engleski

Glauconitic materials from Lower Miocene Macelj-sandstones of the Hrvatsko Zagorje, north-western Croatia

The Lower Miocene Macelj-sandstones, from the western part of Hrvatsko Zagorje, are green in colour with variable amounts of glauconitic grains. The Macelj-sandstones were deposited in shallow-marine environments on foreshore, shoreface and tidal flat enviroments with or without deltas at stream and small river mouths. The detritus load for the marine shoals, which were often separated from the open sea by sandy bars and tidal flats, is debris transported by rivers from areas of active erosion. Periodic but explosive synsedimentary dacite-andesite volcanism made possible tuff accumulation and the presence of great quantities of volcanoclastic material within the epiclastic detritus. The natural sandstone samples were analysed by polarising microscope and by X-ray power diffraction (XRD). After separation, the pure or most pure glauconitic materials were analysed by XRD, chemical analysis and thermal analysis (TG, DTA and DTG). The results show variation, not only in the glauconitic material of the sandstone samples, but also within individual samples. These differences are already noticeable from the microscopic examination of sandstones and after glauconite separation. The amount of smectite layers varies from < 5% to approximately 40% depending on the degree of order and the stage of glauconitic evolution. This is indicated by the contents of K, Al, Fe, adsorbed water and cation exchange capacity as well as XRD power patterns. The detected mineralogical differences of glauconitic material are consistent with the microscopic data which indicates redeposition of an unconsolidated sediment. Chemical differences of magnetically more homogeneous glauconitic material may be explained by differences in chemical and structural features within the evolving series of glauconites which originated in same glauconitization cycle, from probably the same or similar initial substrate. The results obtained from this investigation neither deny nor confirm the results achieved by other ivestigators, inicating that glauconitic material from the examined Macelj-sandstones were probably produced by alteration of dacite-andesite volcano-clastic material in shallow-marine foreshore, shoreface and tidal flat environments.

Glauconitic materials; Smectite; Glauconitization cycles; Shallow-marine environments; Lower Miocene sandstones; Hrvatsko Zagorje

nije evidentirano