Reinterpretation of ‘Middle’ Cambrian stratigraphy of the rifted western Laurentian margin: Burgess Shale Formation and contiguous units (Sauk II megasequence), Rocky Mountains, Canada
Abstract Until recently, research on the renowned fossil animals of the Burgess Shale has advanced to a greater degree than an understanding of the rocks in which they are found. Studies addressing lithostratigraphy and hydrothermal petrography of the so-called ‘inner carbonate belt’ and adjacent Chancellor Group, however, have begun to re-evaluate long-standing hypotheses on the Middle Cambrian of Western Canada. Tectonic activity along the Kicking Horse Rim (buried remnants of Neoproterozoic rifting) during the Cambrian Period had more influence on local sedimentation than previously thought. Notably, large-scale collapse of the Cathedral carbonate platform margin at ∼ 509 Ma BP is evidence of reactivated basement faults. These failures produced listric Megatruncation Surfaces, having near vertical escarpments (> 150 m in height) where they terminate against the platform. The majority of what have been interpreted as shed olistoliths from the Cathedral platform margin is herein shown instead to be carbonate mud mounds. These grew in situ along the face of the Cathedral Escarpment, and are associated with fossiliferous intervals in overlying basinal mudstones at three distinct stratigraphic horizons (two within the Burgess Shale) during the Delamaran and Marjuman stages. The mounds nucleated where deep-seated normal faults intersected the seafloor, building primarily during periods of relative sea level transgression. Mound growth is associated with syndepositional exhalative activity, inferred from stratigraphic relations, sedimentology, and geochemistry. The new name Monarch Formation is proposed for the initial post-collapse beds and mounds deposited against the Cathedral Escarpment (early Glossopleura Zone); the variably-defined term ‘Takakkaw Tongue’ is thereby confined to pre-collapse slope deposits coeval to and correlative with the Cathedral Formation.