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The Kimmeridgian tidal-bar calcarenite facies of western Istria (Western Croatia, Jugoslavia)

Oberjurassische Gezeitenbarren-Kalkarenit-fazies im Westlichen Istrien (Jugoslawien)

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Summary

The Upper Jurassic limestones of western Istria are composed of three main facies: 1. peloid packstone/grainstone deposited in a shallow subtidal environment; 2. oolitic coated bioclastic calcarenite and 3. mudstone with desiccation cracks, originating in a near-shore environment ranging from shallow subtidal to supratidal or meteoric vadose zone. The calcarenite facies is characterized by thick-bedded to massive bioclastic grainstones/rudstones with cross-bedding and, at places, also large symmetrical wave ripples. It is composed of well-sorted tangential ooids and variable amounts of large coated bioclasts ofCladocoropsis and other hydrozoans, corals, pachydont bivalves and test of benthic foraminifers. Almost all the larger bioclasts are coated by micrite envelopes. The Kimmeridgian grainstone/rudstone facies in Western Istria has been interpreted as tidal bar and winnowed carbonate sands.

Zusammenfassung

Im Malm (Kimmeridgium) von W-Istrien können drei Hauptfaziestypenunterschieden werden: Peloid-Packstone-/Grainstone-Fazies des flachen Subtidals; Ooid- und Bioklasten-Gezeitenbarren-Kalkarenitfazies und Austrocknungs-Mudstone-Fazies (flaches Subtidal-Supratidal infolge von Meeresspiegelschwankungen). Im Hangenden der Gezeitenbarren-Kalkarenitfazies finden sich Brekzien aus Küstennähe oder sogar des Auftauchbereichs und/oder Bauxitlagerstätten, die die transgressive Grenze zur Austrocknungs-Mudstone-Fazies des Tithons kennzeichnen. Die wichtigsten sedimentologischen Merkmale dieser Fazies sind dickbankige bis massive Ooid-Grainstones und Ooid-Bioklasten-Grainstones/Rudstones mit klar ausgeprägter Schrägschichtung und starkwelligen Rippelmarken. Außer tangentialen Ooiden enthalten diese Schichten auch unterschiedliche Mengen von groben umrindeten Bioklasten (Cladocoropsis und andere Hydrozoen, Korallen, pachydonte Muscheln, benthische Foraminiferen). Nahezu alle Bioklasten (Rindenkörner) besitzen allseitige Mikritrinden.

Die Texturen und Strukturen, insbesondere die Schrägschichtung und die Wellenrippel, beweisen, daß der Detritus aus Ooiden und Bioklasten transportiert und durch Gezeitenströme und Wellen abgelagert worden ist.

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Tišljar, J., Zagreb, V. The Kimmeridgian tidal-bar calcarenite facies of western Istria (Western Croatia, Jugoslavia). Facies 17, 277–283 (1987). https://doi.org/10.1007/BF02536792

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