Bedding Plane Formation

[TheBitterEnd]

Owing to not being able to keep my gob shut I have ended up having to think hard about bedding planes (it's a laugh a minute here...). Anyway, I have checked the usual references and they seem very thin/vague on exactly how and why bedding planes form. So I thought I'd ask on here on the off-chance any one can cite any work that has been done.

What I would like to understand is the typical sequence of events that causes a layer of bedding to be separate from the next layer. If the ocean floor is just mud and sedimentation stops and then restarts does a separate bedding plane form? For how long does sedimentation have to stop? Does the lower layer have to start to solidify, be compacted or undergo some other process? Is the sea floor often coated with other material, however thinly, during times of non-sedimentation?

I am thinking particularly of bedding within a single cycle, rather than bedding between cycles.

 

[peterk]

This any good?
PUNCTUATED AGGRADATION
The theory that the long-term aggradation of sediment (through geological time) has been via episodic SEDIMENTATION. This is in contrast with the traditional concept of UNIFORMITARIANISM and the continual and gradual build-up of sediments through time. Early studies such as that by Barrell (1917) provided the initial challenge to the longheld paradigm of gradual aggradation. The theory of punctuated aggradation began to gather momentum once more in the early 1980s. Ager (1980: 43) fuelled the debate by referring to sediment stratigraphy as having ?more gaps than record?, and argued that the large disparities between modern sediment deposition (for a specific environment) and ancient calculated deposition was a result of the episodic nature of aggradation. The theory of punctuated aggradation treats each bedding plane as a pause in sedimentation, whereas continual aggradation considers bedding planes as merely signifying a change in diagenesis or texture, and treats the formation as the basic stratigraphic unit, each one a product of a particular environment. The term punctuated aggradational cycle (or PAC) was coined by Goodwin and Anderson (1985), within their hypothesis for episodic stratigraphic accumulation. The hypothesis argues that, allowing minor exceptions, the stratigraphic record consists of thin (1?5 m thick), basin-wide, shallowing-upward cycles. These are sharply defined by surfaces produced by geologically instantaneous relative BASE-LEVEL rises (termed punctuation events). Deposition occurs during intervening periods of base-level stability. A host of depositional environments can be included in the PAC hypothesis (e.g. fluvial, deltaic, shelf, slope, etc.), as PACs are assumed to exist in all depositional environments influenced by rapid base-level rises.

The PAC hypothesis proposes that allogenic processes such as sea-level change are responsible for changes in the stratigraphic record, rather than autogenic processes (e.g. channel migration, etc.) that are held as responsible in continuous aggradation. Autogenic processes are not dismissed entirely, but are treated as localized stratigraphic influences, superimposed on the allogenic processes. The bounding surfaces between the PACs are often traceable laterally for vast distances since they are formed by large-scale allogenic processes. This allows them to be accurate stratigraphic markers in the field. Base-level rise during a punctuation event can be rapid (reaching 1m per 100 years) whereas stratigraphical analysis indicates that the recurrence of such punctuation events can be as frequent as 50,000 years, thus reflecting the rapidity of the base-level rise. Thickness of PACs, though generally thin, varies considerably though long-term aggradation rates remain similar. Goodwin and Anderson suggest that the most likely mechanisms responsible for PACs would include episodic crustal movement, episodic movement of the geoidal surface and global eustatic sea-level changes.
References
Ager, D.V. (1980) The Nature of the Stratigraphical Record, 2nd edition, New York: Wiley.
Barrell, J. (1917) Rhythms and the measurement of geologic time, Geological Society of America Bulletin 28, 745?904. Goodwin, P.W. and Anderson, E.J. (1985) Punctuated aggradational cycles: a general hypothesis of episodic stratigraphic accumulation, Journal of Geology 93, 515?533.

Further reading
Dott, R.H. (1982) SEPM presidential address: episodic sedimentation ? how normal is average? How rare is rare? Does it matter? Journal

 

[Gollum]

can't beat a bit of good bed time reading

 

[TheBitterEnd]

Thanks for taking the time Peter. I had a quick skim of the Dott paper which seems to give some ideas about the kind of things that happens to sediments during and between sedimentation events. I'll try to dig out the others.

