The Światkowa Wielka Tectonic Window belongs to the Grybów Nappe of the Fore-Magura Group of units. This tectonic window is located in the marginal part of the Magura Nappe and is composed of Oligocene — Sub-Grybów Beds as well as the Grybów Marl Formation. These beds have been correlated with the Oligocene deposits of other tectonic windows of the Grybów Nappe in Poland. Our research reveals that the Krosno beds’ shally facies, which occur at the western termination of the Światkowa Wielka Tectonic Window, belong to the Dukla succession. On the basis of calcareous nannoplankton analysis, the Grybów Marl Formation as well as the Krosno Beds belong to the NP23–NP24, and NP24 Zones, respectively. The structure of the Światkowa Wielka Tectonic Window reveals a multi-stage evolution of the Magura Nappe overthrust onto their foreland.