Somethings to keep in mind,

Your muck may not be that thick. You may find that you only have a few inches of the sludge and that what lies beneath it may be more firm clay-like soils. This could help you out. My situation was an 10 foot deep pond that silted in over 50 years and was only 2 foot deep...that was a lot of soupy sludge.

You also need to think about how the pond was constructed. If, for example, your pond basin was littered with gravel veins or sand streaks and it needed a full bowl of clay packed into it...you could jeopardize the water holding ability should you dig through to a porous subsoil. I would have this conversation with a pond expert prior to disturbing the pond bottom. I doubt this is the case, but better safe than sorry.

Fish on!,
Noel