Basin Fluids 1
with eNd values of -6 to -9 and Jura-Cretaceous foreland basin sediments that show a bimodal pattern of heterogeneity . The negative values for Ordovician and older sediments are consistent with sediment derivation from "proximal" Canadian Shield whereas the signatures of the foreland strata reflect the relative proportions of detritus derived from erosion of pre-Devonian strata in the thrust belt, characterized by negative Nd signatures, and volcanic detritus, likely derived from the southern Coast Belt of southwest Canada or possibly terranes west of the Omineca Belt, characterized by relatively juvenile (positive) Nd signatures. However, the enigmatic and unexpected results are the compositions for the Devonian through Jurassic sedimentary rocks. A positive shift of 6 eNd units occurs in the Upper Devonian Sassenach Formation (eNd -9.5) and persists until foreland basin sedimentation in the Late Jurassic. The Devonian to Triassic (Whitehorse Formation) sNd values vary only slightly from eNd -9.5 to -6.7 and requires input from an isotopically juvenile and well-mixed source. A similar isotopic shift is seen in miogeoclinal strata from the northern Canada Cordillera although the magnitude of the shift and the timing is somewhat different as a consequence of the local effects of the Peace River Arch and eruption of REE-enriched volcanic rocks. Three scenarios are possible: 1) sediment input from juvenile fringing island arcs and back-arc basins such as Quesnellia and Slide Mountain Terrane, 2) sediment derivation from the Appalachian orogen of eastern North America and 3) sediment derivation from Ellesmerian sources in the Canadian Arctic. Option 1 is considered unlikely because typical arc magmas, such as basaltic andesite, have much lower Nd concentrations than continental crust (10 ppm vs. 35 ppm, respectively) and mass balance calculations suggest that more than 50% of the material in the samples we analyzed would have to be volcanic in origin. This is difficult to reconcile with quartzose composition of many of the units (Sassenach, Rocky Mountain Group etc.). Option 2 is tantalizing because of the isotopic similarity of the DevonianTriassic of the Cordillera with sedimentary strata of the Appalachian foreland (e.g. Ouachita trough) but is considered unlikely because of the persistence of the TransContinental Arch, a paleogeographic feature that would have blocked drainage westward from the Appalachians. Option 3 is considered the most likely and is supported by our recent analyses of preCarbonifer ous strata from the Innuitian foldbelt of the Canadian Arctic that demonstrate an isotopic link between sediments of western Canada and foreland basin sediments of the Innuitian. Cross-formational flow in west-central Alberta: implications for petroleum migration and entrapment.