DEPOSITIONAL ENVIRONMENT AND PALEOECOLOGY OF THE MIDDLE CAMBRIAN BURGESS SHALE RECONSIDERED—MUD VOLCANISM, SYNGENETIC FAULTING, AND PALEOBATHYMETRY

Structural, sedimentologic, geochemical and paleontologic observations of the Burgess Shale and other Chancellor Group units reveal that syngenetic mud volcanism was a major process during deposition. Evidence for mud volcanism includes: fault-controlled dikes and stratiform lithosomes composed mostly of clinochlore or, less commonly, Ba-rich or illite-rich exhalative mudstone; biotic and sedimentologic changes over short distances along strike; mud injectites into country rock; “Christmas tree” stratigraphic architecture of dikes and exhalative mud flows; concentric normal faults/slumps around exhalative mud sources; mud flow channels and lobes; flow-sheared mud fabrics; mud breccias, chaotic bedding; dilational cracks on bedding surfaces; and exotic clasts, including serpentine lizardite, from deep serpentinization/steatization sources in the lower crust. Reworked clinochlore clasts, and rare fossils in bedded clinochlore, show the hypothesized exhalative muds to be seafloor or near seafloor, not deep-burial diagenetic, phenomena. X-ray diffraction of fossiliferous shales records chlorite-serpentine and clinochlore in addition to illite/muscovite, quartz, and variably calcite, dolomite and minor K-feldspar. Mud dikes hosted in major faults, including the Cathedral Escarpment and Fossil Gully faults, show the faults to be Middle Cambrian, not Laramide, in age. Vertically dragged and sheared strata abutting the Cathedral Escarpment on Fossil Ridge indicate that the“Escarpment” represents a transpressional growth fault. Earlier estimates of Burgess Shale paleobathymetry depend on the measured height of the “Escarpment”, that being 200 m. It is unlikely that the “Escarpment” produced significant sea-floor relief. Therefore Burgess Shale paleobathymetry may have been shallower than previously thought, which would account for in situ macrophytic algae in the fossil assemblages. Mud eruptions, together with “normal” distal shelf processes such as storms, likely generated obrution events necessary to produce the lagerstätten. Enhanced microbial productivity and animal concentrations around associated brine seeps, as occur around modern marine mud volcanos, can account for the patchy distribution of fossil sites in the Burgess Shale and other units on the Kicking Horse Rim.