The term "Laurentian Abyss" is not a recognized geographical term. There is no abyss in the Laurentian Great Lakes or in the surrounding region with that name.

It's possible you're thinking of:

* The deepest part of Lake Superior: This is called "The Basin" and has a depth of approximately 406 meters (1,332 feet).

* The Mariana Trench: This is the deepest known part of the Earth's oceans, reaching a depth of around 10,924 meters (35,843 feet).

Please clarify what you're looking for, and I can help you find the information you need.