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Clay minerals in paleosols of Cretaceous age in Istria, Croatia

Greenish clays often occur as thin layers in Early Cretaceous shallow-water carbonate sediments of Istria where they mark the top of shallowing upwards sequences. When they show evidence of subaerial exposure and pedogenesis they are considered paleosols. The clay mineralogical composition is as follows: (1) randomly oriented I/S mixed layer, (2) regularly oriented I/S mixed layer, (3) illitic material, (4) kaolinite and (5) chlorite. The insoluble residue of limestones situated immediately below greenish clays are dominated by smectite, and contain also kaolinite and illitic material. The very high smectite content in the insoluble residue is probably the result of additions of volcanic dust during carbonate sedimentation. That clay mineral composition in the pelitic material clearly indicates the influence of both pedogenetic and diagenetic processes. It may also indicate additional volcanic dust contribution before and during the subaerial exposure.

Greenish clays; paleosols;Early Cretaceous shallow-water carbonate sediments; clay mineralogy; Istria; Croatia

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