Field investigation, biostratigraphic, paleoecological, and sedimentary microfacies analyses, as well as diagenetic processes characterization, were carried out in the Epirus region (Western Ionian Basin) to define the depositional environments and further decipher the diagenetic history of the Late Cretaceous–Early Paleocene carbonate succession in western continental Greece. Planktonic foraminiferal biostratigraphy of the studied carbonates revealed that the investigated part of the Gardiki section covers the Cretaceous–Paleogene (K-Pg) transition, partly reflecting the Senonian limestone and calciturbidites formations of the Ionian zone stratigraphy. Litho-and bio-facies analyses allowed for the recognition of three distinct depositional facies: (a) the latest Maastrichtian pelagic biomicrite mudstone with in situ planktonic foraminifera, radiolarians, and filaments, (b) a pelagic biomicrite packstone with abundant planktonic foraminifera at the K-Pg boundary, and (c) an early Paleocene pelagic biomicrite wackestone with veins, micritized radiolarians, and mixed planktonic fauna in terms of in situ and reworked (aberrant or broken) planktonic foraminifera. The documented sedimentary facies characterize a relatively low to medium energy deep environment, representing the transition from the deep basin to the deep shelf and the toe of the slope crossing the K-Pg boundary. Micropaleontological and paleoecological analyses of the samples demonstrate that primary productivity collapse is a key proximate cause of this extinction event. Additional petrographic analyses showed that the petrophysical behavior and reservoir characteristics of the study deposits are controlled by the depositional environment (marine, meteoric, and burial diagenetic) and further influenced by diagenetic processes such as micritization, compaction, cementation, dissolution, and fracturing.

The study of microbial mats in extreme environments is of high scientific interest from geological, ecological, and geomicrobiological aspects. These mats represent multilayer bio-structures where each taxonomic group dominates a specific vertical layering distribution resulting from its growth and metabolic activity. In the present study, microbial mats in a hot spring environment from Aedipsos (Euboea Island, Greece) resulting in the creation of thermogenic travertine, were studied through an interdisciplinary approach. The mineralogical composition was determined by optical microscopy, XRD, and SEM-EDS microanalysis, and the identification of Cyanobacteria was made primarily on morphological characteristics. The main mineral phase in the studied samples is calcite and, to a less extent, aragonite, with several trace elements in the mineral-chemistry composition, i.e., up to 1.93 wt. % MgO, up to 0.52 wt. % SrO, up to 0.44 wt. % Na₂O, up to 0.17 wt. % K₂O, and up to 3.99 wt. % SO₃. The dominant facies are lamination and shrubs, which are the most common among the facies of thermogenic travertines of the area. Several layers were identified, (i) a top mainly abiotic layer consisting of calcium carbonate micritic crystals, (ii) a second biotic layer–the Cyanobacteria layer, dominated by the species Leptolyngbya perforans, (iii) a third biotic layer where Leptolyngbya perforans, Chloroflexus and other bacteria occur, and (iv) a deeper abiotic part with several layers where no photosynthetic microorganisms occur. In the upper layers, nineteen (19) species of Cyanobacteria were identified, classified in the orders Chroococcales (37%), Synechococcales (31%), Oscillatoriales (16%), and Spirulinales (6%). Among the identified Cyanobacteria, there are typical thermophilic and limestone substrate species. These Cyanobacteria are found to participate in the biomineralization and biologically-influenced processes, i.e., (i) filamentous Cyanobacteria are trapping calcium carbonate crystals, and diatoms, (ii) extracellular polymeric substances (EPS) create crystal retention lattice contributing to the biomineralization process, and (iii) filamentous sheaths of Cyanobacteria are calcified, resulting in the creation of calcium carbonate tubes.

Caves with hot springs and speleothem deposits are infrequent environments of high scientific interest due to their unique environmental conditions. The selected site is a small open cave with a hot spring and stalactites in the Aedipsos area (NW Euboea Island, Greece), which was studied through an interdisciplinary approach. The mineralogical composition of the speleothems was determined by optical microscopy, XRD, and SEM-EDS microanalysis, and identification of the Cyanobacteria species was made based on morphological characteristics. The main mineral phase in the studied samples is calcite, with several trace elements (i.e., up to 0.48 wt.% Na₂O, up to 0.73 wt.% MgO, up to 4.19 wt.% SO₃, up to 0.16 wt.% SrO and up to 2.21 wt.% Yb₂O₃) in the mineral-chemistry composition. The dominant facies are lamination and shrubs, which are the most common among the facies of the thermogenic travertines of the area. Based on the studied stalactites, twenty-nine different Cyanobacteria species were identified, belonging to the following orders: Synechococcales (28%), Oscillatoriales (27%), Chroococcales (21%) and Nostocales (21%), and Spirulinales (3%). Among them, thermophilic species (Spirulina subtilissima) and limestone substrate species (Chroococcus lithophilus, Leptolyngbya perforans, and Leptolyngbya ercegovicii) were identified. The identified Cyanobacteria were found to participate in biomineralization processes. The most characteristic biomineralization activity is made by the endolithic Cyanobacteria destroying calcite crystals in the outer layer. In a few cases, calcified cyanobacterial sheaths were detected. The presence of filamentous Cyanobacteria, along with extracellular polymeric substance (EPS), creates a dense net resulting in the retention of calcium carbonate crystals.