long range at the western and northern margins of the aforementioned Carnian platform, with tectonic contact. Southeast of the Rezi Dolomite range, bedded or ...
Acta Geologica Hungarica, Vol. 38/2, pp. 111-129 (1995)
Contribution to the U pper Triassic geology of the Keszthely M ountains (Transdanubian Range), western Hungary* Gábor Csillag, Tamás Budai, László Gyalog, László Koloszár H u n g a r ia n G e o lo g ic a l I n s ti tu te , B u d a p e s t
A considerable part of the thick and extensive dolom ite complex of the eastern K eszthely Range, form erly assigned to the H auptdolom it, belongs to the Ederics platform of M iddle to Late Carnian age. The Ederics Formation (Sédvölgy Dolomite Mb.) im m ediately underlies the H auptdolom it. A ccording to analogies with the Balaton H ighland, the Ederics platform came into being in the early Julian. Representing the m arginal facies of the Julian basin, Buhimvölgy Breccia show s the w estern lim it of the platform in the Keszthely Range. Progradation of the platform started in the late Julian; this has resulted in the deposition of an intertidal-supratidal sequence above the reef facies (Ederics Mountain). The U pper Norian Rezi Dolomite Formation of restricted basin facies forms several kilometre long range at the western and northern m argins of the aforementioned Carnian platform , with tectonic contact. Southeast of the Rezi Dolomite range, bedded or thick-bedded d olom ites with D asycladaceae and M egalodontidae represent subtidal environm ent, belongs to the H auptdolom it. This m em ber of the H auptdolom it (Pad-kő Dolomite) is coeval with the m id d le-u p p er p art of the Rezi Dolomite. K ey w o rd s:
Carnian, Norian, paleoenvironm ent, paleogeography, stratigraphy, K eszthely
In troduction
Within the scope of geological mapping of the Balaton Highland, detailed field work in the Keszthely Mountains was carried out in 1982-1983 at the scale of 1:10 000. However, the lithostratigraphic assignment of the Triassic units has become very contradictory for lack of a proper and uniform stratigraphic and palaeogeographic frame. Under- and overlying formations of the Hauptdolomit have appeared on the map as independent small patches inside the widespread area of the Hauptdolomit. After completing the mapping, several authors dealt with stratigraphic and paleo-environmental interpretation (Budai and Kovács 1986; Gyalog et al. 1986). Plotting geological maps on the scale of 1 : 20 000 and describing the core samples caused a réévaluation of the distribution and lithostratigraphy of the Rezi Dolomite (Budai and Koloszár 1987). In addition, a dolomite sequence A ddresses: G. Csillag, T. Budai, L. Gyalog, L. Koloszár: H-1143, Budapest, Stefánia ú t 14, H ungary Received: 26 May, 1995 * Lecture was given in the H ungarian Geological Society on the 16th of March 1993. Akadémiai Kiadd, Budapest
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was detected in the Keszthely Range which is older than the Hauptdolomit, situated below or beside the Carnian Veszprém Marl (Császár et al. 1989). During the final plotting of the maps on the scale of 1: 50 000 (Budai and Koloszár 1990) more steps were taken to clarify the Upper Triassic buildup of the Keszthely Range: - the dolomite complex of the Szabad hill plateau belongs not to the Hauptdolomit but to the Ederics Formation; - dolomites occurring west of this area belong to the Rezi Dolomite. Carnian basin and platform facies of the Keszthely Range The Ederics Limestone of peri-reef facies occurring in the hills above Balatonederics and marls of Carnian basin facies in the Szent Miklós valley were first recorded by Lóczy (1913, 1916). In regard to the stratigraphic and paleogeographic knowledge of the Carnian units of the Keszthely Range, the study of the section of borehole Hévíz-6. (Fig. 1) is of crucial importance (Góczán et al. 1983). On the basis of fieldwork and fossil evidence, Gyalog et al. (1986) assigned the Carnian coral-bearing dolomite of the eastern plateau to the Hauptdolomit. The sequence which is built up by an alternation of limestone and dolomite layers was interpreted as a direct transition between the Ederics Limestone and the Hauptdolomit. Nevertheless, this interpretation is inconsistent with the following facts: - The Carnian age of the dolomite does not contradict the definition of the Hauptdolomit, but its corals (Thecosmilia sp.) and microfacies, showing a reef environment, are closer to the Ederics Limestone than to the Hauptdolomit. - The evidence of boreholes Vát-4 and Vát-5 shows that the dolomite of the eastern plateau underlies the Veszprém Marl, in opposition to former assumptions (Bohn 1979; Gyalog et al. 1986). In the Szent Miklós valley (Fig. 2), outcrops of