Digital Varves

Varves of the Month for 3/1/2011 - 5/31/2011

Connecticut Valley Varves Kelsey Ferguson (Redlands Brick Co.), South Windsor, Connecticut

Scale bar in cm.

Click on image to download original image file

This month's image shows ice-distal varves in an outcrop core collected in South Windsor in the Connecticut Valley of central Connecticut. The core was split and partially dried to reveal differences between sediment layers. Numbers on the right side of the image are AM (American) years in the new North American Varve Chronology (NAVC). These varves were deposited in Lake Hitchcock about 1150 years after ice recession at South Windsor when the receding ice margin was 50 km to the north near South Hadley, Massachusetts. The varves shown here are about 16,800 yr old. The varves at South Windsor very closely match other varve sections (see graph) nearby in Connecticut (Petersen Farm in South Windsor, Scantic in East Windsor, Glastonbury) and also much further north in Massachusetts. All of the very thin varves measured in this core (AM 3985, 3986, and 3992) were also recorded by Antevs in his original varve measurements. To see the matching of varve sections from across New England for the whole NAVC go to NAVC Master Plots.

Plot of North American Varve Chronology varves AM 3950-4050 at sections in Glastonbury, South Windsor (Kelsey-Ferguson - Redlands Brick Co. pit and Petersen Farm), and East Windsor (Scantic), Connecticut and southern Massachusetts record of Antevs (1922). The yellow swath is the part of the plot shown in varve image. (~1.5MB JPG)

Varve thickness was largely controlled by glacial meltwater activity as meltwater moved south away from the glacier but at least some of the sediment in the varves shown here was entrained by glacial meltwater from local tributaries that were non-glacial at the time of varve deposition. Varves in this part of the sequence, unlike varves lower in the section, have only a very faint red tint to their otherwise gray color. This indicates that meltwater entering the lake from the glacier and meteoric sources were no longer contributing significant red sediment to this area of the lake. The varves shown here are much thinner than varves below AM 3300 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 Connecticut.

The summer or melt season layers of each varve (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 or you may see units of darker color when the image is magnified (still lighter than the winter layer) that represent relatively weak bottom currents early in the melt season. The distinct light coarse layers (see AM 3988, 3990, 4006, and 4010 for example) that begin many summers and abruptly truncate winter layers are the result of rapid meltwater release from the glacier and adjacent land surface in the early melt season, perhaps assisted by lake overturning. Dark early melt season layers are prominent in varves AM 3988, 4003, 4004 and 4011. The upper parts of some summer layers have a slightly grayer appearance than lower in the summer layer and most summer layers grade into the winter layer above. Thin highly rhythmic units within some summer layers (see AM 3999, 4005, and 4008) are suggestive of diurnal variations in meltwater flow that deposited silt in daily pulses. Summer layer thickness in this sequence is either similar to or somewhat 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.

One very thick varve (AM 4002) stands out on the image for two reasons. First, it is the thickest varve on the image mostly (but not entirely) because of a single thick graded bed that starts the summer layer. This unit is either a flood event produced when a local lake in a tributary was released by ice recession or it represents are large melting event. This event is also seen in other varve sections and appears to get thicker and coarser further north suggesting that the flood had its source along the receding ice front near South Hadley, Massachusetts. Second, the varve has a very clayey unit in the upper part of the summer layer that can be mistaken for a winter layer. This unit has too much silt in it and appears to be too thin to be a true winter layer. It may represent the beginning of the non-melt season that was interrupted by melting or runoff (storm) events just prior to winter clay deposition. The extreme thickness of the true winter layer of this varve and the appearance of the late summer clayey unit may have been triggered by the introduction of excess volumes of clayey sediment to the lake during this year and the beginning of clay flocculation late in the melt season.

Past Varves of the Month...