STRATIGRAPHIC AND TECTONIC SETTING OF NEOPROTEROZOIC GLACIAL STRATA, SW VIRGINIA, USA

Neoproterozoic breakup of the supercontinent Rodinia left a lasting effect on Earth’s surface in terms of rifted continental margins, volcanic deposits, and opening of ancient ocean basins. Coincident with the breakup, the Earth entered an acute glacial period frequently referred to as Snowball Earth, and for which the Cryogenian Period was named. Understanding the extent of ice cover, the presence of glacial vs. interglacial episodes, and rates of climate change during the Cryogenian is dependent upon studies of sedimentary rocks deposited during the glacial episodes. Glaciolacustrine sedimentary rocks of the Konnarock Formation (KF) in SW Virginia comprise the most complete record of Neoproterozoic glaciation in the eastern US. Despite the significance of the KF to the understanding of Neoproterozoic glaciations, few studies of the formation have been published since the 1990s. The KF is commonly assumed to represent deposits of the Sturtian glacial episode (~717-660 Ma). Its age, however, remains uncertain. The KF overlies bimodal volcanic rocks of the Mount Rogers Formation (MRF) that formed during volcanism and fault basin development in early stages of Rodinian intracratonic rifting. The youngest dated rhyolites in the MRF are ~ 749 Ma, although a small ‘outlier’ of rhyolite mapped within the KF is reported to yield an age of ~751 Ma. The minimum age of the KF is constrained by overlying rift-to-drift sedimentary rocks, which are likely younger than 563 Ma. Therefore, at present the age of the KF is only bracketed between ~750 and 563 Ma.

In an ongoing study of the stratigraphic and tectonic relationships of the KF and MRF, we mapped the KF in contact with two distinct volcanogenic bodies – one of flow-banded rhyolite resembling the ~753 Ma Whitetop member of the MRF, the other consisting of porphyritic rhyolite, tuff, and volcaniclastic sandstone or conglomerate, intruded by basaltic and rhyolitic dikes. Basal rhythmites of the KF are also intruded by mafic dikes, and in one key location, by a hypabyssal felsic dike. Based on field relationships, we hypothesize that the latter volcanic sequence and initial KF deposits were deposited on an uneven rift-basin topography, possibly against the Whitetop Rhyolite uplifted in the footwall of basin-bounding faults. Geochronology will be used to test this hypothesis.