The Ocean Drilling Program drilled Hole 904A on the upper continental slope offshore New Jersey through 230 m of Eocene chalks and an opal-A/opal-CT diagenetic front. In addition to a suite of standard logging tools, a dipole sonic tool was deployed to determine the in situ shear sonic velocity and the elastic properties of these slow, highly porous, sediments. While porosity decreases by 20% across the diagenetic front, the compressional velocity and the density are observed to increase by 30%, the shear sonic velocity and the bulk modulus by about 60%, and the shear modulus by almost 300%. It is shown that shear and compressional velocities are both controlled by porosity and consolidation, and that existing models for the prediction of elastic properties and shear velocity from standard logs are still valid through the diagenetic front. The strong effect of diagenesis on rigidity makes the shear velocity log a highly sensitive indicator of pore shape and pore filling transformations. ¿ American Geophysical Union 1996