File:Dropstone in proglacial lacustrine rhythmitic argillite (varvite) (Konnarock Formation, Neoproterozoic, ~750 Ma; Grassy Branch Outcrop - Rt. 603 roadcut, Smyth County, Virginia, USA) 5 (30474473801).jpg

Dropstone in proglacial lacustrine rhythmitic argillite from the Precambrian of Virginia, USA.

The Snowball Earth Glaciations during the Neoproterozoic were the most significant ice ages that Earth ever experienced - two or three of them occurred in succession. The most extreme models describing Snowball Earth have 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. Each Snowball Earth Glaciation was 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 rock shown above is rhythmically laminated argillite (= a very low-grade metamorphic rock formed by slight alteration of shale). Published research has identified these sediments as proglacial lacustrine turbidites - they appear to be varves (rhythmites). 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.). Demonstrable annual layering is scarce in the sedimentary rock record.

Shown above is a small granite pebble dropstone in argillite. Dropstones are formed when glaciers over bodies of water melt and release their ice-entrained sediment grains. Larger dropstones that fell into fine-grained sediments (= mud, as in the case here) deform the underlying layers and often have arched, draping sediment layers above.

The granite is derived from the Cranberry Gneiss, a Mesoproterozoic-aged (1.0 to 1.3 Ga) basement rock unit in the Appalachian Mountains.

Stratigraphy: Konnarock Formation, Neoproterozoic, ~750 Ma