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Pelitic Materials Associated with Subaerial Unconformities in the Early Cretaceous in Istria, Croatia:) Sediments or Paleosols?

During Early Cretaceous time the Adriatic carbonate platform was built up by the rhytmical alternation of various shallow water related carbonate sediments, which are interrupted by several local and regional subaerial unconformities. They are particularly well expressed in West Istria where, from Hauterivian to the upper part of Late Albian, at least 8 subaerial exposure phases of different duration occur. These unconformities are accompanied by greenish, grey pelitic materials with high clay content whose thickness is paleorelief related and laterally changes from several centimetres up to one meter. They occur as either emersion breccias with clayey matrices or, more often, as clays. The intent of the present study is to investigate these pelitic sediments by geochemical, mineralogical and micromorphological methods, to clear up the origin of the material they are composed of, and to see whether or not they are. These materials have a very high clay content up to 95%. Most samples are composed only of clay minerals with small amounts of quartz and pyrite. The clay fraction (<2mm) consists mainly of smectitic and illitic material, in some samples small amounts of kaolinite and chlorite were detected. According to the infrared studies the smectites are aluminum-rich montmorillonites without iron substitution in the octaeder position; the amount of magnesium is also very low. The 10Ĺ peak of the illitic material is very broad which indicates a very small particle size. REE patterns in clays from the Late Aptian regional emersion may indicate that their main source rocks are not the underlying limestones (e. i. insoluble residue) but probably aeolian derived material.

Emersion layers; Lower Cretaceous shallow-marine carbonates; Paleosoils; Smectitic and illitic clays; Adriatic carbonate platform; Istria (Croatia)

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