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MAPPING THE NORTH CASCADES; HISTORY OF THE PROJECT
by R.W. Tabor, R.A. Haugerud

In the mid 1970s, electric power companies in Washington State were ambitiously building nuclear reactors at several different sites and pursuing site licenses required by the Nuclear Regulatory Commission. The Reactor Hazards Program of the Office of Regional Geology of the U.S. Geological Survey enjoined the branch of Western Regional Geology to begin a long term geologic mapping program in the North Cascades to help sustain the USGS's role as advisor to the NRC on licensing and to help us understand the complex geology of the North Cascades.

In 1975, the North Cascade mapping project was born under the guise of the Wenatchee project, aimed at producing four 1:100,000 scale geologic maps and fundamental geologic data for the Wenatchee 2 degree quadrangle which essentially covers the southern part of the North Cascades. Virgil Frizzell, Jr. and Richard Waitt, Jr. joined Rowland Tabor and a constantly changing array of talented field assistants, all funded by the Reactor Hazards Program, initially directed by Program Manager Carl Wentworth. For several years we were joined part time by John Whetten (U of Washington), Don Swanson (BIGP), Robert Zartman (BIG), and several others. Naturally our mapping has been built on the work of many others, especially students who have trod diligently over the difficult terrain of the North Cascades.

The first 7 years of mapping were mostly in the Tertiary Cascade volcanic cover that laps up on the older terranes of the higher parts of the North Cascades. We thought that we might find a more salubrious record of tectonism in the younger rocks that would help establish a tectonic framework applicable to today's faulting hazards. Two fundamental structures cross the Wenatchee 2-degree quadrangle: the regional Straight Creek Fault and the Olympic-Wallowa Lineament. Our studies helped establish the history of these structures, but neither can be blamed for the modern earthquakes in the North Cascade region.

Location of the project maps

As the mapping team moved into the older rocks to the north,they found that a major part of the geologic problem was one of separating out older collisional structures from high-angle faults. As understanding of the geologic complexities grew, strangely enough, so did recognition of the intractable geologic problems, but in no way are they as great as the problems facing the reactor builders. By 1982, the reactors were on hold, and applications for licenses with the NRC all but ceased.

The Survey soon phased out its Reactor Hazards program, but the Wenatchee project, reborn as the Mount Baker project continued with the mapping. The money then came from the National Geologic Mapping Program and, in the past, from the Washington State Department of Natural Resources. State geologists with the Division of Geology and Earth Resources were busy revising the state geologic map, and the Mt Baker project has been producing some of the appropriate maps . By this time many of our early team members had gone on to more genteel professional work. Quaternary geologist Derek Booth, who joined the North Cascade team in 1978, stayed on through 1986, but moved on to other work about the time that Ralph Haugerud joined the project.

Multiple exotic terranes, thrust-stacked sequences, and deep-seated Eocene metamorphism, all seen through a scrim of Tertiary arc volcanics and plutons and fierce brush, challenged our brains, while topographic relief of 4 to 7 thousand feet challenged our knees. We didn't spend much time thinking about recent faulting, but the fascinating tectonic picture, albeit an ancient one, has come into view.

In the last years of the project, about 1991 to 1995 our field work focused on the Methow area, in the Robinson Mountain and Twisp quadrangles, on the east side of the main North Cascade Range, where Vicki Todd had filled in some of the details. We also had the help of Robert Miller (SJ State U), whose detailed structural mapping in the crystalline rocks along the Ross Lake fault zone helps tie the Methow rocks to the North Cascades. Brian Mahoney (University of Wisconsin) and Joe Dragovich (Washington Division of Geology and Earth Resources) helped untangle crucial stratigraphic relationships in the Twisp quadrangle. The Methow offers insight into overall tectonics because the rocks are unmetamorphosed marine and terrestrial sedimentary rocks with stratigraphic coherence. They were once planar and usually reveal which way was up. Their stratigraphy and structure is more complex than originally recognized, but that tells more about the deformation of the range as a whole. In 1991, project personel began digitizing the geologic maps in progress at that time. To date 7 1:100,000 geologic maps have been completely digitized in ARCINFO mostly utilizing the USGS menu program, Alacarte by Carl M. Wentworth and Todd T. Fitzgibben. The digital maps are already being used by other government agencies for awide variety of programs. Downloadable digital coverages will be available for all of the 1:100,000 scale geologic maps.

Most of the 33,000 square kilometers encompased by the project was complete before the project was officially terminated in 1995. The final touches on Robinson Mountain and the Twisp quadrangles were completed by Ralph Haugerud from 1995 to 1999.

ROCK SAMPLES Representative rocks samples of major North Cascade geologic units are archived with North Cascades National Park in Marblemount, Washington.