overburden vs burthen

overburden and burthen both are nouns.

Word	Noun	Adjective	Verb	Adverb
overburden	Yes	No	Yes	No
burthen	Yes	No	Yes	No

As verbs, burthen is a hypernym of overburden; that is, burthen is a word with a broader meaning than overburden:

Other hypernyms of overburden include burden, weight, weight down.

overburden (noun)	burthen (noun)
an excessive burden	a variant of `burden'
the surface soil that must be moved away to get at coal seams and mineral deposits

overburden (verb)	burthen (verb)
burden with too much work or responsibility	weight down with a load
load with excessive weight