MINERALOGY AND SOURCE MATERIAL FOR THE TOLSONA MUD VOLCANOES IN THE COPPER RIVER BASIN, ALASKA

The focus of this research is to determine the source of the mud being extruded from the Tolsona mud volcanoes, based upon its mineral content. The mud volcanoes lie within the Copper River basin, which is comprised of Pleistocene glacio-lacustrine sediments at the surface and Mesozoic rocks at depth. Therefore, we tested two existing hypotheses for the source of the mud. First, the mud may be the result of diapiric movement from Lower Cretaceous and Upper Jurassic marine sedimentary rocks at depth to the surface as a result of abnormal pressure (Foresman, 1970) and low-grade metamorphic activity. Secondly, the mud may be representative of Pleistocene glacio-lacustrine sediments. Na-Ca-rich groundwater, methane, and nitrogen gas seep from the mud volcanoes. The water chemistry and gas composition have been previously studied (Foresman, 1970; Motyka et al., 1989) and suggest their source is from rocks of Cretaceous or Jurassic age. The presence of unabraded Upper Cretaceous fossil shell fragments in the mud also supports this thesis (Gantz et al., 1962). For this study, samples were collected from each mud volcano, nearby glacio-lacustrine sediments, and exposed Mesozoic sedimentary and metasedimentary bedrock. Samples were dried, crushed, and sieved to <63µm, then analyzed for their mineral composition using a Cu radiation source in a Miniflex x-ray diffractometer (XRD) instrument. Data reduction of the XRD peak locations and intensities suggest the presence of the following minerals: quartz, feldspar, chlorite, and illite. Diffraction patterns of the mud samples most closely resembled that of a Lower Cretaceous/Upper Jurassic rock sample and differed significantly from an Upper Cretaceous rock sample. The glacio-lacustrine sediments contained too much organic matter for successful mineral composition determination with the XRD. Therefore, these preliminary data, when considered with water and gas chemistry, support the initial hypothesis.