Depositional history of Middle Cambrian to Lower Ordovician deep-water sediments, Bay of Islands, western Newfoundland

The Northern Head group is an upper Middle Cambrian to Lower Ordovician base-of-slope sediment apron, deposited downslope from a shallow-water carbonate platform, and now disposed in imbricate thrust slices within the Humber Arm Allochthon, western Newfoundland. The group encompasses the upper Middle Cambrian to upper Tremadoc Cooks Brook Formation and the upper Tremadoc to middle Arenig Middle Arm Point Formation. The discovery of numerous fossil localities has facilitated the erection of a stratigraphic framework, embodying several correlatable members. These subdivisions reflect a natural change in the depositional style upward, from carbonate to shale-dominated.;Gravity transport of carbonate sharply diminished in earliest Ordovician time but hemipelagic carbonate sedimentation persisted into late Tremadoc time.;The change to shale-dominated sediments of the Middle Arm Point reflects a new, low-relief margin upslope. Hemipelagic shale was accompanied by detrital, windblown dolomite and both were reworked by bottom currents. Diminished input of shelf and slope-derived organic carbon, and more vigorous marine circulation, resulted in elevated Eh levels in the depositional environment which are indicated by (1) a pronounced increase in bioturbation, and (2) a new "suboxic" diagenetic regime, distinguished by widespread precipitation of Mn-carbonate and barite, largely within the stability field of hematite. Condensed sedimentation characterizes the middle of the Middle Arm Point, spanning the Tremadoc/Arenig boundary. Shale deposition continued until the collapsing margin was buried by sandstones of the overlying Eagle Island formation during middle Arenig time.;The Northern Head group and contemporaneous Cow Head Group were both part of an active carbonate margin until the Tremadoc. Their depositional and paleoceanographic histories diverged in late Tremadoc time, when a low-relief margin developed upslope from the Northern Head group while active carbonate sedimentation continued upslope from the Cow Head Group. This irregular carbonate margin morphology is suggestive of a basement-related structural juncture separating the two areas.;The Cooks Brook is characterized by abundant platform-derived, gravity-transported carbonate, and hemipelagic black shale, deposited in a deep water, poorly-oxygenated environment. Earliest sediments accumulated at the mouths of submarine canyons but were buried by debris flow conglomerate sheets and carbonate sand turbidites from an active, upslope shallow-water platform margin.