Sedimentology question
Alexander Avtanski
Recently I visited La Jolla, CA, and noticed the very nice layered
sandstone formations close to the beach. Although I don't have
formal education in stratigraphy, it is kind of hobby of mine to
try to interpret the geologic history of an area looking at the
rocks. I took a lot of photos, but still haven't got the time to
organize them. Meanwhile, one thing that I noticed was a very
interesting boundary between two layers, that looks like this
(this photo is not mine, but it's from the same area):
If this doesn't open, the full URL is
http://www.cs.brown.edu/people/psar/photos/SanDiego/tn/LaJolla_Sandstone_Closeup.JPG.html
The bottom layers are fine laminated and fine grained, the thing
on top is more rough grained, with almost no discernible laminae.
This looks like something caused by one layer sliding on top of
another. Is this interpretation correct? It looks like partial
liquefaction took place at some point (there were some intrusions
from the bottom layer to the top one, that looked to me like
water-escape structures). If this is true, this should mean that
the whole thing happened pretty fast - probably caused by an
earthquake or landslide.
The other thing that comes to mind is turbidity current. I've
never seen turbidity current deposits with my own eyes, and I don't
know if it should look like this. The top layer certainly have
some of the features of turbidity current, but I'm a bit uneasy
with the idea...
What do you think? Does anyone knows this particular formation?
Regards,
- Alex
P.S. I hope to put my images on the web soon - I may have more
questions then, :-)
is...@earthlink.net
Alexander Avtanski wrote:
> Hello,
>
> Recently I visited La Jolla, CA, and noticed the very nice layered
> sandstone formations close to the beach. Although I don't have
> formal education in stratigraphy, it is kind of hobby of mine to
> try to interpret the geologic history of an area looking at the
> rocks. I took a lot of photos, but still haven't got the time to
> organize them. Meanwhile, one thing that I noticed was a very
> interesting boundary between two layers, that looks like this
> (this photo is not mine, but it's from the same area):
>
> http://tinyurl.com/4yyva
>
> If this doesn't open, the full URL is
>
http://www.cs.brown.edu/people/psar/photos/SanDiego/tn/LaJolla_Sandstone_Closeup.JPG.html
Cool pictures!
>
> The bottom layers are fine laminated and fine grained, the thing
> on top is more rough grained, with almost no discernible laminae.
>
> This looks like something caused by one layer sliding on top of
> another. Is this interpretation correct? It looks like partial
> liquefaction took place at some point (there were some intrusions
> from the bottom layer to the top one, that looked to me like
> water-escape structures). If this is true, this should mean that
> the whole thing happened pretty fast - probably caused by an
> earthquake or landslide.
I'm assuming that the material in the photograph isn't totally
lithified.
I'd say what you are looking at is soft sediment deformation. When
sediments have been newly depostited they react to overlying stresses
and tend to slide and bulge. Especially when weight accumulates above
the sediments they will have a tendency to slip around until the
pressure conditions lithify the sediments more firmly.
>
> The other thing that comes to mind is turbidity current. I've
> never seen turbidity current deposits with my own eyes, and I don't
> know if it should look like this. The top layer certainly have
> some of the features of turbidity current, but I'm a bit uneasy
> with the idea...
As I understand turbidity currents is that they form from deep ocean
landslides. I don't believe sediments exposed in La Jolla were
deposited deeply. But I could be wrong.
Hope this helps,
Andy
George
<is...@earthlink.net> wrote in message
news:1112819353....@g14g2000cwa.googlegroups.com...
I don't know how deep these deposits were laid down, but they do appear to be
soft sediment deformation to me, so I would agree with ishky here.
Alexander Avtanski
is...@earthlink.net wrote:
> Cool pictures!
As I said, I did not take this picture - I found it on the net, :-)
> I'm assuming that the material in the photograph isn't totally
> lithified.
That's correct. It's very soft sandstone. Is this what makes
you think that it has never being buried at significant depth?
> I'd say what you are looking at is soft sediment deformation. When
> sediments have been newly depostited they react to overlying stresses
> and tend to slide and bulge. Especially when weight accumulates above
> the sediments they will have a tendency to slip around until the
> pressure conditions lithify the sediments more firmly.
>
> As I understand turbidity currents is that they form from deep ocean
> landslides. I don't believe sediments exposed in La Jolla were
> deposited deeply. But I could be wrong.