Thanks for that Gollum 

I'm still interested in the "how long is a piece of string" question of what separates one bed from another though

 

[gus horsley]

Formation of bedding planes varies from rock to rock and on conditions after deposition (eg orogenic episodes which "squeeze" rocks and open up lines of weakness).  In the specific case of limestones (which I'm assuming you're referring to), deposition is often in shallow lime-rich warm waters with an abundance of fauna such as corals, brachiopods, etc.  Seasonal fluctuations can affect rates of deposition and sometimes evaporation (such as with the development of oolites) but quite often there isn't a huge input of land-derived sediments.  Therefore, in the case of Carboniferous Limestones, the deposits tend to be shallow, offshore, relatively still lagoons with a steady input of carbonaceous oozes and animal remains in which reef structures may be preserved.  There may not be much evidence of bedding planes under these conditions.  However, once the deposits have become uplifted and form rocks they are quite brittle and tectonic movements, even very slight ones, would lead to the opening up of beddings and faults by creating or relieving stresses within the rocks.

 

[TheBitterEnd]

Thanks Gus. Interesting that you think that bedding might not be evident until after solidification - I need to ponder on that. I assume the carbonate sedimentation process is occurring somewhere on earth at the moment? If so has anyone sampled it to see what it looks like?

I am fairly familiar with some estuarine silts and they show "bedding" quite markedly. Obviously these are formed under different conditions and are not carbonates...

 

[graham]

Thanks Gus. Interesting that you think that bedding might not be evident until after solidification - I need to ponder on that. I assume the carbonate sedimentation process is occurring somewhere on earth at the moment? If so has anyone sampled it to see what it looks like?

I am fairly familiar with some estuarine silts and they show "bedding" quite markedly. Obviously these are formed under different conditions and are not carbonates...

Estuarine silts presumably show 'bedding' as they are subject to clearly cyclical processes? This is in direct contrast to the steady continuous processes described by Gus for sedimentation in still lagoons. And I imagine that the chemical nature of the sedimints becomes rather more important once lithification (?sp) kicks in as the sediment becomes buried and the pressure increases.

 

[bograt]

I suspect that changes in climate would have a significant effect on the way beds where laid down, different temperatures and atmospheric acid levels would encourage different aqueous life forms, thereby creating different sediments, giving rise to shell beds for example.
This could also go some way to explaining the variations in hardness and solubility we find in adjacent beds, different sized organisms would give different sized sediment particles, the smaller ones would compact tighter, thereby creating a denser bed.

 

[TheBitterEnd]

I'm sure all these things are factors and I imagine it's not a case of one process followed by another. We essentially have a shallowing ocean basin where shallowing is a result of some  mix of sea level change, tectonic uplift and sedimentation. As these changes occur climate, ocean temperature and ocean chemistry are all changing.

These changes will drive and interrupt the sedimentation process. At some periods the ocean floor becomes a land surface and we see paleo-karst surfaces (e.g. Red Wall at Trowbarrow quarry) which are subsequently inundated and undergo further deposition. This is, as I understand it, the cyclic nature of bed formation.

Now, within a given cycle (e.g. the Gordale Limestone Member see this reference) we see separate bedding planes. I suppose what I am trying to understand is: is it just that sedimentation stopped and started  or does the sediment undergo some change before the next bed starts to form (as with the estuarine silts which dry before the next layer; but let's not get side tracked by that I am not saying that carbonate sediments dry out). It's easy when shales are interbedded but when there are no shales?

Gus's point is interesting in that beds may not separate until they form into rock and are stressed but given that undulating beds are known there must be a predisposition to bedding at certain elevations in the strata.

Anyway, thanks for all the replies, I was kind of hoping that some one would point me a a reference or two (PhD or something) on this, ideally where they had looked at processes currently on-going.

 

[Pitlamp]

Just picking up on a point Gus made - "seasonal variations". Wasn't our crustal plate astride the equator during the lower Carboniferous, leading to the same old boring hot weather (with none of the, ahem, "variety" in the weather we now "enjoy" in the Dales)?

I just looked at Penguin's "A Dictionary of Geology" - and found it not very helpful re your specific question.

