The massive 53.6 Ma Flat Creek granitic pluton of the Nisling Plutonic Suite intrudes flat-lying volcanic rocks of the 70 Ma Carmacks Group in the Stikine Terrane of the Intermontane Belt in the Yukon. Specimens ( n = 334) from 22 sites in granite from the ~100 km 2 pluton plus 3 sites in cross-cutting 51.1 Ma Eocene andesitic dikes were tested using alternating field and thermal step demagnetization and magnetic susceptibility and saturation isothermal measurements. Magnetite was the sole important characteristic remanent magnetization (ChRM) carrier. Most granitic and andesitic specimens carried a lower temperature and coercivity normal-polarity A N ChRM and an antiparallel higher temperature and coercivity reversed A R component, but some specimens of both rock types carried just A R and some granitic specimens carried just A N components. Combining the A N and A R directions, the granite pluton yielded a mean direction of declination (D) = 165.2°, inclination (I) = −76.8°, (number of sites [N] = 31, radius of cone of 95% confidence [α 95 ] = 2.2°, precision parameter [k] = 139) and the dikes a mean D = 158.0°, I = −79.5°, (N = 3, α 95 = 6.0°, k = 423). Paleomagnetic contact tests proved inconclusive because of the contemporaneous and dual polarity remanence of the specimens. The pluton’s paleomagnetic pole indicates a nonsignificant northward displacement of 0.8° ± 4.6° and a significant clockwise rotation of 14° ± 10° for the Stikine Terrane relative to the North American craton after ca. 54 Ma. Regression analysis of ≤;54 Ma paleomagnetic motion estimates for all Intermontane terranes against time also shows nonsignificant translation with a significant rotation rate of 0.34° ± 0.11°/m.y. This implies that the Intermontane terranes since ca. 54 Ma have behaved as a quasi-coherent upper crustal plate that rotated atop a North American cratonic lower crust about a proximal near-vertical axis. It is speculated that the rotation was marked by westward extension in southern British Columbia and by eastward compression in the northern Cordillera, together amounting to ~550 ± 160 km of displacement orthogonal to the stable cratonic margin. The compression component in the north, driven by Pacific plate subduction and collision of the Yakutat terrane, is the suggested cause of arcuate orogenic uplift in the Mackenzie Mountains.

Abstract Deposition of the Sauk megasequence of northwestern Canada, from Peace River in British Columbia (north of lat. 56°N) to the Beaufort Sea in the Northwest Territories and Yukon Territory (lat. ∼70°N), occurred along a complex segment of the rifted western margin of Laurentia. This synthesis describes the stratigraphy, depositional history, tectonic setting, and resource potential associated with the development of the Cambrian-Ordovician great American carbonate bank. There have been few detailed studies of the Sauk megasequence in these remote areas of Canada, and much information on platform and equivalent facies remains at a reconnaissance scale. Platform rocks occur as relatively flat-lying strata of the Interior Platform that thin eastward onto the Canadian shield and correlate westward to strata within the deformed belt of the Canadian Cordillera. It is generally accepted that the Sauk megasequence was deposited along the subsiding rifted margin of Laurentia because facies pass from shallow-marine settings to deeper water slope and basinal settings from east to west. Abrupt facies changes, thickness changes, and intercalated volcanic rocks in most successions suggest a setting more complex than a simple passive margin during the development of the Cambrian-Ordovician great American carbonate bank. During Sauk megasequence deposition, positive and negative tectonic features controlled deposition along the continental margin and are represented by distinct present-day physiographic regions. The northern Rocky Mountains region lies between 56 and 60°N and is included in this summary because its geologic history is more similar to regions to the north, instead of to the southern Cordillera. North of 60°N latitude, five main regions span the vast northern Canadian mainland sedimentary basin: Beaufort-Mackenzie, Interior Plains, Mackenzie Arc, Northern Yukon, and Selwyn-Cordillera. The lower Paleozoic succession is poorly known in the “Beaufort-Mackenzie Basin,” a term that typically refers only to uppermost Cretaceous to Holocene Mackenzie Delta sediments. The northern Rocky Mountains region lies north of Peace-Athabasca arch. The arch was a positive area and forms the southern limit to the Macdonald platform and the Kechika trough, a graben system that flanked the platform to the west. The Kechika trough accumulated mainly shallow-water siliciclastic sediments until the Late Cambrian, and Sauk I and II