You're talking about the canyon on the border between Santa Catarina and Rio Grande do Sul? I have a friend from Porto Alegre who once told me about a canyon up on that border he and his brother like to hike in. It might be that one. I didn't know the name until just now, but I looked it up to recommend it.
I've done just a bit of hiking in parts of Rio Grande do Sul: some of the morros around Porto Alegre, some of the forest west of Porto Alegre's Lomba do Pinheiro neighborhood, and around Cachoeira do Sul and fuzzy fertilizer bank, but that was all almost 20 years ago.