SALT DIAPIRIC SOURCE FOR UNUSUAL CLASTS IN KILOMETER-DEEP EDIACARAN CANYONS, FLINDERS RANGES, SOUTH AUSTRALIA

Salt diapirs that intermittently flowed laterally at the surface (allochthonous flow) and kilometer-deep paleocanyons are exposed in the mid-Ediacaran Wonoka Formation in the Flinders Ranges of South Australia. Recent stratigraphic work shows that salt breakout and canyon incision are coeval and due to subaerial exposure and erosion associated with a large sea-level drawdown potentially triggered by the ~580 Ma Gaskiers glaciation. Diapir exposures are no longer characterized by salt, but by polymictic breccia of remnant non-evaporite rock types originally interstratified with evaporites, and subsequently disrupted and entrained during diapiric flow. The basal canyon fill contains polymictic conglomerate previously interpreted to have been sourced from sub-Wonoka and lower Wonoka wall rocks. The origin of dolomite and quartzite clasts as much as several meters in diameter, as well as volcanic clasts remain ambiguous owing to the lack of an appropriate source in the canyon wall strata (if diapiric breccia is not considered). It has been suggested that canyon fill dolomite clasts represent early dolomitization of the canyon wall in an intensely stratified Ediacaran seawater column prior to being redeposited (Husson et al., 2015).

New sedimentologic, petrographic, and geochemical (d¹³C, d¹⁸O, d²⁶Mg, and Mg/Ca) data suggest that all of the quartzite and volcanic clasts and some of the dolomite clasts in the canyon fill were sourced from diapiric breccia. Our evidence includes a comparison of sedimentologic and petrographic characteristics of dolomite and quartzite clasts and the comparable size of clasts in the canyons and diapirs (> 2 m). The geochemical signature of dolomite clasts in diapiric breccia (d¹³C values between -6 and +4‰ and d²⁶Mg values between –1.2 and -2.7‰ DSM3) is similar to dolomite clasts in the canyon fill, and incompatible with the geochemistry of dolomite beds in the Wonoka wall rocks. These results are consistent with recent work documenting the dynamic interaction between the Wonoka canyons and allochthonous salt flow during the Ediacaran, and remove the necessity of an intensely stratified Ediacaran seawater column to reconcile dolomite clasts with primarily positive d¹³C values in the basal canyon fill.