A 110-foot dredging barge sank in stormy Lake Huron early Thursday and a tugboat that was pushing the barge capsized, spilling an unknown amount of diesel fuel but not injuring anyone, authorities said Thursday.

It wasn't immediately clear what caused the Arthur J. to sink around 4:35 a.m., the U.S. Coast Guard said. The commercial vessel went down near the southern end of the lake, just over a mile from the Michigan coast and about 65 miles northeast of Detroit.

A crew from the Coast Guard's Port Huron station accounted for everyone who had been aboard the two vessels, said Chief Petty Officer Kyle Niemi, of the Cleveland district office.

The dredge could carry up to 8,000 gallons of fuel, but the owner said it only had about 1,500 gallons when it sank, along with "a minor amount" of lubricating oil associated with dredge equipment, the Coast Guard said. The tugboat had about 300 gallons of diesel.

It's unknown how much escaped, but a roughly 500-foot sheen was visible on the surface.

By early afternoon, the Coast Guard, St. Clair County's hazardous materials team and other agencies had placed more than 1,400 feet of containment boom on the water to limit the fuel's spread -- a task hampered by choppy waves and driving rain, Petty Officer Lauren Laughlin said.

The sheen appeared to be breaking up and none of the fuel had reached shore, she said. But the St. Clair County Health Department temporarily closed all the county's beaches, Jennifer Michaluk, health education and planning director, told the Port Huron Times Herald.

MCM Marine Inc., based in Sault Ste. Marie, owns the dredge and the 38-foot tug Madison. The company has hired a contractor to handle the cleanup, which the Coast Guard will monitor to ensure federal regulations are followed.

"We want to do everything we can to make things right and try to get our boats back," said Tom Spencer, accountant for MCM. He said he didn't know what caused the sinking but said the weather on the lake was "really nasty."

"We're very grateful our crew is OK. That's the most important thing," Spencer said.

Both vessels were partially submerged in 22 feet of water nearly six miles north of the entrance to the St. Clair River, which links Lakes Huron and Erie. Commercial traffic had not been affected because the stricken craft are outside the shipping channel, but the Coast Guard advised mariners in the area to be cautious.

"The Coast Guard will continue to ensure the safety of responders and the public," said Lt. Justin Westmiller, spokesman for the Detroit office.

One of Detroit's municipal water intake pipes is in the "general vicinity" of the spill but there is no immediate danger that fuel will be drawn into the pipe, said Brad Wurfel, spokesman for the Michigan Department of Environmental Quality. He said the DEQ had contacted city water officials as a precaution.