THE SMITHIAN-SPATHIAN ANOXIC EVENT AND ITS POSSIBLE IMPACT ON THE BIOTIC RECOVERY FROM THE PERMIAN-TRIASSIC MASS EXTINCTION

Pelagic cherts and deep water facies from a variety of locations reveal that anoxic conditions were present in the Panthalassic Ocean across the entire span of the Early Triassic. Encroachment of those waters onto the continental shelves appears to have occurred twice during the Early Triassic. The first, well-documented invasion of anoxic waters began near the Permian-Triassic boundary and reached its maximum extent during the Griesbachian (Wignall and Twitchett, 2002). Anoxic conditions apparently subsided during the Dienerian before a second incursion of toxic waters into shallower environments took place along the western margin of Pangea during the Smithian-Spathian interval. This incursion of anoxic waters was limited to lower latitudes as revealed by the presence of anoxic and suboxic facies within the Union Wash Formation and Virgin Limestone (Moenkopi Formation) of the southwestern United States and the Thaynes Formation of the Phosphoria Basin (central U.S. Rockies). While anoxic waters were apparently present in offshore environments of the Western Canada Sedimentary Basin (WCSB), those toxic waters do not appear to have impinged into the shallower marine environments of the WCSB during this time. Anomalous carbonate precipitates and large microbial bioherms found in Smithian-Spathian facies from the southwestern U.S. imply the presence of alkaline as well as anoxic waters at low latitudes. The emerging picture of the Smithian-Spathian anoxic event is therefore one of limited latitudinal extent: while lower latitude shelves experienced some degree of flooding by anoxic and possibly alkaline waters, higher latitudes appear to have been unaffected. This distribution of toxic environmental conditions may have played a role in determining the timing and shape of the biotic recovery from the mass extinction as it correlates to a noted high- to low latitude trend in the timing of the initiation of the recovery.