A Veszprém Megyei Múzeumok Közleményei 12. (Veszprém, 1973)

Gellai Mária–B.: Úrkút környéki albai mészkő rétegsorok mikrofácies vizsgálata

MICROFACIES STUDY ON THE ALBIAN LIMESTONE SEQUENCE OF ŰRKŰT Two Albian limestone profiles (of boreholes Zs

—4 and Zs

—6) have been studied on the basis of 151 thin sections. Two types of matrix have been distinguished. The granulomorphous one consists mainly of micrite. It shows several sorts according to the main size of grains (max. 10—12 microns) and the regularity of the granulometry. The crystallomorphous matrix consists of a coherent sparite field. Matrix and the shape-units form the texture. The shape-units I have observed are: pellet (intraclast, sensu J. KONDA 1967 and J. FÜLÖP 1966), pseudo-oöid (reworked detritus with micritic incrustation), calcareous sand, detritus and fossils. Related types of texture form a microfacies. The microfacies I have distinguished — as listed in the order of the increasing hydrodynamic energy of the sedimentary environment — are: Granulomorphous

—detritic faciès: more or less intact fossils and fragments; no worn and no coated units; no crystallomorphous field, but scattered calcite spars. Pellet faciès: granulomorphous, crystallomorphous or mixed matrix; pseudo-oöids less than 10%; Orbitolina appears. Pellett

—pseudo-oöid faciès: similar matrix; the uncoated fragments, which transported from the reef and settled just before the burial and consolidation of the sediment, appear always together with the pseudooöids; calcareous sand grains and peeled-off laminae of coated fragments, which transported from the reef and sediment also occur. Calcarenite facies: similar matrix; a part of the sands grains are limestone, the rest are Orbitolina, less frequently bivalves, gastropods and echinoderms; most of them are rounded; the sand grains are coated by idiomorphic calcite. Crystallomorphous facies: sparite of mosaic-like pattern, with small opaque inclusions and limonitic rims; only few grains seem to be biogenic. This facies makes up only two beds. These two profiles represent the Albian limestone of the southern Bakony Mts. This limestone, according to the results of the present study, differs from the Albian limestone sequence of the northern Bakony Mts. The lower part of this sequence contains well-individualized horizons defined by the predominance of Pachyodonta, smaller-Foraminifera or Orbitolina. In the southern profiles these faunal elements occur together. Accordingly the Albian limestone of the southern Bakony Mts. is not identical with the Albian Requienia Limestone (J. NOSZKY jr. 1934 = Zirc Beds, F. HAUER 1862) of the northern Bakony Mts. as was believed before. Therefore I have denominated it Úrkút Limestone, as an independent stratigraphie unit, after the location of its most important outcrops. It was designated as an independent unit also by J. MÉSZÁROS (1967) according to the results of his geological surveying. Mária-Bernadette Gellai