Platy coral patch reefs from eastern Panthalassa (Nevada, USA): Unique reef construction in the Late Triassic

Abstract

Upper Triassic (Lower Norian) reefal buildups near Mina, Nevada, represent some of the earliest scleractinian coral reefs from eastern Panthalassa. The small patch reefs (~ 20–40 m high and ~ 50–150 m wide, obvious metre-scale elevation above the surrounding sediments) are from the Luning Allochthon and grew on an inner to middle ramp or in a deep lagoon. The Mina patch reefs were constructed by several different coral ecomorphotypes (platy, tabular, domal/massive, and branching corals) interpreted to have had zooxanthellate symbionts, and record subtle coral zonation within the reefs. Based on modern coral ecomorphotypes, platy to tabular corals at Mina grew in the lower euphotic zone (stressed by low light conditions), and the massive or domal corals inhabited shallower water (possibly above fair weather wave base) and were stressed by wave energy. Unlike most other Late Triassic reef ecosystems where phaceloid branching corals or calcareous sponges constitute the principal bioconstructors, the platy to tabular corals were the primary builders in the Mina patch reefs. The Mina reefs are also unique because cryptic and cavernous internal environments, epibionts, cryptobionts, and thick microbial crusts are rare or absent. The combination of platy coral dominance with the lack of epibionts/cryptobionts/microbial crusts suggests that the Norian reefs from Mina, Nevada represent a unique form of scleractinian reef construction from the Late Triassic.