Graded bedding

Schematic illustrations of two styles of graded bedding: left: normal grading; right: inverse grading.
Schematic illustrations of two styles of graded bedding: left: normal grading; right: coarse tail grading.

In geology, a graded bed is a bed characterized by a systematic change in grain or clast size from bottom to top of the bed. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones. Such a bed is also described as fining upward.[1] Normally graded beds generally represent depositional environments which decrease in transport energy (rate of flow) as time passes, but these beds can also form during rapid depositional events. They are perhaps best represented in turbidite strata, where they indicate a sudden strong current that deposits heavy, coarse sediments first, with finer ones following as the current weakens. They can also form in terrestrial stream deposits.

In reverse grading or inverse grading the bed coarsens upwards. This type of grading is relatively uncommon but is characteristic of sediments deposited by grain flow and debris flow.[2] A favored explanation for reverse grading in these processes is kinetic sieving.[3] It is also observed in aeolian processes, such as in pyroclastic fall deposits.[4] These deposition processes are examples of granular convection.

  1. ^ Allaby, Michael, ed. (2013). "fining-upward succession". A Dictionary of Geology and Earth Sciences (Fourth ed.). Oxford: Oxford University Press. p. 219. ISBN 9780199653065.
  2. ^ Tucker, M. (2003). Sedimentary Rocks in the Field. Wiley. p. 115. ISBN 978-0-470-85123-4.
  3. ^ Gray, J.M.N.T.; Shearer, M.; Thornton, A.R. (2006). "Time-dependent solutions for particle-size segregation in shallow granular avalanches". Proceedings of the Royal Society A. 462 (2067): 947–972. Bibcode:2006RSPSA.462..947G. doi:10.1098/rspa.2005.1580.
  4. ^ Duffield, W.A.; Bacon, C.R.; Roquemore, G.R. (1979). "Origin of reverse-graded bedding in air-fall pumice, Coso Range, California". Journal of Volcanology and Geothermal Research. 5 (1–2): 35–48. Bibcode:1979JVGR....5...35D. doi:10.1016/0377-0273(79)90031-3.