 

[Brains]

A few thoughts...
My sedimentology lecturer was quite adament that nearly all geologic time was represented by bedding planes, not the beds. Given the rate of sedimentation, geologic time scales and the seen thickness of formations this statement is self evident.
Some beds can be related to single events (greywacke submarine avalanches, tuff beds and lava flows), and others to limited time time events such as bioturbidited uniform micrites, coral reefs, etc.
Given the time scales I would guess that bedding are related to a regional change in sedimentation, eg a delta relocating its outfall, climate change, or even plate tectonics being factors.
Short term factors like seasonality will be minor in the great scheme of things

 

[TheBitterEnd]

Pitlamp - the second page of this reference puts us just south of the equator in the late Devonian.

Brains- thanks for the thoughts which are in agreement with Peter K's post above.

Just hit by a thought - is gravitational compression enough? i.e in periods of no additional sedimentation the sediments will continue to settle and de-water then when sedimentation starts again the sea floor will be a bit more compact and hence a degree of separation between layers exists.

 

[Brains]

Possibly, and carbonate sediments can self cement relatively quickly with the formation of "beach rock" in only a year or two

 

[Pie Muncher]

Bedding planes may represent impurities ie. organic, volcanic, chemical, detrital materials layed down in a single wide spread event, think ash cloud, saline deposits type of thing but also through dissolution, migration and redeposition/crystalisation during compaction, compression and lithification processes, think Cherts in Chalk for example.

 

[Brains]

Cherts layering in chalk is NOT coincident with bedding, but is more likely related to redox fronts, perhaps associated with groundwater.
Stylolites are the dissolutional feature in carbonates, and again are not related to bedding. Erosional features would give an uncomformity, which may locally follow bedding is unlikely to do so over a distance. Surface impurities are a possibility.

 

[Glenn]

Sorry if this has already been described, and I missed it, or did not understand, but what are the "chert nodules" that you see in limestone.

Thanks in advance.

 

[graham]

Sorry if this has already been described, and I missed it, or did not understand, but what are the "chert nodules" that you see in limestone.

Thanks in advance.

Lumps of microcrystalline, cryptocrystalline and microfibrous quartz.

Sometimes a replacement mineral sometimes a different sedimentation environment.

 

[TheBitterEnd]

Glenn ? If you were asking "how come it's in the limestone" I suspect you will get a range of answers because there appear to be various mechanisms and I'm not convinced the "experts" really understand the processes involved.

As Graham indicates it is a silica mineral that is replaced into the limestone after the deposits are buried in the ocean floor. Some theories suggest replacement is by hydrothermal fluids carrying dissolved silica and to explain the presence of nodules some people seem to think there is preferential deposition of the silica around certain fossils.

 

[gus horsley]

Just picking up on a point Gus made - "seasonal variations". Wasn't our crustal plate astride the equator during the lower Carboniferous, leading to the same old boring hot weather (with none of the, ahem, "variety" in the weather we now "enjoy" in the Dales)?

I just looked at Penguin's "A Dictionary of Geology" - and found it not very helpful re your specific question.

There could still have been seasonal variations due to the different configurations of the continents. 

When the carb Lst was being deposited it was on the margins of the closing Rheic Ocean (caused by the collision between Gondwanaland to the south and Laurentia to the north) where a number of features were affected the continental crust such as the genesis of fault-bounded oblique collision basins (where sediments were often pushed from one basin into another), ephemeral openings of MORB-type rifts creating ophiolite complexes and their consequent obduction, compression-affected and sometimes enclosed sedimentary environments (which some of the Carb limestones could have included and were certainly responsible for Permian deposits), etc.  The closure of this ocean and its associated depositional features and oregeny are still not fully understood.  I've studied the effects in Cornwall and the effects of the collision compressed the sedimentary deposits in the area by over 60% in some places.

I know this does little to add to the debate about bedding planes but it might indicate that the picture isn't as straightforwards as it appears.

 

[bograt]

I always thought that Chert and Flint nodules were "Volcanic bombs", thrown out by a volcanoe and solidified in the carboniferous waters.
WRT the earlier query about gravitational compression, I think it probable that compression could have been enhanced by upthrust from underground volcanic activity.