Lithologic-stratigraphical conditions of the Miocene sulphate layer karstification on the north-western slopes of the Khotyn Upland
Authors: Kostiuk Uliana , Ridush Bogdan 
Published: April 2024

The lithologic-stratigraphic features of the sulphate layer karstification in the north-western outskirts of the Khotyn Upland were influenced by sedimentation conditions in the Middle Miocene. The facies characteristics of the gypsum layer in the vicinity of Pohorylivka Village indicate that gypsum facies formed in both deep-water conditions (lamellar gypsum) and nearshore saline conditions (concealed-crystalline facies with traces of gypsified microbial mats). Additionally, the nodular facies are observed replacing the herbaceous gypsum facies in the valley of the Black Stream. Furthermore, the transition from the concealed-crystalline gypsum facies with traces of gypsified microbial mats to a clayey-sandy-carbonate layer is noted, reflected in the morphology of the Pioneer Cave galleries.

The morphology of the cave galleries in the valley of the Chornyi Potik River is characterized by the prevalence of sculpted surfaces, indicating the predominance of epigenic karstification. It is evident in the wide and low profiles of the galleries. However, in some places, the morphology overlays on stereogenic forms (dome-shaped, oval-stretched, round), indicating the influx of water from an artesian aquifer. In other words, initially, the sulfate dissolution occurred under artesian water conditions, and later, erosion began to occur with the influx of unconfined surface water.

The Pionerka Cave galleries' structure has signs of hypogenic and epigenic speleogenesis. Genetically, the cavity is related to contemporary relief forms. The mezomorphology of the upper part of the cave, with various speleogenic forms, indicates the formation under artesian conditions in interaction with ascending waters and with the subsurface waters of a finer drainage system. The most active phase of karstification in the studied area occurred when the tributaries of the Dniester River incised. These led to the expansion of fissures into two large galleries. Due to the presence of a carbonate-clay-sandy layer, which became a detachment surface, the cavity began to be filled with allochtonic debris and blocks. Meanwhile, the cave ceiling continued to form in this gallery under the influence of hypogenic karstification. In the southeast gallery, due to facies differences (the presence of a layer of fine-grained gypsum with traces of gypsified microbial mats), the gallery has a rift-like character with undulating walls and a dome-shaped ceiling.

In general, the investigated areas on the outskirts of the village of Pohorylivka are crucial for reconstructing the conditions of karstification (speleogenesis) for the entire Northern Bukovina region. Our research significantly contributes to the hydrogeological, geomorphological, and geological features of the Prut-Dniester interfluve territory. The lithologic-stratigraphic features of the sulphate layer karstification in the north-western outskirts of the Khotyn Upland were influenced by sedimentation conditions in the Middle Miocene. The facies characteristics of the gypsum layer in the vicinity of Pohorylivka Village indicate that gypsum facies formed in both deep-water conditions (lamellar gypsum) and nearshore saline conditions (concealed-crystalline facies with traces of gypsified microbial mats). Additionally, the nodular facies are observed replacing the herbaceous gypsum facies in the valley of the Black Stream. Furthermore, the transition from the concealed-crystalline gypsum facies with traces of gypsified microbial mats to a clayey-sandy-carbonate layer is noted, reflected in the morphology of the Pioneer Cave galleries.

Keywords: gypsum deposits, karst, lithology, Khotyn Upland.