the Buhimvölgy Breccia are indicative of an interfingering contact between the Carnian basin (Veszprém Marl) and the coeval carbonate platform (Ederics Formation). This situation is similar to that of the Southern Alps where the blocks of Cipit Limestone (Cipit boulders) up to several metres in size (derived from the coeval platform) are found whithin the Cassian beds of basin facies (Biddle 1980; Bosellini and Neri 1991). According to the latest interpretation our lithostratigraphic classification, based on mapping, differs in the following two respects from the "official” one of the Hungarian Commission on Stratigraphy (Haas and Császár 1993): - Within the Veszprém Marl Formation, in addition to the Mencshely Marl, Nosztor Limestone, and Csicsó Marl Members, the Buhimvölgy Breccia Member is also distinguished as a proximal slope facies of the Ederics and Sédvölgy platforms, coeval facies of the upper two basinal members. Megabreccias and
A cta Geologica Hungarica
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Fig. 1 U pper Triassic surface of the K eszthely Mts. кТз - Kossen Form ation; гТз - Rezi D olom ite Form ation; fPT3 - H aup td o lo m it Form ation, Pad-ко D olom ite M em ber; (Тз - H au p td o lo m it Form ation; ST3 - Sándorhegy Form ation; уТз - Veszprém Marl Form ation; Т з - Ederics Form ation
allodapic bioclasts, ooids, oncoids and limestone clasts from 0.5 to 40 cm size (Csillag 1991), originating from the platform, represent this member. - Within the Ederics Formation, the Sédvölgy Dolomite Member has already been defined on the Balaton Highland, whereas the reef limestone is called Ederics Formation. In the Keszthely Range the presence of this dolomite member was only supposed previously, however, later it was found on greater areal extent than the reef limestone.
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Fig. 2 U p p e r Triassic buildup of the E astern part of the K eszthely Range. гТз - Rezi D olom ite Form ation (; ггТз - m iddle m em ber; г;Тз - low er member); г' (Тз - tran sitio n betw een Rezi D olom ite and H au p td o lo m it; (РТз - H au p td o lo m it Form ation ("P ad-кб Dolomite M em ber"); ?Тз F lauptdolom it Form ation; sbT3 - Sándorhegy Form ation, B arnag Member; УсТз - V eszprém Marl F o rm atio n , Csicsó Marl M em ber; Ederics Formation: esT3 - Sédvölgy D olom ite M em ber: %Тз E d erics Limestone M em ber; ee-sT3 - transition b etw een Ederics Lim estone a n d Sédvölgy D olom ite Member; 1. m eg abreccia (Veszprém Form ation, Buhim völgy Breccia M em ber); 2. co rals; 3. tectonic b o u n d ary ; 4. stratigraphic boundary
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Upper Triassic geology of the Keszthely M ts 775
Evolution of the Ederics platform By analogy with the Balaton Highland (Budai 1991; Csillag 1991) and the Veszprém area (Peregi 1979), the Ederics platform probably emerged in the Julian. Likewise, in the Southern Alps the Cassian dolomites became much more extensive in the Middle Carnian, forming the main mass of the Sella, Nuvolau and Lagazuoi platforms. Bosellini (1984, Fig. 1) Masetti et al. (1991, Fig. 1) and De Zanche et al. (1993, Plate 1) clearly illustrated the intensive progradation of the Cassian Dolomite (so-called Upper Cassian Dolomite or Cassian Dolomite 2) in the Middle Carnian (Julian substage). The evolution of the Balaton Highland fits better with this history than with the continuous progradation of the Cassian dolomite from the Ladinian, as advocated by Bizzarini and Braga (1987). In spite of the general analogy in the evolution of the two areas, there are some differences in the lithologic and stratigraphic buildups. The Wengen Formation of the Southern Alps, underlying the San Cassian Formation is absent in the Balaton Highland, and is substituted by the Buchenstein Formation. The sequence, overlying the Julian platform-basin complex is also different in the two regions. In the Dolomites the Dürrenstein Dolomite is deposited on the San Cassian Formation and the Cassian Dolomite, as the intraplatform basins were filled up and the paleotopography was levelled (Bosellini 1982; De Zanche et al. 1993). However, the lithofacies of the Sándorhegy Formation is different, its depositing situation on the Ederics and Veszprém Marl Formation is quite similar (Csillag 1991). In the south-eastern part of the Ederics platform (Ederics hill) the reef margin facies types can be found which were not dolomitized. Further west-northwest, up to the platform margin, the carbonate "bank" is generally composed of dolomite. A considerable part of the dolomite was presumably formed diagenetically late, i. e. the material of the large allodapic blocks (up to 10 m in size) and boulders (5-40 cm) of reef-facies is