The slick, which was about 60 feet (18 meters) wide, was spotted about 1,000 yards (meters) offshore from Goleta State Beach west of Santa Barbara, said Coast Guard spokeswoman Petty Officer Andrea Anderson.
Anderson could not give the approximate length of the sheen.
In May, as much as 2,400 barrels of crude oil were
spilled onto a pristine beach about 15 miles (24 km) west of Goleta when a pipeline ruptured along the coast. But there were no immediate reports of any link between the two incidents.
"We don't have a definite source yet," Anderson said of the oil sheen discovered on Wednesday.
David Zaniboni, a spokesman for the Santa Barbara County Fire Department, told Los Angeles television station KABC that his agency also has not identified the source of the sheen and as a result was not calling the incident an oil spill.
A Coast Guard helicopter was surveying the area of the oil sheen, and other members of the agency went out by vessel to take a closer look, Anderson said.
The Coast Guard urged individuals not to make contact with the sheen and to report any oil sightings to its National Response Center.
Zaniboni told KABC that the area regularly experiences natural oil seepage but that firefighters were called when two kayakers came into shore after crossing into black oil in the water.
"All I can go by is what we were told by the kayakers, and they were saying this was more than they've ever seen," Zaniboni told KABC.
Goleta State Beach County Park and pier remained open, Santa Barbara County officials said.
Plains All American, which operated the coastal pipeline that ruptured and caused the spill in May, said the latest oil sheen could not have come from one of their pipelines since they do not operate any underwater pipes in the area, company spokeswoman Meredith Mathews said.
(Reporting by Alex Dobuzinskis and Dan Whitcomb in Los Angeles; Additional reporting by Rory Carroll in San Francisco,; Editing by Sandra Maler)