the biozone curionii (Lower Ladinian). Although sedimentation was very condensed, it didn’t reach the level of mixing fauna. Synsedimentary and early burrowing processes differentiated 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. The deposition of the studied Hallstatt Limestones is considered to be due to anaerobic oxidation of organic matter, which provided excess alkalinity, inducing carbonate precipitation. Sedimentation took place on differentially-subsided deep swells. After drowning, the swells were covered by pelagic carbonate deposits. Further slight rotation of blocks, along listric faults, may have led to additional differential subsidence of the blocks. Shelf bathymetry and third-order sea-level changes played a significant role in the formation of the Hallstatt beds. In terms of sequence stratigraphy, the studied hiatus concretions and beds are considered genetically linked to rising or high sea-level, formed at the initiation of transgressions, as well as during the time of maximum rate of transgression, in areas where the sediment input was strongly reduced (“condensed section”). Taking into consideration the present location of the Hallstatt Formation, in the context of the Hellenides, an area suitable for the deposition of the Hallstatt Limestones, 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. The widespread Middle Triassic Han Bulog Limestones (ammonoid-bearing pelagic limestones) from Triassic successions of the Eastern Alps (Dinarides, Hellenides) may have formed partly in similar slope environments.