Dear Allen
Good to see the thought you have put into this.
Allen Roy wrote:
> Just to see if I'm on the same page as you let me make a few
> introductory comments.
>
> Since we were not there to see the formation of sedimentary rocks, in
> order to attempt to discover how they were formed (and subsequently
> lithified), we look to analogous depositions occurring now in the
> world around us. As Raymond in Petrology: The Study of Igneous,
> sedimentary and Metamorphic Rock, says on pg. 321, "By studying both
> the processes in present-day environments and the lithofacies formed
> by those processes, sedimentologists are able to infer how similar
> ancient rocks were formed."
>
> It seems to me that it can be said like this:
> 1. A Sedimentary Deposition implies a unique Sedimentary Environment,
> or SD --> SE.
> 2. We are able to find Sedimentary Depositions which have similar
> lithofacies to Sedimentary Rocks,
> or SD ~ SR.
> 3. We assume that a Sedimentary Rock implies a unique Sedimentary
> Environment prior to litification,
> or SR --> SRE.
> 4. Thus we conclude that there existed a Sedimentary Rock Environment
> that was similar to a Sedimentary Environment of today,
> or SRE ~ SE.
>
I would say that 1 and 3 are givens, and that 2 is an observation. When
2 is observed then 4 applies. Two is not always observed however, see
my comments below.
Incidently,this methodology also applies to volcanic rocks as well. It
is supplemented by laboratory and theoretical studies. For example,
much of our understanding of turbidity currents came from studies in the
laboratory. The laboratory is especially important for understanding
intrusive and metamorphic rocks which have formed out of sight deep in
the earth's crust, and to processes of deformation and partial melting
going on in the upper mantle.
> And so geologists spend much time studying the processes of today
> in-order to provide a basis for interpreting the sedimentary rock
> record.
Sedimentologists certainly do spend a lot of time studying modern
environments and depositional processes so as to understand the rock
record. It is great fun, especially when in some nice locality like the
Great Barrier Reef or the Bahamas! Often there is very good
correspondence with what we see in modern sediments and what we find in
ancient ones. However as Paul Wright so aptly said (Wright, V P 1994.
Early Carboniferous carbonate systems: an alternative to the Cainozoic
paradigm. Sedimentary Geology 93: 1-5.), rather than an exact guide to
the past, the present is a yardstick with which we compare the past, so
see the ways in which past processes conformed and differed to the
present. I certainly think that the yarstick is a much better metaphor
than that of the key in how we should use the present to understand the
past. In addition there are many present processes which are difficult
to study but which are inferred to have formed ancient deposits. It is
very difficult to study what is going on underneath a continental ice
sheet, or in a major pyroclastic eruption. Other processes could
happen today but are so rare that we have not observed them, such as
asteroid impacts, large scale cauldron subsidence, or major collapse
of a carbonate continental margin. Perhaps it is just as well.....
There are also many ancient sediments which have no direct counterpart.
Examples included banded ironstones, early mimetic dolostones of the
Proterozoic, and widespread cementstones of the Proterozoic, the muddy
limestones of the Palaeozoic, etc. In your terminology then
5. SR =/= modern SD (mSD)
therefore
6. ancient SD (aSD) =/= mSD
and thus
7. aSE =/= mSE
For these cases we need to rely of theoretical studies, laboratory
experiments, and more general analogues, often from very spatially
restricted environments. In addition, ancient deposits can tell us what
does on in modern environments which are difficult to ignore, such as
under and ice sheet, during an impact or violent eruption. Studing
metamorphic and intrusive rocks also tells us something of the proerties
and processes that occur or have occurred in difficult to study regions
of the deep crust or in magma chambers.
> From this we can see that the greater the similarity between a
> Sedimentary Rock and a known Sedimentary Deposition the more reliable
> the assignment of an Sedimentary Environment.
Exactly right. That is why there is much more debate over things like
banded ironstones than there is over fluvial sandstones.
> It has happened that the interpretation of an environment has
> changed. For instance, the Bouma sequences, which had been
> interpreted as slow deposition of fines in still water, has since been
> reinterpreted as Turbidite deposition (a vastly different,
> catastrophic change in environments). It was found that a greater
> similarity existed between the Bouma sequences and Turbidite
> depositions than with the previously interpreted environment.
The turbidite model was one of the first facies models to be developed,
it was proposed by Daly and Kuenen in the 1930's. Hence it's importance
in the earlier literature of sedimentary environments.
I have not read the early turbidite literature, so I am not familiar
with previous understandings on the depositional environments of
successions that were considered to be turbidites. However I suspect
that they were thought to have been laid down in relatively deep
water. Since thick turbidite successions are also thought to be formed
and preserved only in deep water (below fair weather wave base), the
overall environments did not change. What the turbidite model did do
was provide a process for the deposition of the sediments, a process
which previously had not been recognised.
