The first in a series of contributions on Quaternary deposits in the northern Campine area (Belgium), the present paper discusses the conditions of deposition and processes involved, as well as a reconstruction of sedimentary palaeoenvironments and the stratigraphic position of these strata within the Quaternary sequence. Only the sedimentary characteristics of the lowermost portion of these deposits are outlined; these consist of sandy-clayey sediments, reaching a thickness of more than 40 metres. Five sedimentological units are recognised; each unit accumulated either in an estuarine or in a fluvial-aeolian environment. The succession of palaeoenvironments occurs in an alternating pattern, with fluvial-aeolian deposits following upon estuarine strata. The transitions between these are primarily the result of sea level fluctuations within that particular stratigraphic interval. In contrast, the continuous evolution within estuarine deposits is tectonically induced.