ANCIENT SEEPS IN THE MIDDLE CAMBRIAN BURGESS SHALE: PALEOECOLOGICAL AND EVOLUTIONARY IMPLICATIONS

Field studies in the Middle Cambrian Burgess Shale Formation and Duchesnay Unit (both Chancellor Group, British Columbia) indicate that seeps may have profoundly influenced the composition and distribution of the sediments and biota. Our evidence for seeps includes deposits of iron phyllosilicate ooids, carbonate mud mounds, and the appropriate paleotopographic and tectonic settings. Additionally, studies by others show that extensive dolomitization of adjacent platform carbonates probably occurred contemporaneously with deposition of the Chancellor Group and indicate significant syndepositional migration of hot hypersaline brines. Finally, we note massive sulfide deposits in close proximity to the classic localities in the Burgess Shale, although the age of their emplacement is uncertain. Within the ooids, we found chamosite, a product of thermally-altered berthierine and nontronite, iron-rich clays produced at hydrothermal vents and seeps. Known to inhibit autolytic decay, nontronite may be linked to the unusual preservation of the Burgess Shale organisms. Localized seeps might help account for the extraordinary differences in the abundance and composition of the contained biota over short distances laterally and stratigraphically within the Burgess Shale, the biota representing a composite of seep-related and more “normal” background organisms. If seeps can be linked to the Burgess Shale, and to other Cambrian Burgess Shale-like lagerstätten, might the Metazoa have originated and radiated at hydrothermal vents and seeps in the late Precambrian? Vent deposits that would record this initial diversification would be mostly subducted since and seep deposits are rare and generally unexplored in this context. This would reconcile evidence from gene sequence data (which indicates a pre-Ediacaran origin for the metazoan phyla) with the general absence of pre-Ediacaran metazoan fossils. The Cambrian “Explosion” then may record the adaptive radiation of the already evolved major lineages of the Metazoa beyond vents and seeps to broader marine environments.