engleski

Diagenesis of Miocene pelitic sedimentary rocks in the Sava Depression (Croatia)

Miocene pelitic sedimentary rocks from six wells from the Sava Depression (sub-basin in the south-western part of the Pannonian Basin System) were investigated in order to determine the degree of diagenetic alterations. Qualitative and semiquantitative mineral composition of samples and the content of smectite in illite-smectite (I-S) were determined by X-ray powder diffraction (XRD). Vitrinite reflectance and Thermal Alteration Index (TAI) were measured for estimating palaeotemperatures. Carbonate minerals, clay minerals and quartz are the main constituents of pelitic sediments in the Sava Depression. Feldspars (albite), pyrite, opal-CT, and hematite are present as minor constituents in some rocks. Mineral composition of the rocks is, apart from previously known differences caused by various depositional environments and clastic material provenance, dependant on the degree of diagenetic processes. At elevated temperatures and burial depths minerals formed by alteration processes or precipitated at the surface, like smectite, kaolinite and calcite, were gradually replaced by minerals formed by diagenetic processes, i.e. by illite-smectite, illite, chlorite, Ca-excess dolomite/ankerite and albite. Based on XRD data for smectite, I-S and illite, three stages of diagenetic development have been established. The early stage was characteristic of samples at depths <1.8 km containing smectite, I-S of the random R0 type and detrital illite. The middle stage begins with the appearance of the ordered R1 type of I-S at the depths >1.8 km and temperature above 80°C. At depths >4.6 km with corresponding temperatures >190° C, begins the late stage, characterized by the presence of R>1 I-S with <10% smectite.

diagenesis; illite-smectite; Miocene; Pannonian Basin System; pelitic sedimentary rocks; Sava Depression; Thermal Alteration Index; vitrinite reflectance

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