PALEOSEISMIC IMPLICATIONS OF FLUID-ESCAPE STRUCTURES IN RAISED DELTASAT LOWER-5-ISLANDS - ECONOMY-POINT, NOVA-SCOTIA

Deformed proglacial deltaic sediments at Economy Point - Lower Five Is lands, Nova Scotia, were examined as a possible indicator of postglaci al earthquake activity. The deposits represent an ice-proximal, glacio marine-deltaic environment, that likely experienced postglacial seismi c shaking. Several styles of deformation were recognized that could be attributed to penecontemporaneous deformation and are common to parti cular mechanisms of formation and facies. Ball and pillow structures a ssociated with: (I) loading by rapid sedimentation, or (2) from percus sion by dropstones, are common in the bottomset units. Crumpled, rolle d-up or boudinaged layers were likely formed by several mechanisms, in cluding: (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 postglacial seismic shaking of saturated sediments. Charcoal, excavated above a fluid-escape structure in tops et layers below Holocene sediment at Economy Point, has been radiocarb on dated and indicates that a significant seismic ground-disturbance m ay have occurred there around A.D. 1870 +/- 70 years. Replicate sampli ng is needed to confirm the age of the seismic disturbance. However, i f this approximate age is correct, the deformation may have resulted f rom the 1855 (estimated M 5+) earthquake at New Brunswick, located abo ut 100 km to the west. These preliminary results demonstrate that the physical structure of deltaic sediments can provide a record of neotec tonic activity and contribute valuable data for assessment of earthqua ke occurrence and/or recurrence. Deformation structures associated wit h catastrophic fluidization of topset facies or overlying Holocene sed iments are considered to be the best indicators of postglacial seismic disturbance.