Sedimentology (1995) 42, 665-682 Basin dimensions and morphology as controls on amplification of tidal motions (the Early Miocene North Hungarian Bay) ORSOLYA SZTANO* and POPPE L. DE BOERt “Department of Geology, Eotvos L. University, H-1088 Budapest, Mrizeum krt 4/a., Hungary t Comparative Sedimentology Division, Utrecht University, 3508 TA Utrecht, Budapestlaan 4, The Netherlands ABSTRACT Following the Late Aquitanian sea-level fall, tide-influenced deposition started in the North Hungarian Bay, an embayment in the Paratethys open to the north-east. The relatively narrow, funnel shape of the bay supported amplification of tidal movements, resulting in the generation of strong tidal currents. The length and the depth of the North Hungarian Bay and the connecting seaway through East Slovakia fell into the ‘Tidal Amplification Window’ and thus fulfilled the conditions needed for resonant amplification of semidiurnal (M2) tides. Tide-influenced deposits were formed at both sides of the North Hungarian Bay. They reflect dominant currents in opposite directions and of different strengths at either side of the basin. This difference was the result of bottom-tide interactions. Cyclonic (anticlockwise) residual currents were induced above the asymmetrical central depression in the bay and were superimposed upon the tidal currents, producing an anticlockwise, time-and-velocity asymmetrical current system. The North Hungarian Bay and other examples show that amplification of tidal motions and formation of tide-influenced deposits may occur if basin dimensions pass through the ‘Tidal Amplification Window’. This window represents ideal conditions for resonant or amphidromic amplification of tidal currents. It determines an ideal length/depth or width/depth ratio relative to the wavelength of the astronomical tides. Thus signs of strong tidal influence in fossil basin fills can be used to reconstruct the dimensions (length, depth and width) of such basins. INTRODUCTION The physiography of a marine basin determines the potential controls of the dynamics of the system. The shape, depth and orientation of a basin influence whether, and to what extent, currents generated by winds, tides or both affect sedimentation. The influence of tides is most often recognized in relatively shallow-marine sediments (cf. Johnson & Baldwin, 1986). It is suggested here that extensive tide-influenced sediments, particularly in fossil basins, indicate that they were formed in a meso- or macrotidal setting. Microtidal settings can be recognized in the geological record only if the tidal basin was large enough for considerable water masses to flow in and out during each tidal cycle, and other potential forces (e.g. winds) did not overshadow the effects of the relatively small tidal amplitude. An example of such conditions is found in the present-day Wadden Sea, where a relatively small tidal amplitude combined with a large drainage basin leads to relatively strong tidal currents (Oost & de Boer, 1994). Tidal waves generated in the open ocean can be amplified when they propagate into shallow marine basins (Pugh, 1987). Amplification of the tidal wave depends on the morphology of the receiving basin, which thus determines the tidal range and the strength of the resulting tidal cur- rents. Therefore, in fossil cases, the physiography of a basin can be deduced from indirect evidence 0 1995 International Association of Sedimentologists 665