The morphology of fossil pebbles as a tool for determining their transport processes (Koźmin South lignite open-cast pit, central Poland)

Marek Widera


The Koźmin South lignite open-cast pit is the southernmost exposure of Palaeogene (lower Oligocene) deposits in central Poland. This study is focused on Palaeogene pebbles of the so-called Koźmin Gravels, rede- posited in the Neogene sands. These pebbles are not known from the adjacent territory of central Poland and they vary in petrographic composition, shape and surface texture. The classification of the pebble-size particle form is determined as DI/DL (elongation ratio) and DS/DI (flatness ratio). A great number of pebbles are disc-shaped (oblate-shaped) and blade-shaped or they can be classified as platy, bladed and very bladed pebbles. The oblate–prolate index (OP) is less than –2 for 45% of the particles (typical of beach pebbles), the mean sphericity (yp) is 0.56 (typical of beach pebbles), and the mean roundness (RWt) is 0.73 (typical of beach pebbles).

The data obtained from SEM analysis of quartz pebble surface micromorphology are characteristic of high mechanical energy of the littoral environment. The surface of some pebbles is excellently polished with v-shaped indentations and grooves. The average composition of the heavy mineral fraction occurring with these pebbles is marked by the predominance of zircon (exceeding 70%). All the above-mentioned data, in the light of extensive literature, indicate that more morphological features of the analysed pebbles have been inherited from the littoral/beach environment. Moreover, a detailed petrographic study was very useful for determining the prove- nance of these pebbles. The most characteristic rocks are greyish-blue quartzes. They are known only from the Sudetes Mts., situated on the NE slope of the Bohemian Massif. Thus, the rock fragments were transported at least 300 km by rivers from the Sudetes to the littoral/beach zone of the Palaeogene sea. Then, the residually-marine beach pebbles were redeposited into the Neogene debris flow and/or fluvial deposits. The present-day area of the Koźmin South lignite open-cast pit was tectonically active at that time.

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