EARLY JURASSIC CORALS FROM THE EL ANTIMONIO GROUP OF SONORA, MEXICO, AND THE IMPLICATIONS FOR CORAL RECOVERY AFTER THE END-TRIASSIC MASS EXTINCTION

Within the Sierra de Santa Rosa Formation of the El Antimonio Terrane in Sonora, Mexico, are Early Jurassic (Sinemurian-Toarcian) corals. A global mass extinction occurred at the Triassic-Jurassic boundary and coral reefs collapsed at the end of the Triassic. During this period of global climate change approximately 200 million years ago, sea level was dropping and the Central Atlantic Magmatic Province was releasing prodigious amounts of greenhouse gases and volatiles with increasing ocean acidity. The Jurassic recovery was underway during the first 8-12 million years following the extinction event and reefs were exceedingly rare. Coral diversity was further suppressed during the Toarcian Oceanic Anoxic Event associated with the Karoo-Ferrar large igneous province. Corals did not fully recover until the Middle Jurassic. The dynamics of coral collapse and recovery are relevant to understanding marine deposition and the nature of ecological stresses following the mass extinction. Compared to the Tethys, precious little is known about the recovery in western North America although reefs and corals from the Late Triassic have been well studied. The Sierra de Santa Rosa Formation is divided into lower, middle, and upper members encompassing approximately 1500 meters in thickness. It includes fossiliferous, shallow-to-deep marine sequences in strata tectonically overlying Precambrian metamorphic basement rock and conformably underlying the mostly volcanic Cerro San Luis Formation speculated to be Lower to Middle Jurassic. The Sierra de Santa Rosa Formation has been established as Early Jurassic using bivalves, ammonites and U-Pb dating of detrital zircons. Early Jurassic corals occur at multiple locations in the middle and upper members of the Sierra de Santa Rosa Formation. These coral locations include Montlivaultia haimei and other Sinemurian solitary corals in impure limestone as well as a Toarcian carbonate buildup of colonial and branching corals and subordinate bivalves. Most coral fauna are highly recrystallized and reveal mainly macroscopic details. The analysis of these corals is used to test paleogeographic and tectonic models for northwestern Sonora and adjacent USA. The Sonoran reef-like buildups are perhaps the only Early Jurassic examples in western North America.