GEOLOGICAL SURVEY OF CANADA OPEN FILE 5487 Geology of Quiet Lake and Finlayson Lake map areas, south-central Yukon – An early interpretation of bedrock stratigraphy and structure D.J. Tempelman-Kluit 2012 GEOLOGICAL SURVEY OF CANADA OPEN FILE 5487 Geology of Quiet Lake and Finlayson Lake map areas, south-central Yukon – An early interpretation of bedrock stratigraphy and structure D.J. Tempelman-Kluit 2012 ©Her Majesty the Queen in Right of Canada 2012 doi:10.4095/291931 This publication is available for free download through GEOSCAN (http://geoscan.ess.nrcan.gc.ca/). Recommended citation Tempelman-Kluit, D.J., 2012. Geology of Quiet Lake and Finlayson Lake map areas, south-central Yukon – An early interpretation of bedrock stratigraphy and structure; Geological Survey of Canada, Open File 5487, doi:10.4095/291931 Publications in this series have not been edited; they are released as submitted by the author. ii iii PREFACE Quiet Lake and Finlayson Lake map areas include the heart of the Pelly Mountains. They provide excellent bedrock exposure of the north-central Canadian part of the orogen called the North American Cordillera. They comprise late Proterozoic to early Mesozoic platform and outer shelf strata of ancient North America that were shortened during Mesozoic folding and thrust faulting. These strata are structurally overlain by thrust slices of metamorphosed Paleozoic sedimentary, volcanic and gneissic rocks that originated far from this continent. Both the in-place and overthrust rocks were intruded by plutons of late Mesozoic age. The map areas are bisected diagonally by the Tintina Trench which is underlain by a dextral strike-slip fault with 450 km of post-Cretaceous displacement. This Open File report presents a general description of the regional stratigraphy of the two map areas and an interpretation of structure, metamorphism and igneous history as they were understood in the early 1980s. It builds upon field work dating back to Operation Pelly, a large and wide-ranging reconnaissance survey in central Yukon begun by the Geological Survey of Canada in 1958. The field observations upon which the stratigraphic units are based were principally made between 1973 and 1976. The report describes principal structures and develops a comprehensive structural interpretation with deductions of the relative timing of faults. The mineral deposits are placed in a regional tectonic framework. It is a classic report that extends from field observations to propose correlations of stratigraphy, structural models and imaginative geological hypotheses which can be tested by further geological work. These include the concept of discrete packages of platform stratigraphy overlapped on low-angle thrust faults, and the recognition of internally deformed sheets of allochthonous sedimentary, volcanic and plutonic rock overlying the platform strata. The latter idea resulted in a widely read and acclaimed paper (Tempelman-Kluit, 1979) This report was written in the early 1980s in the traditional „memoir‟ form to accompany 1:250 000 scale maps. At the time of writing the techniques for isotopic dating limited the interpretation of metamorphic rock units and for them this report is no longer relevant. Nevertheless some aspects of the regional tectonic model expounded here have served for 25 years, although many details have contemporary explanation. Following discovery of a copper-zinc sulphide deposits south of Finlayson Lake (1992), a boom in mineral exploration and revision mapping improved the stratigraphic framework northeast of Tintina fault (see Murphy et al., 2006 and references therein). Southwest of Tintina, the regional mapping and descriptions of stratigraphic units (primarily Pelly-Cassiar platform and slope facies) herein remain current. The latter are the prime motive for publishing this unfinished work – to make these descriptions available to subsequent workers. Most of Quiet Lake map area has not seen systematic bedrock mapping during the last 30 years. Steve Irwin Subdivision head Geological Survey of Canada, Vancouver iv GEOLOGY OF QUIET LAKE AND FINLAYSON LAKE (105F, 105G) MAP AREAS, SOUTH-CENTRAL YUKON Abstract The Pelly Mountains in central Yukon expose a cross-section through part of the collision orogen that is the Canadian Cordillera. Elements of this cross-section are the pre-tectonic autochthonous succession, klippen of sheared and metamorphosed allochthonous rocks, thrusts that repeat the autochthonous succession, late tectonic regional metamorphic rocks and associated granite batholiths, and the Tintina fault, a late tectonic dextral strike-slip fault. Late Proterozoic through Triassic shelf strata, six to eight kilometres thick, comprise the autochthonous sequence. They are divided into four