Miocene-Pliocene ice-volcano interactions at monogenetic volcanoes near Hobbs Coast, Marie Byrd Land, Antarctica

Abstract

Paleoenvironmental reconstructions and 40 Ar/ 39 Ar geochronology of seven eroded monogenetic volcanoes near the Hobbs Coast, Marie Byrd Land, West Antarctica provide proxy records of WAIS paleo-ice-levels in MiocenePliocene times. Interpretations, based on lithofacies analysis, indicate whether the volcanoes erupted below, near, or above the level of the ice sheet. Our interpretations differ significantly from previous interpretations as they highlight the abundant evidence for ice-volcano interactions at emergent paleoenvironments but limited evidence of higher-thanpresent syn-eruptive ice-levels. Evidence for subglacial volcanic paleoenvironments is limited to Kennel Peak, a ~8 Ma volcano where a pillow lava sequence extending 25 m above current ice level overlies an inferred glacial till and unconformity. A major complication in the Hobbs Coast region is that the volcanism occurred on interfluves between regions of fast-flowing ice. Such a setting precludes establishing precise regional paleo-ice-levels although the presence or absence of ice at times of eruptions can be inferred.

Chapter 5.4a Marie Byrd Land and Ellsworth Land: volcanology

“…This stage was mostly trachytic in composition and was Subglacial volcanic palaeoenvironments are inferred at two localities: Kennel Peak (Wilch and McIntosh 2007) and Bowyer Butte (LeMasurier 1990d). At Kennel Peak, a c. 125 m-thick section of hawaiitic pillow lavas and interpillow hyaloclastite breccia (8.01 ± 0.87 Ma) unconformably overlies a smooth moulded outcrop of granitic basement rocks (Wilch and McIntosh 2007). A <1 m-thick glacial till occurs locally along the unconformity between the volcanic and basement rocks.…”

“…The vesicularity and steep dips of the interbedded pillow lava and hyaloclastite, and the presence of similar age (K-Ar 8.39 ± 0.66 Ma) subaerial lavas nearby at an outcrop just north of Kennel Peak (LeMasurier 1990d), suggest that the pillow hyaloclastite deposits form part of a subaqueous lava-fed delta sequence, possibly associated with subaerial lava effusion. The pillow lava and hyaloclastite sequence extends only c. 25 m above today's ice surface, suggesting that the syneruptive local palaeo-ice level was at least slightly higher than today's ice level (Wilch and McIntosh 2007). At Bowyer Butte, a volcanic sequence of subaerial lava over thin hyaloclastite beds (1-5 m) resting on a striated granite surface has a low precision K-Ar age of 9.82 ± 1.8 Ma (LeMasurier 1990d).…”

“…LeMasurier (1990d) inferred a subglacial eruptive environment based on the presence of bedded basaltic hyaloclastite and interbedded tillite. Wilch and McIntosh (2007) inferred that the volcanic rocks resulted from subaerial Strombolian and phreatomagmatic eruptions, noting the bedding and cross-stratification, the variably vesicular lapilli tuff and tuff breccia, and the mix of sideromelane and tachylite glass clasts. They noted that some of the granitoid cobbles and boulders were encrusted with a thin (1 cm) coat of basaltic lava, and argued that this is consistent with eruption through a tillite but not consistent with the clasts being part of an interbedded glacial tillite.…”

“…undertaken by Wilch and McIntosh (2007) in the Hobbs Coast Volcanic Field. Such work would be likely to provide additional valuable information on the past extent of the WAIS at the time of the eruptions.…”

“…Monogenetic volcanoes at Bowyer Butte (9.82 ± 0.90 Ma by K-Ar: from LeMasurier and Thomson 1990) and Kennel Peak (8.01 ± 0.87 Ma) provide indications of Late Miocene syneruptive palaeo-ice levels that were 50-200 m higher than today's local ice levels (LeMasurier 1990d;Wilch and McIntosh 2007). Interpretations of regional ice-level changes from these local ice levels is complicated by the fact that both sites are situated on interfluves between glaciers that are descending steeply from inland areas to the Getz Ice Shelf.…”