Grunge is indie rock — look at the years:
In fact, grunge’s roots can be traced back to the 1980s, if not further.
“Deep Six,” the 1986 compilation on Seattle label C/Z Records, is recognized as the first record to document this burgeoning regional sound. It featured a half-dozen local bands: Soundgarden, Green River, the Melvins, Malfunkshun, the U-Men and Skin Yard.
The Melvins, who have essentially never stopped touring or recording with 20 studio albums to their name, were friends and mentors to Nirvana’s Cobain and Krist Novoselic, who hailed from the same rural part of Washington state. As Novoselic once said of the sway Melvins frontman Buzz Osborne held over him: “Buzz was the preacher, and his gospel was punk rock.” The Melvins sonic influence on Nirvana’s first album is unmistakable. – CNN
Punk gave rise to post-punk and then emo (Fugazi).
At the same time, indie rock arose: part punk, part folk, part 60s revivalism (Pink Floyd, 13th Floor Elevators, etc.)
Their goal: to reduce angry punk and metal to humanist, sociable, feelings-oriented music.
With Nirvana, then Opeth, then modern metal, they succeeded.
(Also see Faux culture.)