Calcitized evaporites in the Mesoproterozoic Atar and El Meriti Groups, Mauritania, Africa

Abstract

One of the most profound byproducts of the Great Oxidation Event (GOE) was the onset of oxidative weathering on land and the riverine delivery of sulfate to marine systems. Although the exact nature of marine oxygenation in the aftermath of the GOE remains uncertain, marine sulfate likely remained below 2 mM until the late Mesoproterozoic and may have remained <10 mM through the Neoproterozoic. Prior to the late Mesoproterozoic, precipitation of sulfate minerals were inhibited by low marine sulfate and possibly by elevated carbonate saturation. By ∼1.1 Ga marine sulfate concentrations were sufficient to sustain the earliest widespread evaporite deposition. Low sulfate concentrations, however, influenced both the deposition and diagenesis of evaporite deposits. Here we investigate unusual limestone deposits in the 1.1 Ga Atar Group, Mauritania, that contain both macroscopic and microscopic features that suggest a primary origin as gypsum. Elevated concentrations of carbonate-associated sulfate (CAS), sodium, and potassium are consistent with primary deposition as a sulfate evaporite and subsequent calcification under low water-rock ratios. Isotope and elemental compositions further indicate diagenetic recrystallization under marine to hypersaline marine conditions. Combined, these observations show the potential for recognizing key evaporite facies in the rock record and support a growing recognition of late Mesoproterozoic evaporite deposits.