Final answer:
A deposit rich in broken-up carbonate shells of marine organisms is indicative of a reef environment, which is characterized by warm, shallow marine waters and is conducive to the preservation of fossils.
Step-by-step explanation:
The deposit described here, containing very little sand and mud but a lot of broken-up carbonate shells of marine organisms, suggests that the likely depositional environment was a reef. The presence of broken-up carbonate shells indicates accumulation in a marine setting where organisms with calcium carbonate shells, such as oysters, lived and died. This type of environment is typically associated with warm, shallow marine waters where reefs are known to form and thrive. Moreover, the environment is conducive to the preservation of fossils, as the calcium carbonate from shells can contribute to the formation of limestone, a common sedimentary rock in such environments. The distinctive marine deposits such as coccolithophores and diatomite further indicate a marine rather than continental environment.