>
> Hope this helps,
> Andy
Thanks, :-)
- Alex
Carsten Troelsgaard
"Alexander Avtanski" <avta...@ispwest.com> skrev i en meddelelse
news:d31b2...@enews1.newsguy.com...
> Hello,
>
> Recently I visited La Jolla, CA, and noticed the very nice layered
> sandstone formations close to the beach. Although I don't have
> formal education in stratigraphy, it is kind of hobby of mine to
> try to interpret the geologic history of an area looking at the
> rocks. I took a lot of photos, but still haven't got the time to
> organize them. Meanwhile, one thing that I noticed was a very
> interesting boundary between two layers, that looks like this
> (this photo is not mine, but it's from the same area):
>
> http://tinyurl.com/4yyva
>
> If this doesn't open, the full URL is
> http://www.cs.brown.edu/people/psar/photos/SanDiego/tn/LaJolla_Sandstone_Closeup.JPG.html
>
> The bottom layers are fine laminated and fine grained, the thing
> on top is more rough grained, with almost no discernible laminae.
> This looks like something caused by one layer sliding on top of
> another. Is this interpretation correct? It looks like partial
> liquefaction took place at some point (there were some intrusions
> from the bottom layer to the top one, that looked to me like
> water-escape structures). If this is true, this should mean that
> the whole thing happened pretty fast - probably caused by an
> earthquake or landslide.
I think you hit water-structure right on. Crude sediment above fine has a
logical implication of a relative sealevel drop, but I agree that the
tectonic interpretation of a tectonic raising beach or gradient comes very
natural. A falling sealevel combined with a severe storm is a less likely
alternative.
> The other thing that comes to mind is turbidity current. I've
> never seen turbidity current deposits with my own eyes, and I don't
> know if it should look like this. The top layer certainly have
> some of the features of turbidity current, but I'm a bit uneasy
> with the idea...
>
> What do you think? Does anyone knows this particular formation?
The top sediment is not shown very detailed. A characteristic of a turbedit
is inverse grading, with the crude grains/clasts at bottom, different types
of internal layering/current-ripples and capped with fines when the event
slowly settles. Google Bouma sequence for more details.
Further details on the fines is speculative, but it looks like a light
kaolinitic clay. The kaolinite is the most (chemically) degraded clay
possibly indicating wet/warm climate - a northern machanically degraded clay
would be chloritic. It's not bioturbed which is unusual, but I'll not
speculate on why - any suggestion would involve some extreme from normal
conditions.
Carsten
Carsten Troelsgaard
"Carsten Troelsgaard" <carsten.t...@mail.dk> skrev i en meddelelse
news:4254691c$0$203$edfa...@dread11.news.tele.dk...
>
>It's not bioturbed which is unusual, but I'll not speculate on why - any
>suggestion would involve some extreme from normal conditions.
I come to think of a major volcanic eruption in the neighborhood, loads of
easily degradable ash. If it winds up in a place that is inhabited by a
non-softbottom-loving fauna, there may not be biota to bioturbate.
There is a lot of discrepancies in alternatives for the visible lamination.
Carsten
Alexander Avtanski
There was a heavily bioturbated layer about 50cm below this one.
I'll try tonight to organize and upload my pictures on the net
and will post the links to them.
Thanks to all who replied.
- Alex
is...@earthlink.net
Alexander Avtanski wrote:
> Hello Andy,
>
> is...@earthlink.net wrote:
> > Cool pictures!
>
> As I said, I did not take this picture - I found it on the net, :-)
I must have missed that.
>
> > I'm assuming that the material in the photograph isn't totally
> > lithified.
>
> That's correct. It's very soft sandstone. Is this what makes
> you think that it has never being buried at significant depth?
>
I've visited California twice, the northern coast and the southern
coast. The reading I did before hand never mentioned deep deopsition of
the sediments exposed at the coast.
Andy
Alexander Avtanski
I finally uploaded the images. The URL us this:
http://avtanski.com/images/geo/lajolla
Here is the bioturbated (bioturbed?) layer I mentioned:
http://avtanski.com/images/geo/lajolla/P8.jpg
(you may have to zoom on the image to see details) So, Carsten
was right to look for such a layer, :-)
Is this (#2 on the pic) how a lagoon environment should loook
like? Cross-lamination seem (to me at least) to show that there
was predominant direction of the flow toward right (toward the
sea) - is this so, or it is hard to tell? If so, this should
mean that there was some supply of water - probably some river
emptying through the lagoon (I'm not sure "lagoon" is the
correct term)?
