The syntax depends on indentation and block layout of text. It's very hard to express using the usual grammar tools. In PetitMarkdown, before Jan added indent-aware parsing to PetitParser, I was thinking of doing multi-pass parsing, first recognizing block-level, then inline syntax.
The closest thing to a standard is John Gruber's reference implementation, and that is a sequence of regular expression search-and-replace passes. Not exactly what you'd call a grammar :)