New findings about the facies of the „Pebbly mudstones“ of variegated deposits of Kroměříž Formation (Karpatian, Central part of the Carpathian Foredeep)
Vol.23,No.1-2(2016)
One of the facies of variegated deposits of the Kroměříž Formation (Miocene, Late Karpatian) is represented by pebbly mudstones. The Kroměříž Formation is formed by clays, mudstones, sands, sandstones, gravels and conglomerates. This formation has been deposited along the active margin of the Carpathian Foredeep ahead of the front of the Western Carpathian flysch nappes and is divided into the variegated deposits and the Holešov conglomerates. The variegated deposits are known only from the deep boreholes. In this paper the samples of pebbly mudstones from boreholes Rataje-1, -2 and Kroměříž-1, -2 are described. The studied samples have been subjected to the grain size analysis, petrographic and provenance analyses of gravel clasts and analyses of the chemistry and provenance of detrital garnets. The pebbly mudstone from the borehole Rataje-1 contains ~24 % gravel, ~18 % sand and ~58 % clay + silt and can be classified (according to Nichols 2009) as a gravelly mudrock. The pebbly mudstone from the borehole Kroměříž-1 is formed by ~2.5 % gravel, ~19 % sand and ~78.5 % clay + silt and is classified as a sandy mudrock. The clasts are mostly formed by quartzs (30–35 %), sandstones (20–27 %), metamorphic rocks (10–20 %) a magmatic rocks (10–19 %). The clasts of chert (3–12 %) and limestones (1–9 %) are less common. Other rocks as shales, mudstones and menillites are only accessory (representing about 1 % of clasts each). The presence of the clay and silt matrix and also the mudstone intraclasts and rock extraclasts show mixing of the intrabasinal unconsolidated fine-grained deposits and coarse-grained clastic material from the extrabasinal environment. The clasts have subangular and subrounded shape. The predominant type of detrital garnets is Alm-Prp with pyrope component to 20 %, which represents ~65 % of detrital garnets. The pebbly mudstones have been interpreted as deposits of cohesive debris flows (being a part of the subaqueous fan or delta?). The flysch nappes of the Western Carpathians served as a source for both clasts and detrital garnets, and deposits of the Kroměříž Formation are supposed to represent a prograding wedge into the basin.
Carpathian Foredeep; Karpatian; Kroměříž Formation; borehole core; pebbly mudstones; detrital garnet; provenance
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