Province: Blue Ridge

Smyth, Washington, and Grayson Counties

The Konnarock Formation (1100 meters thick) contains the most convincing evidence of glaciation during the late Proterozoic in southeastern North America (Miller, 1994). The Konnarock was formerly included within the Mount Rogers Formation as the upper part of the formation, but was separated by Rankin (1993) into its own unit because it is believed to rest unconformably on top of the Mount Rogers Formation and may be significantly younger.

The lower part of the Konnarock Formation consists of monotonously layered cycles of maroon argillite (mud/silt) and gray to green fine sand/silt. The rocks here are divided into three main types: coarse laminates (0.5 to 4 cm thick layers); fine laminates (2 mm thick); and massive mudstone. In addition, dropstones (coarser pebbles of mostly Cranberry Gneiss) are found in all varieties of the argillite. The laminates preserve sediment deposited in a lake that experienced seasonal fluctuations in environmental conditions, such as a lake that was surrounded by a glaciated highland. The dropstones are evidence of sediment dropped from icebergs that floated in the lake. Please refer to the Mount Rogers Field Trip (stop 2 and stop 3) for more information.

The upper part of the Konnarock Formation consists of interlayered maroon sandstone and diamictite (massive, structureless unsorted rock). The sandstones may be massive, forming one-meter-thick, structureless beds. Other sandstones are graded and they show signs of internal soft sedimentary deformational features such as convolute bedding and slump folds. The diamictite is often massive, structureless, and contains a conglomerate composed of Cranberry Gneiss grains (some up to 1.2 meters across) and less common volcanic clasts. The diamictite commonly consists of 20-50% clasts that are matrix supported, with the matrix composed of sand grains in a clay-rich matrix. The diamictite is of particular interest because of its interpretation as glacially deposited sediment at the place where the glaciers melted perhaps at the edge of the ancient Konnarock lake. Other varieties of rock found in the upper part of the Konnarock Formation include bedded diamictite (beds are 1-20 cm thick) and laminated diamictite (with beds less than 5 cm). For more information, see the Mount Rogers Field Trip, stop 4, spots 8 and 9.

Above: the coarse laminate showing the alternating gray sand/silt and the maroon mud/silt layers from Rt. 603 between Trout Dale and Konnarock, stop 2 of the Mount Rogers trip.

Right: dropstones of Cranberry Gneiss (below and to the right of the penny) surrounded by both coarse laminate and fine laminate from Rt. 603 at Konnarock, stop 3 of the Mount Rogers trip.

Close up of the diamictite showing clasts of Cranberry Gneiss (light) and rhyolite (dark) in the sandy unlayered maroon host sediment. Mount Rogers Trip, stop 4, spot 9.
Of particular importance is the nature of the lower and upper contacts of the Konnarock Formation. There are no fossils or radiometric dates of the Konnarock Formation itself. However, the underlying Mount Rogers Formation has a radiometric date of 760 m.y., and the overlying Unicoi Formation contains a fossil which dates it in the early Cambrian, making the Unicoi no older than 570 m.y. With nearly 200 million years in between, that leaves quite a bit of uncertainty for the age of the Konnarock!!! Rankin (1993) described evidence that the lower contact with the Mount Rogers Formation (the Wilburn Rhyolite Member is the uppermost unit) is an unconformity. He described an outcrop along Rt. 603 where the bottom of the Konnarock Formation, in contact with the Wilburn Rhyolite, is a conglomerate with clasts of mostly Wilburn Rhyolite but also other rhyolite units and microcline derived from the Cranberry Gneiss. In addition, the map pattern of the lower Konnarock contact indicates that it is in stratigraphic contact with all the underlying units from the rhyolites on down to the Cranberry Gneiss. The contact is interpreted by Rankin to represent a rubbly irregular surface on which the Konnarock was deposited.

The upper contact has been described at Big Hill on U.S. 58 by Rankin (1967), and Rankin and others (1994) and is also visited at stop 4 of the Mt. Rogers trip (although the actual contact is not exposed where we will be). The contact is sharp and parallel. The Konnarock Formation near its top on the Virginia Creeper Trail is characteristically a maroon diamictite. The overlying Unicoi Formation, however, consists of a quartz-pebble conglomerate. The interpretation favored by Rankin is that the contact is a paraconformity. In other words, although the Konnarock and Unicoi have a sharp contact with no structural or orientation difference between them, there is probably a significant amount of time missing at the contact. This is supported by the fact that the two formations differ quite a bit in their sedimentary environments (see the Unicoi Formation page) and that the compositions of the clasts differ, indicating that the sediment source had changed.

Links for additional information:

Mount Rogers Field Trip Stops 2, 3, 4

Mount Rogers Geologic History


Miller, J.M.G., 1994, The Neoproterozoic Konnarock Formation, southwestern Virginia, USA: Glaciolacustrine facies in a continental rift: in Deynoux, M., Miller, J.M.G., Domack, E.W., Eyles, N., (editors), Earth's Geologic Record; Cambridge University Press, Cambridge, p. 47-59.

Rankin, D.W., 1967, Guide to the Geology of the Mt. Rogers area, Virginia, North Carolina and Tennessee: Carolina Geological Society Field Trip Guidebook.

Rankin, D.W., 1993, The volcanogenic Mount Rogers formation and the overlying glaciogenic Konnarock Formation - Two Late Proterozoic units in southwestern Virginia: U.S. Geological Society Bulletin 2029, 26 p.

Rankin, D.W., Miller, J.M.G., Simpson, E.L., 1994, Geology of the Mt. Rogers area, southwestern Virginia Blue Ridge and Unaka Belt: in Schultz, A., and Henika, B. (editors), Fieldguides to southern Appalachian structure, stratigraphy, and engineering geology: Virginia Tech Guidebook No. 10, p. 127-176.

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