LATE JURASSIC PALEOGEOGRAPHY AND ANAEROBIC-DYSAEROBIC SEDIMENTATION IN THE NORTHERN ANTARCTIC PENINSULA REGION

Late Jurassic anaerobic-dysaerobic mudstones crop out on both the Wedd ell Sea (back-are) and Pacific (fore-arc) margins of the northern Anta rctic Peninsula. The only known occurrence on the Pacific margin of th e Antarctic Peninsula is the Anchorage Formation of Livingston Island. This mudstone dominated unit comprises interbedded volcaniclastic san dstones, pyroclastic/epidastic tuffs and radiolarian mudstones. The vo lcaniclastic sandstones are interpreted as representing deposition fro m turbidity currents. The tuffs represent sedimentation by both primar y airfall processes and resedimentation by low concentration turbidity currents. The radiolarian mudstones represent suspension sedimentatio n, and reveal an upward increase in bioturbation with a transition fro m anaerobic-dysaerobic conditions to dysaerobic-aerobic conditions. Th ese facies and the observed vertical change in oxygenation conditions are similar to those seen in the Nordenskjold Formation on the Weddell Sea margin of the Antarctic Peninsula. However, biostratigraphical in vestigations show that the transition from dysaerobic to aerobic condi tions occurred during the late Kimmeridgian-early Tithonian in the Anc horage Formation but late Tithonian or early Berriasian in the Nordens kjold Formation. This diachroneity is related to the palaeogeographica l development of the Antarctic Peninsula magmatic are. A wide epiconti nental sea and subdued are relief in the early Kimmeridgian was follow ed in the Tithonian by are uplift, increasing oxygenation in the fore- are basin, and the development of a restricted basin in the back-are r egion. In latest Tithonian-earliest Berriasian times a substantial are had developed which supplied volcaniclastic sediment to the fore-are basin; only then was the back-are basin undergoing the transition from dysaerobic to aerobic conditions. Anaerobic conditions initiated by r egional upwelling and expansion of the oxygen minimum zone were perpet uated in a silled basin in the back-are area, formed by the emergent a re.