What are the major differences between autochtonous, allochthonous and parautochthonous sediments (in terms of field observations, biotic assemblage diversities and sedimentological characteristics)?
Dear Suman,
The three terms refer to the origin of a sediment:
- autochtonous refers to sediments that are native to its location (e.g. carbonat precipitation in a lacustrine facies),
- allochtonous refers to sediments originating from a place other than where it was found (e.g. glacial dropstones in marine sediments), and
- paraautochtonous refers to sediments of intermediate/mixed characteristics.
As such, these are very general terms and, as a consequence, there are no a priori characteristics which can be applied universally. In the field, you will have to analyze the facies under consideration and find out what specific lithological properties you would expect and what the depositional regime was. If you find clear deviations from these expectations, clear indications of (long distance) transport (e.g. well-rounded pebbles), biotics of different origin, etc., these may be good hints for allochtonous formation.
Best wishes,
David
Dear Suman,
The three terms refer to the origin of a sediment:
- autochtonous refers to sediments that are native to its location (e.g. carbonat precipitation in a lacustrine facies),
- allochtonous refers to sediments originating from a place other than where it was found (e.g. glacial dropstones in marine sediments), and
- paraautochtonous refers to sediments of intermediate/mixed characteristics.
As such, these are very general terms and, as a consequence, there are no a priori characteristics which can be applied universally. In the field, you will have to analyze the facies under consideration and find out what specific lithological properties you would expect and what the depositional regime was. If you find clear deviations from these expectations, clear indications of (long distance) transport (e.g. well-rounded pebbles), biotics of different origin, etc., these may be good hints for allochtonous formation.
Best wishes,
David
Dear Suman,
The above answers are fine for carbonate sediment, but these terms are also applied in terrigenous sediments, e.g. autochthonous vs. allochthonous sedimentation on the continental shelf.
Best regards,
Xiting
In ecology the terms are also applied when referring to different sources of organic carbon, mainly in lakes where primary production is one source (autochthonous) and organic carbon which is washed into the lake by streams is another source (allochthonous). In streams periphyton produce autochthonous carbon, while leaves and needles which fall into the stream is allochthonous carbon. Isotopic analysis can help to distinguish these carbons.
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