[1] D. G. A Whitten with J. R. V. Brooks, Penguin Dictionary of Geology, 1979
The sediment in an organic sediment rock is made of fossils.
Plants remain squashed deep underground over millions of years make an organic sedimentary rock called coal.Coal doesn't look like it is made of sediments.It is also often difficult to seee the plant fossils within it because they have become so compacted over time,the less stable materials have left and all that remains is carbon.
Source: https://answers.yahoo.com/question/index?qid=20091201222522AAFc6hi
And coal is carbonaceous material.