Because the turbidite model was one of the first for deep slope deposits
it was sometimes over applied. Some turbidite sequences are now thought
to represent re-sedimented pyroclastics, storm beds, or contour current
deposits, often closely associated with true turbidity current deposits
> The vast majority of sedimentary environments studied today and
> applied to sedimentary rocks also apply the same rates of deposition
> to the rocks. Thus, by extension, the shear quantity of sedimentary
> rocks implies a record of long ages.
I would agree. This is especially the case where the sediments have to
be grown by organisms, such as fossil reefs, or sediments composed of
organisms that have grown elsewhere and transported in. Other examples
are thick acculations of fine-grained laminated shales, varves, units
such as the Green River Formation, thick cyclic evaporite deposits.....
the list goes on
In addition there are the cases where there is good evidence of time
breaks in between sedimentary or volcanic strata, such as
unconformities, ancient soil horizons, buried karst surfaces,
hardgrounds with borings, encrustations of iron oxides, phosphate,
glaucony, etc.
> There is an increasing awareness that catastrophic events (such as
> asteroid impact and explosive volcanism, etc.) are also a part of the
> geologic record. As these events and associated environmental impacts
> are assessed, depositional facies are determined and studied and the
> rocks restudied and their depositional environment reassessed. Of
> course, there is reluctance to set aside previous interpretations for
> an assortment of reasons.
>
The reluctance especially applies to the early stages of a hypothesis.
I remember in 1981 when as a 4th year student I presented a seminar
reviewing the then new hypothesis of an impact model for the K-T
boundary. There was certainly a lot of scepticism and even hostility.
Now of course it would generally be accepted, even considered boring. I
remember a few years later I was at a workshop where evidence for an
impact ejecta horizon was presented for the first time within the Late
Proterozoic of the Adelaide Geosyncline. There was great excitement,
but no hostility at all. Similar and greater problems faced people such
as Bretz with his 1920's catastrophic flood model for the channelled
scablands in Washington. It was not widely accepted before the late
50's, but now it too is a widely used model for many Quaternary
landforms and deposits round the margins of northern hemisphere ice
sheets, and of course for Mars.
However, we must be careful not to exaggerate the recentness of
acceptance of catastrophic processes. Thick pyroclastic deposits have
always been recognised as indicating large eruptions, meteorite impacts
have been generally recognised from the 60's. The catastrophic flooding
explanation for the Channelled Scablands I think was widely accepted
from the early 70's. The catastrophic model for the KT boundary was
recognised as conceptually possible from the early 80's, and so forth.
God Bless
Jonathan
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Dear Allen
Good to see the thought you have put into this.
Allen Roy wrote:
Just to see if I'm on the same page as you let me make a few introductory comments.I would say that 1 and 3 are givens, and that 2 is an observation. When 2 is observed then 4 applies. Two is not always observed however, see my comments below.Since we were not there to see the formation of sedimentary rocks, in order to attempt to discover how they were formed (and subsequently lithified), we look to analogous depositions occurring now in the world around us. As Raymond in Petrology: The Study of Igneous, sedimentary and Metamorphic Rock, says on pg. 321, "By studying both the processes in present-day environments and the lithofacies formed by those processes, sedimentologists are able to infer how similar ancient rocks were formed."
It seems to me that it can be said like this:
1. A Sedimentary Deposition implies a unique Sedimentary Environment,
or SD --> SE.
2. We are able to find Sedimentary Depositions which have similar lithofacies to Sedimentary Rocks,
or SD ~ SR.
3. We assume that a Sedimentary Rock implies a unique Sedimentary Environment prior to litification,
or SR --> SRE.
4. Thus we conclude that there existed a Sedimentary Rock Environment that was similar to a Sedimentary Environment of today,
or SRE ~ SE.
Incidently,this methodology also applies to volcanic rocks as well. It is supplemented by laboratory and theoretical studies. For example, much of our understanding of turbidity currents came from studies in the laboratory. The laboratory is especially important for understanding intrusive and metamorphic rocks which have formed out of sight deep in the earth's crust, and to processes of deformation and partial melting going on in the upper mantle.
