Fossils from seabed bedrocks: Implications for the nature of the acoustic basement of the Mendeleev Rise (Arctic Ocean)

Abstract The geological information on the structure of the Mendeleev Rise in the Arctic Ocean is scarce due to inaccessibility of this offshore area; therefore its origin is still disputable. Two expeditions of the multi-year programme of deepwater research are devoted to solving this problem. The programme is based on integrated seafloor studies using a research submarine for recovering bedrock samples from seafloor outcrops under subglacial conditions. Sampling was performed in three polygons in the southwestern, central and northern parts of the Mendeleev Rise, where according to seismic data, an acoustic basement crops out at the seabed. Sedimentary and igneous rocks were sampled, with similar rock types in all three of the polygons. The fossils were extracted from sedimentary rocks. The detailed study of crinoids, conodonts, acritarchs, brachiopods, foraminifers, dinocysts, spores and pollen was carried out. Obtained data allowed the dating of 13 samples. Among sedimentary rocks exposed on steep slopes of the Mendeleev Rise , stratigraphic units of three ages were identified: Late Ordovician-Silurian, Middle-Late Devonian and Early Cretaceous. The Upper Ordovician-Silurian rock association mainly occurs in lower parts of the sampled slopes and consists of alternating dolomites, quartzose sandstones and limestones. Carbonate rocks with clastic components similar to those in quartzose sandstones as well as quartzose sandstones with dolomite cement were encountered. Partly dolomitised varieties were observed among limestones; the same spectrum of bioclasts is found in dolomites and limestones. The Middle-Upper Devonian (upper Givetian – Famennian) rock association occur s mainly in upper parts of the slopes and consists of alternating limestones and sandstones. The Lower Cretaceous (Barremian-Aptian) sandstones occur in the upper part of the slope of the Trukshin Seamount (the north of the Mendeleev Rise). New geological data demonstrates that lower and middle Paleozoic rocks of the Mendeleev Rise were deposited in the shallow-water marine environments of the epicontinental basin. Paleozoic deposits form the upper part of the acoustic basement of the Mendeleev Rise. Overlaying Lower Cretaceous sandstones are the basal layer of the stratified Mesozoic-Cenozoic sedimentary cover of the Mendeleev Rise.