A ruptured oil pipeline has contaminated part of a reservoir, a river and a creek and threatened fish and other wildlife in northern Wyoming in one of the largest inland oil spills on record, state and federal officials said today.

Environmentalists, fighting what they see as a major threat to wilderness areas, immediately called the spill a sign of what could happen if Interior Secretary James G. Watt allowed more oil drilling in such remote areas.

"If this kind of thing can occur in such relatively flat and open land, we feel it has ominous implications for wilderness areas," said Bill Cunningham, northern Rockies representative of the Wilderness Society.

Wyoming officials said they had not yet determined the cause of the break in the 12-inch crude oil pipeline which spilled what they estimated to be more than 6,000 barrels, or about 250,000 gallons, of oil 10 miles south of Byron, Wyo.

The rupture apparently occurred during the weekend but was not discovered until yesterday morning, a common problem in remote areas. A passerby saw oil on the surface of Yellowtail Reservoir, about 20 miles east of Byron, and valves feeding the pipe were shut off, a Coast Guard official said.

A Coast Guard official sent to investigate the leak said it had not yet been determined what impact the spill had had on local wildlife but said the volume of oil lost approached the state's last major spill in 1980 into the Platte River.

The 1980 spill, apparently caused by underground telephone cable construction, killed 1,752 muskrats, 19 geese, 19 ducks and destroyed 183 goose eggs, according to the state's game and fish department.

Pete Petera of the game and fish department said the area affected by this week's spill served as a home for mink, muskrats and other fur-bearing animals who could be harmed by the oil. He said the lower Shoshone River, which received the oil spill from Whistle Creek and transported it to the reservoir, did not have many game fish.

The reservoir, however, is full of trout and walleyed pike, Petera said. Although the spill is reported to have extended only 200 yards into the flood prevention reservoir, "I don't think it's going to do the fish a danged bit of good," he said.

David Jossi, a private contractor working with the Department of Transportation in Washington, said there were 115 crude oil pipeline breaks in 1981 which spilled 578,169 barrels of the heavy oil onto American soil. Ninety-nine of the breaks were caused by construction equipment such as backhoes.

A spokesman for the state department of environmental quality said an initial report on this week's spill mentioned construction work in the area of the break. But officials from Wyoming state departments, as well as Coast Guard and Environmental Protection Agency officials, had reported no definite cause for the rupture late today as they continued to drive and fly by helicopter over the area.

A Coast Guard official said a containment boom floated across the mouth of the Shoshone River was keeping oil from spreading further into the reservoir. Another containment device was holding oil at the surface where Whistle Creek and the river meet so that the oil could be easily removed.

Officials of the Marathon Pipeline Co., which operated the pipe system, could not be reached for comment. Cunningham said pipelines laid in remote areas were dangerous not only because spills were difficult to detect but because rocky terrain made them susceptible to landslides and, in some areas, earthquakes. Jossi said transportation department figures show damage from pipeline failures in 1981 totaled about $5.2 million.

A suit brought by Wyoming against the pipeline company involved in the 1980 spill is pending in court. A suit by the pipeline company against the telephone company on whom it blames the rupture has also not been settled.