PRELIMINARY REPORT OF LATE CARBONIFEROUS ONCOIDS FROM THE BRØGGERHALVØYA, NW SVALBARD, ARCTIC NORWAY

Numerous oncoids in the lower part of Wordiekammen Formation (Moscovian) are investigated from the two different localities (Strypbekken and Stupbekken) of Brøggerhalvøya, NW Svalbard. The oncoid-bearing beds are characterized by presence of rounded to ellipsoidal osagid-type meso-oncoids (2-20 mm) and macroids (>20 mm). Oncoids from Strypbekken are dominated by meso-oncoids with lesser amount of macroids (up to 50 mm). On the other hand, those from Stupbekken, consist prevalently of rather larger-sized oncoids (up to 70 mm. Nuclei of the oncoids are mostly green and phylloid algae and some are hard to identify. Cortices are composed of diverse biota. Compactly- or loosely-spaced, encrusting tubiform foraminifera (Palaeonubecularia) are abundant within the cortex. These tubes are marked by their dark micritic wall and display range in diameter (20-100 μm). Well-preserved microbes, which are also significant constituents of the oncoids, are appeared as cluster of radiating filamentous tubes. They commonly co-occur with peloidal and micritic massses with rare indistinct filaments (poorly-preserved microbes?). Colonial, branching hollow tubes (Nansenella?) and irregular-shaped encrusting laminar organisms (Archaeolithophyllum?) are also common. Based on preliminary microscopic observation, dimensions of oncoid appear to have relation to the biotic composition. Smaller oncoids are dominantly formed by foraminifera associated with structureless or slightly-laminated micritic mass (foraminiferal-microbial oncoids). Larger oncoids are mostly built by consortium of microbe, foraminifera, and alga (composite oncoids). Other possible candidate for causing size difference is water-energy level. Meso-oncoids are dominated within wacke- to packstone matrix, whereas, macroids are predominantly found in pack- to grainstone matrix implying the dissimilarity in degree of agitation. Although the biotic composition and associated facies of oncoids are valuable paleoenvironmental proxies, oncoids and oncoidal facies from the late Paleozoic sedimentary successions of Svalbard have been overlooked. Further detailed study of these oncoids and their surrounding facies will enable us to access more precise interpretation and reconstruction of depositional environments of the region.