Sedimentary processes at the base of a West Antarctic ice stream: Constraints from textural and compositional properties of subglacial debris
Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub-ice-stream environment. Piston coring in holes bored by hot-water drilling yielded five 1-3 m long, undisturbed subglacial sediment cores. We analyzed granulometry, composition, and particle morphology in these cores. The UpB cores are composed of a clay-rich, unsorted diamicton containing rare marine diatoms. Sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-thick till layer and not in situ glacimarine sediments. The till does incorporate material recycled from the subjacent poorly indurated Tertiary glacimarine sediments of the Ross Sea sedimentary basin, which extends beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clay-rich source sediments. The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinders interparticle stress concentrations. Our observations are consistent with the conjecture that subglacial deformation of weak, fine-grained tills does not produce significant comminution of till debris (Elson 1988). Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine-grained sediments. Since such weak till layers create a permissive condition for ice streaming, sub-glacial geology may have an indirect but strong control over the location, extent, and basal mechanics of ice streams.
Additional Information© 1998 SEPM (Society for Sedimentary Geology). Received 30 July 1996; accepted 11 September 1997. This project was funded by grant OPP-9219279 from the National Science Foundation. Partial support was provided to R. Scherer by NSF grant OPP-9496169 and the Swedish Natural Science Research Council and to S. Tulaczyk by the Henry and Grazyna Bauer Fellowship. Rowena Lohman provided essential help with laboratory analyses and James Spotila and Miriam Jackson with thorough reviews of an early version of this manuscript. We thank Richard Alley, Lincoln Pratson, and an anonymous reviewer for helpful criticisms, some of which are not answered here. Contribution 8506, Division of Geological and Planetary Sciences, California Institute of Technology.
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