Microfacies study of the Tata Limestone Formation (Aptian) in the northern Bakony Mountains, Hungary

Abstract

The Tata Limestone Formation consists of fine- to very coarse-grained rocks generally with grainstone, rarely with packstone texture. On the basis of a point-count analysis, the rock-types are: bioextrasparite; extrabiosparite; silicified crinoidal limestone; calclithite; and biomicrite with sponge spicules. The most frequent rock-forming biogenic grains are the skeletal detritus of crinoids and molluscs; extraclasts predominate among land-derived grains. The cement is larely of syntaxial rim-type, the rest is mosaic calcite. Three microfacies types have been recognized. They indicate a shallow water environment but each was formed at different depths. The fine-grained packstones (microfacies A) contain sponge spicules and represent the deepest environment. The very coarse-grained grainstones (microfacies C) consists mostly of mollusc debris with micritic envelopes; the molluscs are partly or completely filled with mosaic spar. This microfacies can be interpreted as a shallow subtidal sediment. The medium- to coarse-grained crinoidal grainstones with high extraclast content (microfacies B, which has the widest distribution) are transitional between microfacies A and C. The deposition of the Tata Limestone Formation marks the end of a long pelagic period in the history of the Bakony Mountains. During the Aptian, the area was greatly uplifted. Elevated to different altitudes, the carbonate blocks that emerged played a dual role. They not only supplied detritus to the sedimentary basins but also provided excellent conditions for benthonic life, which became dominated by crinoids.