The tar mat was discovered Saturday morning when a U.S. Army Corps of Engineers dredge contractor hit it, creating a strong odor and oil sheen near Outcast Marina, according to Phillip West, coastal resources manager for the city of Orange Beach.

West said he and response officials are 99 percent positive that the tar mat is a result of the BP oil spill of April 2010. Water samples were taken and sent to Auburn University.

BP PLC, majority owner of the oil well that was the source of the spilled oil, declined comment Monday and referred questions to the oil spill National Response Center.

The corps contractor immediately moved off the site, but the area has not been closed to boaters since the mat is sealed in the sand, according to West on Monday. The water depth in the area near the Pass ranges from 9 to 15 feet, he said.

The size of the mat is not known and it's hard to estimate until coring is done, he said. A tar mat has a crust-like exterior and the oil or gooey tar is in the middle.