If this is correct, then #3a might mark a storm that brought
ocean sediments over the side in the lagoon? Another
interesting thing I noticed was the sharp layer between #3b
and #3c - this one seem to be coated with something that holds
water (notice the plants).
This shows everything in context:
http://avtanski.com/images/geo/lajolla/P5.jpg
The layers from the previous picture are at the bottom
(#2 and #3, notice the green stuff too). Here is a closeup of
the upper part:
http://avtanski.com/images/geo/lajolla/P9.jpg
Here #5 and #7 look pretty much like #2 (the supposed "lagoon"
thingy). Here #5 and #7 looked to me like something I've seen
on a picture before - lenticular bedding, - that is associated
with tidal flats (where it shouldn't show preferred flow
direction). #6a looks like storm deposit to me; the border
between #6 and #7 I dare to assume is caused by ripples that
dried (notice the cracks just below the ripples that I think
might be old mud cracks), then were slowly covered with sediments,
and finally deformed. About #8 - I don't have idea what it
might be, but it looks like deposited in pretty rough water
(surf? how come?).
Here is another closeup:
http://avtanski.com/images/geo/lajolla/P7.jpg
Two questions here. First, the small holes right of #8a look
to me like some mud and sand with organic stuff was swept there
and when the organics "rotted" away. Is this correct?
Some of the supposed water-escape structures are visible here:
I'm talking about #9a. On the other hand, now it doesn't look
to me much like water flowed through it, so it may be purely
deformational... The same thing in context is here:
http://avtanski.com/images/geo/lajolla/P6.jpg
There are a lot of other things that look interesting. Here
are all the images:
http://avtanski.com/images/geo/lajolla/
For the other structures I have a lot of tose half-baked
theories of mine, but I think everybody got bored from me
already, :-) Anyway, if you like looking at such things and
find something interesting on the images, please let me
know.
Thanks,
- Alex
George
"Alexander Avtanski" <avta...@ispwest.com> wrote in message
news:d33un...@enews4.newsguy.com...
> Hello again,
>
> I finally uploaded the images. The URL us this:
>
> http://avtanski.com/images/geo/lajolla
Definitely soft sediment deformation:
http://www.geology.ucdavis.edu/~gel109/SedStructures/SoftSedDef.html
> Here is the bioturbated (bioturbed?) layer I mentioned:
>
> http://avtanski.com/images/geo/lajolla/P8.jpg
Flame and burrow structures?
Robert Flory
can point you to some really wild examples of soft and semi brittle
defformation in aolian sediments. Casper Formation near Chimney Rock.
Bob
"Alexander Avtanski" <avta...@ispwest.com> wrote in message
news:d33un...@enews4.newsguy.com...
George
"Robert Flory" <wyo...@MORETHANWARMmail.com> wrote in message
news:d34poi$es2$1...@news.astound.net...
> If you like soft sediment deformation and every get near Laramie Wyoming, I
> can point you to some really wild examples of soft and semi brittle
> defformation in aolian sediments. Casper Formation near Chimney Rock.
>
> Bob
Here are some good examples of soft sediment deformation at the Garden of The
Gods, In Southern Illinois:
http://www.backpackcamp.com/GOG/GOGIronSwirls.jpg
http://www.backpackcamp.com/GOG/GOGRock2.jpg
Carsten Troelsgaard
"Alexander Avtanski" <avta...@ispwest.com> skrev i en meddelelse
news:d33un...@enews4.newsguy.com...
> Hello again,
>
> I finally uploaded the images. The URL us this:
>
> http://avtanski.com/images/geo/lajolla
>
> Here is the bioturbated (bioturbed?) layer I mentioned:
>
> http://avtanski.com/images/geo/lajolla/P8.jpg
>
> (you may have to zoom on the image to see details) So, Carsten
> was right to look for such a layer, :-)
>
> Is this (#2 on the pic) how a lagoon environment should loook
> like? Cross-lamination
I don't think that it's cross lamination, it's more like a deformation.
The lagoon environment has a lot of different characteristics depending on
climate and coast type.
