OTHER NUMERICAL DATING TECHNIQUES VARVE CHRONOLOGY Lakes can produce annual layers. Usually occur in glacial lakes or those that freeze over in winter. Coarser sediments are deposited in summer. Winter-summer layers are called COUPLETS. Couplets in lakes are known as VARVES. Count the couplets back from the sediment surface to determine numerical age. OTHER NUMERICAL DATING TECHNIQUES VARVE CHRONOLOGY OTHER NUMERICAL DATING TECHNIQUES LICHENOMETRY Lichens are plant-like organisms that grow on rocks. Grow at a measurable rate. By measuring size on items of known date, the size is plotted against size on unknown aged objects. Good for the last 9000 years. OTHER NUMERICAL DATING TECHNIQUES LICHENOMETRY OTHER NUMERICAL DATING TECHNIQUES SURFACE EXPOSURE DATING Numerical dating can also be applied to landscape features. Features such as moraines or alluvial fans can make use of COSMOGENIC ISOTOPES. These isotopes are produced on the surface of newly exposed rocks by bombardment of cosmic rays. COSMIC RAYS are high velocity neutrons. Penetrate rocks to a depth of 2-3 m. Go right thru you. OTHER NUMERICAL DATING TECHNIQUES Alluvial fan Moraine OTHER NUMERICAL DATING TECHNIQUES COSMOGENIC DATING Cosmic rays produce new “cosmogenic” radioactive isotopes. Si, Mg, Al, Fe are converted to 10Be. K, Ca, Cl are converted to 36Cl. Once formed they begin to decay. OTHER NUMERICAL DATING TECHNIQUES COSMOGENIC DATING 10Be has t1/2 of ~ 300,000 years. 36Cl has t1/2 of ~ 1.5 million years. Fills the gap between 14C and K-Ar. Problem exists that new cosmogenic isotopes are always being created. Not constant number like non-cosmogenic isotopes. Also dealing with very small amounts of daughter isotopes. May be lost due to erosion. AGE OF THE EARTH Oldest Earth materials are zircons found in sandstone in Australia 4.1-4.2 by Metamorphic Rocks in NWT, Canada Granites in Greenland 3.96 by 3.7 by Meteorites have U-Pb and Rb-Sr dates of 4.6 by AGE OF THE EARTH Moon rocks dated between 3.3 and 4.6 by Original lunar crust is 4.6 by Moon and Earth formed at about the same time. Earth is 4.6 by old. SEDIMENTARY ARCHIVES GE 142 focuses on sedimentary rocks What makes them different than igneous and metamorphic rocks? SEDIMENTARY ARCHIVES With sedimentary rocks we can consider: Source area Mode of transport Physical processes active Chemical processes active Biological processes active Climate Lithification and Diagenesis These may all influence a single unit SEDIMENTARY ARCHIVES Tectonic Influence Deformation of large areas of the Earth’s crust over large intervals of time SEDIMENTARY ARCHIVES Uplift Size of particles? Thickness of units? Sediment supply? SEDIMENTARY ARCHIVES Subsidence Size of particles? Thickness of units? Sediment supply? SEDIMENTARY ARCHIVES Low Topography Size of particles? Thickness of units? Sediment supply? SEDIMENTARY ARCHIVES Craton Composed of two parts Shield and Platform Shield - exposed ancient crystalline rocks Core of continent Canadian Shield in North America SEDIMENTARY ARCHIVES Craton Platform – covering on shield Flat to gentle dip Undisturbed since preCambrian Stable interior of continents SEDIMENTARY ARCHIVES Orogenic Belts Elongate regions that border craton Usually highly deformed Mountain belt Earthquakes, volcanoes in younger belts Highly deformed or meta. rocks in older belts Exposed igneous intrusive bodies SEDIMENTARY ARCHIVES Accretionary Wedges (Coastal Plains) Large wedges of sediment accumulate on margins of craton Later deformed Accumulate on trailing edge Deformed on leading edge SEDIMENTARY ARCHIVES Tectonic setting determines type of sedimentary rocks Sedimentary rocks can then infer tectonic setting SEDIMENTARY ARCHIVES Environments of Deposition All of the physical, chemical, biologic and geographic conditions under which sediments are deposited. Type of sediment influenced by climate and geography The type of sediment is key to the environment Use modern depositional environments to interpret past UNIFORMITARIANISM or ACTUALISM SEDIMENTARY ARCHIVES Environments of Deposition Sedimentary rocks may be: Extrabasinal in origin Sediments formed from the weathering of pre-existing rocks outside the basin, and transported to the environment of deposition Intrabasinal in origin Sediments form inside the basin; includes chemical precipitates, most carbonate rocks, and coal. By comparing modern sedimentary deposits with ancient sedimentary rocks, the depositional conditions can be interpreted.