The report area, two 15-minute. quadrangles, is in the southern part of Stevens County and the northern part of Spokane County, about 30 miles north of Spokane. Much of the area is underlain by two great Precambrian sections-the Belt Supergroup and the Deer Trail Group. The Deer Trail Group appears to be equivalent to the upper part of the Belt Supergroup, although differences in thickness and stratigraphy suggest that the sites of deposition of the two sections were much farther apart than the sections are now. The Precambrian Huckleberry Formation and Monk Formation unconformably overlie the Deer Trail Group, but not the Belt Supergroup. ~oth Precan:tbri~.n .group~. ~~e. ove~lain by the .C~brian .~ddy Quartzite. Cambrian, Devonian, and Mississippian carbonate rocks are found.above.the Addy Qua.rtzite where.it overlies the Belt Superg~oup.• ..representing three periods of plutonic activity, intrude the Precambrian and Paleozoic rocks. They range in composition from granodiorite to alkali-rich quartz monzonite. The old~~t J~.ttte ~Fl.owery Tr~l9ra.n9~~orite, an isolated pluton near the center of the area. It appears to have been intruded about 200 million years ago. Five others, mainly in the northern part of the area, were intruded about 100 million years ago, and intrusion of the remaining three, the Silver Point Quartz Monzonite and two small satellitic plutons in the southern part of the area, apparently climaxed plutonic activity about 50 million years ago.Andesite of Oligocene(?) age occurs in one small area southwest of Chewelah. Yakima-type(?) flows of the Columbia River Group are preserved at lower elevations in the southern part of the area. Small patches of conglomerate, possibly of Tertiary age, unconformably overlie the Huckleberry -Formation southwest of• Chewelah• and the Starvatioii"'Flat "QuartzMOnZoiiite near Cliff Ridge. Quaternary giaci.8"1 ~~ci • ~ifu~f;J-deposi~~~~~r large parts of the west half of the area at lower elevations.The Belt Supergroup and the Deer Trail Group appear to be confined to different structural blocks that are separated by a major structural discontinuity. The chief structures of the Belt Supergroup block, which underlies most of the eastern part of the area, are a roughly north-south-striking anticline and syncline. Both folds are overturned to the west in the northern part of the area. Six large northwest-striking highangle faults appear to predate the folding and are probably Precambrian. Although the sense of movement on these faults is not well established, five show apparent left-lateral slip, and one shows apparent right-lateral slip. The five could well be dip-slip faults that have displaced rocks on the north sides downward relative to the south sides.The north-south folds and northwest faults of the Belt Supergroup block are apparently truncated by the Deer Trail Group block, which underlies the western part of the area and strikes N. 30°-40° E. The consistency of the different trends of and in, the two blocks and the apparent differences in facie~ and thickne...
Bedrock in north-central Washington comprises (1) weakly to moderately metamorphosed eugeosynclinal rocks of Permian, Triassic, and possibly Jurassic ages, (2) high-grade polymetamorphic rocks-gneiss, schist, and amphibolite-that are, at least in part, more highly metamorphosed derivatives of the rocks of the previous category, (3) Mesozoic and Cenozoic plutonic rocks, and (4) Cenozoic lavas and continental sedimentary deposits.A review of the radiometric ages from north-central Washington suggests a complex history of plutonism and metamorpP.ism beginning in Late Triassic and extending to Eocene time. However, ages of coexisting minerals from single samples typically are moderately to highly discordant. The discordance reaches a maximum along a zone flanking the Okanogan gneiss dome on the west. This dome forms the southwestern extremity of the Omineca crystalline belt, a north-trending orogenic subprovince about 250 km (150 mi) long and 55 km (35 mi) wide in British Columbia and Washington characterized by the presence of Shuswap (Monashee Group) terrane, gneiss domes, and allied metamorphic rocks.The Okanogan gneiss dome and the Shuswap are believed to be products of metamorphism and deformation, in part at least, ofLate Cretaceous age. The development of the gneiss domes indicates that metamorphism within the Omineca reached sufficient intensity at depth to cause incipient anatexis and mobilization of the infrastructure. The discordance west of the gneiss dome and the Omineca belt is attributed to weak thermal metamorphism that developed above and west of the zone of more intense metamorphism.Ages from within the Okanogan gneiss dome range from Late Cretaceous (lead-uranium, zircon) to Eocene (K-Ar, biotite and muscovite; fission track, apatite). Their discordance is attributed to slow cooling after the climax of metamorphism in the Late Cretaceous.Orogeny and plutonism in the north-central area of Washington began almost simultaneously during the Late Triassic with folding of the Permian and Triassic bedded rocks and their intrusion by the 195-million-year-old Loomis pluton. Thermal events subsequent to Late Triassic cannot be linked with specific orogenic deformation, except for deformation associated with the hypothesized Late Cretaceous metamorphism and mobilization of the Shuswap and the gneiss domes within it. This event is temporally associated with westward-directed overthrusting along the Shuksan thrust to the west, attributed to mid-Cretaceous orogeny, and with eastwarddirected overthrusting to the east along the Cordilleran thrust belt, attributed to the Laramide orogeny, which ended in the Late Cretaceous or early Tertiary. The Shuswap terrane and associated gneiss domes appear to occupy the axial zone between convergent thrusts that show an aggregate crustal contraction of possibly 250 km (150 mi).These relations suggest a genetic model as follows: The Permian volcanic and pyroclastic rocks of the region were probably deposited in island-arc and back-arc basins located east of an east-dipping sub...
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