SEGMENTATION OF THE WESTERN LAURENTIAN CONTINENTAL MARGIN: EVIDENCE FOR ACTIVE TECTONISM IN THE NEOPROTEROZOIC-CAMBRIAN ‘MIOGEOCLINE’

Major variations in Neoproterozoic to Cambrian stratigraphy along the western Laurentian continental margin (Norris and Price, 1966) require along strike structural inhomogeneties during Neoproterozoic extension. Neoproterozoic to Cambrian stratigraphic and structural relationships in western Montana and central Idaho require the continental margin was tectonically active. Critical observations include: 1) Cambrian Flathead sandstone variously overlies both upper Belt and middle Belt Supergroup strata, requiring local removal of of 5-8 km of strata prior to Cambrian deposition; 2) cobble to boulder fanglomerate in the Flathead sandstone at Ermont, MT requires significant local topographic relief in Cambrian time; 3) the presence of a unimodal population of 500 Ma zircon in 500 Ma Worm Creek quartzite requires rapid uplift and erosion of the Cambrian Beaverhead plutonic complex in NC Idaho; 4) alkali magmatism in central Idaho at 665-650 Ma and 500-485 Ma (Big Creek-Beaverhead belt) indicates long-lived magmatism; 5) structural relations of the Poison Creek thrust along the Salmon River in C Idaho indicate down-to-west Neoproterozoic extension; 6) detrital zircon from the Cambrian Flathead sandstone yield Neoproterozoic to Cambrian (U-Th)/He ages, suggesting significant exhumation along the continental margin. In addition, the Flathead sandstone is characterized by a remarkably uniform, regionally consistent ca. 1780 Ma unimodal detrital zircon signature and a distinct absence of zircon derived from underlying Belt Supergroup, requiring rapid uplift and erosion of an unknown Paleoproterozoic source.

These structural and stratigraphic relationships suggest the northern Rocky Mountains north of the modern Snake River Plain acted as an independent structural domain from subsiding areas to the south and north during Neoproterozoic rifting, and was characterized by episodic uplift, intermittent sedimentation and sporadic magmatism. These features are quite distinct from the classic miogeoclinal successions in southern Alberta and British Columbia and southern Idaho and Utah. This is consistent with the asymmetric extension postulated by Lund et al (2010).