super-sequences are characterized by complex facies changes. Carbonate deposition was prominent in the Middle Cambrian when bioherms developed on tilted fault blocks in the western part of the Kechika trough. The uppermost Cambrian–Ordovician Sauk III stratigraphy records deposition during postrift passive-margin thermal subsidence; however, during the middle Sauk III supersequence, a distinct east-to-west platform-to-basin setting evolved in response to renewed extension along the margin. The Interior Plains region lies mainly in the Northwest Territories and contains a subsurface record of the Mackenzie platform. In the Cambrian, the area was an epicontinental marine basin consisting of several depocenters that opened to the south and were segmented by positive high elements such as the Mackenzie arch. Lower and Middle Cambrian units are siliciclastic dominated and discontinuous because of the extent of paleogeographic highs. Extensive Sauk III platform carbonates of the Mackenzie platform were deposited beginning in the latest Cambrian. The Mackenzie Arc region is the eastern part of the foreland belt in Yukon and the Northwest Territories. It offers a more continuous stratigraphic record compared with the Interior Plains, but abrupt facies changes from north to south suggest phases of extension. The region contains deformed strata of the Mackenzie platform, the Mackenzie arch, and the eastern part of Selwyn Basin, which extended into the platform as Misty Creek embayment. The Sauk I supersequence is siliciclastic dominated, and carbonates are present only in distal slope settings. Sauk II and III supersequences are carbonate dominated, representing expansion of the Macdonald platform across the Mackenzie arch. Prominent slope facies in the Sauk II and III supersequences suggest an abrupt platform-to-basin setting that also accumulated submarine volcanics during phases of extension. The Selwyn-Cordilleran region extends from southwestern Yukon and northeastern British Columbia to the international border with Alaska. It is an area of deformed incompetent strata that were deposited in the Selwyn Basin, west of the Mackenzie Arc. The region contains autochthonous North American strata east of the Tintina Fault, and parautochthonous and al-lochthonous terranes west of the fault. One parautochthonous terrane thought to be a displaced slice of the North American margin is the Cassiar terrane or Cassiar platform. The Sauk mega-sequence of Selwyn Basin is generally similar to that of the Mackenzie Arc region, although the Sauk I and II supersequences contain additional units of distal basinal siliciclastic facies. The northern Yukon region contains several mountain belts and intermontane basins or lowlands that contain a succession with overall similarities to the North American Sauk mega-sequence, although the record is poorly understood in this remote area. The main tectonic element is a broad area of platform carbonates, the Yukon stable block, which is bordered to the east by the Richardson trough and to the south by the Selwyn Basin. The lower Sauk I supersequence is absent because of uplift of the Ogilvie arch. Block faulting influenced a mostly siliciclastic-dominated Sauk II supersequence, although deposition of carbonate resumed during deposition of the upper Sauk II supersequence and persisted throughout the Sauk III supersequence deposition. Sauk megasequence carbonates that may have been part of the great American carbonate bank underwent abrupt lateral facies changes indicative of syndepositional extension during the Early to Middle Cambrian, and again in the Middle Ordovician. These tectonic complexities, coupled with the lack of detailed studies, preclude a comprehensive sequence-stratigraphic analysis and determination of relative or eustatic sea level changes. The base of the Sauk megasequence is a prominent sub-Cambrian unconformity in most regions; however, eastward (toward the craton), this surface represents a merger of several unconformities, and westward (basinward), various Cambrian units overlie Proterozoic rocks. The base of the Tippecanoe megasequence, although muted and less clear in thicker shelf and offshelf successions, lies in the Middle or Upper Ordovician. The Sauk I supersequence is dominated by shallow-water siliciclastic deposits, many of which are localized in extent, with rare carbonate in distal settings such as the Selwyn Basin. Carbonate units of the middle to upper Sauk I supersequence were most likely deposited basinward of a carbonate platform. The best record of Sauk II carbonate deposition occurs along the margins of the Selwyn Basin or is represented bynarrow tracts associated with block faulting. The basal parts of the Sauk III supersequence are poorly known, but significant Upper Cambrian terrigenous