Barberpole stratification: yet another example of heterolithic banding from the deep marine clastic realm

Heterolithic banding comprising alternating clay-rich and clay-poor layers is common in deep marine clastic strata. However, their origins remain a matter of debate, with potential formative processes related to hybrid event beds, flocculation, sedimentation under transitionally turbulent to cohesive flows, and shear banding. Examples of banding, here termed barberpole stratification, are common in argillite of the Cambrian Moose River Formation in the Meguma terrane, Nova Scotia, Canada, where they host disseminated gold mineralization and belong to a succession of greenschist-facies peri-Gondwanan deep marine clastic rocks. Barberpole units are several meters to tens of meters thick and composed of interstratified claystone and siltstone. Clay-rich and clay-poor alternations occur from lamina to bed scale, but barberpole strata can be more generally subdivided into 2–25 cm thick recurring units, which characteristically include (from base to top): (a) laminated siltstone ± claystone overlain abruptly by (b) interlaminated dark claystone and siltstone, gradationally overlain by (c) massive dark claystone with dispersed silt, capped sharply by (d) massive light grey mudstone with dispersed silt, and gradationally by (e) light grey silty claystone with siltstone lenses. Parts of this idealized succession, typically (a), (b), and (e), are locally missing. There is no difference in mineralogy between light and dark grey claystone layers except for traces of graphite in the dark mudstone, implying the presence of precursor organic matter. These preliminary investigations suggest that the Meguma barberpole bands are akin to heterolithic banded strata from the Britannia Formation of the North Sea and the Windermere Supergroup in Canada, linked to alternating near-bed rheological conditions related to systematic changes in sediment concentration and shear at the base of turbidity currents. The presence of organic material likely promoted clay cohesion during the sedimentation of dark claystone layers, and therein, rheological conditions very near the bed.