Szekessy Vilmos (szerk.): A Magyar Természettudományi Múzeum évkönyve 62. (Budapest 1970)

Nagy, I. Z.: Lower Liassic molluscs from the Mts. Mecsek, Hungary

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 62. PARS MINERALOGICA ET PALAEONTOLOGICA 1970. Lower Liassic Molluscs from the Mts. Mecsek, Hungary By I. Z. NAGY, Budapest The Lower Liassie Sequence oï the Mts. Mecsek The black coal sequence of the Mts. Mecsek, restricted to the Hettangian and Sinemurian, represents, with respect to its evolvement, the so-called Grestenian type. By one of the effects of the Early Labinian orogeneous phasis, the former sedimentary basin became a system of estuaries open towards the east. The bottom of the basin sunk more rapidly in the south than in the north. With a lacustrine facies, the lower group of the coal sequence settled onto the Rhaetian. The middle group (Upper Hettengian) shows inundational and palustrine, the uppermost one (Lower Sinemurian) paralic and low palustrine, characters. As one of the results of the unequal sinking of the basin bottom, the coal sequence is 1000-1200 m thick in the south, and merely 100-200 m deep in the north. The 350-1000 m thick so-called hanging marl sequence (Upper Sinemurian) settled onto this sequence. The hanging marl lies parallel with the coal sequence, that is, its thickness decreases northwards. The coal beds change from the limnic character into the paralic, when followed towards the younger ones. This change is corroborated also by the intercalated, yet comparatively not rich, faunal elements. The cemented fannal banks gradually diminish northwards. Liassic of the Grestenian type, similar to or even deeper than the one found in the Mts. Mecsek, is known only from extra-European regions, thus from Anatolia, the Caucasus, the North American Cordilleras, and the Philippine Islands (E. NAGY, 1969). The characteristics of the material The fossil remains derive from the shale following the coal beds and the hanging marl above the sequence, respectively. The intercalated sterile shale layers yield also floral elements (I. Z. NAGY, 1961). The molluscs are not coincidental with the flora, though oysters settled on floated timber have already been found in the fossil material, e.g. the specimen discovered at Hosszúhetény (Plate I, Fig. 8). The conditions of the undisturbed, indeed, closed state of the basin system are revealed also by the frequent occurrence of pyritization as well as the opened shells connected by the ligament. One of the interesting products of the tectonic movements enacted in the sequence is the "tubular marl" originating from Zobák (Plate I, Fig. 1). Their folding agrees, also as to their order of magnitude, with the similarly Lower Liassic (Hohenheim, Liassic delta) specimens, published by A. H. MÜLLER (Lehrbuch, second edition, vol. 2, 1963, p. 125). The infundibuliform passages and their fine transverse folds are known also as "pscudofossils". DZTXLINSKY & KOTLAKCZYK (1955) also publish phenomena of similar tectonic origin ; they have also reproduced experimentally this sliding-and-folding phenomenon.