Reconstruction of Eocene Depositional Environmets in the Vértes Hills, Based on Microfacies

S. Pálfalvi: Reconstruction of Eocene depositional environmets in the Vértes Hills, based on microfacies analysis

Reconstruction of Eocene depositional environmets in the Vértes Hills, based on microfacies analysis

Sarolta Pálfalvi

Ph.D thesis

Ph.D School for Earth Sciences, Ph.D Programme for Geology-Geophysics

Head of the Ph.D School for Earth Sciences: Dr. Miklós Monostori professor, doctor of Hungarian Academy of Sciences

Head of the Geology-Geophysics Ph.D Programme: Dr. Miklós Monostori professor, doctor of Hungarian Academy of Sciences

Supervisor: Dr. Miklós Kázmér associate professor, Eötvös Loránd University,

Department of Palaeontology

Eötvös Loránd University

Department of Palaeontology

Budapest, 2007

Introduction

The subject of my PhD work was recommended by Dr. Tamás Budai, the head of the Division of Geological Mapping of the Geological Institute of Hungary, as part of its mapping project on the area of the Vértes-Gerecse Mountains. The aim my work was to present the Eocen facies and palaeonvironments, to correlate the studied sequences and to reconstruct the Eocene evolution of the area based on the sedimentological and microfacies analysis of the carbonate successions of the outcrops and deep well cores investigated during the mapping and field works.

The history of the research of the Eocene in the Transdanubian Range has a long past and is quite differentiated, mostly because of the research for raw materials (bauxit and brown coal). Thanking for this, I could utilize a large amount of data from the literature about stratigraphy and paleontology of the area, collected during the past two centuries. In the same time, much less published results were available about the microfacies analysis of the Eocene successions, even for the entire Hungarian Eocene areas. Such works were published by Kecskeméti & Vörös (1975), Haas (1984), Varga (1985), Kázmér (1985, 1993). Concerning the Vértes Hills, Kecskeméti & Vörös (1985, 1986) have performed microfacies studies of several deep well cores were deepened on the western side of the Oroszlány basin, as well as, Bignot et al. (1985) presented several microfacies of the Eocene rocks overlying bauxite at the Bagoly Hill near the village Gánt.The detailed evaluation of the carbonate successions of the Vertes Hills and the correlation of the sequences of the Eocene basins and basin margins has not been published yet, although, there might have been some effort in some internal industrial reports.

Methods

The first phase of my research works began with filed works of the area, commonly with the co-workers (Budai Tamás, Csillag Gábor, Fodor László, Kercsmár Zsolt, Less György) of the Department of Mountain Geologyof the Geological Institute of Hungary. This included the investigation of 11 bigger and numerous smaller outcrops and 9 deep well cores which were available at the depository of deep well cores at Szépvizér.

The most important part of my work was the microfacies analysis of the thin sections. I have completed the investigation of 300 thin sections of measures of 5 x 5 and 2 x 3 cm2. I used the classification of Dunham (1962) and Embry Klovan (1972) for the classification and the naming of the microfacies types based on the grain composition and the rock fabric The percentage contents of the ingredients were determined by the semi quantitative method of BaccelleBosellini (1965). I have studied the quality, size, sorting and roundness of the grains of different origins and the results were presented in tables.

The microfacies analysis have been completed by the study of the documentation of the boreholes. Since there are few thousands of boreholes for coal and bauxit on this area, first of all, I have considered the most important ones which yielded the thickest Eocene successions and tha material of which has been tested. Utilizing these previous information and my results I have re-evaluated several successions of the boreholes. Based on the borehole studies I have compiled geological sections in order to obtain an accurate picture about the occurence, thickness and the relationship of the different formations.

I have made an effort to become acquainted with previous publications of the Eocene formations of the Vértes Hills and I have been continuously following the results of the recent research works. I have studied the relevant publications about the palaeoecological studies of Eocene microfossils and the carbonate ramps, as well. Utilizing all these, I have interpreted the results of the microfacies analysiss and I determined the different sedimentary environments using analogues.

I have correlated the studied sequences and compared the transgressions concluded from the results of the correlation to the eustatic sea level changing published by Haq et al. (1988). As the summary of my results I have outlined the Eocene evolution of the Vértes Hills and its surroundings.

