Smectite to illite diagenesis in early Miocene sediments from the ...

Clay Minerals (1998) 33,523 537

Smectite to illite diagenesis in early Miocene sediments from the hyperthermal western Pannonian Basin R. F. S A C H S E N H O F E R ,

G. RANTITSCH, C. H A S E N H O T T L , AND B . J E L E N *

B. RUSSEGGER

Institutfiir Geowissenschafien, Montanuniversitiit Leoben, A-8700 Leoben, Austria, and *Institut za Geologija, Geoloski Zavod Ljub(jana, Dimiceva 14. Slo-61000 Ljubljana, Slovenia (Received 19 May 1997; revised 30 September 1997)

A B S T R A C T : The smectite to illite transformation in early Miocene sediments was studied in wells and outcrops in the western Pannonian Basin, where magmatic activity caused very high Miocene heat flows (250-400 mW/m2). Although the thermal history is similar, large differences in smectite to illite diagenesis were observed. (1) The boundaries between early/middle and middle/late diagenesis in two wells (Pichla, Radkersburg) and the Maribor area correspond to vitrinite reflectance values of 0.4-0.8 and 1.1-1.5% Rr. Anchimetamorphism starts at N2.1% Rr. In spite of magmatic heating, the smectite to illite transformation can be modelled using kinetic data proposed for areas with a 'normal' burial diagenesis. (2) Smectite to illite reactions are advanced compared to vitrinite reflectance in the Ribnica-SelnicaTrough. This is probably related to fluids with elevated K+ concentrations. (3) Clay mineral alterations lag behind coalification significantly in the Mitterlabilt well. These different correlations indicate that clay mineral thermometry should be used with caution and that factors other than temperature and time can influence clay mineral reactions.

The thermal history of the western Pannonian Basin (Mura Depression, Styrian Basin) is characterized by variable and locally extremely high early Miocene heat flow, which was caused by magmatic activity (e.g. Ebner & Sachsenhofer, 1995) and by the rapid uplift of basement units (e.g. Dunkl & Dem6ny, 1997). Consequently, vertical and lateral coalification gradients in early Miocene sediments are often very high. Because early Miocene sediments are considered to be potential source rocks, a detailed knowledge of the maturity pattern is important for hydrocarbon exploration in this part of the Pannonian realm (e.g. Jelen, 1985/86; Hamrla, 1989; Sachsenhofer, 1994). In this study, the smectite to illite transition in early Miocene samples was investigated to support the reconstructions of the thermal history and to investigate the geological factors which affected clay mineral diagenesis in a geologically complex basin.

A wealth of information has been published in recent years on the smectite to illite diagenesis and its correlation with organic maturity (e.g. Velde & Espitali6, 1989). Numerous studies indicate that many factors, including the starting composition of smectite, fluid composition, and the rock to water ratio, influence the progress of clay mineral diagenesis (e.g. Freed & Peacor, 1989). However, temperature, time and K § availability are considered the most important factors (Hoffmann & Hower, 1979; Pollastro, 1993; Huang et al., 1993). Kinetic models for the smectite to illite transformation were established by several authors (e.g. Dutta, 1986; Bethke & Altaner, 1986; Pytte & Reynolds, 1989; Velde & Vasseur, 1992; Huang et al., 1993) and can be used as calibration tools in thermal history analysis. Hillier et al. (1995) suggested kinetic data for the smectite-illite diagenesis in the Pannonian Basin (Table 1). Their model was calibrated with data from the Great H u n g a r i a n Plain, the

