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PALEOENVIRONMENTAL INVESTIGATIONS IN AN EARLY MIOCENE (EGGENBURGIAN) MARINE FLOODED ESTUARY FROM THE AUTOCHTHONOUS MOLASSE ZONE

Oleg Mandic1, Mathias Harzhauser2 & Reinhard Roetzel3

1 Institut für Paläontologie, Universität Wien, Althanstrasse 14, 1090 Wien; oleg.mandic@univie.ac.at
2 Naturhistorisches Museum Wien, Geolog.-Paläontolog. Abteilung; mathias.harzhauser@nhm-wien.ac.at
3 Geologische Bundesanstalt, Tongasse 10-12, 1031 Wien; rroetzel@cc.geolba.ac.at

Early Eggenburgian marine successions transgressing the SE Bohemian Massif reflect a stepwise landward shift of the paleoshoreline. The marine sediments onlap either directly the ancient relief formed by crystalline rocks or prograde into an extended estuarine-fluvial system coming from NW. During the Oligocene and Lower Miocene this river met the Alpidic Molasse Sea in southern direction, via a "L"-shaped (E-W and N-S striking), asymmetric (E tilted) tectonic valley termed Horn Basin. Fluvial gravel and sand of the St. Marein Freischling Formation was deposited in the relief of the Bohemian Massif at that time. A first transgressive impulse culminated in vast mudflats and oyster bioherms of the Mold Formation forming along the coasts of the Horn Basin. Melanopsids, potamidids and theodoxids indicate brackish conditions and a still active fluvial influx. A narrow estuarine system was already established in the Horn Basin. Finally, the transgrading sea flooded the ancient Horn river alluvial plane completely. The brackish-estuarine biotopes of the Mold Formation were gradually replaced and fully marine conditions became installed. The Loibersdorf Formation which includes the holotypus of the Eggenburgian document these fully marine environments within the Horn Basin.

Several excavation campaigns of the Institute of Paleontology Vienna focused on the fauna of the marine transgression. Especially the extremely rich shell accumulations in the base of the Loibersdorf Formation were studied in detail. Due to the poor outcrop conditions an artificial outcrop was made exposing the transition from the estuarine-brackish clay of the Mold Formation into the fully marine sand of the Loibersdorf Formation. The taphonomy of the coquinas was documented in vertical sections as well as in expansive horizontal sections. The exact position of fossils and pebbles was transferred to a transparent plastic foil. Numbers of specimens were counted in quadrants and taphonomic features of each shell were documented carefully.

Two coquina types in the basal part of the Loibersdorf Formation could thus be separated very clearly. The lower one bears a dense layer of Oopecten gigas, a stratigraphic index fossil of the Early Eggenburgian. Pebbles and poorly sorted sand indicate very shallow marine conditions in a well agitated coastal environment. Similar high energetic environments were settled by sediment laying, thick-shelled pectinids throughout the Miocene. A considerable number of articulated valves hint at a rather autochthonous occurrence of the pectinids. The close position to the shoreline is also indicated by shells of the potamidid Granulolabium plicatum and the trochid Paroxystele amedei. Littoral environments are also favored by the mytilid Crenomytilus? aquitanicus and balanids, which are abundant elements in the basal coquina.

The second coquina type, which is embedded in medium sand, differs distinctly in its composition and displays a much higher diversity (59 species). It is predominated by bivalves such as Pitar lilacinoides, "Laevicardium" kuebecki and Glycymeris fichteli. Among the gastropods, Protoma cathedralis, Babylonia eburnoides, and Paroxystele amedei are most important along with several naticids.

In one of the quadrants 168 bivalve shells could be identified on the species level. From these only 23 specimens are represented by articulated valves, whereas the other 145 specimens are disarticulated. The articulated valves could be attributed exclusively to Pitar lilacinoides and Glycymeris fichteli. 35% of the glycymerids but only 2% of the Pitar lilacinoides specimens are documented by articulated valves. This points to a two-stepped story of coquina formation. The Pitar lilacinoides - "Laevicardium" kuebecki coquina probably formed as result of heavy agitation during a storm event and could thus be classified as tempestite. Later the fine sand which buried the shell bed was settled by deep burrowing glycymerids. The shell bed, however acted as unconformity which could hardly be passed by the large-sized, robust Glycymeris fichteli and thus caused a "condensation" of glycymerids. This interpretation is also strongly supported by the in-situ occurrence of glycymerids and rare specimens of Panopea menardi in the overlaying sand.

Hence, the transgression of the Eggenburgian Sea is documented by a fining and deepening upwards sequence. Pectinid-dominated paleo-communities formed close to the shoreline during the early transgression. During the landward shift of the coast, sandy sublittoral environments in 10-50 m deep water formed which were settled mainly by infaunal bivalves. Sporadic storm events triggered the accumulation of shells in tempestites, which later acted as barrier for digging bivalves such as Glycymeris fichteli.