Chemostratigraphy of the Late Mesoproterozoic Atar Group, Taoudeni Basin, Mauritania: Muted isotopic variability, facies correlation, and global isotopic trends

Sedimentary strata of the Atar/El Mreiti groups, Taoudeni Basin, Mauritania, are characterized by carbon isotope values that fall largely between −2.5‰ and +4.0‰, and contain stratigraphic trends that are strikingly similar to those recorded worldwide in strata with depositional ages from ∼1.2 Ga to 1.1 Ga. Chemostratigraphic results are consistent with recent Re–Os dates of ∼1.1 Ga for Atar/El Mreiti group strata (Rooney et al., 2010) and support an emerging dataset that identifies the Late Mesoproterozoic as an isotopically distinctive interval in Earth history. The relatively low-amplitude carbon isotope variability limits the use of chemostratigraphy in detailed correlation during this time interval. In this study, we combine chemostratigraphic data with key chronostratigraphic horizons and detailed analysis of depositional facies to construct a rigorous and testable model for the intrabasinal correlation of Atar and el Mreiti group strata. Data support a depositional model that results in broadly uniform stratigraphic thicknesses across the West African craton, with predominantly stromatolitic facies of the Atar Group (Mauritania) and the Hank and Dar Cheikh groups (Algeria) representing deposition on shallow-water craton margins, and clayey-carbonate and shale facies of the El Mreiti Group (Mauritania) representing deposition on a broad, shallow-water, epicratonic platform. Within this stratigraphic framework, regional differences in carbon isotope composition are interpreted to reflect spatial differences in water chemistry likely associated with local carbon cycle dynamics in cratonic environments that have limited exchange with the open ocean. Similarly, distinct differences in trace element composition recorded in multilayered marine cements within deeper-water stromatolitic reef facies are interpreted to reflect vertical differences in water chemistry associated with redox-gradients in the water column.We use this multifaceted approach to test a recent hypothesis that suggests correlation of Atar/el Mreiti group strata with glaciogenic strata of the Vazante Group, São Francisco craton. Critically, preservation of shallow-water carbonate and evaporite strata across more than 1200 km of the predominantly epicratonic Taoudeni Basin—with minimal evidence for high-amplitude sea level change—is consistent with inferences of late Mesoproterozoic highstand conditions in a global greenhouse climate. Such characteristics are inconsistent with suggestions of a time-equivalency with glaciogenic strata of the Vazante Group, São Francisco craton, despite broad similarities in Re–Os geochronology and carbon isotope composition. Together, these results emphasize both the utility and limitations of carbon isotope stratigraphy to inform hypotheses regarding global correlation and biospheric evolution.

► Mesoproterozoic C-isotopes represent distinct feature in Earth history.
► Muted isotopic variability require additional use of stratigraphic/geochronologic information.
► Geochemical data indicate distinct water masses, both laterally and vertically.
► Combined dataset indicates no correlation of Atar-Vazante groups, despite Re–Os dates.