> seem (to me at least) to show that there
> was predominant direction of the flow toward right (toward the
> sea) - is this so, or it is hard to tell? If so, this should
> mean that there was some supply of water - probably some river
> emptying through the lagoon (I'm not sure "lagoon" is the
> correct term)?
It'll do.
> If this is correct, then #3a might mark a storm that brought
> ocean sediments over the side in the lagoon?
Could be.
> Another
> interesting thing I noticed was the sharp layer between #3b
> and #3c - this one seem to be coated with something that holds
> water (notice the plants).
It could suggest a larger erosional unconformity with a crude lag deposit,
or that the sediment right below it is clayey and not permeable.
> This shows everything in context:
>
> http://avtanski.com/images/geo/lajolla/P5.jpg
>
> The layers from the previous picture are at the bottom
> (#2 and #3, notice the green stuff too). Here is a closeup of
> the upper part:
>
> http://avtanski.com/images/geo/lajolla/P9.jpg
>
> Here #5 and #7 look pretty much like #2 (the supposed "lagoon"
> thingy). Here #5 and #7 looked to me like something I've seen
> on a picture before - lenticular bedding, - that is associated
> with tidal flats (where it shouldn't show preferred flow
> direction). #6a looks like storm deposit to me; the border
> between #6 and #7 I dare to assume is caused by ripples that
> dried (notice the cracks just below the ripples that I think
> might be old mud cracks), then were slowly covered with sediments,
> and finally deformed.
I like to think of it as a marsh where animals spoiled the turf.
> About #8 - I don't have idea what it
> might be,
I have a suggestion below, but it's from what I('m supposed to) 'know', not
from what I see. The internal structures are 'wrong' in that it doesn't
really show barriere or shore-facies. #P5 showes the recent alteration of
the sediment, so I take that some structures may not be original.
> but it looks like deposited in pretty rough water
> (surf? how come?).
>
> Here is another closeup:
>
> http://avtanski.com/images/geo/lajolla/P7.jpg
>
> Two questions here. First, the small holes right of #8a look
> to me like some mud and sand with organic stuff was swept there
> and when the organics "rotted" away. Is this correct?
Could be.
> Some of the supposed water-escape structures are visible here:
> I'm talking about #9a. On the other hand, now it doesn't look
> to me much like water flowed through it, so it may be purely
> deformational... The same thing in context is here:
>
> http://avtanski.com/images/geo/lajolla/P6.jpg
>
> There are a lot of other things that look interesting. Here
> are all the images:
>
> http://avtanski.com/images/geo/lajolla/
>
> For the other structures I have a lot of tose half-baked
> theories of mine, but I think everybody got bored from me
> already, :-) Anyway, if you like looking at such things and
> find something interesting on the images, please let me
> know.
Hi Alex
About the 'green stuff'. It's likely greensand, but Gerhard Einsele refer to
green pigmentation, verdissement, in one of three ways: glauconitization
(starved/outer shelf, cold water), verdinisation (inner shelf/delta-front
zones, 20-60m, varm climate (but clayey)) and chloritization.
I generally miss what I can interpret as migrating sands like bars and
submarine dunes, and this causes me to suggest a somewhat sedimentstarved
(marine rather than fluvial) environment. I don't know the hinterland, but
if it's a wide coastal plain and a modest precipitation, an inundation of it
may produce a wide flat shelf with low sediment input, and a tidal activity,
though with the same amplitude as the present, that passes a higher volume
of water across the plain, and constitute a stronger shaping factor on the
sediments than what you see at present. Such an environment may produce
organic rich sediments and develope lagoons protected by barrieres closer to
the shelf edge/deeper water. This suggestion is what is called a highstand
or transgressive system tract in a sequence-stratigraphic context. I have
decided that the dark matter is finegrained and clayey, and if it's not, I
may be way off in my interpretation.
I'm puzzled on a lot of accounts, particularly the straight laminated strata
we've been talking about. On P#7, below the 6-mark: it would have to be
predominantly clayey. Even the light layers could not be sand as I know of
no mechanism that disperses sand this way - unless we change to a whole
other environment: the surf zone of the beach front. That could make the
dark matter 'heavy minerals' and not organic matter. The lamination between
8a and 9a is capped with soft-body deformation .. that's not a logic
combination, but one may see a possible water conduite at 9 o'clock from 8a.
