THE GROTTO CREEK SUCCESSION: A UNIQUE RECORD OF LATE TRIASSIC – EARLY JURASSIC ENVIRONMENTAL AND BIOLOGIC CHANGE

The end-Triassic mass extinction (ETE) was one of the largest in the Phanerozoic. It is provisionally linked to the emplacement of the Central Atlantic Magmatic Province and the subsequent environmental and climatic responses that led to a severe deterioration of the terrestrial and marine biospheres. However, our current understanding of this mass extinction is hampered by: 1) Geographically biased data that primarily derive from the ancient Tethys Ocean; and 2) studies that only have focused on the interval immediately surrounding the mass extinction. To address these limitations, we present the initial results from our ongoing, multi-disciplinary study of long-term environmental change across the Late Triassic – Early Jurassic as recorded in the McCarthy Formation exposed at Grotto Creek in Wrangell Mountains of Southcentral Alaska.

The succession exposed at Grotto Creek consists of more than 550 meters of well-preserved marine strata that were deposited on the Wrangellia Plateau in the open, tropical Panthalassic Ocean. Analysis of the sedimentary facies and architecture of the McCarthy Formation indicates that it represents a distal ramp environment dominated by hemipelagic deposits and reworked shallow-water sediment deposited in lobe complexes. Ammonite, bivalve, hydrozoan, and conodont biostratigraphy indicates the Grotto Creek section is a relatively complete succession that contains all the stages from the Triassic Norian Stage to the Jurassic Pliensbachian Stage. Importantly, the succession contains many primary volcanic ash beds, and the first U-Pb age dates from two of these beds bracket the Norian-Rhaetian Stage Boundary and yield age estimates of 209.84+0.14 and 208.08+0.23 Ma, respectively. The preliminary organic carbon isotope (δ¹³C_org) record contains major features consistent with the records in the Tethys and elsewhere: negative excursions are found at the Norian-Rhaetian Boundary and around the ETE, and a positive excursion that occurs in the Hettangian. In summation, the succession at Grotto Creek offers a unique long-term window into environmental and biological changes that occurred in the Panthalassic Ocean across the Late Triassic – Early Jurassic interval and holds great promise to refine our understanding of this particularly turbulent time in Earth History.