main divisions bounded by unconformities or a depositional hiatus. Oldest are Eocambrian and Lower Cambrian shale and siltstone, capped by an argillaceous limestone, which together form the Ketza group. Unconformably above it are Upper Cambrian and Ordovician slate, phyllite and alkaline basalt of the Kechika Group, and Silurian and Devonian siltstone, dolostone and quartz sandstone, of the Askin group. Profound and rapid facies variation characterizes this second division. Also part of this division is the shale-dominated Harvey group, which is time-equivalent to the Kechika and Askin groups. Unconformably above are Upper Devonian and Mississippian black slate and marine felsic volcanic rocks - the Seagull group. The fourth division is thin and restricted in distribution. It includes a Permian and an Upper Triassic siltstone separated by a disconformity. The autochthonous succession extends to, and is like that in, the Cassiar Mountains; it has equivalent units in the northern Rocky Mountains. Broadly it is equivalent to the outer shelf of the North American miogeoclinal wedge. Three assemblages of ductile deformed rocks are thrust over the "in place" succession from the southwest. Although each assemblage is dismembered so that its internal stratigraphy is unknown, the three are not mixed, and exhibit a consistent structural order. Lowest is a mylonite schist, that represents Triassic (and older?) immature clastic rocks and intermediate volcanics. Next is an assemblage of Late Paleozoic amphibolite and serpentinite with flaser limestone. Highest are granodiorite gneiss and schist, the sheared and metamorphosed remnant of Paleozoic intrusive rocks. K-Ar ages show that all three assemblages were sheared and metamorphosed during the Late Triassic and Early Jurassic. Four northeast directed thrusts repeat the "in place" beds beneath, and northeast of, the transported strata. Cumulative shortening is on the order of 100 kilometres. Each thrust carries a slice two or three kilometres thick and is itself deformed by newer thrusts, folds and tear faults. The thrusts, interpreted as splays of one basal detachment, formed in sequence from southwest to northeast during the Late Jurassic and Early Cretaceous. On the southwest the structurally imbricated "in place" strata and the transported rocks above them, were regionally metamorphosed about the late Early Cretaceous to form the Big Salmon and Mink complexes. At the same time partial melting produced granites like the Quiet Lake, Nisutlin and Big Salmon batholiths. The metamorphic rocks are exposed in large structural culminations centred roughly on the batholiths; these fold early thrusts, giving a structural relief near eight kilometres. Some granites may be cut by thrusts below and the metamorphic culminations may be severed by the basal detachment. Tintina fault is the locus of 450 kilometres of dextral slip as shown by offset of the autochthonous succession. Movement followed intrusion of most or all of the late tectonic granites. Movement may have been ceased by Early Eocene when fluvial conglomerate was laid down along the fault. Because its slip coincides with shortening in the Selwyn and Mackenzie mountains the Tintina is considered to be a listric branch of the same detachment surface on which shortening was accomplished. It is viewed as a giant tear fault in the detached slab. A model in which the Pelly Mountains are the product of Middle Jurassic oblique collision of a volcanic arc with the then-western margin of ancient North America fits these data. Sheared metamorphic rocks, which are interpreted as the arc‟s subduction complex, were obducted in the Late Jurassic. Cretaceous imbrication and dextral translation of the autochthonous rocks are responses to obduction, while metamorphism and plutonism reflect heating of the overridden and imbricated "in place" slab. v FORWARD This final report from fieldwork for the Quiet Lake-Finlayson reconnaissance mapping project has been changed from its original intended 'memoir' format to an 'open file'. The open-file series allows publication of this nearly finished manuscript with the author's original interpretations. A casualty of the change is that stratigraphic unit descriptions which were intended to formalize 12 new formations are here presented as informal formations (because an open file is not recognized as a formal publication by the North American Stratigraphic Code). Despite a long delay in publication this final report contributes new information to the understanding of Yukon geology. The five maps at 1:50 000 scale and stratigraphic descriptions of units comprising the Pelly-Cassiar platform in Yukon were not previously available. The description of structures from field observations supported by annotated