As part of the previous research, we have examined red fillings of the Upper Triassic megalodontid bivalves at a selected location in the Julian Alps, in the Pod Peski Valley. Based on optical and cathodoluminescent microscopy and X-ray fluorescence analysis (XRF), we confirmed four generations of shell infillings. Since one of the basic questions of previous research, mainly related to the origin of the last generation of the sediment infill in megalodontids remained unanswered, we decided to further examine this area. We described and in detail sampled the rock sequence a few meters below and above the »main« layer with the above-mentioned megalodontids. Already during the previous sampling campaign, we noticed that solution voids appear in the surrounding rocks. These voids are filled with reddish sediment infill, similar to that in the megalodontids. In order to further study neptunian dykes and their impact on fillings in the last generation, we investigated in more detail five additional dykes in the surrounding area. Based on selected samples, we performed a detailed microfacies analysis of sediment sequence and neptunian dykes.
The studied section of Dachstein limestone mainly consists of intraclastic peloidal packestone to wackestone with larger bioclasts (megalodontides, gastropods) and solution voids. In a few samples, we observed a presence of Thaumatoporella. Stromatolites and emersion breccias with black intraclasts (»black pebbles«) appear in the upper part of the section. These intraclasts also appear in some places in red paleokarst solution voids. In thesolution voids, we observed a multi-generational filling with alternation of calcite cement and red sedimentary fillings. The investigated neptunian dykes were generally divided on the basis of dimensions and location of occurrence. Two of them, located directly on the »main« layer with red-filled megalodontids, contain planktonic foraminifera, which indicates the Middle Jurassic or younger age. The next two studied dykes are located just above the »main« layer, one of which contains clasts with calpionellis, characteristic of the Late Jurassic / Early Cretaceous . The last dyke we explored is located a few tens of meters away from the »main« bedding plane and is much larger than all the previous ones. It is a hundreds of meters long, with diverse sedimentary fillings indicating several separate generations of filling. In a few sample from this dyke we identified Early Cretaceous planktonic foraminifera.
Based on microscopic analysis, the reddish paleokarst sedimentary fillings in the solution voids were found to be remarkably similar to the sedimentary fillings of the first and second phases in the megalodontid bivalves. The fillings are part of the more complex paleokarst system observed in the Dachstein limestone. More important findings, related to the last generation, were also found in the neptunian dykes, where optical microscopy also showed similarity between the last sediment filling in megalodontids and some neptunian dykes on the »main« bedding plane. For ending of our research, we discovered a Santonian - Maastrichtian sedimentary filling with globotruncanid foraminifers in the upper part of the succession in one of the solution voids.
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