LANDSLIDES IN GLACIAL LAKES: A FUNDAMENTAL PROCESSES GOVERNING THE STRATIGRAPHY OF GLACIAL LAKES IN MOUNTAINOUS TERRAINS

In mountainous terrains sediments deposited in glacial lakes frequently provide the underpinnings for both prehistoric and historic settlement. The composition and internal structure of these sediments affect their strength and hydraulic properties which in turn dictates how they have eroded during the Holocene and their utility for various human activities.

Ice-proximal and ice-distal glacio-lacustrine sediment facies, e.g. varved silt/clay, have been recognized for some time and widely mapped across areas once flooded by glacial lakes. In recent years detailed mapping of glacial sediments in the mountain valleys of northern Vermont reveals the importance of a glacio-lacustrine facies dominated by landslide deposits: Stratified Diamict. These deposits consist of interlayered diamict and fine sand and/or silt. In areas close to shore diamict dominates, whereas farther from shore the frequency and thickness of silt and/or fine sand layers increase. Sedimentary structures within the diamict are rare, but sometimes a poorly developed bedding-parallel fabric is present. Bedding in the silt and fine sand layers occurring between diamict layers can be coherent, but is often disrupted by faults, shear zones, and folds. Deformation is sometimes restricted to the contact with the overlying diamict, but frequently affects whole strata. This facies is most common in steep-sided valleys (where 10’s of meters of sediment has accumulated. Sediments oftentimes not only completely fill former lake basins, but continue as gently sloping surfaces of diamict extend ing well above projected lake elevations.

Sediments accumulating on steep slopes are inherently metastable. Glacial till in the Green Mountains is very silt-rich owing to the micaceous bedrock. When recently-exposed, unvegetated till on the steep slopes surrounding glacial lakes was water-saturated it frequently failed flowing into the adjacent lakes as debris flows. Silt, suspended following the landslide, settled forming thin partings between diamict layers. Sand, sourced from either inflowing streams or the retreating ice sheet, also frequently failed, failures triggered by spring melt, storms, or debris flows. Modern slope failures have preferentially occurred along the sloping silt layers resulting in translational slides.