One diamictite and two rifts: Stratigraphy and geochronology of the Gataga mountain of Northern British Columbia

Neoproterozoic glacial diamictites and rift-related volcanics are preserved throughout the North American Cordillera, yet the nature and timing of both glaciation and rifting are poorly constrained. New geochronological, geochemical, and stratigraphic data from the Cryogenian Gataga volcanics and bounding units at Gataga Mountain, in the Kechika Trough of northern British Columbia, better constrain the age of these rift-related volcanics and suggest that they erupted during glaciation. At Gataga Mountain, three informal sequences are exposed; a basal quartzite, the Gataga volcanics, and an overlying mixed carbonate-siliciclastic succession. The basal quartzite is dominated by cross-bedded sandstone with an intertidal facies assemblage including bidirectional cross-stratification and mud-cracks, indicative of non-glacial deposition. The overlying Gataga volcanics are over one kilometer thick, comprising both mafic and felsic units, with volcaniclastic breccia and interbedded sedimentary units including iron formation and matrix-supported diamictite with exotic clasts. Magmatic ages in the upper Gataga volcanics span 696.2 0.2 to 690.1 0.2 Ma, and detrital zircon from the underlying non-glacial quartzite provide a maximum age constraint on the onset of glaciation <735.8 0.6 Ma. We interpret interfingering beds of matrix-supported diamictite with exotic clasts within the Gataga volcanics to record sub-ice shelf sedimentation and volcanism during the Sturtian Glaciation. Although volcanic facies are consistent with eruption in a sub-ice to sub-aqueous (below ice shelf) environment, we acknowledge the difficulty of distinguishing sub-glacial from sub-aqueous explosive volcanic facies. Overlying the Gataga volcanics, a mixed carbonate-siliciclastic succession contains minor basalt flows that are geochemically distinct from the underlying volcanic rocks. Based on chemostratigraphic and lithostratigraphic similarities, we suggest that this sequence is correlative with Ediacaran strata to the north. Together, we suggest that the stratigraphy and geochemical signature of volcanic rocks at Gataga Mountain records two episodes of Neoproterozoic extensionrelated sedimentation and volcanism, the first indicated by the Cryogenian Gataga volcanics and interbedded sedimentary strata and the second by the overlying Ediacaran carbonate-siliciclastic succession with interfingering basalt.