ABSTRACT

Ash layers in the Lower Tithonian Longing Member (Ameghino Formation) on the Antarctic Peninsula are derived from the fallout of tephra associated with explosive eruptions. These layers are preserved in a complete and continuous sedimentary record formed in a deep, anoxic marine environment, with high background sedimentation rates allowing rapid burial and preservation. Absence of bioturbation precluded the thin ash beds from mixing with enclosing sediment. The average interval between explosive eruptions in the volcanic arc of the Antarctic Peninsula during the Early Tithonian is estimated at between 700 and 1000 years, the highest frequency of explosive volcanic events ever defined in ancient marine rocks. This frequency is comparable to that of explosive eruptions in some presently active volcanic arcs. This indicates that deposition in some deep marine settings can produce records sufficiently complete to determine accurate frequencies of explosive eruptions in ancient magmatic arcs. The apparent lull in the volcanic activity in the Antarctic Peninsula during this time, inferred from dating of volcanic rocks from the arc itself, is not supported by the marine record in the adjacent basin and must be due to a bias of sampling.