Digital Varves

Varves of the Month for 6/1/2011 - 7/31/2011

Connecticut Valley Varves: Core at Scantic, East Windsor, Connecticut

Scale bar in cm.

Click on image to download original image file

This month's image is three spliced high resolution images that show transitional (proximal to distal) varves in a core collected at Scantic in East 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 450 years after ice recession at East Windsor when the receding ice margin was 26 km to the north near Chicopee, Massachusetts. The varves shown here are about 17,300 yr old (See: NAVC calibration) The varves at Scantic very closely match other varve sections and sequences (see graph) from nearby in Connecticut (Petersen Farm (PET) in South Windsor, Kelsey Ferguson (KF) Brickyard in South Windsor, Glastonbury (GL) and Antevs (1922) original normal curve) and also much further north in Massachusetts (Antevs (1922) normal curve). The very thin varve in this image (AM 3479) was not 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 3400-3500 at sections in South Windsor (Kelsey-Ferguson - Redlands Brick Co. pit and Petersen Farm), and East Windsor (Scantic), Connecticut and the Connecticut and southern Massachusetts records of Antevs (1922). The yellow swath is the part of the plot shown in the core image. Note that the scale for Massachusetts varve thickness (right side) is different and the Massachusetts varves are much thicker than the Connecticut varves as a result of being closer to the receding ice front when they were deposited. Click to download a high resolution version of this graph (~3.4 MB 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 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 are primarily gray (unlike varves lower in the section that are red) and have only a very faint red tint in the clay beds of some varves (AM 3467-3469 and 3489-3490). (You may have to view the image at full resolution to see the slight red tint.) 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. In the slightly red-colored varves the red color is in the winter clay bed and primarily near the top of the clay units. This may mean that the red component of the sediment (probably a hematite pigment) is finer than most of the clay mineral particles making up the lower and grayer part of the winter layer.

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 (especially for example AM 3469-3472, 3477, and 3491-3496), although there are not as many distinguishable units as lower in the section where the varves are much thicker and strictly ice-proximal. The upper parts of some summer layers may have a slightly grayer appearance than lower in the summer layer and many summer layers grade into the winter layer above. Summer layer thickness in this sequence shows a strong positive correlation to winter layer thickness. 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 sediment discharges to the lake.

Past Varves of the Month...