That could make the organic matter the container of water, not clay.
The top massive sandbodies at 8 and 9 would be the outer protective barriere
displaced onto the lagoon from further off shore, as the relative sealevel
rises - this is in contrast to a rule-of-thumb that says: deeper water,
finer sediment.
Carsten
Alexander Avtanski
Thank you for the detailed reply. I just hope this discussion is
as much fun to you, as it is to me.
Carsten Troelsgaard wrote:
> > [ ... ]
> > Is this (#2 on the pic) how a lagoon environment should loook
> > like? Cross-lamination
>
> I don't think that it's cross lamination, it's more like a
> deformation. The lagoon environment has a lot of different
> characteristics depending on climate and coast type.
Maybe I'm not using the correct term (cross-lamination) here. What
I meant is that some layers seem to go on angle and end into other
layers. For example, look at the highlighted detail on this image:
http://avtanski.com/images/geo/lajolla/P8a.jpg
There are other places where this happens. What I noticed was
that the angle is always sloping downward to right (west), and
that's why I assumed predominant water flow toward west, followed
by periods of almost no flow. Maybe I'm seeing just what I want
to see. :-)
There certainly seem to be deformation too, as you said.
> About the 'green stuff'. It's likely greensand, but Gerhard
> Einsele refer to green pigmentation, verdissement, in one of
> three ways: glauconitization (starved/outer shelf, cold
> water), verdinisation (inner shelf/delta-front zones, 20-60m,
> varm climate (but clayey)) and chloritization. I generally miss
> what I can interpret as migrating sands like bars and submarine
> dunes, and this causes me to suggest a somewhat sedimentstarved
> (marine rather than fluvial) environment. I don't know the
> hinterland, but if it's a wide coastal plain and a modest
> precipitation, an inundation of it may produce a wide flat
> shelf with low sediment input, and a tidal activity,
A theoretical question here: If one wants to figure out if this
was salt water or fresh water environment, what should be done?
Chemical analysis?
> though with the same amplitude as the present, that passes a
> higher volume of water across the plain, and constitute a
> stronger shaping factor on the sediments than what you see at
> present. Such an environment may produce organic rich
> sediments and develope lagoons protected by barrieres closer
> to the shelf edge/deeper water. This suggestion is what is
> called a highstand or transgressive system tract in a
> sequence-stratigraphic context. I have decided that the dark
> matter is finegrained and clayey, and if it's not, I may be
> way off in my interpretation.
It is indeed finegrained, so you seem to be right again. :-)
> I'm puzzled on a lot of accounts, particularly the straight
> laminated strata we've been talking about. On P#7, below the
> 6-mark: it would have to be predominantly clayey. Even the
> light layers could not be sand as I know of no mechanism
> that disperses sand this way - unless we change to a whole
> other environment: the surf zone of the beach front. That
> could make the dark matter 'heavy minerals' and not organic
> matter.
Another view of the same thing is this:
http://avtanski.com/images/geo/lajolla/P9.jpg
Here it seems that layers #5 and #6a are divided by erosional
surface. So, it seem possible that in #5 the dark matter is
organic, and in #6a it is heavy minerals, and #6a could be
indeed surf zone. Unfortunately I didn't get any "samples" -
it was beautiful and I didn't want to scratch and dig.
> The lamination between 8a and 9a is capped with soft-body
> deformation .. that's not a logic combination, but one may
> see a possible water conduite at 9 o'clock from 8a. That
> could make the organic matter the container of water, not
> clay. The top massive sandbodies at 8 and 9 would be the
> outer protective barriere displaced onto the lagoon from
> further off shore, as the relative sealevel rises - this
> is in contrast to a rule-of-thumb that says: deeper water,
> finer sediment.
Interesting idea. I still have problems imagining the
movement of sediment when the sea level changes, so this never
occurred to me I guess I need some practice, :-)
Thank you very much,
- Alex
Alexander Avtanski
> "Robert Flory" <wyo...@MORETHANWARMmail.com> wrote in message
> news:d34poi$es2$1...@news.astound.net...
>
>>If you like soft sediment deformation and every get near Laramie Wyoming, I
>>can point you to some really wild examples of soft and semi brittle
>>defformation in aolian sediments. Casper Formation near Chimney Rock.
