THE SPHERULE LAYER IN THE PALEOPROTEROZOIC GRÆNSESØ FORMATION IN MIDTERNÆS, SOUTH GREENLAND

The single known outcrop (~ 4 m long) of the spherule layer in Midternæs was relocated, and ~13 kg and ~1.2 kg of sample were collected from the layer and the overlying strata, respectively. The spherule layer has a minimum thickness of 145 cm, and float of this layer extends along strike for ~200 m to the southwest. The outcrop is partially obscured by rubble, and it appears to dip (<15°) to the southeast. The rock is a medium dark grey (very pale-to-grayish orange on weathered surfaces) dolomixtite: subrounded-to-angular clasts of medium dark grey-to-black chert (up to 10 cm across) and grayish orange-to-pale yellowish brown dolostone (up to 110 x 40 cm in size) are supported by a fine-grained calcitic dolomite matrix that comprises ~40-50% of the rock. The larger chert clasts are tabular and elongate, and their long axes are typically aligned roughly parallel to the contacts of the bed. Evenly dispersed in the matrix are grayish-black spherules (~400-1800 μm in diameter) and subrounded-to-angular, coarse sand-to-granule-sized chert grains, each of which comprises about 15-20% of the rock, and very fine-to-coarse sand-sized, subangular epiclastic quartz (~1-2% of the rock). Obvious volcanic material has not been observed. The spherule layer is massive, and no obvious size gradation of clasts was observed. Microscopically, the spherules are mostly spherical and ovoid in shape, and rare teardrop and dumbbell shapes are present. Under crossed polarizers, the spherules typically show fans of radial-fibrous aggregates that consist mainly of chalcedony and sericite. The fans typically show variable orientations within a given spherule; some fans radiate inward from the margins of the spherules. One spherule contains a possible infilled vesicle. Very coarse sand-sized blocky particles are present, and several of them show a possible flow foliation as defined by minute sericite laths. Single-grain X-ray diffraction (XRD) analysis confirms the spherule mineralogy, and it indicates the presence of zircon, dravite, anatase, and rutile in the heavy mineral (r>2.96 g/cc) separates. Most zircons are pink, but some are white opaque, and of the latter, five have thus far shown asterism on Debye-Scherrer XRD patterns. Field, petrographic, and XRD analyses support the hypothesis of an impact-related origin for the spherule layer.