© 2020, Saudi Society for Geosciences.The Eocene İslambeyli Formation outcrops at the lowest sedimentary unit of the northern part of the Tertiary Thrace Basin. This formation consists mainly of siliciclastics with increasing carbonate contribution in the upper part in the Pınarhisar region (NW Thrace). Mineralogically, this formation is derived from the quartz-rich sedimentary and felsic igneous provenance in an active continental margin (ACM) tectonic setting. The siliciclastics are composed of mainly dioctahedral Ca–smectite with minor amounts of illite, chlorite, and kaolinite, plus non-clay minerals quartz, K-feldspar, plagioclase, muscovite, biotite, calcite, zeolite (heulandite–clinoptilolite), and gypsum. Primary fragments of the İslambeyli Formation transported from the metamorphics of the Paleozoic Strandja Massif were widely altered according to the alteration indices of the massif and resulted in the enrichment and depletion of some elements (e.g., increase in Th/U ratio, negative Eu anomaly). The feldspar volume and total content of Na2O + K2O values of the formation are clearly lower than those of the massif. Mg and Ca in the smectite structure were probably obtained from seawater in a shore and shallow marine-reef environment during and after the deposition of the İslambeyli Formation.