Micropetrographic characteristics of the studied mudrock and sandstone samples (a: 1/7; b: 1/10; c–e: 1/4; f: 1/3). (a, b) Textural features of the marl samples, including oriented organic matter fragments (i). Note enrichment of framboidal pyrites (ii, “black spots”) in a close relationship with organic matter. (c–f) Silt- to sand-sized detrital grains together with organic matter (i) seams and reworked mudrock (iii) fragments in sandstones. Intergranular pores are filled by blue epoxy resin. Dominant framework grains: quartz (iv), carbonate lithics (v), and plagioclase feldspar (vi). Abbreviations: PPL = plane polarized light; XPL = crossed polars.