The glass ramp of Wrangellia: Late Triassic to Early Jurassic outer ramp environments of the McCarthy Formation, Alaska, U.S.A.

Abstract

The marine record of the Triassic–Jurassic boundary interval has been studied extensively in shallow-marine successions deposited along the margins of Pangea, particularly its Tethyan margins. Several of these successions show a facies change from carbonate-rich to carbonate-poor strata attributed to the consequences of igneous activity in the Central Atlantic Magmatic Province (CAMP), which included a biocalcification crisis and the end-Triassic mass extinction. Evidence for a decline in calcareous and an increase in biosiliceous sedimentation across the Triassic–Jurassic boundary interval is currently limited to the continental margins of Pangea with no data from the open Panthalassan Ocean, the largest ocean basin. Here, we present a facies analysis of the McCarthy Formation (Grotto Creek, southcentral Alaska), which represents Norian to Hettangian deepwater sedimentation on Wrangellia, then an isolated oceanic plateau in the tropical eastern Panthalassan Ocean. The facies associations defined in this study represent changes in the composition and rate of biogenic sediment shedding from shallow water to the outer ramp. The uppermost Norian to lowermost Hettangian represent an ∼ 8.9-Myr-long interval of sediment starvation dominated by pelagic sedimentation. Sedimentation rates during the Rhaetian were anomalously low compared to sedimentation rates in a similar lowermost Hettangian facies. Thus, we infer the likelihood of several short hiatuses in the Rhaetian, a result of reduced input of biogenic sediment. In the Hettangian, the boundary between the lower and upper members of the McCarthy Formation represents a change in the composition of shallow-water skeletal grains shed to the outer ramp from calcareous to biosiliceous. This change also coincides with an order-of-magnitude increase in sedimentation rates and represents the transition from a siliceous carbonate-ramp to a glass ramp ∼ 400 kyr after the Triassic–Jurassic boundary. Sets of large-scale low-angle cross-stratification in the Hettangian are interpreted as a bottom current–induced sediment drift (contouritic sedimentation). The biosiliceous composition of densites (turbidites) and contourites in the Hettangian upper member reflects the Early Jurassic dominance of siliceous sponges over Late Triassic shallow-water carbonate environments. This dominance was brought about by the end-Triassic mass extinction and the collapse of the carbonate factory, as well as increased silica flux to the ocean as a response to the weathering of CAMP basalts. The presence of a glass ramp on Wrangellia supports the hypothesis that global increases in oceanic silica concentrations promoted widespread biosiliceous sedimentation on ramps across the Triassic to Jurassic transition.