Tonian (1000-720 Ma)
Beck Spring Dolomite (Death Valley, USA)
The Beck Spring Dolomite Member of the Pahrump Group was deposited between 770-740 Ma (Macdonald et al. 2013) on the rifted western margin of Laurentia and stratigraphically rests between the older Crystal Spring and younger Kingston peak members. This cap carbonate is characterized by cherty grey dolostone, occasionally brecciated with roll-up structures, oncoids, pisolites, ooids, and very common stromatolite structures throughout (Marian and Osborne, 1992; Corsetti and Kaufman, 2003). No underlying diamictite occurs; however, the eroded surface of the upper Crystal Spring Member suggests rapid erosion prior to the deposition of the Beck Spring Dolomite(Mbuyi and Prave, 1993), plausibly caused by glacial meltwater floods. Substantial evidence for a glaciogenic origin is mostly δ13C excursions that trend from -2 ‰ to 5‰ from basal to mid-section and then down to -4% in the uppermost section (Corsetti and Kaufman, 2003). A negative to positive δ13C excursion, as well as the Beck Spring Dolomite’s stratigraphic position and color is broadly consistent with other cap carbonates deposited following the Sturtian Neoproterozoic glacial episode (Corsetti and Lorentz, 2006).
Assem Limestone, Tambien Group, Northern Ethiopia
The Assem Limestone lies within the Tambien Group, a 5-km thick, “mixed carbonate-siliciclastic succession” (Swanson-Hysell, et al.). These Sturtian diamictites cap the Tambien Group in northern Ethiopia and are in contact with the Tsaliet Group. Contact between the two is transitional, composed of volcanic ash that was deposited during the formation of the Tambien group. According to work conducted by Beyth (2001) and Hoffman & Li (2009), “the easternmost Negash synclinorium is cored by a diamictite with extrabasinal clasts interpreted to correlate with diamictites of the ca. 717–662 Ma Sturtian glaciation” (Swanson-Hysell et al., 2015). Additionally, the Assem formation contains “abundant stromatolite and microbialaminite lithofacies interbedded with lenses of grainstone and intraclast breccia” in the westernmost exposure while to the east, in the Tsedia and Chehmit synclinoria, the Assem Formation is dominated by wavy to parallel-laminated limestone micrite (Swanson-Hysell, 2015).
Given the depositional context coupled with chemical properties and textural qualities indicate the rock is a transgressive cap-carbonate sequence that contain negative d13Ccarb intervals that correspond to a recovery from “an earlier pre-Sturtian cooling event, perhaps related to the Kaigas glacial interval”. If such findings are correct, the Assem Limestone would be the oldest cap carbonate in the Arabian-Nubian Shield (Miller, 2009).