Neoproterozoic stratigraphy of the Zavkhan terrane of Mongolia: The backbone for Cryogenian and early Ediacaran chemostratigraphic records

Abstract

The Neoproterozoic Tsagaan-Olom Group is exposed in the Zavkhan Terrane of southwestern Mongolia and hosts unique geochemical, paleoclimate, and paleontological records that have become central to our understanding of this pivotal interval of Earth history. New sedimentological, stratigraphic, geochronological, and geochemical data provide context for and further develop these records. Detrital zircon provenance indicates that Neoproterozoic strata of the Zavkhan Terrane were derived from basement with age peaks between 1950 to 2100 and 2400 to 2600 Ma. At ∼800 Ma, the Zavkhan Terrane transformed from an active arc and back-arc complex to a rifted ribbon continent with passive margins on both sides. Deposition was accommodated by extension, which is recorded with syn-sedimentary normal faulting and alluvial fan deposition in the Zavkhan and Khasagt formations. Passive margin sedimentation in the overlying Tsagaan-Olom Group begins with the glacigenic Maikhan-Uul Formation, which consists of two massive diamictite units separated by clast-poor graded beds of the middle member. Detrital zircon at the base of the middle member of the Maikhan-Uul Formation were dated with U-Pb chemical abrasion isotope-dilution thermal ionization mass spectrometry and constrained its age to <729.8 ± 1.4 Ma. This, along with chemostratigraphy and Re/Os geochronological constraints from the overlying Taishir Formation, supports our correlation of the Maikhan-Uul Formation with the ∼717 to 660 Ma Sturtian glaciation. The Taishir Formation was deposited on a carbonate ramp in four large-scale sequence tracts that thin to the southwest. The Taishir Formation preserves a large negative δ¹³C excursion referred to as the Taishir excursion that covaries in carbonate and organic carbon isotopes in limestone sections. A dolomitization front at the top of the Taishir Formation also results in depleted δ¹³C values, however, these are related to local processes and do not represent a global Trezona excursion. Although δ¹³C values in the Ol Formation are highly variable along strike, 0.70756 initial strontium isotope values in limestone of the upper Ol Formation are consistent with earliest Ediacaran values. A sandstone-filled karst surface at the top of the Shuurgat Formation that overlies the Ol Formation defines the top of the Tsagaan-Olom Group and is interpreted to mark a major unconformity. Carbon and strontium isotope values in the uppermost Shuurgat Formation are also consistent with early Ediacaran values and suggest that most of the late Ediacaran Period is missing in the Zavkhan Terrane of Mongolia. Carbon isotope profiles from sections preserved as limestone and dolostone display large differences and indicate that isotopic data from dolomites should be used with caution. With our new data and correlations, we construct composite Cryogenian and Ediacaran carbon and strontium isotope curves from limestone-dominated successions in Mongolia, and then integrate additional geochronological and geochemical data sets from around the globe.