Clearing the way for a landmark Sonoma County conservation deal, the state Coastal Conservancy board Thursday approved a $10 million contribution toward the purchase of Preservation Ranch, a 19,652-acre property that sprawls across the county’s northwest corner.

A national conservation group has reached an agreement to buy nearly 20,000 acres of timberland in northwestern Sonoma County, a move that derails the long-disputed, forest-to-vineyards conversion project pushed by CalPERS, the giant state workers pension fund.