Instruments and methods using borehole logging and electron backscatter diffraction to orient an ice core from Upper Fremont Glacier, Wyoming, USA

Abstract

While glacier fabric reflects the accumulated strain, detailed azimuthal information is required to link the microstructure to the flow, and this is not easily gathered at depth. Borehole logging provides a way to obtain a log of azimuthal orientation of tilted stratigraphic features that can be used to orient the core with respect to glacier flow. We demonstrate this using acoustic borehole logs and the ice core from a 162m borehole in Upper Fremont Glacier, Wind River Range, Wyoming, USA. We measured the dip of tilted dust and bubble layers in the actual ice core, identified them on the borehole log, then used their strike to orient the core sections containing them. We examined the crystal orientation fabric of our samples, using electron backscatter diffraction in a scanning electron microscope. When we compared the orientation of the tilted layers in some samples with the fabric, we found that the normal to the foliation and the c-axes maxima both pointed in the direction of maximum shear stress. This illustrates the usefulness of borehole logs for orienting ice cores after removal from the borehole, and for developing a better understanding of fabric development.