Dredged bedrock samples from the Amerasia Basin, Arctic Ocean

Between 2008-2012, as part of the U.S. Extended Continental Shelf project in the Amerasia Basin, Arctic Ocean, 17 dredges were successfully collected sampling the first rock outcrops in the Chukchi Borderland and surrounding regions for the purpose of describing the geologic nature of the bathymetric features in this area. Multiple lines of evidence indicate that the specimens were collected from submarine rock exposures and were not samples of ice rafted debris, common in the ice covered waters of the Arctic Ocean. Using the USCGC Healy, each dredge was collected along very steep slopes (>35 degrees) measured with high resolution multibeam swath bathymety data. Each haul yielded samples of similar lithologies and identical metamorphic grade with manganese crusts on the surfaces exposed to seawater and fresh surfaces where the rocks were broken from outcrop. High tension pulls on the dredge line also indicated sampling of bedrock exposures. Dredged samples from a normal fault scarp in the central Chukchi Borderland consisted of Silurian (c. 430 Ma) orthogneisses that intruded older (c. 487-500 Ma) gabbros and luecogranties that were all metamorphosed to amphibolite grade (Brumley et al., 2011). Samples from the northern Northwind Ridge consisted of metasediments (greenschist facies) interpreted to have been deposited in a proximal arc setting with detrital zircon U-Pb age peaks at 434, 980 Ma with lesser peaks between 500-600, 1100-2000 Ma, and rare 2800 Ma grains (Brumley et al, 2010). Other dredges in the region of the Northwind Ridge yielded deformed and metamorphosed calcareous sandstones and low-grade phyllites (O'Brien et al., 2013). Taken together these rocks indicate a relationship to the Pearya Terrane of northern Ellesmere Island and S.W. Svalbard that were thought to represent a Cambro-Ordovician volcanic arc terrane that was involved in Caledonian orogenesis (Brumley et al., 2011). These findings constrain plate tectonic reconstruction models and bring into question long held ideas that the Chukchi Borderland was made up of an undeformed platformal sequence that was part of the Laurentian passive margin from Proterozoic through Jurassic time (e.g. Grantz et al., 1998). Dredges collected along fault scarps that border the edges of the Nautilus Basin yielded undeformed but highly altered volcaniclastic rocks that were deposited in a shallow water setting and contain primary potassium feldspar phenocrysts that are not associated with mafic magmas. Also in this region, several dredges contained undeformed Late Cretaceous (112, 88-80 Ma) basalts (Andronikov et al., 2008; Mukasa et al., 2009) interpreted to have been derived from a continental lithospheric source similar to continental flood basalts from other regions (Mukasa et al., 2009). These dredged rock samples not only have implications for the Extended Continental Shelf projects of Arctic nations, but add greatly to the body of geologic information about the history of the Arctic Ocean and provide the first ground truth as to the nature of the bathymetric features within the Amerasia Basin.