Abstract: Cores of pelitic sediments (Eocene-Miocene) of the drillings Puchkirchen 1 and Geretsberg 1 (Molasse Basin, Upper Austria) have been studied to determine the mineralogical and chemical changes taking place during burial diagenesis. Mineralogical and chemical investigations of the bulk samples show that the deepest samples of the profiles are derived from a different source area. In particular, there is an increase in kaolinite and chlorite with depth and a decrease in quartz related to the initial sedimentology and provenance.
Investigations of the <2 µm and <0.2 µm fractions of the profiles Puchkirchen 1 and Geretsberg 1 reveal the diagenetic overprint of the mineral constituents: The gradual illitization of mixed-layer illite-smectite, also reflected in an increase of K2O and Al2O3, is displayed most prominently in the <0.2 µm fraction. The source for the Al and K is the dissolution of K-feldspar (<2 µm fraction), as indicated in many previous studies.
The I-S mixed-layer phases are randomly interlayered to a depth of 1600 m; from there on a regular interstratified I-S phase appears in coexistence with the randomly interlayered I-S mixed layer. The randomly oriented phase is still present in major amounts to depths of 2500 m, presumably as a result of the low geothermal gradient (2.9 °C/100 m) in the Molasse Basin.
The calculation of the structural formula of the end members illite and smectite from this series of I-S mixed-layer phases gave the following results: Smectite:K0.14X+0.44(Al1.10Mg0.46Fe0.36Ti0.01)Si4.03O10(OH)2Illite:K0.44X+0.19(Al1.26Mg0.42Fe0.38Ti0.01)(Si3.52Al0.48)O10(OH)2 The end-member interlayer charge for the smectite component (+0.58) is higher than reported for typical smectites (+0.32 to +0.47). It is suggested that the I-S phases of the Molasse Basin are probably intergrowths of 3 layer-silicate members: illite, low-charged smectite and high-charged smectite. The determined smectite end-member composition represents, therefore, an average for a variable 2-component smectite system. The charge-differences of the 2 smectites would likely reflect the differences in source material, which in turn would have led to the formation of different early, highly smectitic I-S phases in the sedimentary basin.