Abstract Facies of Campanian chalk in the Paris basin are shown to change as a function of accommodation cycles. Fourteen facies—identified in the field at the Chaux-de-Boran and Précy-sur-Oise quarries at Précy-sur-Oise near the Oise River, north of Paris, and studied by thin-section and SEM (scanning electron microscope) analysis—make up four well-defined facies associations. Three nannofacies compose the background chalk sedimentation. Cherts are more abundant in the progradational phases of 5–10-m-thick, higher-frequency cycles. Depositional environments were generally below fair-weather wave base and built up from below storm wave base. The tops of some shallowing-upward cycles were reworked by current activity, perhaps near to fair-weather wave base. Patterns of chert distribution and abundance, early-cemented ftrmgrounds, and evidence of storm-current reworking allow interpretation in terms of progradation, aggradation, and sediment bypass. Two sedimentological models illustrate the contrasting conditions under increasing and decreasing accommodation. Sedimentation took place over a low-relief profile far from any shoreline, with low-angle clinoforms prograding from storm-winnowed platform areas that lay above storm wave base into slightly deeper lows. Symmetrical to asymmetrical cycles of facies associations, corresponding to progradational-aggradational cycles, define genetic stratigraphic units. These stratigraphic units resulted from cycles of increasing and decreasing accommodation of at least two frequencies. Major hardgrounds record sediment bypass at the turnarounds from longer-term decreasing to increasing accommodation. A simple seismic model shows that the abundance and distribution of chert may explain amplitude changes observed on regional seismic data. Progradational intervals with higher concentrations of chert and capped by hardgrounds should create continuous reflections on a seismic line. These reflections should be of variable amplitude, depending upon the spacing between individual chert layers.