Chemical Sedimentation in the Callison Lake Formation, Yukon, Canada: A Window into Tonian Seawater Carbonate Chemistry

The Tonian (ca. 760-740 Ma) Callison Lake Formation of Yukon, Canada, records mixed siliciclastic and carbonate sedimentation in an evolving extensional or transtension marine basin associated with the break-up of the supercontinent Rodinia. These strata are subdivided into four informal members (heterolithic, talc, ramp, and transitional), two of which are characterized by relatively thick accumulations of interbedded Mg-clay and microbial dolostone. Previous models for Mg-clay authigenesis in the Callison Lake Formation highlighted pH as a master variable, but the driving mechanisms for elevating basinal pH remained ambiguous. Here, we combine detailed sedimentological observations with mineralogical data and major/trace element geochemistry from carbonates and mudrocks of the Callison Lake Formation to characterize the local provenance and geochemical conditions within the Callison Lake basin. These data indicate that the Callison Lake Formation received fine-grained sediment from an evolved source region with a relatively significant contribution (>20%) from a mafic terrain. Two-component mixing models between permissible source regions can explain all of the trace element geochemical data, even enrichments in redox-sensitive trace metals. Sedimentological observations coupled with carbonate trace element geochemistry suggest Mg-clay authigenesis was most likely driven by basinal restriction and evaporation. We couple this observation with chemical constraints on Mg-clay nucleation, alkalinity requirements associated with evaporative concentration, and recently published Tonian seawater compositions derived from fluid inclusions in evaporites. Together, this approach provides new constraints on the secular evolution of global seawater carbonate chemistry, all of which yield scenarios characterized by a large DIC pool and relatively high pCO₂.