A coral reef in the Telkwa Range, British Columbia: the earliest Jurassic example

Abstract

An end-Triassic mass extinction profoundly affected reef ecosystems that flourished in the Late Triassic Tethys seaway. The collapse of Late Triassic coral–sponge reefs was followed by an Early Jurassic (Hettangian–Sinemurian) perturbation interval with a near-global absence of reefs and sharp reductions in diversity. A Jurassic (Sinemurian) reef in the Hazelton Group of central British Columbia appears to fill the gap. Its paleoecology and composition show it to be the first large-framework example in North America and perhaps the world. It demonstrates that the reef-building Triassic coral, Phacelostylophyllum, survived the extinction event and was constructing reefs in Early Jurassic time during a global reef eclipse. The reef is a 48 m thick bioherm that grew within the island-arc complex of Stikinia. Following a decrease in volcanism, reef building began with bivalves growing upon water-lain tuffs. The reef was dominated by large dendroid–phaceloid corals, primarily Phacelostylophyllum rugosum, a species known from the Upper Triassic of Italy, which produced extensive constructional framework. Other fossils include bivalves, solitary and colonial corals, and a variety of dwellers and reef destroyers. Three stages of growth are present. During siliciclastic deposition, the reef mound grew into a bioherm with steep relief and flanking beds. Two intervals of arrested growth marked by pyroclastic lenses and hard grounds punctuated the reef's history. The reef was finally overwhelmed by volcaniclastic sediment, was uplifted, and developed paleokarst. The reef is unique in understanding the dynamics of recovery after the end-Triassic mass extinction.