>>
>>Bob
>
>
> Here are some good examples of soft sediment deformation at the Garden of The
> Gods, In Southern Illinois:
>
> http://www.backpackcamp.com/GOG/GOGIronSwirls.jpg
>
> http://www.backpackcamp.com/GOG/GOGRock2.jpg
Both Wyoming and Illinois are quite far from where I live (San Jose,
CA), but who knows, I may visit the places some day. These would
be definitely in my travel plan then.
The "IronSwirls" image is absolutely amazing! And I thought that
the folds at Coyote Hills (a hilly area near Fremont, CA) are wild...
Regards,
- Alex
George
I don't know what the distance between Wyoming and Illinois has to do with
anything. I was just trying to give examples of softr sediment deformation.
Yes, the "Ironswirl" is quite amazing, and I highly recommend it to anyone who
plans to make a trip to Southern Illinois (a lot of geologists make that trip in
order to visit the Southern Illinois fluorspar district).
Carsten Troelsgaard
> Hi Carsten,
>
> Thank you for the detailed reply. I just hope this discussion is
> as much fun to you, as it is to me.
It is.
> Carsten Troelsgaard wrote:
> > > [ ... ]
> > > Is this (#2 on the pic) how a lagoon environment should loook
> > > like? Cross-lamination
> >
> > I don't think that it's cross lamination, it's more like a
> > deformation. The lagoon environment has a lot of different
> > characteristics depending on climate and coast type.
>
> Maybe I'm not using the correct term (cross-lamination) here. What
> I meant is that some layers seem to go on angle and end into other
> layers. For example, look at the highlighted detail on this image:
>
> http://avtanski.com/images/geo/lajolla/P8a.jpg
>
> There are other places where this happens. What I noticed was
> that the angle is always sloping downward to right (west), and
> that's why I assumed predominant water flow toward west, followed
> by periods of almost no flow. Maybe I'm seeing just what I want
> to see. :-)
The above is more like it. This single set could well be tidal, it takes
more to indicate a permanent stream. The particular heterolithic (mixed clay
and sand) sediment is characteristic for the tidal environment. When a sandy
feature gets a claydrape, it is protected from being entrained when currents
resume after high tide.
> There certainly seem to be deformation too, as you said.
>
> > About the 'green stuff'. It's likely greensand, but Gerhard
> > Einsele refer to green pigmentation, verdissement, in one of
> > three ways: glauconitization (starved/outer shelf, cold
> > water), verdinisation (inner shelf/delta-front zones, 20-60m,
> > varm climate (but clayey)) and chloritization.
I left out a posibillity that it could be a recent algal overgrowth.
> I generally miss
> > what I can interpret as migrating sands like bars and submarine
> > dunes, and this causes me to suggest a somewhat sedimentstarved
> > (marine rather than fluvial) environment. I don't know the
> > hinterland, but if it's a wide coastal plain and a modest
> > precipitation, an inundation of it may produce a wide flat
> > shelf with low sediment input, and a tidal activity,
>
> A theoretical question here: If one wants to figure out if this
> was salt water or fresh water environment, what should be done?
> Chemical analysis?
Finding shells or other macrofossil (or characteristic tracefossil) will be
your best chance - the freshwater species would be dicernible from braccish
or marine ones. Braccish environments are often well populated but with a
very low in diversity. On a microscopic level you'r better off. The presence
of pyrite indicate marine conditions. I put your intelligent question to one
of my professors, and I can give you the same answer: well, it's difficult
to tell.
> > though with the same amplitude as the present, that passes a
> > higher volume of water across the plain, and constitute a
> > stronger shaping factor on the sediments than what you see at
> > present. Such an environment may produce organic rich
> > sediments and develope lagoons protected by barrieres closer
> > to the shelf edge/deeper water. This suggestion is what is
> > called a highstand or transgressive system tract in a
> > sequence-stratigraphic context. I have decided that the dark
> > matter is finegrained and clayey, and if it's not, I may be
> > way off in my interpretation.
>
> It is indeed finegrained, so you seem to be right again. :-)
I feel naked without my hands on the stuff.
> > I'm puzzled on a lot of accounts, particularly the straight
> > laminated strata we've been talking about. On P#7, below the
> > 6-mark: it would have to be predominantly clayey. Even the
> > light layers could not be sand as I know of no mechanism
> > that disperses sand this way - unless we change to a whole
> > other environment: the surf zone of the beach front. That
> > could make the dark matter 'heavy minerals' and not organic
> > matter.
