Abstract

Samples of gravity cores recovered from Yakutat Bay and Glacier Bay in southeast Alaska were examined in thin section. The samples, representing rhythmically laminated plume deposits, current-derived silts and sands, and iceberg-rafted diamicts, exhibit microstructures indicative of vertical deformation such as loading and water escape, as well as boudinage and other features, which are indicative of simple shear deformation.

Narrow zones in many of the fine-grained core intervals show subtle patterns of preferred clay-mineral orientations (plasmic fabrics). We interpret these intervals as being low-strain simple shear zones. They all show a consistent and regular internal configuration that is formed by sets of conjugate shear planes. The setting and conditions in the Alaskan fjords allow the assumption that the shear zones are exclusively associated with penecontemporaneous movements of the unconsolidated sediment on subaqueous slopes. Analyzing characteristics and trends relating to symmetry and strain intensity, we argue that these are diagnostic identifiers for sliding mass movements. We present a conceptual model in which deformation resulting from gravity-driven sliding is described. We suggest that strain signatures can be used to recognize types of shear zones and be helpful in distinguishing between, for example, gravity-driven deformation and subglacial deformation in sedimentary records.