SPECULATION ON GLACIAL VARVE DEPOSITION IN THE NORTHEASTERN US

In the Mohawk(NY), Champlain(NY), and Connecticut(NH-VT) valleys varve thickness and lithology are controlled by summer sediment supply and dispersal. Underflow/interflow currents moved from ice-proximal (high sand/silt supply) through ice-distal areas (low sand/silt supply) progressively less influenced by glacial meltwater. This trend also occurs at one location as ice recedes, leading to 1-5 mm varves entirely derived from meteoric runoff. Thick (10 to >100 cm) ice-proximal varves have massive silt/sand units suggesting rapid settling from meltwater plumes, and rippled and cyclically laminated silt/sand units. Ice-distal varves (0.5-3 cm) have laminated silty summer beds thinner than winter clays due to starvation of sand/silt away from the glacier. Draping of summer layers on valley sides suggests that under/interflows occupied 75% of water columns. Submerged bedrock/till highs partitioned basins resulting in ponding of silt/sand up valley. Reduced current velocities down valley resulted in thick (3-15 cm) clayey summer layers and winter clays thinner than in other areas due to summer clay settling. Prominent graded sand/silt beds with clay caps at the bottoms of summer layers represent spring or nival floods that disturbed the tops of winter clays. Graded beds at summer layer tops may represent fall overturning. Floods produced by sudden release of water from tributary lakes created extra-thick (30X summer), graded, faintly bedded units. Sudden lake level drops resulted in abrupt thickening and coarsening of varves as tributary erosion increased sediment supply that was dispersed over a reduced lake area. Winter clays have relatively uniform thickness across a basin. Rapid clay deposition in early winter is suggested by SEM images of floccules, and concentrations of nematode trails and ice-rafted sediment on clay tops. High suspended clay and dissolved Ca, and low temperatures may have promoted flocculation, despite low ionic strength. Pervasive ice-rafted debris (stones and pellets) in NY varves have not been found in New England, perhaps due to a combination of cleaner glacial ice and lower calving rates in a shallower lake.