Stratigraphy, Architecture and Morphodynamics of an Early Neoproterozoic Alluvial Plain: The Nelson Head Formation of Brock Inlier, NWT, Canada

English Abstract:

The Nelson Head Formation is a ca 1.0 Ga sandstone exposed within several western Canadian Arctic inliers, including the Brock Inlier, where it is superbly exposed along extensive canyon cliffs. We present stratigraphic and architecture data consisting of line drawing, logging and paleocurrent analysis of 3D sections. Fluvial deposits record vertical and lateral alternation of coarse-grained, tan-yellow pebbly sandstone (mainly quartz arenite) and, recessive, maroon-red, fine-grained silty sandstone.

Strata are organized in 2 to 8 m thick bedsets that continue for >800 m along strike and >300 m down-dip. Trough-cross bedding is dominant, forming compound sets that reach 4 m in thickness. Subordinate planar-cross bedded deposits form sets up to 50 cm thick, and preferentially occur in the upper parts of bedsets. Soft-sediment deformation structures include common ball-and-pillow, convolute bedding, heavy mineral sags and fluid-escape conduits. Bedsets comprise lower, large inclined sets and upper, cross-cutting lithosomes with planar to concentric fill. Coarser units are somewhat thinner but are laterally more homogeneous up-section. Lower in the section, paleoflow indicators point to northwestern to northeastern transport, with high dispersion in individual units and upward flow rotation in some cases. Up-section, paleoflow is more focussed to the northwest.

Finer-grained units are micaceous (sericitic) and their maroon-red colour reflects an abundance of iron-oxide cement. Bedding units are 10 to 70 cm thick, planar-bedded, and continue for >600 m along strike and >300 m down-dip. Finer units are normally graded, thicker and more laterally continuous lower in the section. Ripple-cross lamination, climbing in places, is common and alternates with minor wavy plane-parallel lamination. Rare trough-cross bedding occurs in dm-thick sets in the lower parts of thicker units. Paleoflow is consistently north-northwest with low dispersion.

Correlative exposures of the Nelson Head Formation in the Minto Inlier of Victoria Island and the Katherine Group of the Mackenzie Mountains comprise remnants of continental-scale fluvial system that drained vast portions of the Canadian Shield. The formation is additionally one of very few Proterozoic sandstones where fluvial sinuosity has been documented.