Overturned Nicola and Ashcroft strata and their relation to the Cache Creek Group, Southwestern Intermontane Belt, British Columbia

Abstract

The Lower Jurassic, Ashcroft Formation contains a thick section of carbonaceous marine shale and a few graded sandstones. Along the south and east margins of the Ashcroft Basin, Ashcroft strata rest unconformably on calc-alkaline and alkaline volcanic flows and sediments of the Upper Triassic, Nicola Group. On the west margin Nicola and Ashcroft strata lie against mélange of the Cache Creek Group. This contact is faulted in some places, but it may be a depositional unconformity elsewhere.South of Cache Creek village, overturned allochthons of Nicola strata were placed on top of Ashcroft beds in Early Jurassic time before Ashcroft sediments were lithified. Turbidity currents flowed southeast contemporaneous with sliding or thrusting of allochthons.Near the Guichon Creek Batholith, the Ashcroft Formation contains a disconformity that separates Sinemurian–Pliensbachian from Callovian strata. However, in the western part of the Ashcroft basin strata appear continuous from Sinemurian–Pliensbachian to Callovian. The Guichon Creek Batholith was emplaced into Nicola strata along the eastern edge of the Ashcroft Basin about 200 Ma ago (late Sinemurian*) and was quickly unroofed to provide granitic debris to the basin.The Ashcroft Basin appears to have been an early Mesozoic outer arc basin. It formed seaward of calc-alkaline magmatism and landward of and possibly on top of a mélange. Middle or Late Triassic radiolaria found in the Cache Creek show that deformation of the mélange took place as late as Late Triassic time. Arc-directed thrusting and sliding may be gravity processes due to elevation of the outer arc ridge during subduction.

Logan Medallist 1. Seeking the Suture: The Coast-Cascade Conundrum

“…As noted earlier, the southernmost Cache Creek rocks were thrust eastward over Quesnellia in the Late Jurassic ( Fig. 3; Travers 1978), and in a probably related but structurally deeper event, westernmost Quesnellia was deformed and intruded by Late Jurassic (157-148 Ma) tonalite gneiss Price and Monger 2003).…”

“…Furthermore, the Bridge River terrane is at least as young as Callovian (~164 Ma) in chert-rich facies and may grade into clastic facies as young as earliest Cretaceous (≥ 130 Ma; Mahoney and Journeay 1993;Cordey 1996), whereas no strata younger than latest Early Jurassic (~180 Ma) are known from the Cache Creek terrane. Although both the Bridge River and Cache Creek terranes probably originated in Panthalassa, the former evidently faced open ocean until trapped in the Early Cretaceous (≤130 Ma) behind the arc rocks in the southwestern Coast Mountains, whereas in northern British Columbia the Cache Creek terrane was thrust southwestward over Stikinia in the earliest Middle Jurassic (~174 Ma) (Mihalynuk et al 2004), and in southern British Columbia was thrust eastward over Quesnellia probably in the Late Jurassic (≤ 160 Ma) (Travers 1978).…”

Geophysical interpretation of the gneiss terrane of northern Washington and southern British Columbia, and its implications for uranium exploration

“…Chronologically equivalent (in part) rocks immediately northwest of the study area, and perhaps extending into its northwestern corner in Canada, are part of the Cache Creek Group. These rocks were previously believed to be the northwestern counterpart of the Anarchist Group, Mount Roberts Formation, and Covada Group of the study area (Fox and others, 1977), but recent descriptions and interpretations indicate that the Cache Creek Group in its type area near Kamloops, British Columbia, is stratally disrupted melange (Monger, 1977(Monger, , p. 1846Travers, 1978), containing lithologic elements of Early Mississippian to Late Triassic age, and is locally metamorphosed within the blueschist facies (Monger, 1977(Monger, , p. 1844. These features suggest that the Cache Creek rocks may mark the position of a formerly convergent plate boundary.…”

Mesozoic granitoid magmatism in southeast British Columbia: Implications for the origin of granitoid belts in the North American Cordillera

“…Intrusion of the Middle Jurassic granitoids overlaps the final stages of amalgamation of superterrane I (Figure 8). Amalgamation commenced in the Late Triassic with the eastward subduction of oceanic crust, now represented by the Cache Creek terrane, beneath the Quesnel terrane [Travers, 1978]. Imbrication of this subduction zone in the Early to Middle Jurassic resulted in the obduction of the Cache Creek terrane and the accretion of the Stikine terrane.…”