Papers

Turbidites: Why are they important?

Economic value

  Turbidites are extremely important for the worldwide economy.  they contain enormous amounts of hydrocarbon deposits.  And in other areas of the world, turbidites contain precious metals such as gold.

oil and gas deposits

  Oil and gas deposits in turbidites come from organic material that was trapped in the flowing current.  Accumulated sediment as more turbidity currents stack one on top of another, pressure, temperature, and time turn the organic material into hydrocarbon deposits.  Kerogen is an organically rich compoutn that can be found in turbidites.  For example, the Teradomari Formation is one of the main sources of petroleum in central Japan.  this formation consists mainly of trurbidity currents.  These are dark brown, bioturbated, silty, and clayey mudstones (Watanbe and Akiyama, 1998).  The mudstones from these deposits contain kerogen, which are the chemical compounds that make up a portion of the organic matter in sedimentary rocks.  it is from kerogens that we get our fossil fuels, oil shales, and petroleums.  There are four different types of kerogens.  These are listed below:

• Type I kerogens are derived from lacustrine algae and forms only in anoxic lakes and other unusual marine environments.  These break down to form liquid hydrocarbons.

• Type II kerogens are derived from terrestrial plant resins, animal decomposition resins, marine algae, from pollen and spores.   These also break down in anaerobic conditions, tending to produce both gas and oil.

• Type II is high in sulfur.

• Type III kerogens are derived from terrestrial plant matter that was broken down by bacteria.  Most of the biomass, however, is derived from the bacterial and not from the plant matter itself.  This kerogen produces coal and gas.

• Type IV kerogens are mainly decomposed organic matter, but have no tendency to produce hydrocarbons.

gold deposits

  Gold deposits have been discovered in turbidites around the world.  These are auriferous, meaning the pyrites contained in these deposits contain gold.  The sedimentary rocks from which these deposits come are mainly shales and slates.  There are other types of deposits.  These contain conglomerates, sandstones, and greywacke, which have a higher than normal amount of gold.  After deposited, auriferous turbidites are folded, faulted, metamorphosed, granitized or invaded by granitic rocks.  The current theory as to why these turbidites contain gold is because of magmatic hydrothermal processes.  The theory states that as a granitic body cools, it differentiates with material moving up and crystallizing.  The last materials to move up and out are the liquids containing the metals.  These liquids move along the fractures and bedding planes until they crystallize into the ore containing veins that we see today (Keppie et al., 1986).

References Cited

Keppie, J. Duncan, Boyle, and R. W., Haynes, S.J.\1986\ Turbidite-Hosted god Deposits, In Geological Association of Canada Special Paper 32.

Watanabe, Harumi, Akiyama Moshahiko, \1998\ Characterization of organic matter in the Miocene turbidites and hemipelagic mudstones in the Niigata oil field, Central Japan, Organic Geochemistry, v. 29, no. 1-3 p. 605-611