Sedimentology and Stratigraphy of the AND-1B Core, ANDRILL McMurdo Ice Shelf Project, Antarctica

During the 2006–2007 austral season, the ANDRILL McMurdo Ice Shelf Project recovered a core 1285 m-long (AND-1B) from Windless Bight in the McMurdo Sound region. This core contains a range of lithologies, including both siliciclastic and volcanic diamictites, sandstones and mudstones; diatomites; and volcanic ash/tuff and one phonolitic lava fl ow. The succession has been subdivided into eight lithostratigraphic units and 25 subunits, based on lithological abundances. Eleven lithofacies have been recognized, ranging from open marine diatomites and mudstones to turbidites to ice-proximal massive and stratifi ed diamictites. More than 60 glacimarine sequences have been recognised, bounded by glacial surfaces of erosion. Three distinct facies stacking patterns are present, showing evidence of glacial advance/retreat/advance with varying degrees of preservation. Carbonate and pyrite are the dominant secondary phases in the core. The pyrite overprint is especially notable in volcanic sediments below ~400 metres below seafl oor (mbsf), where it often obscures stratifi cation and sediment texture. INTRODUCTION AND OVERVIEW This chapter presents the results of sedimentologic description/study and lithostratigraphic subdivision of the AND-1B core. The detailed core descriptions are shown as graphic logs at a scale of 4 m/page and are available online at http://www.mna. it/english/Publications/TAP/terranta.html (Section 'Downloadable fi les'). Summary logs, at a scale of 100 m/page, are included here as fi gure 1. Additional summary logs, at scales of 650 m/page and 1285 m/ page, have been generated and are used elsewhere in this report by other discipline teams. The core has been divided into eight lithostratigraphic units on the basis of major changes in lithology recognised during core description. This division draws attention to the relative importance of prominent lithologies, such as diamictites, diatomites, and volcanic sediments and rocks. The eight major lithostratigraphic units are divided further into a total of 25 lithostratigraphic subunits on the basis of smaller-scale lithologic changes. In addition to changes in the relative abundances of diamictites, diatomites, and volcanic sediments/rocks, changes in the relative abundances of sandstones and mudstones have been used to defi ne lithostratigraphic units. Eleven recurring lithofacies have been recognised in the AND-1B core on the basis of a lithology or an association of lithologies, bed contacts and bed thicknesses, sediment texture, sedimentary structures, and, to a lesser extent, colour. The entire facies assemblage is interpreted to record marine to subglacial environments of deposition. Facies 1 (diatomite) is interpreted to record pelagic sedimentation in an open marine environment, although sometimes affected by iceberg rafting. Facies 2 (mudstone, primarily massive) is interpreted to record environments dominated by hemipelagic suspension settling, which may be either distal or proximal to grounded ice. Siltstone and sandstone laminae within Facies 2 may represent the minor infl uence of distal sediment gravity fl ows, winnowing by bottom currents, or settling from turbid plumes; future research may reveal which of these, if any, carries a signal of glacial proximity. Coarser sediments (poorly sorted sands and lonestone clasts) record the infl uence of ice rafting, at least episodically. Facies 3 (interstratifi ed mudstone and sandstone) is similar to Facies 2 but with an increased importance of sandstone interbeds. This interbedding is interpreted to record the combination of hemipelagic sedimentation and distal to proximal sediment gravity fl ows, sourced either from grounding line processes or volcanic/ tectonic activity. Ice rafting and traction currents may have supplied or modifi ed the coarser-grained component in Facies 3. © Terra Antartica Publication 2007 186 L. Krissek et al. Fig. 1 – Graphical logs of the sedimentary sequence recovered at AND-1B, showing lithostratigraphic subdivisions. © Terra Antartica Publication 2007 187 Sedimentology and Stratigraphy of the AND-1B Core