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Correlation of the illite-smectite diagenesis in the Sava depression with other sub-basins of the pannonian basin system

The aim of the present study is correlation of changes of mineral composition with increasing burial depths in pelitic sediments from the Sava Depression, sub-basin in the south-western part of the Pannonian Basin System (PBS), with those from the neighbouring sub-basins in the PBS. Transition from R0 to R1 type of illite-smectite (I-S), as well as from R1 to R>1 type I-S can be used for determining thermal maturity of source rocks and oil generation potential. Investigations of the illitization process of smectite have been recently performed in the Croatian part of the PBS (Grizelj et al., 2011). In the neighbouring sub-basins the process was well documented earlier by Franců et al. (1990), Hámor-Vidó & Viczián (1993), Hillier et al. (1995), Tanács & Viczián (1995) and Sachsenhofer et al. (1998). Based on XRD data the transition from R0 to R1 type I-S in the Sava Depression started at depths <1.8 km and temperature above 80°C. Slightly higher temperatures (around 100°C) and greater depths of R0 to R1 transformation have been determined in neighbouring sub-basins (110°C at 3.2-3.5 km in the Vienna Basin, 115°C at 2.0 km in the Transcarpathian Depression, and 135°C at 2.0 km in the Maribor area, at 2.5-3.0 km in the Great Hungarian Plain. Only in the Zala Basin and Drava Basins transition started at lower depths (1.0 km). A possible reason for the difference in temperature in the Sava Depression and the neighbouring sub-basins are influence of longer exposition to elevated temperatures and potassium availability on the process of illitization in the Miocene pelitic rocks in the Sava Depression. Transition from R1 to R>1 type I-S in the Sava Depression started at depths >3.0 km and temperature >170°C. This transition begins in the Great Hungarian Plain at >4.0 km, in the Zala Basin and Drava Basin at 3.0 km, in the Transcarpathian Depression at 3.0 km and temperature 160-170°C, and in the Maribor area at 135-175°C. Significant differences in the rate and degree of progression of the illitization process of smectite are observed in sub-basins of the PBS. These differences are caused by different thermal histories of the sub-basins, as indicated by the differences in thermal gradients between comparable areas. The thermal gradient for the Sava Depression is 40°C/km, for the Great Hungarian Plain 35°C/km, for the Vienna Basin 25°C/km, and for the Transcarpathian Depression 55°C/km. REFERENCES: Franců, J., Müller, P., Šucha, V. & Zatkalíková, V. (1990) Organic matter and clay minerals as indicators of thermal history in the Transcarpathian Depression (East Slovakian Neogene Basin) and Vienna Basin. Geologiský Zborník, Geologica Carpathica, 41/5, 535–546. Grizelj, A., Tibljaš, D., Kovačić, M. & Španić, D. (2011) Diagenesis of Miocene pelitic sedimentary rocks in the Sava Depression (Croatia). Clay Minerals, 46, 59–72. Hámor-Vidó, M. & Viczián, I. (1993) Vitrinite reflectance and smectite content of mixed-layer illite/smectites in Neogene sequences of the Pannonian Basin, Hungary. Acta Geologica Hungarica, 36/2, 197–209. Hillier, S., Matyas, J., Matter, A. & Vasseur, G. (1995) Illite/smectite diagenesis and its variable correlation with vitrinite reflectance in the Pannonian Basin. Clays and Clay Minerals, 43, 174–183. Sachsenhofer, R.F., Rantitsch, G., Hasenhüttl, C., Russegger, B. & Jelen, B. (1998) Smectite to illite diagenesis in early Miocene sediments from the hyperthermal western Pannonian Basin. Clay Minerals, 33, 523-537. Tanács, J. & Viczián, I. (1995) Mixed-layer illite/smectites and clay sedimentation in the Neogene of the Pannonian Basin, Hungary. Geologica Carpatica, Ser. Clays, 4, 3–22.

Pannonian Basin System; Sava Depression; illite-smectite; diagenesis

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