>
> Another view of the same thing is this:
>
> http://avtanski.com/images/geo/lajolla/P9.jpg
>
> Here it seems that layers #5 and #6a are divided by erosional
> surface. So, it seem possible that in #5 the dark matter is
> organic, and in #6a it is heavy minerals,
It probably isn't heavy minerals, but light organic matter doesn't really
belong there. And there is a host of other facies that belong to the beach
facies association that is missing. The erosional surface counts in favour
for a surf.
> and #6a could be
> indeed surf zone. Unfortunately I didn't get any "samples" -
> it was beautiful and I didn't want to scratch and dig.
> > The lamination between 8a and 9a is capped with soft-body
> > deformation .. that's not a logic combination, but one may
> > see a possible water conduite at 9 o'clock from 8a. That
> > could make the organic matter the container of water, not
> > clay. The top massive sandbodies at 8 and 9 would be the
> > outer protective barriere displaced onto the lagoon from
> > further off shore, as the relative sealevel rises - this
> > is in contrast to a rule-of-thumb that says: deeper water,
> > finer sediment.
> Interesting idea. I still have problems imagining the
> movement of sediment when the sea level changes, so this never
> occurred to me
A swing in relative sealevel changes the way the sediments distribute
itself. Sequence stratigraphy sorts generic sediments from one swing into
one basic unit. Simple(as principle), but the sequence itself is still
influenced by outside factors like wave-climate, sediment availabillity etc.
The larger classes are observable on seismics, and the knowledge makes you
able to predict high probability targets in oil(or water)-drilling.
> I guess I need some practice, :-)
> Thank you very much,
Thank you for a chance to expand mine. The eqvivalents I know in my
neighbourhood are different.
Carsten
Carsten Troelsgaard
"Carsten Troelsgaard" <carsten.t...@mail.dk> skrev i en meddelelse
news:42577e0d$0$188$edfa...@dread11.news.tele.dk...
>
> "Alexander Avtanski" <avta...@ispwest.com> skrev i en meddelelse
> news:d36kk...@enews1.newsguy.com...
> I feel naked without my hands on the stuff.
snip
> It probably isn't heavy minerals, but light organic matter doesn't really
> belong there.
Looking again on P#5. Judging anything by color is misleading. The zonation
of 9, coloring the surface first yellow then dark.. the dark grains of the
original light-gray sand would not be organic matter dispersed, but
iron-containing heavy minerals (or what, anybody? wouldn't finegrained
organics just ash off instead of starting a diagenetic process - the
iron-minerals would still have to be present as a base for the colorant
anyway). If the sandbody of 9 is emplaced by one energetic event, I would
expect the winnowing and sorting to separate light organics off the sand.
Organic matter in the top-soil may reduce perkolating water and perform the
yellow staining .. here, it's the morning dew that could make the place
habitabl for green algea too.
Or put another way: if so much organic material survived, there probably
wouldn't be structures left for sheer bioturbation of the substrat.
It does put a question to the presence of organic matter in the overall
black layers (which was my spine reaction). Seing dark layering as grains
from volcanic lava or as a recent alteration of sand with a high contents of
it sets another scene.
With this in mind, look at P#6 and the lamination of 6. The longer the surf
can winnow and sort, the darker/heavier the layer it leaves. 7 'looks like'
flaser or lenticular bedding, but that would necessarily make the 'dark'
stuff something else (tidal flaserbedding is a heterolith that 'needs' the
clay to bind the more sandier and easier moving ripples) than the dark stuff
in 6. Taking the consequence of this 1) takes a tidal environment off the
equation, or 2) takes the surf-zone at 6 to be a quiet muddy deposit, or 3)
the dark stuff is indeed different in the two layers.
If 1): I'm out of experience.
If 2): I'm confused.
If 3): If the darkness is a function of freshwater perkolating
through(sandy), to make a differentiated habitat for algal growth/leached
minerals, then the lighter stuff at 7 and below takes the right proportions
of a clayey sediment binding ripples. Above 7 could be a deeper clayey part
(?). The large clasts at 8 can be a meandering chanal that undercuts
lagoonal/coherent bloks of sediment though the dark/light analogy of the
lithology seems reversed.
