By: magic-gigapans
Image: Callan Bentley, Mid-Atlantic Geo-Image Collection
License: Creative Commons Non Commercial ⧉

Uploaded: 14 May 2020
Last Updated: 19 May 2020

1.22 gigapixels
52,060 x 23,404 pixels
173.5 in X 78.0 in at 300dpi
228 pixels per inch

Rhythmites are rhythmically-bedding sedimentary deposits, deposited in a sedimentary basin with some regularly-shifting condition like tidal cycles or day/night changes. The gigapan shows an outcrop of the Konnarock Formation, a bit east of Konnarock, Virginia, in the Blue Ridge province. They are Proterozoic in age. These rocks are interpreted as lake sediments that show rhythm in their seasonality: summer and winter alternating between fine clay (winter) and coarser silt and sand (during the summer, when the ice has thawed and the water feeding into the lake is flowing faster). Bright pink, grainy samples are arkose, a comositionally immature sandstone that corresponds to times when the turbulence/energy of the depositional water went up temporarily. You can find a few cobbles of arkose in this debris pile. While most of these sediments are reddish (indicative of relatively oxidizing conditions during deposition), there are also a few greenish layers, which indicate periods of lower oxygenation in the lake. Another thing to notice is the pronounced foliation that has been imparted to the clay-rich layers (but not so much to the silt-rich layers), causing the cobbles to break off in a "rounded stairstep" pattern. Higher up in the stratigraphic sequence, these rhythmites begin to show dropstones, iceberg-rafted clasts that drop out into the calm waters mid-lake where these much finer sediments were being deposited. Dropstones impinge on the pre-existing muddy layers they drop into, and subsequent muddy layers are draped over top of them. Above the dropstone-bearing rhythmites, the Konnarock Formation transitions to a full-on diamictite, interpreted as glacial till that has been lithified. In other words, the sediments record the advance of ancient glaciers into a Mesoproterozoic lake basin. These glacial deposits have been correlated with the widespread "Snowball Earth" episode(s) of glaciation during the Proterozoic Eon. Two cm-scale pencils are positioned in the image to provide a sense of scale. Also, see if you can find the small "Jack O Lantern" eraser that is hidden somewhere in the gigapan.