Illite/smectite diagenesis in the autochthonous Miocene claystones of the Carpathian Foredeep
Institute of Geological Sciences, Polish Academy of Sciences, Senacka 1, 31-002 Kraków, Poland
Key words: illite/smectite, mixed-layer clay minerals, diagenesis, Carpathian Foredeep
The conversion of smectite to illite has been studied by X-ray diffraction techniques in the Miocene claystones of the Carpathian Foredeep. The analysed samples of claystones were taken from the four boreholes (Jodłówka-4, Buszkowiczki-4, Załazie-2 and Ryszkowa Wola-7) located in the eastern part of the Polish segment of Carpathian Foredeep. The autochthonous Miocene sediments in Jodłówka-4 are buried under the 1500m thick Carpathian overthrust. The other three boreholes are situated north of the front line of the Carpathian overthrust.
In three boreholes (Jodłówka-4, Buszkowiczki-4 and Załazie-2), diagenetic evolution of illite/smectite has been observed as a general trend of decreasing % smectite in mixed-layer minerals with depth (from 70%S to 30%S), starting from a certain level (onset of illitization). In Ryszkowa Wola-7 the onset of illitization has not been reached. In Jodłówka-4 and Załazie-2, the transition from random (R0) to ordered structures (R1) has been observed. The trends of illitization were correlated with present-day temperatures calculated on the basis of gamma-ray logs. Assuming that present-day geothermal gradient is similar to thermal conditions at the time of maximal burial of sediments and comparing the known temperatures of the R0/R1 transition (110 - 120°C) with the values estimated for the two boreholes, the erosion in the investigated region was calculated: 400 - 700m for Jodłówka-4 and 500 - 780m for Załazie-2.
In Jodłówka-4, the gradient of diagenesis (D% smectite/100m below the onset of illitization) is lower than in Buszkowiczki-4 and Załazie-2. These differences are interpreted as related to different temperature of Miocene sediments beneath the Carpathians as compared to the parts of the basin not affected by the overthrusted cover. The temperature differences, in turn, might be due to more rapid subsidence of the Foredeep sediments beneath the Carpathians caused by load of nappes, relative to the northern parts of the basin. Alternative explanation of the temperature differences are variations in thermal conductivity of the two regions related to the differences in lithology: the Carpathian rocks are characterised by lower clay content than the Miocene sediments of the Carpathian Foredeep, their thermal conductivity then should be higher, and, consequently, geothermal gradient and degree of diagenesis in the Miocene beneath the Carpathians should be lower than in more northern parts of the basin.