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 ÒSlushball EarthÓ, 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 Òfreeze-fryÓ 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 (36¼ 40.900Õ North, 81¼ 33.988Õ 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.