ABSTRACT: Paleoseismic implications of fluid-escape structures in raised deltas at Lower Five Islands

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Abstract

Deformed proglacial deltaic sediments at Economy Point - Lower Five Islands, Nova Scotia, were examined as a possible indicator of postglacial earthquake activity. The deposits represent an ice-proximal, glaciomarine-deltaic environment, that likely experienced postglacial seismic shaking. Several styles of deformation were recognized that could be attributed to penecontemporaneous deformation and are common to particular mechanisms of formation and facies. Ball and pillow structures associated with: (1) loading by rapid sedimentation, or (2) from percussion by dropstones, are common in the bottomset units. Crumpled, rolled-up or boudinaged layers were likely formed by several mechanisms, including: (1) intralayer or intra-unit compaction or stretching; (2) in association with gravity slumping, mainly in the foreset units; and (3) from loss of support due to post-depositional fluid-escape, mainly seen in association with deformation exposed in the topset units. The fluid-escape structures are attributed to the expulsion of groundwater and sediment slurries during post-glacial seismic shaking of saturated sediments. Charcoal, excavated above a fluid-escape structure in topset layers below Holocene sediment at Economy Point, has been radiocarbon dated and indicates significant seismic ground-disturbance may have occurred there around A.D. 1870+/-70 years. Replicate sampling is needed to confirm the age of the seismic disturbance. However, if this approximate age is correct, the deformation may have resulted from the 1855 (estimated M 5+) earthquake at New Brunswick, located about 100 km to the west. These preliminary results demonstrate that the physical structure of deltaic sediments can provide a record of neotectonic activity and contribute valuable data for assessment of earthquake occurrence and/or recurrence. Deformation structures associated with catastrophic fluidization of topset facies or overlying Holocene sediments are considered to be the best indicators of postglacial seismic disturbance.

Bibliography of Canadian Geomorphology