Integration of new data in sedimentology, micropaleontology and carbon isotope analysis of the Paleocene-Eocene boundary sequence in northern Belgium enables differentiation of a series of biotic and tectonic events, generally associated with the brief episode of global warming, known as the Initial Eocene Thermal Maximum (IETM, formerly LPTM), and to establish their succession in time. The onset of the carbon isotope excursion (CIE), positioned at the base of the Tienen Formation, is shown to precisely coincide with the onset of a massive Apectodinium abundance, and to be coincident or very close to the development of deeply incised paleovalleys. The organic carbon isotope curve recorded in the lagoonal settings of northern Belgium, presents a complex structure, marked by multiple anomalies and, consequently, is not believed to result from a single instantaneous event. The most negative values (-26\%) are recorded in the Apectodinium-dominated interval (75\%) in the lower part of the Tienen Formation. However, the range of the Apectodinium acme (50\%) suggests that the IETM probably lasted until the end of the deposition of the Tienen Formation. The kaolinite influxes at its base and within its middle part are proved to be unrelated to the CIE. The integration of the dinoflagellate cyst and the carbon isotope data has led to the establishment of a refined chronostratigraphic framework, allowing the depositional history of the southern North Sea Basin to be deciphered. During pre-IETM and post-IETM times widely uniform marine depositional regimes existed throughout the area because of tectonic quiescence. This strongly contrasted with the IETM interval, which is marked by restricted complex lagoonal conditions and differential subsidence. In spite of the extreme warm climate and the tectonic instability of the area, vegetation was relatively stable across the IETM and marked by low extinction and first-occurrence rates. © 2003 Geological Society of America.