The interaction of recovery and environmental conditions: An analysis of the outer shelf edge of western North America during the early Triassic

Abstract Biotic recovery from the Permian-Triassic mass extinction was complex and uneven, with prior studies typically revealing a long, drawn-out recovery that was either delayed for some interval, or was subdued and stepwise in nature. Examples of rapid recovery at the outcrop scale cloud this narrative and point to the importance of environmental stresses in determining the timing and shape of recovery. The Union Wash Formation at the Darwin locality (east-central California) contains at least 3 recovery intervals that terminate with the onset of deleterious conditions and therefore offers a means to examine the relationship between environmental stress and recovery. The first recovery is manifested as bioturbated (ii = 5–6), shallow marine micritic limestones that contain small diameter (2–4 mm) Thalassinoides burrows that form complex networks. This interval is overlain by 650.5 m of laminated mudstone that signal deleterious environmental conditions. The second recovery interval begins directly above seafloor precipitate-bearing micritic limestones that make up the lowermost 130 m of the upper member of the Union Wash Formation and is marked by the occurrence of a 3 m-thick interval containing sphinctozoan sponges, an intervening ~1 m-thick laminated (ii = 1) micritic limestone, and an overlying, 5 m-thick bivalve-rich and bioturbated calcareous siltstone (ii = 4–5) that contains Chrondrites trace fossils. This interval is overlain by 18 m of laminated green shale (ii = 1) that signifies another incursion of anoxic waters. The third recovery interval is represented by a 30 m-thick unit that includes beds of micritic limestone, fossiliferous limestone containing transported fossil grains (including bivalves, microgastropods, sphinctozoan sponges, microgastropods, flat clams, and terebratulid brachiopods), flat pebble conglomerate and vermicular limestone. The results of this study, therefore, point to an uneven recovery within the Union Wash Formation, in which an initially robust recovery in the lower member deteriorated across the remainder of the study section. Areas exposed to persistent environmental stress, therefore, demonstrate protracted and complex recoveries. Study localities located within the habitable zone instead exhibit accelerated recoveries during the post-extinction interval and are indicative of an environmental bias in the form of persistent favorable conditions. The most accurate measures of post – extinction recovery, therefore, are those that examine biotic trends over a broad geographic area, and, as a result, incorporate the role of environmental stress in determining when and how life recovered from the Permian – Triassic mass extinction.