Geology of the SE 1/4 of the Twin Lakes Quadrangle, Ferry County, Washington PDF Download

Author: Corey Y. Fullmer
Publisher:
ISBN:
Category : Geology
Languages : en
Pages : 146

Get Book

Book Description
"The Covada Group, which is exposed in the Twin Lakes quadrangle, is composed of intensely folded and faulted eugeoclinal metasediments. Complexly interfingering and intergrading units of graywacke, phyllite, quartzite, black slate and greenstone are the main lithologies. Eastward dipping isoclinal folds and normal listric faults are the dominant structures. The limiting age of the Covada Group is uncertain, but part of it is considered Lower Ordovician. However, more intensely metamorphosed units may include rocks as young as Triassic, or possibly as old as Precambrian. Crustal shortening during Late Triassic to Middle Jurassic thrust Western Province eugeoclinal strata onto Eastern Province miogeoclinal rocks of comparable age. This east-west compressional phase severely deformed the rocks of the Covada Group by condensing them into a series of large-scale isoclinal folds that generally strike North 10° West to North 30° East. Dips are generally steep and often vertical. Evidence suggests that the Covada Group rocks are suspect in origin, and were subsequently swept against, and accreted to, the North America Cordilleran margin sometime during the Middle Mesozoic. In the Western Province, greenschist-facies Covada Group rocks are separated from sillimanite-facies metamorphic rocks of the Kettle Gneiss dome by a low-angle fault. Evidence suggests that the core of the Kettle dome was emplaced by Post-Eocene regional doming. The low-angle fault appears to be the result of either folding of an older flat thrust (similar to the type that brought the eugeoclinal block in contact with the miogeoclinal block), or the eugeoclinal cover rocks became decoupled from the rising Kettle dome, and were transported down slope, in one consistent direction to the margin, as a result of gravity sliding"--Document.