POTENTIAL TRIGGERS OF LARGE PREHISTORIC LANDSLIDES IN PRESUMPSCOT FORMATION, SOUTHERN MAINE

The Presumpscot Formation is a sensitive glaciomarine mud that is prone to failure and common in the most populated areas of Maine (southern and coastal locations). Lidar topographic data recently revealed over 200 large landslides (approximate average area 25 acres/10 hectares) of unknown age that are correlated with the Presumpscot Formation. The ages of 24 landslides/landslide complexes in southern Maine were estimated through radiocarbon analysis of vegetation buried by, caught up in, or deposited on top of the landslides. Prior to this study it was hypothesized that these landslides likely happened in an unstable landscape during early post-glacial times (before 10,000 calendar year BP), but the majority of sampled landslides occurred in the last 4,000 calendar years. Landslide activity can be further subdivided into clusters of events from 3,000-4,000 and 500-1,000 calendar years BP. Landslides in sensitive clay are usually attributed to slope de-buttressing (commonly from river cut bank erosion), wet conditions, or earthquakes. Several landslides in this study can be explained by river cut bank erosion, but the clustering of events and lack of significant river erosion at many sites suggests a more regional trigger. The contributions of paleoclimate and paleoseismicity to landslide activity in southern Maine will be considered.