This is a chart
I created in about 16 hours. It contains the majority of major music styles present in the United States during the 20th century. There is some extra information here and there, and in some places, there may not be enough. However, I estimate over 80 individual genres are represented in this map.
It's a rather complicated flow chart format, but just follow the arrows and you'll be fine. Dates represent approximate time of creation.
If you've ever been interested in an observation of musical genres and how they each influenced development of others, this chart will give you a decent amount of information. The Wikipedia has a significant amount of supplemental information which can tell you more about any of the genres I have listed.