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Abstract

The ANDRILL McMurdo Ice Shelf initiative recovered a 1285-m-long core (AND-1B) composed of cyclic glacimarine sediments with interbedded volcanic deposits. The thickest continuous volcanic sequence by far is ~175 m long and is found at mid-core depths from 584.19 to 759.32 m below seafl oor. The sequence was logged, and initial interpretations of lithostratigraphic subdivisions were made on ice during drilling in late 2006. Subsequent observations, based on image, petrographic, and scanning electron microscopy-energy dispersive spectroscopy analyses, provide a more detailed, revised interpretation of a thick submarine to emergent volcanic succession.The sequence is subdivided into two main subsequences on the basis of sediment composition, texture, and alteration style. The ~70-m-thick lower subsequence consists mostly of monothematic stacked volcanicrich mudstone and sandstone deposits, which are attributed to epiclastic gravity fl ow turbidite processes. This subsequence is consistent with abundant active volcanism that occurred at a distal site with respect to the drill site. The ~105-m-thick upper subsequence consists mainly of interbedded tuff, lapilli tuff, and volcanic diamictite. A Late Miocene (6.48 Ma) 2.81-m-thick subaqueously emplaced lava fl ow occurs within the second subsequence. This second subsequence is attributed to recurring cycles of submarine to emergent volcanic activity that occurred proximal to the drill site. This new data set provides (1) the fi rst rock evidence of signifi cant Late Miocene submarine volcanic activity in the Ross Embayment during a period of no to limited glaciation, and (2) a rich stratigraphic record that elucidates submarine volcano-sedimentary processes in an offshore setting.

Lithostratigraphy from downhole logs in Hole AND-1B, Antarctica

“…However, this relation does not appear to hold for the underlying interval, 728-734 mbsf, where high-susceptibility beds are not refl ected in the lithostratigraphy. Magnetic susceptibility does not follow the iron content (from XRF-CS data), indicating that magnetite is not the principal iron-bearing mineral in this interval, perhaps related to the diagenetic growth of pyrite (Di Roberto et al, 2010). There is no great change in the source of these volcanic-rich claystones and sandstones, suggesting that the original magnetic mineralogy might be somewhat homogeneous, and that dissolution of the magnetic minerals has played a role in shaping the susceptibility profi le.…”

“…3). Higher NGR and K values mark layers where K-rich volcanic components (e.g., lapilli tuffs) dominate over the siliciclastic component (Di Roberto et al, 2010). In addition, potassium is preferentially mobilized during alteration, thus drops to lower K and NGR values probably mark intervals that are more altered than the intervening material.…”

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“…Evidence for Late Miocene (ca. 6.5 Ma) submarine to emergent volcanism was found in close proximity to White Island (Di Roberto et al, 2010). Farther south, Black Island, Minna Bluff, Mount Morning, and Mount Discovery are all major eruptive centers belonging to the Erebus volcanic prov-ince.…”

Alteration of volcanic deposits in the ANDRILL AND-1B core: Influence of paleodeposition, eruptive style, and magmatic composition

“…An ~175-m-thick and continuous volcanic succession is between 584.19 and 759.32 m below the seafl oor (mbsf). This volcanic lithostratigraphic unit (LSU5) is also characterized by the absence of diatomite and by a wide range of siliciclastic sediments (Krissek et al, 2007;Di Roberto et al, 2010). LSU5 includes different lithologies that were interpreted to record the evolution of a volcanic complex from submarine to shallow water and/or emergent conditions with related glacio-volcanic gravity fl ow processes (Krissek et al, 2007;Di Roberto et al, 2010).…”

“…This volcanic lithostratigraphic unit (LSU5) is also characterized by the absence of diatomite and by a wide range of siliciclastic sediments (Krissek et al, 2007;Di Roberto et al, 2010). LSU5 includes different lithologies that were interpreted to record the evolution of a volcanic complex from submarine to shallow water and/or emergent conditions with related glacio-volcanic gravity fl ow processes (Krissek et al, 2007;Di Roberto et al, 2010). These volcanic rocks show different degrees of alteration that increase with depth (Di Roberto et al, 2010).…”