Stromatactis calcilutite mounds are important constituents of a reef complex in the southern Gaspe Peninsula. They reach thickness up to 115 m and had paleoslopes of about 40 degrees . Stromatactis constitutes up to 30 percent of the rock volume. The microfacies of the calcilutite consists a pelletoidal network surrounded by uniform mudstone. Sponge spicules are ubiquitous. By analogy with textural features of a well-documented sponge. Malumispongium hartnageli (Clark 1924) which occurs slightly above the mounds, it is suggested that the pelletoidal network of the mound calcilutite represents a former sponge network that underwent very early diagenetic change and cementation. Stromatactis would have originated as early marine cementation of growth cavities in the sponge network and of cavities created by decay of local uncemented sponge tissues.--Modified journal abstract.