Rhythmite

A brown rock or sediment face with horizontal layers, 18 of which are clearly visible. Some of the layers are obviously thicker than others - presumably the result of differences in annual deposition rates due to seasonal variations.
Pleistocene age varves at Scarborough Bluffs, Toronto, Ontario, Canada. The thickest varves are close to 2 cm thick.

A rhythmite consists of layers of sediment or sedimentary rock which are laid down with an obvious periodicity and regularity. They may be created by annual processes such as seasonally varying deposits reflecting variations in the runoff cycle, by shorter term processes such as tides, or by longer term processes such as periodic floods.

Rhythmites serve a significant role in unraveling prehistoric events, providing insights into sea level change, glaciation change, and Earth's orbital variations which serve to answer questions about climate change.

Annually-laminated rhythmites

Annually-laminated deposits (varves) are rhythmites with annual periodicity: annual layers of sediment or sedimentary rock are laid down through seasonal variations that result from precipitation, or from temperature, which influences precipitation rates and debris loads in runoff. Of the many rhythmites found in the geological record, varves are among the most important and illuminating to studies of past climate change. Varves are amongst the finest resolution events easily recognised in stratigraphy.[1]

Periodically laminated rhythmites

This photo shows a canyon cut into the surrounding flat soil with 32 distinct horizontal layers of soil, each clearly demarked from the layer below. Above the canyon a farm house can be seen in the distance - the farm house provide the perspective that helps the viewer establish that the cut is over 40 deep. The bottom of the cut is filled with tumble weeds.
Distinct layers of Touchet beds in the "Little Grand Canyon" near Lowden, Washington in the Walla Walla valley

Rhythmites may be deposited with periodicities other than annual. The geologic record captures both more frequent events (e.g., tides) and less frequent events (glacial floods).

Tidal rhythmites

Geologic tidal rhythmites display layered Carboniferous Period beds which record tidal cyclical events such as semi-diurnal, diurnal or neap tide, spring tide cycle that demonstrate marine influence in sediments that were previously interpreted as purely continental.[2][3] The geologic record captures layered beds comparable to those found currently in sediments in the Bay of Fundy in Canada and the Bay of Mont Saint-Michel in France.[4] The Storm Mountain area of Big Cottonwood Canyon, Utah, has rhythmites which record sea-level sedimentary deposit fluctuations consistent with the cycle of the tides. Tidal rhythmites are also known from other geological periods and times, such as the late Precambrian.[5]

Proglacial rhythmites

One common mechanism is the episodic flooding which results from glacial dam bursts. In one such example geologists estimate that the Missoula Floods cycle of flooding and reformation of the lake took an average of 55 years and that the floods occurred approximately 40 times over the 2,000-year period between 15,000 and 13,000 years ago. Distinct rhythmites with an approximately 55-year periodicity have been observed.[6][7]

Glacial epicycle rhythmites

Sea-level changes which correspond to the glacial periods also show up as extremely long-term rhythmites. As an example, the ice surge in the Quaternary resulted in changes in sea level of 127 meters to 163 meters. The regression and transgression of the sea level from waxing and waning glaciers have been identified in the rhythmites of the Pennsylvanian and Permian periods.[8]

See also

The Wikibook Historical Geology has a page on the topic of: Tidal rhythmites and dating

References

  1. ^ Hambrey, M. J.; Harland, Walter Brian (1997). Earth's pre-Pleistocene glacial record. Cambridge earth science series. ISBN 978-0-521-22860-2. Retrieved 7 September 2009.
  2. ^ Kuecher, Gerald J.; Woodland, Bertram G.; Broadhurst, Frederick M. (1 September 1990). "Evidence of deposition from individual tides and of tidal cycles from the Francis Creek Shale (host rock to the Mazon Creek Biota), Westphalian D (Pennsylvanian), northeastern Illinois". Sedimentary Geology. 68 (3): 211–221. doi:10.1016/0037-0738(90)90113-8. ISSN 0037-0738.
  3. ^ Archer, Allen W; Kuecher, Gerald J; Kvale, Erik P (1995). "The Role of Tidal-Velocity Asymmetries in the Deposition of Silty Tidal Rhythmites (Carboniferous, Eastern Interior Coal Basin, U.S.A.)". SEPM Journal of Sedimentary Research. 65: 408–416. doi:10.1306/d42680d6-2b26-11d7-8648000102c1865d.
  4. ^ B.W. Flemming and A. Bartholoma (1995). Tidal signatures in modern and ancient sediments. Blackwell Science, Oxford. ISBN 9780865429789.
  5. ^ Williams, G. E. (January 1989). "Late Precambrian tidal rhythmites in South Australia and the history of the Earth's rotation". Journal of the Geological Society. 146 (1): 97–111. doi:10.1144/gsjgs.146.1.0097.
  6. ^ "Walla Walla Level 1 Watershed Assessment" (PDF). WRIA 32 Watershed Plan. Walla Walla Watershed Management Partnership. Retrieved 1 September 2009.
  7. ^ Bjornstad, Bruce (2006). On the Trail of the Ice Age Floods: A Geological Guide to the Mid-Columbia Basin. Keokee Books; San Point, Idaho. ISBN 978-1-879628-27-4.
  8. ^ Washburn, Albert Lincoln (1997). Plugs and Plug Circles: A Basic Form of Patterned Ground, Cornwallis Island, Arctic Canada - Origin and Implications. Geological Society of America, Incorporated. ISBN 978-0-8137-1190-4. Retrieved 7 September 2009.

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