TRIASSIC METAZOAN-ALGAL REEFS ORIGINATED IN A MESOPHOTIC TO DYSPHOTIC SLOPE SETTING BEFORE MIGRATING INTO SHALLOW, PLATFORM-MARGIN ENVIRONMENTS

Mesophotic to dysphotic (~30–1000 m) marine environments are hypothesized to serve as refuges and sites of ecosystem origination during intervals of global warmth. To assess this hypothesis, we investigated the paleoenvironmental context of Triassic metazoan-algal reef recovery following the end-Permian extinction. The Permo-Triassic extinction and recovery interval is an important test case because shallow-marine reef builders of the Paleozoic suffered complete extinction, Early Triassic oceans were characterized by extreme warmth, and the Middle Triassic taxa that rebuilt reefs included the first scleractinian corals, the primary constructors of modern reefs. Middle Triassic reefs thus reflect the origin of modern coral reef ecosystems. However, the pattern and paleoenvironmental context of their recovery remain poorly constrained. The oldest-known Triassic metazoan-algal reef occurs on the slope of the Great Bank of Guizhou (GBG), an isolated carbonate platform in South China. Facies mapping and petrographic analysis indicate that reefal boundstone—composed of probable algal fossils (Tubiphytes) and early marine cement—initiated during the Spathian (upper Lower Triassic) in a mesophotic to dysphotic middle slope environment (~100–300 m water depth). The earliest metazoan reef framework builders (sphinctozoan sponges) first occurred in the lower Anisian (lower Middle Triassic; ~1–2 Myr later) in up to ~400 m of water depth. Within this interval, the coeval platform margin lacks reefal boundstone and instead contains low biodiversity algal and molluscan grainstone, indicating restriction of the reef to deeper settings. During the middle to late Anisian (~1–2 Myr later), the reef facies migrated into shallow water, occupying an outer-margin to upper-slope paleoenvironment, while the middle to lower slope shifted to detrital forereef sedimentation. Reef migration into shallow water postdated evidence for global cooling and coincided with the first occurrences of framework-building calcareous algae and early scleractinian corals. The first Triassic shallow-water reefs were thus derived from precursors in deeper marine environments of the Early Triassic hothouse, suggesting that mesophotic to dysphotic slope environments could serve as refuges and cradles of metazoan ecosystem evolution during intervals of extreme warmth.