Biostratigraphic zonation based on foraminifera and algae in the Triassic deposits of the north sector of the Eastern Carpathians

Triassic carbonate deposits which represent the topic of our research belong to a system of Alpine nappes (Median Dacides) that form the Crystalline Mesozoic Area of the Eastern Carpathians. From bottom to top, these tectonic units are the following: the Infrabucovinian Nappes, the Subbucovinian Nappe, the Bucovinian Nappe and the Transylvanian Nappes. The Bucovinian Nappe has the highest extension among the Central-East-Carpathian Nappes system, supporting the remains of the Transylvanian Nappes that occur only as isolated klippe floating within the Hauterivian-Albian wildflysch Formation of the Bucovinian Nappe. The lower tectonic units of the Bucovinian Nappe, respectively of the Infrabucovinian Nappes and of the Subbucovinian Nappe, occur discontinuously, outcropping in several halfwindows. Triassic sedimentation started by an Induan detrital level, consisting of conglomerates and sandstones. The Induan deposits are overlain by Olenekian-Rhaetian carbonate deposits. The detrital Induan completely lacks microfauna and subsequently, the zones approached in our research are only defineded in the overlaying carbonates. The Anisian-Ladinian dolomites and limestones that form almost exclusively the Infrabucovinian and Subbucovinian Nappes offer little micropaleontological information, and with no stratigraphical significance. The limestones of the Bucovinian Nappe and especially those of the Transylvanian Nappes have revealed a paleontological content, rich enough to define Triassic zones. As a result, the biostratigraphic zonation in the studied area especially relys on biofacies data, achieved by microscopic studies of carbonate Triassic deposits of these two last units. Six zones on foraminifera and algae assemblages are defined in the Olenekian-Rhaetian. The Meandrospira pusilla Zone characterizes the Olenekian stage. The zone occurs in Bucovinian and Transylvanian facies. It was separated in the stratified limestone level, overlying the detrital Induan in Haghimas Syncline (Bucovinian facies). The massive dolomites of the Bucovinian Nappe provided a poor paleontological content consisting of a few foraminifera of no stratigraphical value, which makes impossible the definition of the zone in the lower Anisian. The Meandrospira dinarica Zone corresponds to Pelsonian and Lower Illyrian of the Bucovinian Nappe. It characterizes the lower half of the algal limestones, overlying Lower Anisian massive dolomites. The Diplopora annulata Zone characterizes the Upper Illyrian substage and Ladinian stage of the Middle Triassic. The zone has been identified in both Bucovinian and Transylvanian Nappes. The mentioned zone defines the upper half of the white algal limestones in Bucovian facies and the white-gray limestones in Transylvanian facies. The Ophthalmidium exiguum Zone corresponds to Carnian and was only separated in Transylvanian limestone facies. The Glomospirella friedli and Miliolipora cuvillieri zone characterizes the Norian. The zone was separated only in the Transylvanian Nappes of the Rarau Syncline, where it occurs in two distinct lithologies: nodular red limestones and white limestones. The Angulodiscus tenuis Zone corresponds to Rhaetian Transylvanian facies. It was established in the red and grey limestones of the Rarau Syncline. Comparing the zones established by various authors in the Tethysian domain with the zones separated in this study in the Triassic carbonate deposits of the Eastern Carpathians, one will notice similarities and identities, especially at the Olenekian and Anisian stage. Starting with the Upper Triassic, the index-species here proposed are completely different of those that define the zones of the West Carpathians, Bulgaria, or the entire Carpathian-Balkan region. This proves the existence of some particular conditions, specific to sedimentation, within the north region of the Eastern Carpathians.