SNOWBALL EARTH RHYTHMITES
The Snowball Earth Glaciation during the Neoproterozoic was the most significant Ice Age that Earth ever encountered. The most extreme model describing Snowball Earth has glacial ice completely covering all continents and all oceans, even at the equator. Some models, called “Slushball Earth”, have Earth’s equatorial oceanic areas not completely frozen over. The Snowball Earth Glaciation was apparently followed by a super-greenhouse climate. The resulting sedimentary record of these “freeze-fry” events typically consists of glacial tillites and overlying cap carbonates. These units are preserved at many localities on Earth.
The sample shown below has well-preserved horizontal laminations in argillite (a very low-grade metamorphic rock between shale and slate). Note that the laminations are rhythmic in their thicknesses and weathering pattern. These are rhythmites. Published research has identified these sediments as proglacial lacustrine turbidites - they appear to be varves. Varves are common sedimentary units in Pleistocene proglacial lake settings. This is a Snowball Earth equivalent. The rhythmicity reflects seasonal changes (summer-winter-summer-winter, etc.)
Locality: Grassy Branch outcrop, northern side of Rt. 603, adjacent to Grassy Branch, ~2.5 to 3 miles east of Rt. 603-Rt. 600 intersection, southern Smyth County, western Virginia, USA (36º 40.900’ North, 81º 33.988’ West).
Proglacial rhythmitic argillite (field of view ~9.1 cm across) from the Konnarock Formation (Neoproterozoic, 650 to 700 m.y.) in southern Smyth County, western Virginia, USA.