Thesis

The Vértes Hills and the Oroszlány-Pusztavám Basin

1. I have determined 7 main facies and 21 microfacies types in the Middle-Late Eocene Szőc Limestone occuring at the NW margin of the Vértes Hills. Based on the microfacies analysis I have concluded, that in the Early Bartonian, on the NW part of the Vértes Ridge a low angle carbonate ramp (Szőc Limestone) (Pálfalvi 2004) developed, which could be subdivided into 3 parts: inner ramp above fair weather wave base (FWWB), mid-ramp between FWWB and storm wave base (SWB), and outer ramp under SWB (Fig. 1/b).
2. On the inner ramp I have differentiated 3 main depositional environments. (1) in the vicinity of the rocky coast: extraclast-red alge-echonoderm-molluscan rudstone, floatstone or packstone (1A microfacies type); (2) sea grass community: Acervulina-red algae-echinoderm-molluscan packstone/grainstone (B2 microfacies type); (3) bioclastic sand shoals: Acervulina-red alage grainstone (B3 microfacies type). Around FWWB paraaotochtonous Nummulites bank bank was formed (C2 microfacies type). Mid-ramp was dominated by larger foraminifera (floatstone/rudstone, E1-2 microfacies type). The changes of the dominant taxons - Nummulites perforatusN. millecaputDiscocyclina, Operculina - refer to different palaeodepth (and different hidrodinamic energy and light condition). Probably on small palaeohigh coral-algal patch reef was formed, which was surrounded by rhodolith-bearing sediments (F1-3 facies type). On the outer ramp Discocyclina-Operculina floatstone was deposited (E3-4 microfacies type). In the basin glauconitic, planktic foraminiferal-calcarenite wackestone (packstone) was formed.
3. I have observed, that the main influencing palaeoecological factors were depth, light condition, hidrodinamic energy, substrate, nutrients., and sedimentation rate. On the inner ramp the sea grass community records high energy, well-lit, nutrient rich conditions, high carbonate production and high sedimentation rate. The mid-ramp was characterized by oligotrophic environment, gradually deepening water with decreasing energy and light intensity and with the occurence of more and more flat and thin larger foraminifera, under the influence of occasional storms. The outer ramp was low energy, poorly illuminated environment with low sedimentation rate.
4. I have concluded that syn-sedimentary deformation (Kercsmár 2005) and the different sedimentations of the inner-mid-ramp and the basin indicated the increasing of the slope angle of the carbonate ramp at the NW margin of the Vértes Ridge, and it was accompanied by slumping and sediment gravity flows of the nummulitic limestones on the mid and outer ramp (fig. 1/c). This process led to the formation of a distally steepened ramp.
5. I have concluded that the preserved bioclastic grainstones (G1-3 microfacies types) of the distally steepened ramp (fig 1/d) were deposited on the mid and outer ramp in high energy environment, as the result of resedimentation by sediment gravity flows or storm activity (Pálfalvi 2005).

Dad Ridge

1. I have concluded that the Dad Ridge was an isolated carbonate platform formed on an inherited topographic high. In the Early Bartonian on the SE slope of the ridge a (probably distally steepened) caronate ramp was formed with inner ramp facies (A1-B microfaicies types) and Nummulites bank (C2 microfacies type) similar to the ones formed on the NW slope of the Vértes Ridge.
2. As a result of the Early Bartonian relative sea level rise inner ramp facies were backstepping and Nummulites bank was formed above them, and on the slope allochtomous nummulites tests were deposited (D1-3 microfacies types) due to sediment gravity flows or storm reworking (fig 1/a). The sediments deposited on the platform top was eroded by the ‘infraoligocene denudation”. As a result of the microfacies analysis of slope deposites I have supposed that the inner ramp was drowned due to relative sea-level rise and on the platform top around FWWB paraautochtonous Nummulites bank was formed, and the nummulites test resedimented from this area.

Csákberény and Gánt Basins

1. I observed that the successions at Csákberény and Gánt are much thicker than the ones in the Oroszlány Basin (Pálfalvi et al. 2006). It was caused by the forced subsidence of the Csákberény area, due to local syn-sedimentary deformation, and the sedimentation that could keep in pace with the subsidence due to the high sedimentation rate, resulted from high siliciclastic influx, and in the case of the Kincses Formation, from the high carbonate production.
2. In the Forna Formation I differentiated 2 main facies. Around Csákberény and on the western part of the Gánt Basin sedimentation took place in a restricted lagoon and mixed siliciclastic-carbonate successions with molluscs and miliolids (wackestone/packstone) were deposited. At the beginning of the Late Lutetian-Bartonian transgression, the area of the Gánt bauxite mines was exposed, and in the karstic depression “blue-hole type” sedimentation (Carrannante et al. 1994) took place due to ingression. Palustrine sedimentation with intraclasts and root-related structures and lacustrine sedimentation with Charophyta took place (fig 1/e). Further relative sea-level rise resulted in lagoonal environment on the whole area.
3. In the Kincses Formation above the extraclast-bearing basal layers I differentiated 5 microfacies types according to their dominant fossiles (Miliolina, Alveolina, coral, green algae, small Reusella), which were deposited in a lagoon with seasonally changing salinity. In the Kincses-Csolnok transitional successions Nummulites-, Discocyclina- and Operculina-bearing sediments (floatstone, packstone) were deposited in open marine sublitoral environment. Here I could observe the intercalation of allochtonouus bioclastic (Miliolina. Alveolina) grainstone deposits due to storm reworking.

