With a little-noticed amendment to the Marine Mammals Protection Act. Congress yesterday knocked out one of three major contenders to transport Maskan oil to the northern tier states.

The amendment, sponsored by Sen. Warren Magnuson (D-Wash.), passed the Senate Tuesday and the House yesterday by voice votes. It prohibited the construction of any crude oil facility on Washington's Puget Sound, east of Port Angeles.

The effect is to scuttle the Trans-mountain pipeline project with a terminal at Cherry Point, Wash., proposed by a consortium including Atlantic Richfield and other major oil companies. Magnuson said the Cherry Point facility would create oil spill hazards in the pristine waters of the sound.

Although bills to establish an orderly selection of a pipeline route have been introduced in both Houses, no committees had taken action. Key staffers on the House side learned only by reading the Congressional Record that one of the major alternatives had been unexpectedly eliminated.

The amendment does not, however, preclude the building of the Northern Tier Pipeline which would transport the oil from a terminal at Port Angeles on the Straits of Juan de Fuca adjacent to Puget Sound. Magnuson said he personally opposes the Northern Tier proposal, but would allow state officials to make the decision.

While the Cherry Point proposal would have used existing pipeline through Washington and Canada at a cost of about $150 million, the Northern Tier proposal would involve a new pipeline through undisturbed land in five northern states at a reported cost of $1 billion. Both would terminate in Minnesota.

A third alternative involving a terminal at Kitamat, British Columbia, through Canada to Minnesota, was proposed but recently withdrawn by a consortium including Ashland and Murphy oil company subsidiaries.

However, the plan could be revived later, a Magnuson spokesman said.

"The big question now is whether the Northern Tier proposal - favored by the Minnesota (congressional) delegation - or the Kitamat proposal - favored by Montana and North Dakota - will be approved," a Magnuson aide said yesterday.

A pipeline is needed because Prudhoe Bay oil, which began flowing through the Alaska pipeline this summer, will create a surplus of 500,000 barrels a day on the West Coast. The oil is to be shipped through the Panama Canal until a west-to-east pipeline system is set up.

A northern pipeline would handle only part of the surplus. The rest would be transported through a southern line from Long Beach, Calif., to Texas, as proposed by Standard OIl of Ohio. The Sohio plan, however, has run into stiff environmental opposition in California.

The pipeline issue has been heated in the state of Washington. The legislature passed a bill in its last session favoring Northern Tier-Port Angeles proposal. The bill was vetoed by Gov. Dixy Lee Ray who advocated expanding existing facilities at Cherry Point.

Magnuson said yesterday he had eliminated the Cherry Point proposal because "that is what the state law provides." The Washington coastal zone management program supports a facility "at or west of Port Angeles."

Although he did not prohibit it in his amendment, Magnuson said he opposes the Northern Tier plan as well as the Cherry Point facility because of the possibility of oil spills in the vicinity of the Puget Sound.

"The Puget Sound is the only remaining pristine unpolluted inland sea in the world. The state of Washington should not assume the risk of spills for oil it doesn't need. We shouldn't use the sound or the Juan de Fuca Straits" for a pipeline terminal.

The Magnuson amendment has been forwarded to the White House for signature as part of the Marine Mammals Act. However, a hearing on the northern pipeline issue was scheduled this morning before a House Interior subcommittee. As word spread about the Magnuson move last night, House members were preparing to reassess the situation.