REGIONAL GEOLOGICAL SETTING

The Upper Triassic through Cretaceous succession of the Queen Charlotte Islands is probably the most complete Mesozoic biostratigraphic reference section for the northeast Pacific region. The Triassic-Lower Jurassic portion of this sequence encompasses the Wrangellia terrane succession (Jones et al. 1977), which is a thick accumulation of Upper Triassic massive oceanic basalts (Karmutsen Formation) conformably overlain by deep-water clastic facies and fringing reef carbonates (Sadler Limestone), with local latest Triassic to earliest Jurassic age tuffaceous beds (Peril and Sandilands formations). The overlying Lower Jurassic strata (Maude Group) are comprised of continentally-derived clastic detritus. These rocks are in turn overlain by Middle to lower Upper Jurassic volcanic and volcaniclastic strata (Yakoun and Moresby groups) that provide evidence of the initiation of proximal andesitic arc volcanism (see detailed summaries in Sutherland-Brown 1968; Cameron and Hamilton 1988; Woodsworth and Tercier 1991; Haggart 1987; Haggart et al. 1995).

The bulk of Cretaceous strata on the Queen Charlotte Islands are assigned to the Queen Charlotte Group (Sutherland Brown 1968; Haggart 1991, 2004) (Figure 2). The Longarm Formation ranges from Valanginian to Aptian in age while the Haida Formation ranges from Albian to Turonian. Longarm Formation strata accumulated in the western parts of the basin while Haida Formation rocks were deposited in more easterly parts of the basin. Both of these units are characterized by basal conglomerate and coarse-grained sandstone and represent basal transgressive sequences, which fine up-section into deeper water facies. Longarm Formation strata grade upward into outer-shelf mudstone and shale, locally with calcareous concretions, assigned to the informal Hotspring Island formation, of Barremian-Aptian age. Haida Formation strata grade upward into siltstone, mudstone, and shale, previously named the Haida Shale Member but now assigned to the informal Bearskin Bay formation of Albian-Cenomanian age. Localized distal turbidite deposits assigned to the Skidegate Formation represent outer-shelf and distal fan facies that accumulated in deeper parts of the basin during Cenomanian to early Turonian time. Coarse-grained fan-delta and submarine-fan complexes of the Honna Formation prograded into the basin from the east during later Turonian to Coniacian time (Higgs 1990, 1991; Haggart 1991). Latest Cretaceous deposits include upper Santonian to upper Campanian shelf muds of the informally named Tarundl formation (Haggart 2004; Haggart et al. 2009). Deep-basin Cretaceous facies are apparently absent on the islands and may have been truncated by late Neogene strike-slip movement (Haggart 1991).

The principal biostratigraphic groups utilized in correlation of the Cretaceous strata of the islands are the ammonites and bivalves, including the work of McLearn (1972), Jeletzky (1977), Riccardi (1981), and Haggart (1995); the succession of established faunal zones is summarized in Figure 2. Recent work with radiolarians (Haggart and Carter 1993; Carter and Haggart 2006) has shown that this group also holds promise as a potential biostratigraphic correlation tool for Cretaceous deposits. Planktic foraminifera are also valuable biostratigraphic tools in these strata (Patterson and Fowler 1996; Prokoph et al. 2000, 2001; Patterson et al. 2004, 2005; Dalby et al. 2009).

Cretaceous strata of Queen Charlotte Islands accumulated in a forearc basin west of an active magmatic arc and deposition was essentially continuous within the basin from at least Valanginian through Campanian time (Haggart 1991). The basin appears to have been open to the proto-Pacific Ocean on the west (Haggart 1991, 1993), resulting in generally open-marine conditions. Given its position west of the active arc, and assuming an onshore westerly wind pattern, volcanic strata and tuffs are rare within the succession. The overall stratigraphic succession reflects continuous basin subsidence for much of Cretaceous time, with an eastward-migrating shoreline (Haggart 1991). Earlier studies (Yorath and Chase 1981; Fogarassy and Barnes 1991) suggested that the Aptian represented a time of uplift and erosion in the Queen Charlotte Islands region, but subsequent detailed stratigraphic studies have demonstrated that deposition was continuous within the basin during this interval (Haggart 1991; Haggart and Carter 1993).

A second pulse of plutonism in the Queen Charlotte Islands region occurred during Paleogene and possibly persisted through earliest Neogene time (Anderson and Reichenbach 1991; Hickson 1991; Hamilton and Dostal 1993). Thermal metamorphism associated with the Jurassic and Paleogene plutonism has detrimentally impacted the quality of fossil preservation in many areas (see Orchard and Forster 1991). In addition, the region is structurally complex (see Thompson et al. 1991; Lewis et al. 1991) making geologic correlation difficult in these intensely-deformed geological units. When available, biostratigraphic control is thus of great value in helping to unravel the original stratigraphic relationships of rocks and interpret the geological history.