siliciclastic rocks in some regions suggest a phase of uplift. The Sauk III supersequence is otherwise characterized by widespread deposition of carbonate platform and equivalent slope and basin deposits at the edge of platforms during widespread transgression and continued margin subsidence. The vast area of northwestern Canada has both hydrocarbon and mineral resource potential in rocks of the Sauk megasequence, although exploration is limited by remoteness and lack of infrastructure. Early Paleozoic intracratonic and platform-to-basin settings of the margin accumulated both potential source and reservoir rocks for fluid hydrocarbons. The Colville Hills area of the Interior Plains region has the most significant discovery of gas and some condensate within Cambrian strata. This basal Cambrian siliciclastic play potentially exists across the Interior Plains. Porous reservoir facies of Cambrian–Ordovician platform carbonates constitute another potential play, and abundant source rocks are present within equivalent basinal facies. Although mountainous regions have little hydrocarbon potential because of their geologic history, some Cambrian–Ordovician rocks host mineralized zones, particularly in the Selwyn-Cordillera region such as the metallogenic belt of Zn-Pb-Ag sedimentary exhalative deposits of the Anvil district along the western margin of the Selwyn Basin.

Abstract Triassic rocks of the Interior Platform and eastern Cordillera of Canada have long been recognized as an interesting and variable sequence of marine strata occupying an elongate belt that extends from the United States border on the south to 69°N latitude and Beaufort Sea on the north (Fig. 4G.1, 4G.2). Triassic deposits are best developed in Western Canada Basin of British Columbia and Alberta, where they occupy three main physiographic provinces, Rocky Mountains on the west, Rocky Mountain Foothills, and Interior Plains to the east. Triassic rocks also occur in Liard River area of southern Yukon Territory and within northern Yukon Territory and District of Mackenzie in the British, Barn, Richardson, Selwyn, Wernecke, and northwestern Ogilvie Mountains. However, because much more published information on Triassic rocks in Western Canada Basin is available, and because of the greater development and economic importance of the system there, most of this report will be directed toward the region south of latitude 60°N. Triassic rocks of the Rocky Mountains, Foothills, and Interior Plains comprise over 1200 m (Fig. 4G.1) of westward-thickening, siliciclastic and carbonate rocks and lesser amounts of evaporites. Contained marine faunas range in age from Griesbachian to Norian. The Triassic rocks of northern Yukon Territory display similar lithofacies but lack evaporites. The strata form a marine wedge deposited along a topographically low, tectonically stable continental shelf and shoreline, a continuation of Permian conditions at the passive western margin of the North American Craton. They form part of what is referred to as

Abstract The Carboniferous System in Western Canada Basin (Fig. 4E.1-4E.3) is a thick succesion of strata deposited on the downwarped and downfaulted western margin of the ancestral North American plate, the central to western cratonic platform, and southern Yukon Fold Belt. This succession, representing the upper Kaskaskia sequenceand lower Absaroka sequence of Sloss (1963), comprises two main lithofacies assemblages. The lower assemblage is basinal shale and generally thickens southwestward or basinward (see Fig. 4 E . 9 - 4 E . 1 3) . Upward and northeastward, it passe into an upper assemblage of platform and ramp carbonates (Fig. 4E.4, 4E.5, 4E.9-4E.13) and sandstone-dominated siliciclastic facies, deposited in deep-water slope to continental settings. Both assemblages consist of numerous formations, some separated by regional disconformities. Subaerial erosion during the Late Carboniferous, Permian, and subsequent periods removed large parts of the succession, particularly in the Interior Plains, the region west of the Rocky Mountain Front Ranges, and the Cordillera between southwestern District of Mackenzie and northern Yukon Territory. Where the Carboniferous remains, it is generally unconformably overlain byeither Permian or Mesozoic strata. Carboniferous formations are preserved in twomain regions. The southern one, which includes much of the eastern Cordillera andsouthern to western Interior Plains, extends from southwestern Manitoba to southwestern District of Mackenzie. The northern area includes the eastern Cordillera ofnorthern Yukon Territory and northwestern District of Mackenzie (Fig. 4E.1). Betweenthese regions, erosional remnants are present in the Mackenzie and Selwyn Mountains of east-central Yukon and west-central District of Mackenzie.

