REDOX STRUCTURE OF MESOPROTEROZOIC SHALE DERIVED FROM SULFIDE MINERALS AND MINERAL TEXTURE

It has long been inferred that black shale requires deposition under persistently anoxic conditions. In the Phanerozoic, this idea has been challenged by the occasional presence of bioturbation, which suggests that many shale-rich environments may not have been as oxygen-deprived as previously thought. In the Proterozoic, shallow-water organic-rich black shale deposits similarly contain evidence for wave activity, again suggesting the potential for more variable redox conditions during deposition. Here we use a combination of iron sulfide mineralogy and morphology to further investigate differences in bottom water oxygenation between coeval epicratonic and pericratonic black shale deposits of the late Mesoproterozoic (~1.1 Ga) Atar and El Mreiti groups, Taoudeni Basin, Mauritania. The presence of co-occurring pyrite and marcasite, for instance, has been related to fluctuating oxygenation of the overlying water column, and the formation of pyrite framboids are generally restricted to weakly reducing conditions in the vicinity of the redox boundary.

In this study, iron sulfide mineralogy, mineral fabric, and the relative distribution of these features were compared between coeval formations from the Taoudeni Basin, Mauritania, and used to decipher a more complete history of oceanic redox occurring in the same depositional basin. Fifty-two polished thin sections were analyzed using reflected light microscopy—12 from two cores within the pericratonic Atar Formation (Atar Group), and 40 from within a core from the epicratonic Tourist Formation (El Mreiti Group). Distributions of iron sulfide minerals and textures imply the presence of predominately anoxic and stable depositional environments for the Atar Formation, whereas more variably euxinic environments occurred in the Tourist Formation. These data are consistent with isotopic and trace metal data that suggest that epicratonic environments of the Touirist Formation coincided broadly with a regional chemocline, whereas pericratonic environments of the Atar Formation reflect deposition persistently beneath this regional chemocline.