PALYNOFACIES ANALYSIS IN EASTERN NORTH AMERICAN LAKES APPLIED TO STUDIES OF PALEOSEISMICITY

Evidence of paleoseismicity can be difficult to recognize in stable near-craton environments, particularly where glaciation has obscured terrestrial evidence, thus limited long-term data are available to interpret seismic hazards. Lakes in intracontinental settings provide a sedimentological archive that includes mass-transport deposits (MTDs) and soft sediment deformation structures that may be related to regional paleoseismicity. Cores containing such sediment structures may be used to provide material for palynofacies analysis, which can corroborate or independently identify sediment movement.

Subaqueous sediment slumps include organic matter that in a vertical column may be distinguishable from bounding layers in having palynofacies typical of nearer shore sedimentation. Palynofacies analysis using a particle frequency approach comparing terrigenous and near-shore organic matter, non-pollen palynomorphs, and amorphous organic matter was applied to test samples from lakes suspected or known to have paleoseismic slumping deposits. A sharp increase in abundance of phytoclasts and near-shore material relative to amorphous organic material at the 32 cm interval in core SP14 from the deep basin of Sluice Pond, Massachusetts, records a likely MTD. A Bayesian age model was calculated for core SP14 using chronostratigraphic constraints from ²¹⁰Pb, two AMS ¹⁴C dates, ¹³⁷Cs onset and peak, Ambrosia horizon, total lead peak, and total vanadium peak. Within the errors of the age model the suspected MTD tentatively correlates with the 1755 Cape Ann earthquake. Similarly, a marked increase in the ratio of phytoclasts to amorphous organic matter compared to bounding layers occurs at the top of an MTD identified in core 15-07 and within sub-bottom acoustic profiles from Lac Dasserat, western Quebec. Pollen records from the cores can also help constrain the timing of the mass transport events, assisting in regional comparisons of lake records.