STATE OF WASHINGTON DEPARTMENT OF NATURAL RESOURCES BRIAN J. BOYLE, Commissioner of Public Lands ART STEARNS, Superv isor DIVISION OF GEOLOGY AND EARTH RESOURCES RAYMOND LASMANIS, State Geologist GEOLOGY OF THE WENATCHEE AND MONITOR QUADRANGLES, CHELAN AND DOUGLAS COUNTIES, WASHINGTON By Randall L. Gresens BULLETIN 75 1983 For sale by Department of Natural Resources, Olympia, Washington Price $6.00 FOREWORD The first mining claims were staked for gold in the Wenatchee area in 1885. Prospectors flocked into the region and, as a result, the City of Wenatchee was incorporated in 1892. The lure of gold has again attracted the attention of the public and exploration companies. A staking and leasing rush of major proportions is taking place during 1983. The Geology and Earth Resources Division of the Department of Natural Resources is fortunate to be able to publish this timely report by the late Randall L. Gresens, a professor at the University of Washington. The author was employed by the division to complete the geologic mapping of the Wenatchee and Monitor quadrangles, in the summers of 1975-1982, and to write a bulletin to accompany the maps. The division was in possession of his manuscript, completed field maps, and cross sections at the time of his tragic accident in the summer of 1982. Randy's report presents new hypotheses regarding the age and origin of hydrothermal alteration, as well as associated ore minerals at the L-D gold mine (also known as the Lovitt, Gold King, and Golden King). The report expands and elaborates on the geologic units, especially sedimentary formations of the Wenatchee area. His attention to detail and lucid style of writing make this report extremely useful to present and future workers in the area. The staff of the division will miss Randy both as a friend and as an esteemed colleague. We are proud to present this document to the geologic community and the public as a working monument to Randy's ability as a geologist. Because the published report and maps could not receive the author's final scrutiny, the division accepts responsibility for any errors that may have been introduced as a result of the editing process. Special thanks go to Professo r E. S. Cheney, of the University of Washington, for his assistance in reading the manuscript and locating the figures needed for this volume. Raymond Lasmanis Washington State Geologist III ERRATA Bulletin 75, Geology of the Wenatchee and Monitor quadrangles, Chelan and Douglas Counties, Washington, by Randall L. Gresens Under "Description of Units" on Plates 1, 2, and 3, the description of the Swauk(?) Formation reads "Swauk(?) Formation of late Eocene Age." It should read "Swauk(?) Formation of early Eocene age." CONTENTS Page Introduction......................................................... 1 Purpose and scope of the investigation........................... 1 Regional geologic setting. .... ...................... ........... 1 Pre-Late Cretaceous metamorphic rocks--Swakane Biotite Gneiss........ 2 Swakane Biotite Gneiss in the Wenatchee quadrangle... ........... 3 Foliation in the Swakane Biotite Gneiss..................... 4 Cataclasis in the Swakane Biotite Gneiss.................... 4 Origin and age of the Swakane Biotite Gneiss..................... 4 Sedimentary and extrusive igneous rocks of Cenozoic age.............. 5 Swauk Formation. .. .. • . 5 General description. ........................................ 5 Swauk(?) rocks in the Wenatchee and Monitor quadrangles..... 5 Chumstick Formation.............................................. 7 General description ......... .......................... ... 7 Chumstick rocks in the Wenatchee and Monitor quadrangles.... 8 Lower Ch ums ti ck rocks.................................. 8 Tuff units in lower Chumstick rocks ••• ••• ••.•••••• 10 Mission Creek tuffs •.•••..•••••• .••• ••• .••••• 10 Yaxon Canyon tuffs........... ............... 11 Fairview Canyon tuff •••• ••••••••.••• ••.•••••• 12 Horse Lake Mountain tuffs •••.•••.•••••••••••• 12 Miscellaneous tuffs of uncertain correlation •.••••••••.•••••••..•••••••••••. 12 Nahahum Canyon Member of the Chumstick Formation ••••••• 13 Chumstick Formation east of the Entiat fault ••••. •••••• 15 Significant differences between the Swauk(?) and Chumstick Formations............ ... ............... .................. 15 Wenatchee Formation ...•••...••...•...••. •• ••••••.•• •••••...•.• • .. 19 Occurrences west of the Entiat fault ••.••••••••••.••••• ••••• 20 Type sec ti on.. .... ............ 20 Sandstone and shale member ••.••••.••••••••••.••••• 20 Conglomerate member ••••••••••••••.••••••••••••••.• 20 Geomorphic expression •••• ••••••••. •••.••• •••• ••••. 21 Reference section .••••••••••••••••••.•.••••••.••••••••• 21 Dry Gulch reference section •.