The lessening of economic sanctions against Iran in a deal announced this week will keep downward pressure on crude oil prices, commodities trader Dennis Gartman said Monday.

"It'll be some while. They're not going to come online tomorrow. They're not going to come online next week," he said. "But the market believes and understands, and I think rightfully expects that by the turn of the year we're going to have Iranian crude oil coming back to the market."

(Read more: Iran deal unlikely to help much at the gas pump)

On CNBC's "Fast Money," the editor and publisher of The Gartman Letter noted activity in term structures for crude oil, specifically West Texas intermediate going into contango.

Contango occurs when futures prices for a commodity are greater than the current spot price.

(Read more: Contango: CNBC Explains)

"Prices have been under pressure," Gartman said. "Prices may well continue to be under pressure for some while going into the future."

Crude oil prices could test $85 per barrel levels, he added.

"There's a gob of $110 crude oil out there," Gartman said. "There's probably a good deal less $80 crude."

(Read more: Keep buying stocks at all-time highs: US Trust)

Gasoline prices at the pump could also be lower by the end of 2013, he said.

"I bet we take RBOB under $2.50," he said. "I bet we take the public's gas at the pump under $2.75 gas relatively easily after the turn of the year."

—By CNBC's Bruno J. Navarro. Follow him on Twitter @Bruno_J_Navarro.