ALBERT

Description: Lonestone abundances in CRP-1 were investigated using three methods: core examination at Cape Roberts Camp, analysis of digital core images and follow-up core examination. For all images of split-core, we determined size and depth of every detectable lonestone larger than 3 mm. Lonestone abundance decreases exponentially with clast size. Although no significant depth-dependent variations in lonestone size distribution were detected, a strong 0.5-0.7 m abundance periodicity, of unknown origin, is evident within diamicts. Lonestone volume percentage was estimated from size distribution: most size classes contribute approximately the same volume to the total. Sizes 〉16 mm have rare enough lonestones that their counts are nonrepresentative when based on short intervals of split core. This problem does not affect total counts significantly, but the volume analysis needs to be confined to 〈= 6 mm lonestones to avoid instability induced by rare and nonrepresentative larger lonestones. If lonestone abundance can be used as an indicator of glacial proximity, then our CRP-1 lonestone abundance logs confirm the overall character of previously inferred variations in relative distance to the ice margin. Large-scale changes in lonestone abundance also reflect the CRP-1 sequence stratigraphy, with individual sequences generally characterised by basal lonestone-rich diamict overlain by lonestone-poor sands and muds. The relationship between glacial proximity and lonestone abundance within diamicts and within sand-mud intervals is, however, less certain. For example, two or three gradual lonestone increases may indicate regressions during glacial advances, in contrast to the more common CRP-l pattern of dominantly transgressive sequences.

Description: CoreScan images were examined to determine the origin of breccias in the CRP-1 core. Breccias occur throughout the core. but are dominant deformation features in the upper 85 m. Breccia textures, boundaries and texture arrangements suggest that in situ fracturing and horizontal planar shearing are important deformation mechanisms in the upper part of the Miocene section. Forceful injections of silt and clay into fractures point to dewatering of overpressurised sediment. Breccias located below 55 metres below the sea floor (mbsf) occur associated with soft-sediment deformation, which is absent in younger intervals of the core. Deformation styles interpreted from the breccia textures, their downcore distribution, and the relations of breccias with sequence boundaries and lithologies, such as diamictites and graded beds, suggest brecciation occurred as a result of subglacial shearing and mass-movement processes. These mechanisms were also proposed for breccias and soft sediment deformation features observed in other McMurdo Sound cores. Subglacial shearing was a possible cause for the development of two thick brecciated intervals at ~44 and at ~79 mbsf, whereas slope failure and redeposition was probably the cause of brecciation below ~85 mbsf.