Originally prepared for the GSA Thompson Field Forum that ran from Terrace to Prince Rupert, British Columbia, this guide describes the geology along the Skeena River transect of the Coast Mountains batholith, the largest Cordilleran batholith of western North America and one of the largest continental-margin batholiths in the world. The last guide to this area was published in 1983 and this new volume is the only modern overview of the last decades of work. The authors use the transect as a basis to examine the growth of the Coast Mountains batholith as a whole, emphasizing commonalities and variations with the batholith and how these traits may reflect magmatic processes that create convergent-margin batholiths.

Abstract Detailed sedimentological and stratigraphic analyses of a c. 1500 m thick, siliciclastic-dominated slope succession in the Neoproterozoic Isaac Formation at the Castle Creek study area (southern Canadian Cordillera) reveals the occurrence of four well-preserved mass-transport complexes (MTCs) composed principally of slide/slump and debris-flow deposits. The stratigraphically lowest of these complexes is about 60 m thick and crops out for >2.5 km laterally, consisting of slide and debrite. The slide has an irregular erosive base with ramp-and-flat geometry. This is overlain locally by boulder-sized blocks of slightly to moderately deformed strata, bounded by shear surfaces. The slide is overlain by a debrite that pinches and swells laterally, consisting of matrix-supported conglomerate with common metre-scale clasts of mudstone and coarse-grained sandstone embedded in a mudstone-rich matrix with dispersed, pebble quartz grains. Based on its stratigraphic position at the base of the slope, vertical stacking of slide-debrite, lithological distribution, considerable thickness and lateral extent, this MTC is interpreted to be associated with a major episode of continental slope instability and submarine mass-wasting. The close association between the MTC and underlying/overlying mixed carbonate-siliciclastic strata suggests that sea level most likely exerted a key control on sediment supply, which ultimately led to the emplacement of this MTC.