Comparison of geochemical and distinctive mineralogical features associated with the Kinzers and Burgess Shale formations and their associated units

Abstract

This paper compares two North American occurrences of fossils that exhibit Burgess Shale-type preservation (BST): the Middle Cambrian Burgess Shale Formation of southeastern British Columbia, and the Lower to Middle Cambrian Kinzers Fm of southeastern Pennsylvania. Redox-sensitive trace elements (Mo, Ni/Co, V/Sc) indicate that palaeo-oxygenation conditions varied with respect to faunal assemblage and location. Ogygopsis-bearing fauna in both the Burgess Shale and Kinzers formations are associated with a dysoxic metal signature. Burgess Shale-type preservation in the Kinzers Fm was deposited in a fluctuating oxycline, whereas the BST-fossil beds of Fossil Ridge and Mt Stephen (Burgess Shale Fm) yield a geochemical signature that consistently indicates oxic conditions. Thus, sustained low-oxygen conditions do not correlate with higher quality of fossil preservation at these sites. Ogygopsis is associated with dysoxic conditions in both localities, and may therefore be a faunal indicator of such environments in Middle Cambrian strata. Carbonate units associated with both the Burgess Shale and Kinzers formations contain large, generally concordant, near-surface cavities that contain herringbone calcite cements that show no significant enrichment in the potential calcite inhibitor ions such as reduced Fe, Sr, S, Pb, and Zn. The abundance of micro-inclusions of dolomite suggests that Mg may have been the calcite inhibitor ion responsible for herringbone cement in these Cambrian occurrences. Another distinctive mineralogical feature is the presence of doubly-terminated, euhedral, zoned, authigenic quartz as a conspicuous part of a carbonate host-rock replacement assemblage. The association of both herringbone calcite and zoned quartz crystals in units associated with the Burgess Shale and Kinzers formations suggest a common feature in the depositional and diagenetic environment of these two regions in which Burgess Shale-type fossil preservation occur; syn-depositional brine flow is the likely cause of these distinct mineralogical features. Brine seeps in the depositional environment may have provided nutrients for stable high-abundance faunal communities in a slope setting, thereby accounting for the abundance of fossil material. Furthermore, high-salinity brine in the pore waters of carcass-bearing Cambrian muds may have further limited shallow burrowing, scavenging, and decomposition rates, accounting for the exceptional preservation in these two localities.