The flank beds surrounding a Niagaran reef-platform at Lapel, Indiana consist in their steeply dipping, upper portion of grain-supported crinoidal dolarenite, while their middle and lower portions are made of layers of mud-supported crinoidal dolarenite decreasing in thickness radially and alternating with interreef fossiliferous dolosiltite. Petrographic investigation shows that the grain-supported crinoidal layers have textures indicating deposition by a combination of wave action and slumping, while most of the mud-supported crinoidal layers display graded bedding and other typical textures characteristic of turbidites and of fluxoturbidites generated by intermediate mechanisms between slumping and turbid flow. No well-defined horizontal grading occurs in any of the turbidites mostly as a result of the relatively short distance of transportation. Microscopically the effect of the turbidity currents is expressed by constantly opposed variations between the frequency of the coarse crinoid debris brought in and that of the silt-size detrital minerals and of the pyrite which characterize the quiet calcisiltite environment. The clasticity curves do not display characteristic variations because the maximum size of crinoid debris appears biologically controlled and that of the detrital minerals related only to interreef conditions. This example demonstrates for the first time that turbidity currents may play locally a major role in the genesis of flank beds in the Niagaran reefs of the Great Lakes area.