The U.S. Coast Guard says crews have mostly finished cleaning up a Lake Huron beach after an oil spill at Michigan's Cheboygan State Park.

The oil was discovered Tuesday, coating a 25-by-300-yard stretch of beach near the tip of Michigan's Lower Peninsula. A sheen also was spotted on nearby Duncan Bay.

Coast Guard Lt. j.g. Kaitlyn Williams said the sheen had dissipated Thursday and the onshore cleanup was 80 percent complete.

Williams says six barrels of contaminated sand have been removed, along with several dozen bags of oil-soaked sticks and other debris.

The Coast Guard has taken samples from a tug and a carferry to determine if either vessel was the source of the spill. The samples have been sent to a lab for analysis.

The park remains closed.