An Ediacaran iron formation: New evidence for ferruginous late Neoproterozoic seawater

Abstract

It is widely believed that the Earth's oceans became increasingly oxygenated during the late Neoproterozoic, most notably after the end of the Marinoan glaciation approximately ∼635 million years ago. However, recent geochemical analyses from various Ediacaran sediments suggest that some deep ocean basins were instead anoxic and ferruginous [Fe(II)-enriched] throughout the Ediacaran and possibly into the Cambrian, suggesting a more complex global redox structure than previously thought. Acceptance of this idea has been hindered by an apparent absence of Ediacaran iron formations (IF); chemical sediments that were common under such Fe-rich conditions in earlier Archaean-Palaeoproterozoic oceans. Here, we report detailed sedimentological, stratigraphic, petrographic and geochemical data from an Ediacaran IF and associated rocks, including ‘iron-rich’ black shales, siltstones and chert deposits of the Arroyo del Soldado Group in Uruguay. The Arroyo del Soldado Group is a mixed siliciclastic-carbonate succession interrupted by two major episodes of basin flooding and sediment starvation characterized by the deposition of IF and cherts. On the basis of stacking patterns, two stratigraphic sequences are identified within the group. Sequence A consists of continental shelf siliciclastic facies (Yerbal Formation) grading upward into a carbonate ramp system (Polanco Limestone Formation). Separated by a major erosional unconformity, sequence B is composed of coarse-grained alluvial conglomerates and sandstones (Barriga Negra Formation) capped by finer marine siliciclastics (Cerro Espeulitas Formation). The Yerbal and the Cerro Espuelitas formations, which constitute the main subject of this study, are characterized by retrogradational stacking patterns with fining-upward units deposited during transgressive system tracts. Significantly, the top of both units are characterized by distinct fine-grained siliciclastics and chemical sediments, consisting of banded siltstones, cherts and IF for the Yerbal Formation, and ‘iron-rich’ shales interbedded with black shales that pass upwards into thick chert deposits for the Cerro Espuelitas Formation. Significantly, the Yerbal Formation is a pre-Gaskiers glaciation deposit (ca. 600–590 Ma), while the Cerro Espuelitas Formation is post-Gaskiers (ca. 570–560 Ma) in age, providing insight into the palaeoceanographic conditions immediately before and after the last major Precambrian glacial event. The IF and cherts have coherent rare earth element and yttrium (REY) patterns and display the essential shale-normalized characteristics of marine precipitates. Rare earth element and yttrium signatures and mixing calculations show that they differ from Archaean and Palaeoproteorzoic IF in that high-temperature hydrothermal input did not influence their chemistry. Instead, we suggest that low-temperature hydrothermal input may account for the geochemical signatures displayed. By analyzing the REY signature of chemical precipitates (cherts and IF) and evaluating the redox chemistry of the marine water column within a sedimentological and sequence stratigraphic framework, our results confirm that ferruginous conditions dominated the pre-Gaskiers but also the post-glaciation deep-water chemistry, and that global ocean oxygenation may not have occurred well into the upper Ediacaran or even during the Cambrian, as recently proposed by Canfield et al. (2008).