Metazoan-algal benthic ecosystems enhance automicritic slope boundstone: The Triassic Great Bank of Guizhou carbonate platform, Xiliang margin, China

ABSTRACT Carbonate platforms with automicritic boundstone slopes differ from detrital platforms in their architecture and reservoir properties. The controls on the occurrence of automicritic boundstone slopes are poorly constrained, but they have generally been associated with depleted reefs and other metazoan-algal benthic ecosystems. To test this association, we investigated automicritic boundstone occurrence across the end-Permian extinction and Triassic biotic recovery on the Xiliang slope of the Great Bank of Guizhou (GBG), an isolated carbonate platform in southern China. Our findings indicate that automicritic boundstone accumulation was not enhanced by depleted Early Triassic benthic ecosystems. Instead, detrital sediment dominated the GBG slope, and automicritic boundstone was absent until the late Spathian (latest Early Triassic), 4–5 million yr after extinction. Beginning in upper Spathian strata, automicrite forms small (<1.5 cm) and sparse (<15% estimated volume) masses in slope boundstone, coinciding with an early stage of benthic ecosystem recovery. When a metazoan-algal reef subsequently formed during more advanced recovery in the middle Anisian (lower Middle Triassic), automicrite occurrences were more abundant (up to 40% estimated volume) and larger (up to ∼10 cm). The proportions of automicrite and metazoan-algal fossils within samples are also positively correlated (Spearman’s ρ = 0.70; P < 0.0001). Our findings suggest that the controls on slope automicrite accumulation and metazoan-algal benthic ecosystems are similar or that benthic metazoans and algae enhance, rather than inhibit, automicrite accumulation. Our results imply that concept-driven predictions of automicritic boundstone occurrence should be modified—automicritic slopes are not more likely to occur during geologic intervals with depleted benthic ecosystems.