Approximately 3,000 Ar, Sr, and Pb isotopic age determinations for Canadian Cordilleran rocks have been cataloged, categorized as to reliability and significance, and plotted on histograms, distribution maps for different time intervals, and space-time plots to show the magmatic evolution in this 2,300-km portion of the Circum-Pacific Mobile Belt. The history revealed is episodic, with stable distribution patterns within episodes and distinct lulls and changes in distribution between the episodes. From 230 to 214 Ma (during Late Triassic time), extensive mafic volcanism occurred in the Wrangell, Quesnel, and Stikine terranes. Volcanic-related ultramafic complexes are found scattered through the two latter terranes. Large calc-alkaline granitic plutons are known only in a belt crossing Stikinia in northern British Columbia. At the same time, blueschists formed in the Cache Creek accretion wedge. From 214 to 200 Ma (end of Triassic and part of Early Jurassic time), Early to Middle Jurassic arc magmatism began in Wrangellia and in the northern Quesnel, Stikine, and Yukon terranes. A distinct magmatic event is recognizable only in southern Quesnellia. Magmatism was absent on the North American craton. The Cache Creek and Quesnel terranes were definitely linked, Stikine and Cache Creek terranes were probably linked, and a regional metamorphic episode was completed in the Yukon Terrane by this time. From 200 to 155 Ma (late Early to early Late Jurassic time), magmatism was extensive in the Wrangell, Quesnel, Stikine, and Yukon terranes. Magmatism over-lapped into North America only east of southern Quesnellia after about 180 Ma. By the middle of this time interval, the southern Quesnel-Slide Mountain-North America linkage was complete, and major deformation and metamorphism had affected the Omineca Belt in British Columbia. Early to Middle Jurassic magmatism in southern Wrangellia (Vancouver Island) is distinctly older than the Middle to Late Jurassic magmatism that occurred in central Wrangellia (Queen Charlotte Islands). From 155 to 140 Ma (during Late Jurassic time), a few last-gasp plutons of the late Early to early Late Jurassic episode and other rocks with partially reset 155- to 145-Ma dates occur in the Wrangell, Quesnel, and Stikine terranes. Late Jurassic magmatism (160 to 140 Ma) occurred in the Alexander Terrane (Saint Elias region). From 145 to 138 Ma (latest Jurassic and beginning of Early Cretaceous time), plutonism occurred in the Endako area of central British Columbia (Francois Lake suite) but is virtually unknown elsewhere. From 135 to 125 Ma (during Early Cretaceous time), there was a magmatic lull of major significance present throughout western North America. From 110 to 90 Ma (middle Cretaceous time), widespread plutonism occurred across all terranes. Dual culminations are evident: the Coast Plutonic and Ominica belts. Before this time all sutures except those outboard of Wrangellia had been closed. From 80 to 70 Ma (during Late Cretaceous time), a narrow, sinuous belt of magmatism persisted, mostly in the southeastern Coast Plutonic Belt, southwestern Yukon Territory, and scattered across the Skeena and Stikine arches. From 70 to 60 Ma (latest Cretaceous to Paleocene time), a distinct lull in magmatism occurred. Rare plutons of this time interval are known in the Coast Plutonic Belt, on the Skeena Arch, and in the southern Intermontane Belt. From 55 to 45 Ma (latest Paleocene to Middle Eocene time), widespread and voluminous magmatism occurred in all terranes. The early Cenozoic volcanic front crossed the Coast Plutonic Complex from its east side in the south to its west side in the north. Associated thermal and tectonic effects were strong even into the Omineca Belt, producing large reset metamorphic areas in the Coast and Omineca belts. This was a short-lived event, synchronous from southern British Columbia through the Yukon Territory. West of the volcanic front, offshore of Wrangellia, Metchosin volcano growth was underway at this time. Late in this time interval, the 50?