engleski

Subaerial exposure surfaces within Early Cretaceous carbonate deposits of Selina quarry, Istria

The Istrian peninsula, the north-western part of the former Adriatic Carbonate platform (AdCP), represents a region with well-preserved stratigraphic records that offer important insights into the significant events of the AdCP geological history. The Istrian succession is mainly composed of shallow water carbonates accumulated from the late Middle Jurassic to Eocene, and in a lesser extent of Eocene carbonate and clastic rocks, and Quaternary terra rossa and loess deposits. This succession can be divided into four sedimentary units or megasequences of carbonate deposits bounded by important discontinuities - emersion surfaces with different durations. The object of this study are two subaerial exposure surfaces within the 27 m thick Lower Cretaceous succession (Upper Barremian–Upper Albian) in Selina Quarry, near Sveti Lovreč. The purposes were to determine the composition and the provenance of the clay sediment and describe depositional environments. Additionally, micropaleontological/microfacies analyses of carbonate rocks were performed on samples taken from the bottom and top of the subaerial exposure surfaces to determine the age and duration of the subaerial exposures. Mineralogical X-Ray powder diffraction analyses revealed that mixed layered illite-smectite (I-S), illite and smectite are the main constituents of clay sediment, which is at the sampling place up to 1.4 m thick, found at the older subaerial exposure surface. The content of illite increases upwards as well the illite content of I-S. In its lower part I-S contains 30─40% illite component while in the higher part of clay sediment illite content in I-S is ~70%. Other determined minerals are calcite and potassium feldspar. The modal optical analysis of the heavy mineral fraction revealed the presence of opaque minerals (pyrite, marcasite and goethite) and titanite, zircon, tourmaline, garnet, biotite, epidote, while in the light fraction calcite, dolomite and muscovite were found. Sediment at the younger subaerial exposure surface, beside clay minerals (dominant illite and minor illite rich I-S), contains gypsum, jarosite and traces of potassium feldspar. It is possible that those minerals are deposited during oscillating transgression or in the main emersion phase. Considering the isolation of the AdCP during subaerial exposure surfaces (Aptian/Albian and Albian), the source of the material could be volcanic, aeolian or insoluble limestone residue (resedimented). Based on the dominance of illite and illite-smectite over smectite, it could be assumed that at least part of the material had volcanic origin, which was first altered to smectite and later by paedogenesis/diagenesis to I-S and illite. Microfacies analyses of samples from this locality represent various shallow water carbonate platform microfacies types. Determination of stratigraphic position of the investigated Late Barremian–Late Albian succession was based on the identification of microfossil assemblages (benthic foraminifera and calcareous algae Dasycladales) and used their taxa as index fossils. Considering the obtained results, the first subaerial exposure surface is recognized as the Late Aptian–Early Albian regional exposure, while the second one is formed during oscillating transgression in the Late Albian.

subaerial exposure surface, Early Cretaceous (Late Barremian–Late Albian), clay sediments, Selina Quarry, Istria

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