Oil spill crews laid out boom to protect marshes and bird-nesting areas on Monday after an oily substance washed ashore along a remote area of Louisiana's coast.
Samples of that substance will be tested, but it is not suspected to be residual oil from the Deepwater Horizon spill last April.
The substances has washed ashore along some eight miles of Elmers Island, local officials said.
The discovery came a day after the Coast Guard said it received reports of a "dark substance" spread out over 100 miles south of Grande Isle, La. The agency said on Sunday that the goop appeared to be caused by river sediment and contained very little oil.
An analysis found only trace amounts of petroleum hydrocarbons, oil and grease — all at levels well within the Louisiana Department of Environmental Quality's clean water standard, the Coast Guard said in a statement.
Sediment carried down the Mississippi River and possibly agitated by dredging is believed to have caused the dark substance, according to the Coast Guard.