sapropel

noun

1. mud consisting chiefly of decomposed organic matter formed at the bottom of a stagnant sea or lake.

noun

1. an unconsolidated sludge consisting of the decomposed remains of aquatic organisms, esp algae, that accumulates at the bottoms of lakes and oceans

1. An unconsolidated ooze consisting mainly of putrefied plant remains and found in anaerobic areas at the bottom of swamps, lakes, and shallow seas.