Cambrian Tectonic Setting and Fluid Source for the Burgess Shale Seep-related Ecosystem  

Palaeogeographic reconstructions of the Middle Cambrian tectonic plates show an extensive subduction zone that consumed thousands of kilometres of oceanic crust as Laurentia moved northward throughout the Paleozoic.  While the Middle Cambrian margin is commonly described as “passive”, new evidence shows early north-south (shortening-axis) compressive deformation of Middle Cambrian age overprinted by NE-SW (shortening-axis) Laramide-age compressive fabrics.  Structural evidence includes zebra-spar dolomite filled tensile fabrics within southerly-directed bedding parallel thrusts (Eldon Formation, Mount Wapta) that are cross-cut by NE-directed calcite-filled reverse fault fractures.  Within early-dolomitized Eldon Formation laminites from Fossil Ridge scree, reverse faults are filled with coarse ferroan dolomite.  At the renowned Walcott Quarry, southerly-directed reverse faults, consistent with Middle Cambrian kinematics, cut the phyllopod beds.  Cross-cutting NE-striking tensile faults and NW-striking low-angle normal faults can be interpreted as macro normal-slip C-planes, accommodating Laramide NE-directed simple-shear strain. 

At the topographic level of the Walcott Quarry, an excavation named the Adkins Quarry revealed the ‘Escarpment’ contact, where the Burgess Shale is deformed – it is dragged vertically against the Cathedral Formation.  This “smearing” is related to syn-sedimentary faulting (basin downthrown) at the contact of the Cathedral and Burgess Shale formations; there is no offset upsection.  Normal south-directed slip in this zone contains Reidel-tensile gashes filled by coarse hydrothermal dolomite.  Immediately north of the Cathedral fault at the Walcott Quarry, NW-striking, high-angle faults contain hydrothermal Mg-chlorite (clinochlore)-filled shear fabrics and northerly-striking clinochlore filled gash fractures, which indicate synhydrothermal right-slip on NW-striking fault elements.  The NW-striking faults project NE toward NW-striking faults along the access trail which contain strike-slip shear fabric.  The NNW strike of the Monarch and Kicking Horse hydrothermal dolomite-hosted Zn-Pb deposits, also fits the right-slip model for the Cathedral fault system. 

We propose that the Cathedral ‘Escarpment’ is a non-erosional fault, probably related to a deep-seated right-slip wrenching that was active during N-S far-field compression above the subducting oceanic plate.  Dehydration in this subduction zone triggered deep melting of alkaline lherzolitic peridotite mantle sources and some of these melts may have been serpentinitzed by metagenic water in the mid-crust.  Large volumes of magnesium-charged brine then utilized the deep-seated Cathedral fault pathway to migrate to the sediment-water interface within the Burgess Shale basin.  Serpentinization can be held responsible for bulk volumes of serpentinite mud (much of the Burgess Shale Formation) which also utilized the Cathedral conduit system.