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Stratigraphical distribution of Jurassic brachiopods in the Bakony Mountains (Hungary)

Voros A., Dulai A.

Volumina Jurassica

2006

Vol. 4, no. 1

247-248

The Jurassic brachiopod fauna of the Bakony Mountains (Transdanubian Central Range) is well-known: several thousands specimens were collected from the different stages. The collecting work encompassed the whole Jurassic sequence except the Callovian and Oxfordian which are mainly represented by radiolarite. Brachiopods were collected together with ammonites and this offered an exceptional possibility to record their stratigraphic distribution. The ooidal-oncoidal, shallow-water Hettangian Kardosret Limestone yielded 10 (mainly terebratulid) species. The Lower Sinemurian Pisznice Limestone and the cherty, crinoidal Isztimer Formation yielded 46 species of rhynchonellids, spiriferinids and terebratulids. The gradual diversification continued: the Upper Sinemurian Hierlatz Limestone is extremely rich in brachiopods and 66 species were found at three localities. The Pliensbachian is the best studied stage (101 species); more than a dozen sections were collected in four different lithologies: red, condensed, manganiferous limestone; Hierlatz Limestone; crinoidal cherty limestone; Ammonitico Rosso type limestone. The four brachiopod orders (Rhynchonellida, Athyridida, Spiriferinida and Terebratulida) are profusely represented but the bulk fauna is dominated by the species Linguithyris aspasia. The red limestone facies after the Early Toarcian anoxic event yielded only very few specimens of the above mentioned species. The Aalenian Ammonitico Rosso type limestone and greyish siliceous limestone are totally devoid of brachiopods. The same litologies in the Lower Bajocian yielded only a few brachiopods. The Middle and Upper Bajocian shows more diverse lithology (pelagic limestone; red, manganiferous limestone; Hierlatz-like limestone; radiolarite) with the bloom of brachiopods from the Humphriesianum Zone until the end of the Bajocian. Red, Ammonitico Rosso type limestones are widespread in the Kimmeridgian but brachiopods are extremely rare. This limestone is predominant in the Tithonian but Hierlatz-like limestone and white micritic limestone also occur. Brachiopods (mostly pygopids) are especially abundant and diverse in the Hierlatz-like limestones.