MG-RICH BRINE SEEPS IN THE BURGESS SHALE LAGERSTÄTTE CONSTRAINED BY DOLOMITE U–PB GEOCHRONOLOGY

In the southern Rocky Mountains of western Canada, the Burgess Shale Formation (Middle Cambrian) contains world class examples of the earliest arthropods, molluscs, annelids, and chordates; often with the preservation of soft-bodied material as carbonaceous remains. Over a century of research has been conducted since Charles D. Walcott’s discovery of the lagerstätte in 1909 (a sedimentary rock that contains well-preserved fossils with extraordinary abundance and diversity), with several other “Burgess Shale-type” deposits later documented worldwide. Nevertheless, the controls on the occurrence and distribution of lagerstätten remain enigmatic, particularly why such incredible concentrations of biota thrived at these times and in these localities. Coeval and adjacent to the Burgess Shale Formation, a shallow-water succession of carbonate sedimentary rocks has been extensively altered by fault-controlled, hydrothermal dolomitization, with local clinochlore ((Mg,Fe)₅Al(Si₃Al)O₁₀(OH)₈), talc (Mg₃Si₄O₁₀(OH)₂), and magnesite (MgCO₃) mineralization along the margin of the carbonate platform. The timing of such hydrothermal activity, however, is contentious, with interpretations that range from the Middle Cambrian to the Late Cretaceous.

Here we show, using U–Pb geochronology, that the timing of dolomitization in the southern Rocky Mountains was syn- to early post-depositional (Middle Cambrian to Early Ordovician). Surface-breaching extensional faults along the margin of the carbonate platform facilitated the mixture of Mg-rich crustal fluids with seawater, with local soft-sediment deformation and mud volcanism in paleo-topographic lows on the seafloor. Although chemotrophic activity within these deposits was likely minimal, the brine seeps increased nutrient availability that, in turn, promoted microbial productivity. This resulted in the concentration of heterotrophic megafauna along the peripheries of the brine seeps. Such highly saline pore fluids would have also inhibited bioturbation and the scavenging of organic material in the sediment, thus promoting Burgess Shale-type preservation. The results of this study challenge the common models of lagerstätten development, while also providing valuable insights into fundamental processes of hydrothermal dolomitization that can be applied to sedimentary basins worldwide.