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Transcarpathian Basin and the Vienna Basin (Fig. la). The aim of this paper is to investigate the smectite to illite transformation and its correlation with vitrinite reflectance in early Miocene sediments of the Styrian Basin, the Mura Depression and the Ribnica-Selnica Trough (Fig. lb), which were affected by extremely high Miocene heat flows. These early Miocene sediments crop out west of Maribor in the Kozjak/Possruck area and were drilled in several wells. Kinetic data of Dutta (1986), Velde & Vasseur (1992) and Hillier et at. (I995) are applied to model the smectite to illlite transformation in the Pichla 1, Radkersburg 2 and Mitterlabill 1 wells (Fig. lb). The kinetic data of Dutta (1986) are used, because they represent one of the first attempts to describe the rate of the smectite to illite transformation, those of Velde & Vasseur (1992), because they are well established in the literature (e.g. Elliott & Matisoff, 1996), and those of Hillier et al. (1995), because they were calibrated in sub-basins of the Pannonian realm. The wells were selected because their thermal histories are well known and because they are characterized by extremely high palaeoheat flows (250 400 roW/m2). Thus, these wells provide a good test of the kinetic data above, which were calibrated in regions with a 'normal' range of geothermal gradients (24-55 ~ to sediments in a hyperthermal basin. GEOLOGICAL

SETTING

The study area is situated at the eastern margin of the Alps and belongs to the western Pannonian Basin System (Fig. la), which is subdivided into several sub-basins filled with thick Miocene sediments (e.g. Styrian Basin, Mura Depression). The subdivision of the Miocene follows the

Paratethyan biostratigraphic stages (see Fig. 2 for a correlation of Paratethys stages to the standard time scale according to R6gl, 1996). Stratigraphy

The pre-Tertiary basement is formed by low- to high-grade metamorphic rocks and in some places by Mesozoic carbonates. Oligocene and/or early Miocene tonalites in the Pohorje Mountains (Fig. lb) are genetically related to calc-alkaline plutons, which occur along the Periadriatic Lineament (Exner, 1976; Laubscher, 1983; von Blankenburg & Davies, 1995; Fig la). A second magmatic event produced dacitic dykes, sills and stocks in the early Miocene (Karpatian/early Badenian?) times (Faninger, 1970). Its centre is located in the western Pohorje area (Fig. lb). Huge latitic to trachyandesitic shield volcanoes, which are now nearly totally buried by younger sediments, are evidence for contemporaneous (Karpatian/early Badenian) magmatic activity in the Styrian Basin (Ebner & Sachsenhofer, 1995). Neogene sedimentation started during Ottnangian times with the deposition of non-marine sediments (Jeten et al., 1992; Ebner & Sachsenhofer, 1995; Fig. 2). The Karpatian stage is characterized by a marine ingression. Conglomerates, marls, shales and sandstones were deposited during this time period. Early Miocene sediments crop out all around the preTertiary Kozjak/Possruck Mountains (Fig. lb) and dip below Badenian sands, silts and marls east of Maribor. Note that early Miocene sediments in the Mitterlabill 1 well, which is located only 2 km west of a magmatic vent, are overlain by several hundred metres of Karpatian and early Badcnian volcanic rocks (Fig. 3c). The Pichla 1 well is located close to the southern margin of the buried Miocene volcano, but did not drill volcanic rocks (Fig. 3a).

TABLE 1. Parameters used in kinetic modelling (Ea: activation energy; A: frequency factor).

Dutta (1986) Velde & Vasseur (1992) I st reaction 2nd reaction Hillier et al. (1995)

Ea (kJ tool I)

A (Ma J)

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FIG. 1. (a) Location map showing position of study area within the Pannonian Basin System. PL: Per/adriatic Lineament, V: Vienna Basin, GHP: Great Hungarian Plain, TC: Transcarpathian Basin. (b) Sketch map of the study area and position of studied wells. RS: Ribnica-Selnica Depression.

526

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Vitrinite reflectance patterns and thermal history Temperature histories calibrated with vitrinite reflectance data for three wells (Pichla 1, Radkersburg 2, Mitterlabill 1) are presented in Fig. 3 a - c (Sachsenhofer & Littke, 1993). They suggest Karpatian/early Badenian heat flows between 250 and 400 mW/m 2, which were probably

caused by shallow magma chambers (Sachsenhofer, 1994). Maximum Karpatian/early Badenian temperatures reached 150-250~ Thereafter, heat flows decreased significantly and, in spite of deep late Miocene burial, temperatures decreased also. A final magmatic event occurred in the eastern Styrian Basin and the Mura Depression during Plio-Pleistocene times (Fig. 2), but had little influence on the regional heat flow pattern (Ebner & Sachsenhofer, 1995).

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