You searched for: “abyssolith
abyssolith (s) (noun), abyssoliths (pl)
A large mass of intrusive igneous rock having an exposed surface area of more than forty square miles, with no apparent base or floor of older rock: In their oil research, the scientists described the abyssolith layers of rock in which the fossil fuel could be found.

The term abyssolith is part of the vocabulary used in the science of petrology.