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

Szabó, I. ; Ravasz, Cs.: Investigation of the Middle Triassic volcanics of the Transdanubian Central Mountains, Hungary

Fig. 5. Position of the tuffaceous beds within the Upper Anisian-Lower Ladinian sequence in the area of Sóly-őskü (III). — 1. Bedded limestone. — 2. Tuffaceous clay. — 3. Thick-bedded dolomite. — 4. Cherty dolomite. — 5 Bedded dolomite. — 6. Algal dolomite. — 7. Sampling place altered. It is this sequence that yielded the Upper Anisian ammonites of G. ARTHABER (1911) and G. DIENER (1906). The presence of thin layers of green altered tuff within the thick-laminated limestones, immediately overlying the Upper Anisian thick bedded dolomites, is indicative of tuff eruptions synchronous with the deposition of the Lower Ladinian sediments. Simultaneously with the decrease in the intensity of volcanism, some changes in facies manifested by the increasingly dolomitic character of the sediment, can also be observed. The thickness conditions of the pyroclastics deposited in sea water seem to have been controlled by the variation of bottom morphology of the sedimentary basin. Petrography By pétrographie and stratigraphie analogies, these pyroclastic rocks were held for diabasic tuffs. On the basis of our investigations they have proved to be potashtrachytic and rhyolitic tuffs (Table 2, Fig. 6), whose pétrographie description, arranged by stratigraphie units and by areas of distribution within the individual units, is given below. Collectively, the rock material of the tuffaceous layers, intercalated within the Upper Anisian sediments in exploratory shaft XVIII at Pécsely, can be termed : altered lithoclastic-trachytic crystal tuff. The small amount of pyroclastic matter supplied by the eruption fell into sea water and has yielded the clastic fraction of the syngenetic clayey, calcareous sedimentary deposits or, after having been enriched, it has brought about layers of purely volcanic origin. The lowermost tuffaceous layer of the occurrence represents, at the same time, the least altered, purest trachytic tuff horizon. Macroscopically, the rock is of pale greyish-green colour, fine-grained, poorly stratified, with ochre-yellow, iron-stained bands (sample T-155). Under the microscope, it shows the following picture : the argillaceous matrix (illite) of the rock is made up for the most part by mineral fragments, the rest being constituted by detritus of trachytic lava matrix. More than two-thirds of the crys-