Continental siltstones of the Mesoproterozoic Copper Harbor Formation, Michigan contain macroscopic structures of a size and morphological complexity commonly associated with fossils of eukaryotic macroorganisms. A biogenic origin for these structures would significantly augment the Proterozoic continental fossil record, which is currently poor, and also add to a growing body of sedimentological and geochemical data that, albeit indirectly, indicates the presence of life in continental settings early in Earth's history. These three-dimensional structures occur abundantly within a single cm-scale siltstone bed. Along this bedding plane, these structures are generally circular-to-ovoid, range up to several centimeters in diameter, and most specimens possess a transecting lenticular element. Structures exhibit sharp, well-rounded external margins and, in contrast to the surrounding aluminosilicate-rich matrix, are calcitic in composition. Surrounding sedimentary laminae are deflected by and cross cut the structures. A fossiliferous origin is considered but rejected and an authigenic concretionary origin is favored based on these characters. However, a concretionary origin does not exclude the possibility of a biogenic precursor that served as a locus for early diagenetic calcite precipitation. This study highlights the need for careful analysis of morphological, mineralogical, distributional, and sedimentological characteristics when considering the origin of enigmatic structures; morphological complexity alone is an insufficient criterion for assignment of biogenicity. The unusual morphology of these concretions augments known concretion morphologies generally comparable to unusual fossil forms, and draws into question the biogenicity of similarly cryptic Proterozoic structures including, in particular, those of the 2.0 Ga Francevillian B Formation of Gabon.
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