Abstract Improvements of geophysical acquisition and processing techniques and coring data give a new insight into the architecture of large subaquatic dunes. A field of dunes, with amplitudes ranging from 0.7 to 9.4 m. has been studied in the Bay of Bourgneuf. near the Loire inlet, France. Results show that the bedforms have steep lee sides and distinct asymmetry, and that the internal structure consists of steeply-dipping cross-strata truncated by several orders of bounding surfaces. The true dip angle of the laminae, measured on a core collected from a 3.6-m-high dune, suggests the existence of sandflow or sandfall processes, similar to those previously described in eolian dunes or small intertidal bedforms. The combined effect of waves and tidal currents is proposed to explain the strong asymmetry of the structures and the origin of bounding surfaces. Even if these dunes can be considered as tidal bedforms, seismic and core data suggest that the internal structure is probably the result of episodic, rather than cyclic, processes. If compared to the sand waves described by Visser from sheltered environments, this type of sandbody should be difficult to interpret in the stratigraphic record, because it does not display any "tidal criteria" (tidal bundles, mud couplets).