Digital Varves

Varves of the Month for 2/1/2011 - 2/28/2011

Connecticut Valley Varves at North Hatfield, Massachusetts

Scale bar in cm.

Click on image to download original image file

This image shows ice-distal varves from a drill hole at North Hatfield in the Connecticut Valley of central Massachusetts. The summer or melt season layers (light, lower part of each annual layer) of varves in the sequence are labeled with their numbers in the new North American Varve Chronology (NAVC). These varves were deposited about 1250 years after ice recession at North Hatfield when the receding ice margin was about 80-90 km to the north near Bellows Falls, Vermont. The varves shown here are about 14,900-15,000 yr old. The varves at North Hatfield very closely match other varve sections (see graph) across the valley at Amherst (Umass core), further north in New Hampshire and Vermont (LCB, Canoe Bk.) and in the Merrimack Valley (MER) of southeastern New Hampshire as is shown for the century of the varves in the image (AM 5900-6000). To see the matching of varve sections from across New England for the whole NAVC go to NAVC Master Plots.

Plot of the NAVC (AM) varve years 5900-6000. Click on image for larger version (~4MB JPG)

The pattern of sedimentation was complex, being controlled by both glacial and non-glacial (meteoric) processes. Varve thickness was largely controlled by glacial meltwater activity as meltwater moved south away from the glacier but most of the sediment in the varves shown here may have been entrained by glacial meltwater from local tributaries that were non-glacial at the time of varve deposition. None of the varves in this part of the sequence, unlike varves lower in the section, have red sediment in them. This indicates that meltwater entering the lake from the glacier and meteoric sources were no longer contributing significant red sediment. Glacially-derived red sediment is impossible at this point in time since ice has receded to New Hampshire and Vermont where there are no red source rocks. The varves shown here are much thinner than varves lower in the sequence that were deposited when the receding ice front was much closer and more of the sediment was derived from a glacial source that was partly red sedimentary rock in northern Massachusetts.

Summer or melt season layers (light-colored units) are composed of a stack of micrograded units of fine sand and silt. Some of the varves have sandy (lighter) layers marking the beginning (bottom) of each summer layer. The coarse layers that begin many summers are the result of rapid meltwater release from the glacier and adjacent land surface in the early summer, perhaps assisted by lake overturning during the main summer melting event. Occasionally there are relatively thick fine sand layers within the summer layers that represent the influx of sandy sediment from tributaries during rainstorm or land surface snowmelt events. The upper parts of most summer layers have a slightly grayer appearance reflective of more clay and silt vs. sandier units below. Summer layers also appear to grade into their overlying winter layers. Gradation from summer to winter is reflected by many fine sand and silt partings that interbed with clay in the bottoms of winter layers suggesting that clay deposition is initiated before the close of the melting season when melting or rainstorm events can still disperse coarser sediment to the lake floor. The highly rhythmic nature of thin units within the summer layers is suggestive of diurnal variations in deposition. Summer layer thickness in this sequence is much greater than winter layer thickness and also varies much more than winter layer thickness. In this image summer and winter layer thicknesses appear to vary proportionally with the thickest summer layers overlain by the thickest winter layers and the thinnest summer layers overlain by the thinnest winter layers. This suggests that winter layer thickness is controlled by the volume of clay introduced to the lake during the summer and is dependent, like summer layer thickness, on summer discharge volumes to the lake.

Past Varves of the Month...