And so geologists spend much time studying the processes of today in-order to provide a basis for interpreting the sedimentary rock record.Sedimentologists certainly do spend a lot of time studying modern environments and depositional processes so as to understand the rock record. It is great fun, especially when in some nice locality like the Great Barrier Reef or the Bahamas! Often there is very good correspondence with what we see in modern sediments and what we find in ancient ones. However as Paul Wright so aptly said (Wright, V P 1994. Early Carboniferous carbonate systems: an alternative to the Cainozoic paradigm. Sedimentary Geology 93: 1-5.), rather than an exact guide to the past, the present is a yardstick with which we compare the past, so see the ways in which past processes conformed and differed to the present. I certainly think that the yarstick is a much better metaphor than that of the key in how we should use the present to understand the past. In addition there are many present processes which are difficult to study but which are inferred to have formed ancient deposits. It is very difficult to study what is going on underneath a continental ice sheet, or in a major pyroclastic eruption. Other processes could happen today but are so rare that we have not observed them, such as asteroid impacts, large scale cauldron subsidence, or major collapse of a carbonate continental margin. Perhaps it is just as well..... There are also many ancient sediments which have no direct counterpart. Examples included banded ironstones, early mimetic dolostones of the Proterozoic, and widespread cementstones of the Proterozoic, the muddy limestones of the Palaeozoic, etc. In your terminology then
5. SR =/= modern SD (mSD)
therefore
6. ancient SD (aSD) =/= mSD
and thus
7. aSE =/= mSE
For these cases we need to rely of theoretical studies, laboratory experiments,
and more general analogues, often from very spatially restricted
environments. In addition, ancient deposits can tell us what does
on in modern environments which are difficult to ignore, such as under
and ice sheet, during an impact or violent eruption. Studing metamorphic
and intrusive rocks also tells us something of the proerties and processes
that occur or have occurred in difficult to study regions of the deep crust
or in magma chambers.
From this we can see that the greater the similarity between a Sedimentary Rock and a known Sedimentary Deposition the more reliable the assignment of an Sedimentary Environment.Exactly right. That is why there is much more debate over things like banded ironstones than there is over fluvial sandstones.
It has happened that the interpretation of an environment has changed. For instance, the Bouma sequences, which had been interpreted as slow deposition of fines in still water, has since been reinterpreted as Turbidite deposition (a vastly different, catastrophic change in environments). It was found that a greater similarity existed between the Bouma sequences and Turbidite depositions than with the previously interpreted environment.The turbidite model was one of the first facies models to be developed, it was proposed by Daly and Kuenen in the 1930's. Hence it's importance in the earlier literature of sedimentary environments.
I have not read the early turbidite literature, so I am not familiar with previous understandings on the depositional environments of successions that were considered to be turbidites. However I suspect that they were thought to have been laid down in relatively deep water. Since thick turbidite successions are also thought to be formed and preserved only in deep water (below fair weather wave base), the overall environments did not change. What the turbidite model did do was provide a process for the deposition of the sediments, a process which previously had not been recognised.
Because the turbidite model was one of the first for deep slope deposits it was sometimes over applied. Some turbidite sequences are now thought to represent re-sedimented pyroclastics, storm beds, or contour current deposits, often closely associated with true turbidity current deposits
The vast majority of sedimentary environments studied today and applied to sedimentary rocks also apply the same rates of deposition to the rocks. Thus, by extension, the shear quantity of sedimentary rocks implies a record of long ages.I would agree. This is especially the case where the sediments have to be grown by organisms, such as fossil reefs, or sediments composed of organisms that have grown elsewhere and transported in. Other examples are thick acculations of fine-grained laminated shales, varves, units such as the Green River Formation, thick cyclic evaporite deposits..... the list goes on
In addition there are the cases where there is good evidence of time breaks in between sedimentary or volcanic strata, such as unconformities, ancient soil horizons, buried karst surfaces, hardgrounds with borings, encrustations of iron oxides, phosphate, glaucony, etc.
There is an increasing awareness that catastrophic events (such as asteroid impact and explosive volcanism, etc.) are also a part of the geologic record. As these events and associated environmental impacts are assessed, depositional facies are determined and studied and the rocks restudied and their depositional environment reassessed. Of course, there is reluctance to set aside previous interpretations for an assortment of reasons.The reluctance especially applies to the early stages of a hypothesis. I remember in 1981 when as a 4th year student I presented a seminar reviewing the then new hypothesis of an impact model for the K-T boundary. There was certainly a lot of scepticism and even hostility. Now of course it would generally be accepted, even considered boring. I remember a few years later I was at a workshop where evidence for an impact ejecta horizon was presented for the first time within the Late Proterozoic of the Adelaide Geosyncline. There was great excitement, but no hostility at all. Similar and greater problems faced people such as Bretz with his 1920's catastrophic flood model for the channelled scablands in Washington. It was not widely accepted before the late 50's, but now it too is a widely used model for many Quaternary landforms and deposits round the margins of northern hemisphere ice sheets, and of course for Mars.
However, we must be careful not to exaggerate the recentness of acceptance of catastrophic processes. Thick pyroclastic deposits have always been recognised as indicating large eruptions, meteorite impacts have been generally recognised from the 60's. The catastrophic flooding explanation for the Channelled Scablands I think was widely accepted from the early 70's. The catastrophic model for the KT boundary was recognised as conceptually possible from the early 80's, and so forth.
God Bless
Jonathan
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