bed
depositional surface
firmgrounds
glossifungites
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Beds are enclosed or bounded by sharply defined upper and lower surfaces or bedding planes. These surfaces are probably the easiest physical features of sedimentary rocks to identify in outcrop. They are used to subdivide successions of sedimentary rock into their beds and are traditionally used to determine the relative order and timing of the accumulation of the sediments forming the beds. Concurrently the character of the bedding planes be they eroded, cemented, bored, bioturbated, or depositional surfaces is used to aid in the interpretation of these sedimentary rocks. To this end Allen (1983) has described, using fluviatile sediments as an example, that there at least four kinds of bounding sufaces: concordant non-erosional (normal bedding) ; discordant non-erosional ; concordant erosional; and discordant erosional contacts.
Never the less, the origin of the bedding plane can be enigmatic. The hypothesis presented here is that most of these bedding planes are probably surfaces formed by the erosion of unconsolidated sediment that collected at the sediment surface. The weight of the sediment, just beneath the sediment surface, causes this sediment to dewater, compact and become cohesive.
If this sediment surface is subjected to the erosive force of:
• Storm waves
• Fast flowing currents of water (say in tidal or fluvial channels)
• Turbid flow of a density current
then the bedding plane surface will cut down into sediment, truncating the less cohesive sediment of the surface and exposing a surface of the firmer cohesive sediment below.
Should sedimentation resume over this "firm ground" surface, it is proposed that the density difference between the loose uncompacted sediment and underlying firm ground would be expected to form a bedding plane.
In contrast if the sediment accumulation is curtailed and the surface not subject to immediate deposition of further sediment, the surface may be burrowed by glossifungites.
Another potential outcome is that if these surfaces are exposed to the photosynthetic effects of cyanobacteria, elevated salinities or upwelling ground waters, they become cemented at or close the sediment water interface. If these surfaces are exposed for any length of time they may be colonized and bored by marine organisms.
Bedding planes can also be the lower surfaces of sediment bodies that are carried into the depositional setting by turbidities and/or crevasse splays.
Reference
Allen, J. R. L. 1983, Studies in fluviatile sedimentation: bars, bar complexes and sandstone sheets (low sinuosity braided streams) in the Brownstonews (L. Devonian), Welsh Borders. Sedimentary Geology, 33, 237-293.
Einsele G., Ricken W., and Seilacher A., (editors), 1991, "cycles and events in stratigraphy", Springer-Verlag, Berlin, Heidelberg, New York 1991. 955p.
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