–45–36-Ma Catface–Leech River event(s) of southern Wrangellia occurred. There is also time overlap with a diffuse Massett magmatic event in the Queen Charlotte Islands, and with Baranoff Island and Yakutat–Saint Elias region magmatism. Initial 87 Sr/ 86 Sr ratios and petrographic characteristics of Canadian Cordilleran igneous rocks are reviewed in the time frame just described. These reflect the nature of underlying crust, contemporaneous lithosphere thickness, and distance from the subduction zone. Comparisons with other parts of the Circum-Pacific Magmatic Belt shows both out-of-phase magmatism (Japan and southwestern Alaska) and perfect matching of some episodes (Sierra Nevada). Major magmatic episodes correspond to times of increased westward motion of North America with respect to hot spots or to times of increased convergence between western North America and the Farallon Plate.

Abstract The Tatonduk River section is located in the Nation Arch, a wedge-shaped area in eastern Alaska bounded by the Alas-ka-Yukon Territory border and the Dawson, Tintina, and Glenn Creek fault complexes (Fig. 1). The Yukon River flows north through the western portion of this wedge. The Tatonduk River flows west from the Yukon Territory, joining the Yukon River 13 mi (21 km) north of the village of Eagle. Scheduled commercial air service is available to Eagle from Fairbanks, and there is road access to Eagle from May through September. Access to the Tatonduk River section is via riverboat or helicopter. There are no established trails into or within the Taonduk River valley. The high gradient and boulder bed of the river require a very shallow-draft boat with sufficient power to maintain control in the strong current. Shoaling near the mouth and multiple channels upstream demand skill in identification of the main channel. This region falls partly within the boundaries of Yukon-Charley National Rivers Preserve, supervised by National Park Service personnel based in Eagle, and by the Doyon Regional Corporation, based in Fairbanks. Current land management policies allow access to the Tatonduk River valley; however, written permission is advisable for fossil collection, and helicopter usage in specific areas and periods is restricted.

Abstract The Tatonduk River section is located in the Nation Arch, a wedge-shaped area in eastern Alaska bounded by the Alas-ka-Yukon Territory border and the Dawson, Tintina, and Glenn Creek fault complexes (Fig. 1). The Yukon River flows north through the western portion of this wedge. The Tatonduk River flows west from the Yukon Territory, joining the Yukon River 13 mi (21 km) north of the village of Eagle. Scheduled commercial air service is available to Eagle from Fairbanks, and there is road access to Eagle from May through September. Access to the Tatonduk River section is via riverboat or helicopter. There are no established trails into or within the Taonduk River valley. The high gradient and boulder bed of the river require a very shallow-draft boat with sufficient power to maintain control in the strong current. Shoaling near the mouth and multiple channels upstream demand skill in identification of the main channel. This region falls partly within the boundaries of Yukon-Charley National Rivers Preserve, supervised by National Park Service personnel based in Eagle, and by the Doyon Regional Corporation, based in Fairbanks. Current land management policies allow access to the Tatonduk River valley; however, written permission is advisable for fossil collection, and helicopter usage in specific areas and periods is restricted.