General observations

1. I could determine 3 transgressions in the Oroszlány-Pusztavám Depression, as well as in the Csákberény Depression, as the result of the correlation of the studied sequences.
2. I compared the studied sequences to Eocene sequences of other part of the Transdanubian Range (Kollányi et al. 2003; borehole Balinka-285, Horváth-KollányiNagy-Gellai 1989;Kollányi & Gellai Tatabánya Basin, Kercsmaár 2005), and to the eustatic curve published by Haq et al. (1988), and I could observe that the first transgression in the Late Lutetian and the third transgression in the Late Bartonian can be correlated with the global eustatic sea-level rises. However , I could observe some differences in the case of the second transgression in the Early Bartonian, due to flexural subsidence (Tari et al. 1993) caused by syn-sedimentary deformation.

Conclusions

The Late Lutetian-Bartonian sediment pattern was determined by two NE striking elevated ridges (Vértes and Dad Ridge), and two parallel depressions (Oroszlány Depression to the NW and Csákberény-Gánt Depression to the SE). On the ridges carbonate deposition, in the depressions siliciclastic sedimentation (Dorog, Csernye, Forna, Csolnok Formations) took place.

In the Oroszlány-Pusztavám Basin in the Late Lutetian (Kollányi et al.) deposition began after long-lasting exposion with continental, alluvial and coal-bearing sediments of the Dorog Formation, then it was covered by brakish to normal marine molluscan marl belonging to the Csernye Formaion. At the same time the Vértes Ridge was exposed. Around Csákberény sedimentation took place in a restricted to open lagoon in brakish to normal marine environment, and at Gánt first in “blue holes” due to ingression then in lagoon similar to at Csákberény. In the Csákberény Depression subsidence was much more considerable than in the NW side of the Vértes Ridge due to local syn-sedimentary deformation, and the vicinity of the land caused higher siliciclastic influx. This resulted in much thicker sequences in the Csákberény Depression than in the Oroszlány Depression (Pálfalvi et al. 2006).

In the Early Bartonian (Kollányi et al. 2003) the beginning of the deposition of the larger foraminifera-bearing clayly marl (Csolnok Formation) was influenced by transgression. The glauconitic layers occuring in the lower part of the sequences record maximum flooding. Syn sedimentary deformations, observed in outcrops also in the Vértes Hills (Kercsmár et al 2006), contributed to the relative sea-level rise / subsidence of the area.

On the NW margin of the Vértes Ridge in the Early Bartonian a low angle carbonate ramp developed (Pálfalvi 2004). The inner ramp was characterized by high carbonate production and high sedimentaion rate in high energy, well lit environment with high nutrient content. The oligotrophic environments of the mid and outer ramp from FWWB to the deep sublitoral was occupied by larger foraminifera, that could provide carbonate production along the whole homoclinal ramp. On the slope of the Dad Ridge inner ramp environments and Nummulites bank similar to the ones on the Vértes Ridge was formed. In the Csákberény-Gánt Depression the Miliolina-bearing limestone of the Kincses Formaion was deposited in a low-energy lagoon, that was the heteropic facies of the Szőc Limestone.

During highstand relatively high carbonate production due to the rapid radiation of the Nummulites perforatus and N. millecaput resulted in ramp progradaion towards the basin. At the same time on the slope of Dad Ridgeallochtonous Nummulites tests, originated from the nummulites bank formed around FWWB, were deposited due to gravity flows or storm activity.

During the Middle-Bartonian eustatic sea-level fall (Haq et al. 1988) the ramp progradation was continuing. The siliciclastic influx that was considerable in the NE basins of the Transdanubian Range could be observed only in thin layers in the Oroszlány-Pusztavám Basin due to the greater distance of the denudation area. As a result of this fact and the subsidence caused by syn-sedimentary deformation, the influence of the global eustatic sea-level fall was not so effective on the studied area as on the NE part the Transdanubian Range.

In the second half of the Bartonian global sea-level rise (Haq et al. 1988) and local tectonic subsidence indicated the backstepping of the carbonate ramp, and the slope was characterized by sediment gravity flows (fig. 1/c). As the result of the increasing slope angle a distally steepened carbonate ramp (fig 1/d) was formed on the NW margin of the Vértes Ridge, and resedimentation took place on the mid and outer ramp. The Vértes Ridge, which was a barrier between the Oroszlány and Csákberény Depressions till this time, became entirely flooded and normal marine larger foraminifera- At the end of the Middle Eocene due to the considerable global sea-level fall (Haq et al. 1988) the slope deposits of the distally steepened ramp prograded above the basin deposits of the Csolnok and Padrag Formations. Probably this event was marked by the deposition of the Szőc Limestone above the Kincses-Csolnok transitional successions in the Csákberény Depression.

At the beginnibg of the Late Eocene (Báldiné Beke 1984) deposition went on at the Antal-Hill on the distally steepened ramp of the NW margin of the Vértes Ridge. Carbonate sedimentation was occasionally interrupted by volcanic activity of the Velence Hills. As a result of the “infraoligocene denudation” there is no further information about the Eocene sedimentation of the studied area.

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