U–Pb geochronology and Hf isotope systematics of detrital zircon from the basal part of the late Ediacaran sedimentary succession of the Moldova-Podillya basin (SW Baltica): Implications for glacial vs. alluvial origin

Abstract

The Neoproterozoic is marked by two long-lasting global Snowball Earth glaciations—the Sturtian (ca. 715–660 Ma) and the Marinoan (ca. 639–635 Ma)—as well as the shorter-lived, potentially regional Gaskiers glaciation at ca. 580–579 Ma. The subsequent Ediacaran and early Cambrian periods are generally interpreted to be characterized by a warm climate without low-latitude, sea-level glaciations. However, a number of locally, and, sometimes regionally developed sedimentary units worldwide have been interpreted to record sea-level glaciations during this time interval. Some of these units crop-out poorly, were affected by deformation and metamorphism, lack definitive sedimentologic textures and structures, and are poorly dated. Their sedimentological characteristics, as well as age constraints, are thus often insufficient to determine whether they have a glacial origin and if their ages fit into well-defined time intervals marked by cold climate indicators. Detrital zircon geochronology may help distinguish between the heterogeneous and extensive provenances that are indicative of glaciation, and more homogenous, and local provenances that are more typical of alluvial settings. In this paper, we use trends in detrital zircon age distribution patterns from the lowermost late Ediacaran sedimentary succession of the Moldova-Podillya basin in Baltica to constrain the provenance of the Volyn Group sediments. Here, the origin of diamictites within the Hrushka Formation has long been a topic of debate, centered on either a glacial or alluvial origin. Detrital zircons from two sandstone samples and one conglomerate sample from the Bakhtyn Beds of the Hrushka Formation, and two sandstone samples from the Lomoziv Beds of the unconformably overlying Mohyliv Formation were dated using U-Pb zircon LA-ICP-MS techniques. Their age patterns are compared with published data for the overlying Cosăuți (Yampil) and Bernashivka beds as well as new dates for the immediately underlying Paleoproterozoic granites to test for a proposed glacial origin for the Bakhtyn Beds. The areal distribution of the Volyn Group sediments, inferred from drill-core data, is used to constrain the evolution of the sedimentary basin. The detrital zircon results indicate that terrigenous material in the Moldova-Podillya sedimentary basin was sourced from the Precambrian basement during deposition of the Bakhtyn to Cosăuți beds. This is consistent with an alluvial rather than a glacial origin. We infer that detrital zircon geochronology provides a largely untapped potential for testing a glacial origin for units that are otherwise poorly genetically characterized and dated.