POSSIBLE NON-MARINE EUKARYOTE FOSSILS IN THE 1.1 GA COPPER HARBOR FM., MICHIGAN

Geometrically distinctive, apparently biogenic, cm-scale features have been found on the bedding plane of a siltstone within the ca. 1.1 Ga Copper Harbor Fm., the lowest rift-filling sedimentary unit of the Midcontinent Rift. The Copper Harbor Fm. includes conglomerates, sandstones, and finer-grained rocks representing alluvial fan, braided stream, and ephemeral lake environments within the axial valley of the Rift. Calcified stromatolitic mats have long been recognized in the upper part of the Copper Harbor Fm. The newly discovered features were found in a siltstone between two stromatolitic horizons at Horseshoe Harbor, upper Michigan. In plan view, the features are circular to elliptical and 0.3-1.0 cm in diameter. Many have a transecting lenticular element that creates a ‘theta’ geometry. These septa lie at random orientations in the slabs in which more than 70 of the features were found. In cross section, the features are funnel-shaped, narrowing to a blunt tip marked in some specimens by a mm-scale crystal of calcite. Calcite also cements the fine-grained material inside the features. Petrographic and XRD analyses indicate that the rest of the interior material is fine quartz and feldspar plus minor clay (halloysite) and iron oxide, as found in the surrounding rock. SEM images of the material inside and outside the features show no obvious differences in grain size or texture. These features are unlike any common sedimentary or diagenetic structures. If they are biogenic, they most likely represent either body or trace fossils of eukaryotes. They are too large to have been formed by single prokaryotes, and their consistent geometry seems improbable for colonies of individuals. Marine eukaryotes were diverse by Mesoproterozoic time, but there are few reports of terrestrial eukaryotes of this age. Strother et al. (Nature 2011) recently documented morphologically complex eukaryotic fossils from the 1.1-1.2 Ga Torridonian sequence of Scotland, similar both in age and depositional setting to the Copper Harbor Fm. The Copper Harbor stromatolites indicate that in spite of a high-energy environment, there was an active lacustrine ecosystem in the valley of the Midcontinent Rift at 1.1 Ga. The newly discovered features point to a greater degree of terrestrial biodiversity than previously recognized.