Abstract

Late Jurassic anaerobic-dysaerobic mudstones crop out on both the Weddell Sea (back-arc) and Pacific (fore-are) margins of the northern Antarctic Peninsula. The only known occurrence on the Pacific margin of the Antarctic Peninsula is the Anchorage Formation of Livingston Island. This mudstone dominated unit comprises interbedded volcaniclastic sandstones, pyroclastic/epiclastic tuffs and radiolarian mudstones. The volcaniclastic sandstones are interpreted as representing deposition from turbidity currents. The tuffs represent sedimentation by both primary airfall processes and resedimentation by low concentration turbidity currents. The radiolarian mudstones represent suspen­sion sedimentation, and reveal an upward increase in bioturbation with a transition from anaerobic-dysaerobic conditions to dysaerobic-aerobic conditions. These facies and the observed vertical change in oxygenation conditions are similar to those seen in the Nordenskjöld Formation on the Weddell Sea margin of the Antarctic Peninsula. However, biostratigraphical investigations show that the transition from dysaerobic to aerobic conditions occurred during the late Kimmeridgian-early Tithonian in the Anchorage Formation but late Tithonian or early Berriasian in the Nordenskjold Formation. This diachroneity is related to the palaeogeographical development of the Antarctic Peninsula magmatic arc. A wide epicontinental sea and subdued arc relief in the early Kimmeridgian was followed in the Tithonian by arc uplift, increasing oxygenation in the fore-are basin, and the development of a restricted basin in the hack-arc region. In latest Tithonian-earliest Berriasian times a substantial arc had developed which supplied volcaniclastic sediment to the fore-arc basin; only then was the back-arc basin undergoing the transition from dysaerobic to aerobic conditions. Anaerobic conditions initiated by regional upwelling and expansion of the oxygen minimum zone were perpetuated in a silled basin in the back-arc area, formed by the emergent arc.

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