Acta Geologica 38. (1995)

1. szám - J. Haas - A. Balog: Facies characteristics of the Lofer cycles in the Upper Triassic platform carbonates of the Transdanubian Range, Hungary

Faciès characteristics of the lofer cycles 3 sedimentological features, which could not be attained even in the much better exposed Alpine, Carpathian and Dinaridic sections. The results of our studies in the Transdanubian Range, mainly concerning the characteristics and cause of the cyclicity and the platform evolution, have been published in several papers (Haas and Dobosi 1982; Schwarzacher and Haas 1986; Haas 1982, 1987, 1988, 1991, 1994). The aim of the present paper is to characterize the facies which make up the cycles, and to display various types and subtypes of the basic facies. Based on facies analysis of the studied formations a general facies model was compiled, showing sites of deposition of the distinguished facies types. Previous sedimentological description and genetic interpretation of the Lofer cycles Sander (1936) first described the cyclothems of Dachstein Limestone as the alternation of laminated dolomitic limestone and thick-bedded limestone, to which he gave the name "Lofer facies" after the Loferer Steinberg. Sander considered the cycles as caused by sea level changes, but he did not find any evidence of subaerial exposures. He thought that the dolomitic laminae formed below wave base, at a depth of more than 200 m. Schwarzacher (1948, 1954) investigated the characters of cyclicity and recognized the megacyclic organization of the elementary cycles. Fischer's study (1964) on Triassic Lofer cyclothems in the Alps provided new ideas for the investigation of Upper Triassic carbonate formations. He defined the basic features of Lofer cyclothems as follows: 1. Disconformity at the base 2. Clayey red or green basal layer, which is often found only in solution cavities in the underlying rock (A-member) 3. Intertidal member consisting of algal mats and formations with shrinkage phenomena (B-member) 4. Thick-bedded subtidal member consisting of calcarenite and calcilutite with a rich marine biota (C-member). Fischer identified the genetic features of cycle members and interpreted them on the basis of carbonate sedimentological observations in the Persian Gulf. He stated that the typical structural and textural elements of cyclothems were formed by near surface diagenesis (dolomitization, desiccation, weathering). He described shrinkage structures (prism-cracks, sheet-cracks, birdseye pores). For a type of carbonate rock characterized by shrinkage pores ordered in lines, shrinkage cracks and sheet cracks he introduced the term "loferite", of which he recognized four different subtypes: algal mat loferite, pellet loferite, homogeneous loferite and loferite conglomerates. Analyzing the reasons for cyclicity Fischer confirmed Sander's and Schwarzacher's theory about the annual rhythmicity of mm-thick laminae of Acta Geologic a Hungarica

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