A UNIQUE SNAPSHOT IN TIME: COMPLEX CORALS IN AN EARLY TOARCIAN REEF, SIERRA DE SANTA ROSA FORMATION, SONORA, MEXICO

Recent studies of a Toarcian reef, dated with U-Pb analysis of detrital zircons, in the upper member of the Sierra de Santa Rosa Formation of Sonora, Mexico reveal two massive biohermal carbonate deposits containing framework-building corals. The Sierra de Santa Rosa Formation is divided into three informal members: the upper, middle and lower. The carbonate structures situated in the upper member of the formation consist of interbedded mudstone, calcareous sandstone, fossiliferous limestone with bivalve assemblages, and large, diverse, paleoecologically complex corals. The reef-building corals are dominated by phaceloid, cerioid and branching multiserial species, that are currently under taxonomic study. These represent the culmination of the recovery phase after the end-Triassic mass extinction. The reef is possibly the only known example of a Toarcian coral reef in North America. The roughly 20 meter thick massive coral limestone is in the upper unit and is not well defined. A fault at the approximate center divides the limestone and therefore may represent two separate bioherms. An exposure in the Sierra del Álamo confirms the geochronology, with volcanic activity in well-separated tuff beds in the middle part of the Sierra de Santa Rosa Formation. Rock lithology varies between the Sierra de Santa Rosa and the Sierra del Álamo regions, and ammonites have not been reported in detail from the first locality so it had been difficult to establish accurate relative time-stratigraphic correlations of the reef. Recent U-Pb analysis of detrital zircons below, within the fault, and above the reefal mounds in combination with the tuff deposits in the Sierra del Álamo provide evidence of contemporaneous magmatic activity that affirmed the reefal mounds have an early Toarcian age of 180 ± 2 Ma.