The fire near Biscayne Drive was reported around 12:40 p.m. The initial estimate was five to 10 acres but that estimate was scaled back to five acres, Giannini said.
The fire started away from pavement, ruling out a spark from a passing vehicle or a cigarette tossed out a vehicle's window as a cause, Giannini said.
Most of the 79 firefighters who responded to the fire remained at the scene at about 4:45 p.m. mopping up and some will remain overnight watching for hotspots, Giannini said.
Two air tankers and a helicopter were released from fighting the fire. Firefighters from San Rafael, Cal Fire, Novato and the Marin County Fire Department responded, Giannini said.
There are residential units for park employees in the area and other residential units are located at China Camp Village at the east end of the park near San Rafael, Giannini said.
No injuries have been reported and no structures were damaged. The fire was 100 percent contained around 4 p.m., Giannini said.