limestone on the fore-reef slope (Buhimvölgy Breccia). Because of the poor exposure it cannot be decided whether these are megabreccias or small platform-marginal patch reefs, a question which has also been raised for the Dolomites (Wendt and Fürsich 1980). In the next period of platform development peritidal (?dolomitic) sedimentation with early diagenetic dolomitization took place, according to evidence from borehole Vállus Vát-2 which has penetrated the Sédvölgy Dolomite sequence in a thickness of 200 m. Supratidal breccias, palaeo-karst fissure infills (Plate 1,1) and "bioturbation" structures, probably caused by roots, indicate supratidal depositional environment. However, the thickness of the Ederics Limestone, reaching several hundred metres, leads to the conclusion that a reef must have survived in the southern zone of the platform. The third evolutionary period of the Ederics platform is marked by a 100 to 150 metre thick sequence of limestones alternating with dolomites. This Acta Geologica Hungarica
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Fig. 3 Section of the borehole B alatonederics Bet-1 w ith log of calcite/dolom ite ratio 1. calcite; 2. dolom ite; 3. lim estone; 4. dolom ite; 5. soil; 6. breccia; 7. "pulverized" dolom ite; 8. in traclasts; 9. tectonic zone
A d a G eologica H u n g a ric a
Upper Triassic geology of the Keszthiiy M ts 1 1 7
sequence, overlying the pure reef-limestone of the Ederics hill indicates a peri-reef environment with high energy in the reef zone (borehole section Balatonederics Bet-1). The dolomite layers, deposited in the peritidal zone, become more frequent upwards; in the upper 40 m of borehole section Bet-1 only dolomite is found (Fig. 3). This trend could be explained with the advance of the platform margin, following the former reef area replaced by shallow-lagoon facies of the back-reef (Fig. 4). This process is assigned to a strong progradation, began around the Julian/Tuvalian boundary (Góczán et al. 1983), as is the case in the Balaton Highland (Csillag 1991) and in the Dolomites (De Zanche et al. 1993). No overlying units are known in the central part of the Ederics platform in the Keszthely Range. Limestones and marls of the Sándorhegy Formation (Barnag Member), with several centimetres large oncoids can be detected only in a faulted zone on the western margins of the Szabad hill plateau (Szent Miklós valley). This special facies could be traced as an overlying unit above prograding platforms both in the Balaton Highland (Budai 1991) and in the Bakony Range (Sümeg-Sáska), as well. In the northern foreground of the Keszthely Range (borehole Héviz-6) bioclastic, large-oncoidic marls overlying the Ederics Formation show a backstepping platform margin during the Late Tuvalian (Fig. 4). The relationship between the Ederics and the Sédvölgy platform is an open question. Up to now there are no useful data to decide if the Ederics Formation around Veszprém and in the Keszthely Range belongs to a large single platform, or to separate ones. Upper Noriati units of the Keszthely Range The Rezi Dolomite of restricted basin facies is much more extensive in the Keszthely Range than it was previously believed. According to the latest mapping records and data interpretation, the Rezi Dolomite and the Kössen Formation are found in the following four areas of this region (Fig. 1): 1. Western part of the Keszthely Mts.: Rezi basin and the surrounding areas; 2. Eastern part of the Keszthely Mts.: between Vállus and Balatongyörök; 3. Northeastern part of the Keszthely Mts: East of Szent Miklós valley; 4. Isolated patches on the SW margin of the Keszthely Range. 1. The surroundings of Rezi Modern classification of the Rezi Dolomite Formation has been based mainly on the geological sections of this area (Budai and Koloszár 1985, 1987). Even here, we found the extent of the Rezi Dolomite greater than is shown in the map compiled by Budai and Koloszár (1987, Fig. 1), in accordance with the views of Szentes (1953). Acta Geologica Hungarica
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SW Ba la ton Highland
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Fig. 4 G eneralized sketch of the C arnian lithostratigraphic units in the su rroundings of the Keszthely Mts.: fT3 - H auptdolom it Form ation; Sandorhegy Form ation: ьТз - Barnag M ember; shÎ3 - H enye Dolomite M ember; Veszprém M arl Form ation: ' сТз - Csicsó M arl Member- 'ьТз ; Bubimvölgy Breccia M em ber; Ederics Formation: esT3 - Sédvölgy Dolomite M em ber ; %T3 - Ederics Limestone M ember; 1. dolomite2. limestone; 3. dolom itized lim estone; 4. m arly limestone; 5. marl; 6. large (1-3 cm) oncoids; 7. m egabreccia
G. Csillag et al.
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