Origins of event beds in the Jurassic 'Calcari Grigi' Formation, Venetian Alps, Italy

Abstract

Three undulating, large-scale bedforms occur within a 20km wide lagoon which was probably generated by synsedimentary tectonic processes in a Jurassic shallow water rimmed platform (Trento Platform, Italy). Bedform 1 resembles hummocky cross stratification and consists of domes and troughs which display a downcurrent increase in wavelength and decrease in amplitude. Bedform 2 is composed of low-angle undulating foresets whose downcurrent structural variations over 300 m of stratigraphic exposure mimic smaller-scale climbing-wave ripple lamination. Bedform 3 consists of three large-scale stacked 'giant ripples'. Internal characteristics (shell fabrics) of bedforms reflect a complex storm action that was ineffective as sediment transport mechanism but produced an 'in situ' reorientation of shells through strong shear stress and local pressure pulses on and below the lagoonal floor. These bedforms were generated by tsunamis as evidenced by: 1) the action of surface waves; 2) a great lateral extent of exposures; 3) the restriction of bedforms to the same stratigraphic horizon (correlated by means of 'event correlation'). Bedform 1 was produced by first-order impulse-generated waves progressing discontinuously along the deepest area of the lagoon; bedforms 2 and 3 reflect bottom return flows. The trajectory system has been tentatively explained by the hydrodynamic configuration of impulse waves propagating at supercritical conditions around a lateral obstacle.