The Glass Ramp of Wrangellia: Reconstruction of the Late Triassic to Early Jurassic Outer Ramp Environments of the McCarthy Formation, Alaska

Abstract

Following the end-Triassic mass extinction, widespread biogenic ramps on Pangea became glass ramps, which are characterized by siliceous sponge meadows. This transition may represent a global increase in oceanic silica concentrations resulting from the weathering of the Central Atlantic Magmatic Province, but evidence is limited to the continental margins of Pangea. The McCarthy Formation gives us the oceanic perspective, as it represents Norian to Hettangian sedimentation on Wrangellia, an isolated terrane in the Panthalassan Ocean. I therefore analyzed the facies and architecture of the lower and upper member of the McCarthy Formation at Grotto Creek (Wrangell Mountains, Alaska). The McCarthy Formation records an outer ramp where reworked shallow-water sediment was deposited on lobe complexes. When the supply of reworked sediment was reduced, hemipelagic sedimentation predominated. Bottom currents were active when the upper member was deposited and produced a sediment drift on the outer ramp in the Hettangian. The reworked bioclasts transition across the member boundary from calcareous shell fragments to siliceous spicules, which represents a transition from a siliceous carbonate-ramp to a glass ramp in the Hettangian. This transition coincides with an order of magnitude increase in sedimentation rates, which shows how an increase in shallow-water sediment production increases the sediment supply to the outer ramp. The presence of a glass ramp on Wrangellia supports the hypothesis that a global increase in oceanic silica concentrations promoted widespread biosiliceous sedimentation on ramps.