Precambrian Research 182 (2010) 402–412 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres Radiometric and stratigraphic constraints on terminal Ediacaran (post-Gaskiers) glaciation and metazoan evolution Craig L. Hebert a , Alan J. Kaufman a,b,∗ , Sarah C. Penniston-Dorland a , Aaron J. Martin a a Geology Department, University of Maryland, College Park, MD 20742-4211, United States b Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, United States article info Article history: Received 5 October 2009 Received in revised form 7 July 2010 Accepted 15 July 2010 Keywords: Ediacaran Period U/Pb zircon ages Chemostratigraphy Glaciation Fauquier Formation Catoctin Formation abstract Radiometric constraints on mid-Ediacaran Period glaciation (Gaskiers) in Newfoundland narrowed the known temporal gap between widespread ice ages and the evolution of complex metazoans to several million years. To further evaluate this claim we studied an Ediacaran glacial diamictite at the base of the Fauquier Formation of northern Virginia, and discovered a conformable relationship between the post- glacial cap carbonate and overlying volcanic rocks of the Catoctin Formation. U/Pb zircon age constraints for the rift-related volcanic flows suggest initial emplacement around 571 million years ago. Application of the Catoctin age to the Fauquier succession indicates the occurrence of an ice age about 10 million years younger than the 582 Ma Gaskiers event, supporting the view of multiple Ediacaran Period glaciations. Furthermore, the age constraint from eastern Laurentia falls within radiometric uncertainty of fossil- iferous strata in Avalonia, indicating that the Fauquier glaciation was coincident with early metazoan evolution. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Current radiometric and stratigraphic constraints suggest that the oldest complex Ediacaran fossils are separated by millions of years from the youngest recognized Neoproterozoic ice age deposits (Narbonne and Gehling, 2003). The timing of these events is important since extreme climate change has been considered a bottleneck to metazoan evolution (Hoffman et al., 1998; Hoffman and Schrag, 2002). In Newfoundland, for example, only ∼1.5 km of strata separate Gaskiers Formation diamictites (ca. 580 Ma: Bowring et al., 2003) from morphologically simple discs and com- plex fronds of Ediacaran aspect in the overlying Drook Formation. The relatively short stratigraphic distance between climatic and biological events suggests either rapid post-glacial origination or diversification of metazoans, or that Gaskiers refrigeration repre- sented no substantive evolutionary barrier. Due to the lack of temporal constraints on most terminal Neo- proterozoic glacial deposits, the true distribution of Gaskiers ice age deposits is uncertain, leading some to suggest that this Ediacaran Period glaciation was regional and hence unrelated to global Cryo- genian events (i.e. ‘snowball Earth’: Hoffman et al., 1998). Before radiometric constraints were available for the Gaskiers diamictites, ∗ Corresponding author at: Geology Department, University of Maryland, College Park, MD 20742-4211, United States. Tel.: +1 301 405 0395; fax: +1 301 405 3597. E-mail address: kaufman@geol.umd.edu (A.J. Kaufman). these glacial deposits on the Avalon microcontinent were typically equated with those of the Roxbury Formation (Squantum ‘Tillite’) preserved in the Boston Basin, USA, which is dated by U/Pb zir- con analysis of a volcanic cobble near the base of the unit and of detrital zircons in underlying sandstone to be younger than ca. 595 Ma (Thompson and Bowring, 2000). If Squantum glaciation were contemporaneous with arc magmatism in the Boston Basin as suggested by these authors, then the Gaskiers and Roxbury ages are incompatible, suggesting the possibility of multiple Ediacaran Period ice ages (Fig. 1). Other potential Ediacaran-age diamictites include those in the Mortensnes Formation of Norway (Halverson et al., 2005), the Egan Formation in Australia (Corkeron and George, 2001), the Han- kalchough Formation in China (Xiao et al., 2004), and the Serra Azul Formation in Brazil (Alvarenga et al., 2007), although none of these examples are constrained by radiometric dates. Similar to the Gask- iers Formation (Myrow and Kaufman, 1999), post-glacial carbonate lithofacies (i.e. ‘cap carbonates’) above these examples preserve negative  13 C anomalies, which, although of variable magnitude, have been provisionally correlated to a profound Ediacaran carbon cycle perturbation recognized worldwide and termed the Shuram event (Fike et al., 2006; McFadden et al., 2008). Our studies in northern Virginia document a unique con- formable relationship between glacial diamictite and cap carbonate of the Fauquier Formation with volcanic rocks of the Catoctin For- mation, a widespread flood basalt recognized throughout eastern North America that was emplaced during the prolonged rifting of 0301-9268/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2010.07.008