The Late Jurassic deposits of the Boulonnais area (N-France) represents the proximal lateral-equivalent of the Kimmeridge Clay Formation; they accumulated on a clastic-dominated ramp subject to synsedimentary faulting in relation with the northward propagation of the Atlantic rifting. Within the terrigenous accumulations, some carbonate objects are visible at various conspicuous levels: oyster patch reefs and fine-grained carbonate beds, either continuous, or more or less nodular. Preliminary studies demonstrated that the carbonate beds of the Bancs Jumeaux Formation as well as the carbonate matrix of the oyster patch reefs are of diagenetic origin. In this paper, we extend the study to many other limestone beds of the Boulonnais with mud- or wackestone texture, examining facies and microfacies through various techniques as well as geochemical data (O, C and S stable isotopes, major and trace elements). We conclude that all examined carbonate bodies are of early diagenetic origin and that they precipitated at, or close to, the sea bed, from seawater mixing with ascending fluids containing isotopically light carbon of organic origin. Fluid circulation was probably induced by the extensional block-faulting segmentation of the northern margin of the Boulonnais Basin in Late Jurassic times. Fluid seepages were either channelized along fault planes or more diffuse, as illustrated by the model we propose. •Fine-grained carbonate beds and patch reefs in clastic-dominated sediments.•These carbonate bodies formed during earliest diagenesis.•Carbonate precipitation induced by cold seeps.•Fluid expulsions controlled by synsedimentary tectonics.