Mud is not that simple
Monday, 17. December 2007, 19:23:05
Sediment grainsize is an important factor in sedimentology, and tells a lot about the environment in which the gravel, sand or mud was deposited. It tells us about current flow velocities, distance from shore and water depths. It tells us stories about sea level change as the sediment change from coarse grains to fine grains or from fine grains to coarse grains.
Mudstones were long thought to record low-energy conditions of offshore and deeper water environments. Geologists thought that constant, rapid water flow prevented mud's constituents, silts and clays, from coalescing and gathering at the bottoms of rivers, lakes and oceans. As mudstones make up the majority of the geological record - they constitute up to two-thirds of the sedimentary record - it is important to get things right. Mudstones are, however, arguably the most poorly understood type of sedimentary rocks.
New research based on laboratory experiments shows muds will accumulate even when currents move swiftly. The findings appear in the journal Science of 14 December 2007. During their experiments the researchers found that mud beds accumulate at flow velocities that are much higher than what anyone would have expected. The mud accumulates slowly at first, in the form of heart- or arrowhead-shaped ripples that point upstream. These ripples slowly move with the current while maintaining their overall shapes.
A key issue in mudstone sedimentation is flocculation. Flocculation is a process by which individual particles of clay aggregate into clotlike masses or precipitate into small lumps. Such small, loosely held masses or aggregates of fine particles are called floccules. Flocculation enhances the deposition rate of fine-grained sediments, and its understanding is critical for modelling the behaviour of mud in sedimentary environments.
The observations from the experiments do not support the notion that muds can only be deposited in quiet environments with only intermittent weak currents. Another interesting point is the formation of ripples. That ripples have not been observed in sediments so far is probably because they get extremely flat during compaction. Floccule ripples are spaced 30 to 40 cm apart, and ancient sediments of this origin will therefore likely appear parallel-laminated.
Understanding the mechanisms of mudstone deposition is a.o. relevant for oil and gas exploration, maintenance of water reservoirs, and several branches of engineering. The authors think that a re-evaluation of the sedimentary history of large portions of the geologic record nay be necessary.
A big question is of course whether the rest of the geological research environment agree with this viewpoint.
• http://www.sciencemag.org/cgi/content/abstract/318/5857/1760
• http://www.sciencemag.org/cgi/content/full/318/5857/1734
• http://www.terradaily.com/reports/As_Waters_Clear_Scientists_Seek_To_End_A_Muddy_Debate_999.html
PS of 18 December 2007: See Brian's post at Clastic Detritus
Mudstones were long thought to record low-energy conditions of offshore and deeper water environments. Geologists thought that constant, rapid water flow prevented mud's constituents, silts and clays, from coalescing and gathering at the bottoms of rivers, lakes and oceans. As mudstones make up the majority of the geological record - they constitute up to two-thirds of the sedimentary record - it is important to get things right. Mudstones are, however, arguably the most poorly understood type of sedimentary rocks.
New research based on laboratory experiments shows muds will accumulate even when currents move swiftly. The findings appear in the journal Science of 14 December 2007. During their experiments the researchers found that mud beds accumulate at flow velocities that are much higher than what anyone would have expected. The mud accumulates slowly at first, in the form of heart- or arrowhead-shaped ripples that point upstream. These ripples slowly move with the current while maintaining their overall shapes.
A key issue in mudstone sedimentation is flocculation. Flocculation is a process by which individual particles of clay aggregate into clotlike masses or precipitate into small lumps. Such small, loosely held masses or aggregates of fine particles are called floccules. Flocculation enhances the deposition rate of fine-grained sediments, and its understanding is critical for modelling the behaviour of mud in sedimentary environments.
The observations from the experiments do not support the notion that muds can only be deposited in quiet environments with only intermittent weak currents. Another interesting point is the formation of ripples. That ripples have not been observed in sediments so far is probably because they get extremely flat during compaction. Floccule ripples are spaced 30 to 40 cm apart, and ancient sediments of this origin will therefore likely appear parallel-laminated.
Understanding the mechanisms of mudstone deposition is a.o. relevant for oil and gas exploration, maintenance of water reservoirs, and several branches of engineering. The authors think that a re-evaluation of the sedimentary history of large portions of the geologic record nay be necessary.
A big question is of course whether the rest of the geological research environment agree with this viewpoint.
• http://www.sciencemag.org/cgi/content/abstract/318/5857/1760
• http://www.sciencemag.org/cgi/content/full/318/5857/1734
• http://www.terradaily.com/reports/As_Waters_Clear_Scientists_Seek_To_End_A_Muddy_Debate_999.html
PS of 18 December 2007: See Brian's post at Clastic Detritus
WIDGETIZE