Digital Varves

Varves of the Month for 1/1/2009 - 1/31/2009

Mine fire reclamation site, Olyphant, Pennsylvania

Scale bar in cm.

Click on image to download original image file

This month's varves (images A and B) are sub-till varves exposed in a deep excavation at a mine fire reclamation site in Olyphant, northeastern Pennsylvania. Oriented sample blocks of the varves were collected by Duane Braun at Bloomsburg University and given to Jack Ridge at Tufts University to study the lithology and paleomagnetism of the varves (Download PDF: Paleomagnetism of Olyphant varves) Varves at the site were discovered beneath thick till from the last glaciation and probably represent a lake impounded and then overrun by the advance of the glacier. Bedding in the varves is highly compacted by overriding ice and undulates on a scale of 10's of centimeters over large (1-4 m) dropstones from local joint blocks of the Pocono and Pottsville Sandstones. Small high-angle faults that occur throughout the section are also probably related to differential compaction. The top of the varve section is interbedded with diamicton layers, which appear to be ice-proximal debris flows, and is highly deformed directly beneath the overlying till.

A striking feature of the Olyphant varve section is abundant ice-rafted sediment. The ice-rafted debris occurs in the form of dropstones, pellets of diamicton, and pellets of white silt. Ice-rafted debris is most abundant high in the section where it became difficult to trim paleomagnetic samples because of abundant dropstones. A cavity left by a dropstone (c) is on image A, while a dropstone made of tan sandstone is in the center of image B. The most abundant drop sediment is pellets of dark to medium gray sandy diamicton that are flattened to varying degrees. A large diamicton pellet occurs in the top of image A. The lithology and color of the diamicton pellets matches the color and lithology of local till. The diamicton pellets appear to be till that either adhered to the base of the glacier or was frozen in basal ice and was later rafted by icebergs. The flattened pellets of white silt, which are composed of glacial rock flour, are a conspicuous component of the very dark winter clay bed in image B along with darker diamicton pellets.

The lithology and thickness of varves at the site vary as is shown in the two example images. The thin (1.0-1.5 cm) varves in image A (winter beds marked with ‘w') that are rich in ice-rafted sediment were collected from near the top of the section. Each varve has light gray layers at its base with sparse drop sediment suggesting rapid deposition as compared to darker units above (remaining summer and winter layers) that have a higher concentration of drop sediment. Frequently drop sediment appears to have accumulated at the upper surface of the winter layer. This suggests that the "winter", or non-melt season, layer was deposited rapidly and early in the non-melt season prior to the freezing of the lake surface for the winter. Supporting this interpretation is the common interruption of the winter clay beds by thin silt layers. The silt layers appear to represent meltwater events formed at the end of the summer but after the initiation of clay deposition.

The thick (4 cm) varve shown in image B appears to have sparser drop sediment than the varves in image A except for the prominent stone in the center of the image. However, this can be misleading because the annual load of drop sediment is dispersed through a much thicker varve than those in image A and the drop sediment may have been disaggregated by stronger currents. As in image A the base of the varve in image B is light silt that gives way to darker units above with dispersed fine drop sediment (coarse sand grains and granules, small pellets) and occasional interruptions by light silt beds. The winter layer has diamicton pellets as well as conspicuous flattened white silt pellets. A high-angle fault running through the varve is likely related to differential compaction.

Varves at the Olyphant section are not numbered according to a chronology and it is unlikely that their thickness measurements will ever be useful for varve correlation. The bedding in the section is disturbed by mass movement and high-angle faults, and the thickness of individual varves is inconsistent across the section. At this point in time there are no known contemporaneous sections of varves in the region that could be matched to this section. Also, sub-till varve sections are usually thin and relatively rare given the generally short intervals of time they represent and erosion by overriding ice. Making matters worse, bulk sampling of the Olyphant section to separate fine organics for radiocarbon dating would likely produce invalid results because of coal in the varves derived from Pennsylvanian bedrock units of the region. Not only are there coal fragments occurring as drop sediment but the very dark color of the winter clay beds is likely due in part to pulverized carbonaceous material from coal-bearing bedrock units. We are currently collecting paleomagnetic signals from sub-till varves across the northeastern U.S. that may lead to the construction of a record of secular variation of paleomagnetic declination. This record may eventually be pieced together to provide a chronology of Late Wisconsinan ice advance across the region (like the one for L. Wisconsinan ice recession) but many more paleomagnetic samples will be needed for this goal to be achieved.

Past Varves of the Month...