Lower Silurian mud-rich fenestrate bryozoan–sponge buildups of Anticosti Island (Québec, Canada) display facies ranging from fenestrate bryozoan cementstone to crinoid and fenestrate bryozoan hash mudstone–wackestone with distinctive polymud fabric, sponge spicules, and stromatactis. These buildups consist of a massive core with thinner stratiform counterparts that show no significant compositional variation, raising the question as to how such a deposit could have accreted. These features are similar to those mentioned for many Paleozoic carbonate mounds, in particular the Lower Carboniferous Waulsortian or Waulsortian-type mounds. Here, the combination of petrographic, fluorescence spectrometric, and geochemical evidence (carbon and oxygen stable isotopes, rare earth elements + yttrium) shows that the accretionary mechanisms of the Anticosti buildups were the results of marine cement precipitation near the sea floor for the fenestrate cementstone facies, and of early calcification of sponge-derived organic matter (organogenic lithification) within the sediment for the crinoid and fenestrate bryozoan hash mudstone–wackestone facies. Thus, the accretionary mechanisms of Anticosti fenestrate bryozoan–sponge buildups are essentially diagenetic, and conventional mechanisms invoked for reef growth, such as frame building, or sediment trapping, or binding based on the preserved fauna or flora, do not constitute an appropriate explanation for such Paleozoic mud-rich buildups. These conclusions may well shed light on the more general ongoing debate as to the origin and accretionary mechanisms of carbonate mudmounds.