THE INFLUENCE OF RIFT GEOMETRY ON CONTINENTAL SEDIMENT DISPERSAL PATTERNS

Lateral variations in Neoproterozoic-Cambrian sedimentary architecture and basin configuration along the western Laurentian continental margin include distinct changes in stratigraphic architecture, sediment thickness, facies distribution and subsidence patterns that suggest along-strike structural inhomogeneities that profoundly influenced continental sediment dispersal. Southern Idaho and Utah contain a thick Neoproterozoic to Ordovician miogeoclinal succession developed during protracted, multi-stage rifting. In east-central Idaho and Montana, Neoproterozoic to Cambrian strata are thin or absent, sediment dispersal patterns are complex, and the region contains recurrently emergent paleotopographic highs (Lemhi and Montania arches) and localized, structurally controlled 650-500 Ma alkalic magmatism. In southern British Columbia and Alberta, two distinct stages of rifting produced thick Neoproterozoic deep marine turbidite assemblages and Cambrian shallow water strata. These segments differ in the degree of crustal extension, subsidence, block rotation and crustal thickness, and are separated by northeast trending transfer zones.

The margin-parallel pattern of alternating thick miogeoclinal successions separated by thin, stratigraphically complex sedimentary successions is characteristic of asymmetric continental extension. Variations in rift geometry apparently control continental sediment dispersal patterns. Neoproterozoic-Cambrian strata deposited in thick miogeoclinal successions in the Utah and British Columbia segments contain distinct populations of Grenville (ca. 1100 Ma) zircon derived from eastern Laurentia. In contrast, the thin Neoproterozoic-Cambrian succession in Montana is entirely devoid of Grenville zircon, suggesting that an upper plate geometry led to an elevated topography that deflected continental-scale fluvial systems to the north and south. Conversely, Cambrian strata in Montana are dominated by a unimodal 1782 Ma zircon peak that is geographically restricted and difficult to resolve in adjacent miogeoclinal strata. Discrete sedimentary linkage between the different margin segments are indicated by the derivation of ~500 Ma zircon from shallow plutonic rocks in east-central Idaho in the Idaho/Utah miogeoclinal succession.