New map charts region's rocks

December 18, 2001 12:00 am

By JAYSON JACOBY

Of the Baker City Herald

Northeastern Oregon might just have missed having its own Crater Lake.

A pair of state geologists doing field work in the late 1990s for a new map discovered that Tower Mountain, a 6,850-foot peak west of Anthony Lakes, actually is a caldera a collapsed volcano.

The volcano probably fell in on itself about 25 million years old, said Mark Ferns, field geologist at the Oregon Department of Geology and Mineral Industries office in Baker City.

Ferns did the field work for the new map of the La Grande quadrangle along with Ian Madin of the Geology Department and William Taubeneck, professor emeritus at Oregon State University.

Although there may have been a lake or lakes in the Tower Mountain caldera at one time, Ferns said, theres none there now because the floor of the caldera probably rose over the millennia.

That rising apparently didnt happen or wasnt as extensive at Crater Lake or at Oregons other famous caldera, the Newberry volcano south of Bend.

Newberrys caldera contains two lakes, East and Paulina.

Ferns said geologists knew from earlier research that Tower Mountain was a volcano.

But no one had ever discovered the caldera, which is about 10 miles across, he said.

Ferns said the existence of the caldera was a surprise, but it shouldnt have been.

There are volcanoes of similar age, which erupted the same types of rocks, in both Washington and Nevada, he said.

The Tower Mountain caldera wasnt the only surprise geologists discovered during their several years of research.

Ferns said they also found that some of the rocks between Baker City and La Grande are younger than geologists previously believed.

For example, a lava flow just west of Ladd Canyon is about 3 million years old, Ferns said.

Geologists had mapped all the rocks in that area as part of the Columbia River Basalt group, rocks that erupted between about 17 million and 15 million years ago.

The Ladd Canyon lava flow is part of what geologists have termed the Powder River Volcanics Field, which includes several small volcanoes most of which erupted between about 15 million and 6 million years ago, Ferns said.

Among those volcanoes are Frasier Mountain and Mount Harris, he said.

Geologists also uncovered the solution to a longtime mystery about tales of gold in the Blue Mountains between La Grande and Pendleton.

Some experts had scoffed at those stories, because the mountains in that area are built from Columbia River basalts, rocks where gold rarely if ever is found, Ferns said.

But he and the other geologists found previously unmapped deposits of sedimentary rocks in the area, including some gravels that contained placer gold.

You can actually do some panning on parts of Five Points Creek just west of La Grande, Ferns said.

The La Grande quadrangle map covers 1,680 square miles, a little more than 1 million acres. The area stretches from the northern part of Baker Valley through the Grande Ronde Valley, and west to the Pilot Rock area. The quadrangle includes portions of Baker, Grant, Union and Umatilla counties.

The map itself shows the types of rocks across the entire area, including their ages.

The 54-page text accompanying the CD-ROM version also includes information about geothermal energy potential and places to hunt for semi-precious gemstones such as agates, Ferns said.

The CD-ROM costs $10, and a paper version of the map, which includes copies of the text included with the CD-ROM, costs $15.

To view the CD-ROM version youll need a PDF-viewing program such as Adobe Acrobat Reader.

Copies are available at the Geology Departments office, 1510 Campbell St. (corner of Campbell and Cedar streets).