Microfacies analysis and diagenetic features of the Eocene nummulitic accumulations from northwestern Transylvanian Basin (Romania)

Microfacies analyses of twenty-five samples collected from three outcrops located in the northwestern part of the Transylvanian Basin offered new insights into the structural architecture and diagenetic features of the nummultic accumulations in this region. The data from thin-section observations revealed the fact that diagenetic processes (mainly pressure dissolutions) played a crucial role in the fabric development, porosity evolution and general morphology of these build-ups. A high degree of compaction was observed with respect to the abundance of stylolitic structures, fissure sets, and numerous fragmented nummulitic particles within the matrix. Such features allowed the identification of several phases of stylolitization which affected the nummulitic accumulations during burial diagenesis. Moreover, these compactional traits can provide clues for understanding the ratio variations between N. perforatus A (microspheric) and B (macrospheric) forms in such deposits. Combining our results with previous data we better constrained the autochthonous and/or para-autochthonous provenance of the nummulithoclasts. Our results offer a new perspective into the development and preservation of nummulitic accumulations deposited in the Eocene tropical/sub-tropical shallow-marine settings of the central Neotethyan realm.