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U.S. Geological Survey Map I-1661

1987

by R.W. Tabor, V.A. Frizzell, Jr., J.T. Whetten, and R.B. Waitt, D.A. Swanson, G.R. Byerly, D.B. Booth, M.J. Hetherington, and R.E. Zartman

Generalized geologic map of the Chelan Quadrangle. Click here for enlarged version with explanation (72K). Availability of Chelan geologic map Summary of geology View of geologic map and link to digital data Reference list and sources of data maps

INTRODUCTION

Bedrock of the Chelan 1:100,000 quadrangle displays a long and varied geologic history (fig. 1). Pioneer geologic work in the quadrangle began with Bailey Willis (1887, 1903) and I. C. Russell (1893, 1900). A. C. Waters (1930, 1932, 1938) made the first definitive geologic studies in the area. He mapped and described the metamorphic rocks and the lavas of the Columbia River Basalt Group in the vicinity of Chelan as well as the arkoses within the Chiwaukum graben (fig. 1). B. M. Page (1939a, b) detailed much of the structure and petrology of the metamorphic and igneous rocks in the Chiwaukum Mountains, further described the arkoses, and, for the first time, defined the alpine glacial stages in the area. C. L. Willis (1950, 1953) was the first to recognize the Chiwaukum graben, one of the more significant structural features of the region.

The pre-Tertiary schists and gneisses are continuous with rocks to the north included in the Skagit Metamorphic Suite of Misch (1966, p. 102-103). Peter Misch and his students established a framework of North Cascade metamorphic geology which underlies much of our construct, especially in the western part of the quadrangle.

Our work began in 1975 and was essentially completed in 1980. The Chelan 1: 100,000 sheet is the northeast quarter of the Wenatchee 1:250,000 quadrangle and is the second release in a series of the geologic maps which will cover this larger quadrangle and the Concrete 1:250,000 quadrangle to the north. A preliminary open-file report (Tabor and others, 1980) preceded this map.

Responsibility for the mapping of the Columbia River Basalt Group lies mostly with Swanson and Byerly. Tabor, Frizzell, Whetten, Booth, and Hetherington mapped most of the other bedrock area; of that, most of the Chiwaukum graben was mapped by Whetten. Waitt mapped most of the unconsolidated deposits.

A considerable number of new radiometric age analyses are included in this report. The K-Ar biotite and hornblende ages were measured by Tabor and Hetherington, and the zircon and apatite fission-track ages were determined by Frizzell. Zartman determined U-Th-Pb zircon ages in support of the field investigations, and contributed to the interpretation of all geochronologic data.

Field assistants who gave much in the line of camp and mapping duty were Jay Coburn and Ron Tal, 1976; Bill Gaum and Kim Marcus, 1977; Brett Cox, Sam Johnson, Elizabeth Lincoln Mathieson, and Nora Shew, 1978; Joe Marquez, 1979, 1980; and Jim Talpey, 1979. Dennis Sorg produced numerous clean mineral separates.

Helicopter pilots John Nelson, Doug Bucklew, the late Jack Johnson, and Tim Bormin performed miraculous feats in rough terrain and weather. We appreciate their efforts to get us to the rocks.

No geologic study proceeds without considerable discussion and argument. We are particularly indebted to M. Clark Blake, Erik Erikson, Kenneth Fox, Ronald Frost, the late Randal Gresens, Clifford Hopson, James Mattinson, Robert Miller, Victoria Todd, and Robert Yeats for keeping us thinking.

Generalized geologic map of the Chelan Quadrangle. Click here for enlarged version with explanation (72K). Availability of Chelan geologic map Summary of geology View of map and link to digital data Reference list and sources of data maps