Margot McMechan

Reply to Discussion: Vreeland Diamictites - Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia

Hoffman et al. (1998b) developed a modified ‘snowball’ Earth hypothesis to explain the association of Neoproterozoic low-latitude glaciation with the deposition of “cap carbonate” rocks bearing highly depleted carbon isotopic values. According to this hypothesis, a runaway albedo feedback caused the world’s oceans to freeze for millions of years. During this time interval, the continental ice cover was thought to have become thin and patchy because of the virtual elimination of the hydrological cycle (Kirschvink, 1992; Hoffman et al., 1998b). The Vreeland diamictites record vigorous erosion by an extensive ice sheet at or near the pressure melting point (McMechan, 2000). Their deposition and that of several other thick (>800 m) Neoproterozoic glaciogenic deposits, including the Mineral Fork diamictites in Utah (Christie-Blick, 1983), Rapitan diamictites in northwestern Canada (Yeo, 1981), the Sturtian diamictites in South Australia (Young and Gostin, 1989), the Hangeerqiaoke diamictites in northwest China (Wang et al., 1981), the Port Askaig diamictites in southwest Scotland (Spencer, 1971), and the Chuos diamictites in northwest Namibia (Hoffman et al., 1998a), show that substantial areas of vigorous contential glaciation occurred in the Neoproterozic. The existence of these deposits is incompatible with the hypothesized thin and patchy continental ice cover of the ‘snowball’ Earth model (Christie-Blick et al., 1999; McMechan, 2000). Rather than taking the sedimentary record as evidence of warmer climates and partially open oceans, Hoffman (in Hoffman and Schrag, …

Structural Geometry in the Carbon Creek Area of the Rocky Mountain Fold and Thrust Belt, Northeastern British Columbia

ABSTRACT The integration of seismic, surface and well data from the foothills of the Carbon Creek area, northeastern British Columbia show that the Foothills are thin-skinned with structures having formed above a major detachment in the Upper Devonian-Carboniferous Besa River Formation. Middle Devonian and older strata underlying the Besa River detachment were not deformed during Cretaceous-Eocene contractional deformation. Detachment folds, thrust faults and related faultbend folds and tip folds dominate the structural style of Carboniferous to Triassic strata with local detachments within the Upper Carboniferous, lower Triassic, and upper Triassic indicated by structural discordance. A major regional detachment in the Jurassic Fernie Formation separates large-scale detachment folds and local duplexes in overlying strata from underlying structures. The duplexes have resulted in excess shortening of Fernie Formation and Jurassic-Cretaceous Minnes Group strata relative to overlying strata that is most reasonably balanced by two local triangle zones or tectonic wedges rather than one more regional zone. A thick sequence (up to 6 km) of pre-Windermere Proterozoic sediments, underlies the western extension of the Peace River Arch in the Foothills of the Carbon Creek area. The southeastward disappearance of the older Proterozic succession probably reflects a transverse step in the older Proterozoic sedimentary basin that likely coincides with the Hay River Fault. The Sukunka Uplift, an outboard high to the Peace River Embayment, had several hundred metres of east-side down motion in the Carbon Creek area during the late Paleozoic. The change from thick-skinned reactivated normal faults (northern Foothills) to thin-skinned reactivated normal faults at the north end of the Sukunka Uplift suggests the orientation of late Paleozoic normal faults may have influenced the deep structural style of the fold and thrust belt. End_Page 407------------------------