Mud Volcanism and Exhalative Brines, Burgess Shale, Fossil Ridge  

Exhalative deposits and associated putative chemosynthesis-based communities have been described from several localities in the Burgess Shale, but not from the classic outcrops on Fossil Ridge.  We report the discovery of a 30 m thick sequence of dolostones interbedded with a dark green to black weathering, non-fossiliferous lithofacies that resembles the brine seep lithosomes on Mount Stephen and The Monarch.  The study interval occurs within the Odaray Shale Member above the Upper Ehmaniella Quarry.  The dark lithofacies returned MgO values >30 wt% and, based on x-ray diffraction, is mineralogically nearly pure clinochlore.  One distinctive bed is c. 1 m thick and can be traced laterally for at least 100 m.

Three clinochlore-bearing conduits cross-cut dolostones of the Cathedral Formation near its contact with the Burgess Shale.  The largest conduit, with a diameter of c. 0.5 m and an exposed length of 10 m, is roughly lateral to the Walcott Quarry.  The conduit is filled with a pebble breccia of laminated clinochlore clasts and trends towards an embayment in the platform before disappearing into the outcrop 15 m from the “Escarpment” face.  Two narrower and shorter vein-like conduits angle upwards to flat-bottomed embayments roughly lateral to the Raymond Quarry and Upper Ehmaniella Quarry, respectively.  Clinochlore is a Mg-rich product from hydrothermal alteration of mafic minerals and its occurrence here cannot be explained by regional low-grade metamorphism of basinal mudstones.  Likewise, late-stage diagenetic emplacement of clinochlore bodies is improbable because there is syndepositional slumping in some clinochlore beds as well as reworked rounded clinochlore pebbles in some clastic beds. 

The substantial volume of clinochlore evulsed onto the seafloor at Fossil Ridge is consistent with serpentinite-sourced mud volcanism.  Large, circular, slump features may represent post-eruption depressurization and collapse of the mud volcano caldera.  Modern submarine mud volcanoes are typically associated with brine seeps, brine pools and chemosynthetic communities.  In our fieldwork experience, clinochlore deposits in the Burgess Shale occur at or near the contact of platform and basin and are fringed by concentrations of animal fossils.  At Fossil Ridge, the slope of the modern mountainside exposes the clinochlore-rich interval close to the NNW–SSE trending segment of the Cathedral fault, which lies within, and is parallel to, the ridge.  The lower quarries are successively further away from the buried “Escarpment”, so any clinochlore lithosomes associated with these fossils remain unexposed. 

The discovery of exhalative clinochlore on Fossil Ridge indicates that the seep model provides a general palaeoenvironmental explanation for the distribution and composition of fossil assemblages within the Burgess Shale.