•••.••••••••••••••••. 21 Chopper Hill reference section ••••••••.••••••••••• 21 Stemilt Canyon reference section •••• ••••• •••.••. •• 21 Additional occurrences of the unit •••• •• •••••.••••••••. 22 Occurrences east of the Entiat fault ••.••• ••••••.•••• ••••.•• 23 Reference section .•...•..••...•.. ..•.• •.••.•.•••.•.. •. 23 Blue Grade reference section ••. •.••• ••. ••••• ••• ••• 23 Additional occurrences of the unit •••••••••.••• ••• ••••• 23 Depositional environment of the Wenatchee Formation ••••••.•• 24 Sedimentary rocks of Miocene age .•.••••••• •••• ••••••. ••• •• ••••••• 25 Columbia River Basalt Group •••• •••.••.• ••••.•••••••••••••• ••• •• •• 25 V CONTENTS Page Interflow sediments •••••••••• •• •••••••••••••••••••••••••••••••••• 25 Rocks of uncertain age •.••...••.. •. ...•••...••••..•. •.•.•••.• . ••• 27 Intrusive igneous rocks of Cenozoic age •••••••••••••.•••••••••••••••• 28 Horse Lake Mountain intrusive complex ••••••• •••••••••••• •••• •• ••• 28 Rock types..... • . 29 Canyon Number One composite sill and stock •••••••••••••••••• 30 Twin Peaks intrusive breccia and associated si ll-dike complex ••••.••••••••••• ••• •••• • •.• ••.••••••••••••••• •••••• 32 Bear Gulch composite sill •••••••••••• •••••••• ••••••••••••••• 34 Martin's Ranch sill •••• ••••••••••••••••••••••••••••••••••••• 34 Fairview Canyon sill •.••..•.•.•• •.•••.• ••••• •••.•.•.•.••.••. 35 Mi see 11 aneous si 11 s and dikes... • • • • • • • • • • • • • • • • • • • • • • • • • • • • 35 Contact effects of the Horse Lake Mountain intrusive complex..... .............................................. 36 Xenoliths in Horse Lake Mountain rocks ••••• ••••••••••••••••• 36 Alteration of rocks of the Horse Lake Mountain intrusive complex • • ••••.•••••••• •. •••••••••••••••••••••.•• •••••• •••• 36 Age and emplacement of the Horse Lake Mountain intrusive complex....... ........ ................................ .. 37 Wenatchee pinnacles ..•.••.•.• ••••.•.•••••.•.. •. ••......••.•.••••. 38 Felsic rocks of the Wenatchee Pinnacles •• ••••••••••••••••••• 38 Mafic rocks of the Wenatchee Pinnacles •••••••••••••••••••••• 40 Eocene gabbro •••• •••••••••••••••••••••••••••••••••••••• •••••••• •• 41 Lamprophyre of uncertain age •••• ••••• ••••••••••• •••••••••• •••• ••• 42 Hydrothermally altered rocks •••••••••••••• •••• •••• ••••••••••••••• •••• 42 Silicified zone of the L-D mine area ••••••••••••••••••••••••••••• 43 Silicified zone of Number Two Canyon ••• •••••••••• •••••••••••••••• 44 Hydrothermally altered rocks near Horse Lake Mountain •••••••••••• 44 Age of hydrothermal activity ••••••••••••••••••••••••••••••••••• •• 45 Deposits of Quaternary age •••••••••••••• •••• ••••••••••••••••••••••••• 46 Mass wasting and fluvial deposits composed primarily of basaltic material............................................ .. 46 Deposits related to glaciation •••••••••••••••••••• ••••••••• •••• •• 48 Flood deposits ••••••••••••••••••••••• ••••••••••••••••••••••• 48 Unconsolidated sand of flood or eolian origin ••••••• •••••••• 52 Loess deposits ••• ••••••••• •••••••••••••• ••••••••• ••••••••••• 52 Conglomerate of unknown origin •••••••••••••••••••••••••••••••••• • 53 Volcanic ash ••••••••••• •••••••• •••••••••••• ••• ••••••••••••••• •••• 53 Structural geology ............... ................................... 53 Post-Swauk, pre-Chumstick deformation •••••••••• ••••• ••••••••••••• 54 Eagle Creek anticline and associated folds and faults •••••••••••• 54 Post-Wenatchee, pre-Columbia River basalt deformation •• •••••••••• 55 Pitcher syncline and structure at Dry Gulch ••••••••••••••••• 55 Interpretation of the Pitcher syncline and Dry Gulch structure ••••.• .••.•.... • . •.•••••••.•••....•••. 56 Northern continuation of thrust faults at Dry Gulch •••• ••• •• 59 Thrusting across the Entiat fault ••••• ••••••••••• ••• •••••••• 60 Displacement on thrust faults •••• •••••••••• •••••• ••••• •••••• 60 VI CONTENTS Page High-angle faulting •••• ••••••••••• •••••• ••••••••••••••••••• • 62 Complicated structure at upper Squilchuck Canyon ••••• •••• •.• 62 Deformation coeval with emplacement of the Horse Lake Mountain intrusive complex •••••• •••••••••••••••••• ••• 63 Miscellaneous structures of uncertain age •••••••••••••••••• ••••• • 64 Minor faults ................................................ 64 Unusual fold structure in the Nahahum Canyon Member of the Chumstick Formation ••• ••••••••••••••••••••••••••••• 65 Internal shearing within the Chumstick Formation •••••••••.•• 66 Regional tilt and erosional stripping of Wenatchee Formation...................................................... 66 ColTITlents on the bounding faults of the Chiwaukum graben •••••••••• 66 Entiat fault ••• ••••••••••••••••••• •••• ••• •••• •••• •••••• ••• •• 66 Leavenworth fault....................