Hm.
Carsten
B.C. Reed
Hi
The Sediments you are looking in La Jolla are Upper Cretaceous Turbidites
that are part of the Point Loma Formation and the overlying Cabrillo
Formation. There are six facies present 1) shelf and lagoonal, 2) slope and
basin-plain(?) mudstone, 3) outer-fan lobe sandstone, 4) middle-fan
channel-fill sandstone, 5) middle and inner-fan interchannel and channel
margin thin-bedded turbidites and mudstone, and 6) inner-fan channel-fill
conglomerate and sandstone. The facies associations are part of a deep sea
fan that was deposited by westward flowing sediment gravity flows which
transported sediments from the batholithic and pre-batholithic metamorphic
rock from the Peninsular Ranges.
The La Jolla cove area contains the thinning and fining upward megasequences
in middle-fan channel fills. For more information I recomend reading an
article in the AAPG Bulletin titled Paleogeography and Sedimentology of
Upper Cretaceous Turbidites, San Diego, California, by Tor H. Nilsen and
Patrick L. Abbott.
Brian Reed
Alexander Avtanski
B.C. Reed wrote:
> [ ... ]
>
> The Sediments you are looking in La Jolla are Upper Cretaceous Turbidites
> that are part of the Point Loma Formation and the overlying Cabrillo
> Formation. There are six facies present 1) shelf and lagoonal, 2) slope and
> basin-plain(?) mudstone, 3) outer-fan lobe sandstone, 4) middle-fan
> channel-fill sandstone, 5) middle and inner-fan interchannel and channel
> margin thin-bedded turbidites and mudstone, and 6) inner-fan channel-fill
> conglomerate and sandstone. The facies associations are part of a deep sea
> fan that was deposited by westward flowing sediment gravity flows which
> transported sediments from the batholithic and pre-batholithic metamorphic
> rock from the Peninsular Ranges.
>
> The La Jolla cove area contains the thinning and fining upward megasequences
> in middle-fan channel fills. For more information I recomend reading an
> article in the AAPG Bulletin titled Paleogeography and Sedimentology of
> Upper Cretaceous Turbidites, San Diego, California, by Tor H. Nilsen and
> Patrick L. Abbott.
>
> Brian Reed
Hello,
I just came from the library - they had it, I made a copy and I'm now
reading the paper!
Thanks,
- Alex
P.S. I'm still amazed how complete and well organized are the public
libraries here (San Jose, CA). Every time I see their slogan -
"Your tax dollars at work", I'm really glad that I pay taxes! :-)
Carsten Troelsgaard
> Is this (#2 on the pic) how a lagoon environment should loook
> like?
According to B. C. Reed, no. His information certainly puts some of the
observations into perspective. The sedimentary setting (middle fan channel
fill) is out of reach of surface wave and current influence, and it utilizes
the whole specter of the combined structural output of grainsize,
sedimentvolume, current velocity and mode of sedimentation (coherent
massflow, 'liquified grainflow' and 'ordinary' current-structuring only
dependant on speed and grainsize).
If you checked up on the Bouma sequence, an ordinary turbedite is a rather
modest feature 10-15 cm thick that is not easy to reconcile with the profile
at hand. The profile is none the less a stack of individual events that we
ought to be able to take apart in individuals.
I'll give it another shot, and you may correct me from what you find in Tor
H. Nilsen and Patrick L. Abbott's book.
I'll take that #P5 the profile as you say is predominantly sand and silt as
massive layers of clay could be expected to compact and deform overlying
strata over time. The strata are straight and has not been imposed late
deformation. The comment serves to suggest that layer 1 is another massive
sandbody (like 4) and not the soft basin-bottom, as well as the
parallel-lamination is not clay either.
The wavy layers at 2, 5, 7 and round 3 is in part low current-velocity
cross-stratified ripples (Bouma Tc), but I doubt that they can be considered
individual coherent units: At #P8 four 'coins' belov the coin, there is a
wavy but parallel-laminated layer one 'coin' thick. If this is laminated
clay, it could terminate an event, but it's followed by low velocity ripples
and thus likely is a high-velocity parallel lamination and a small turbedite
in itself.
Each turbidite ideally has a high velocity parallel-laminated buttom,
current-ripples above and capped by laminated or structureless clay - but if
we only look at distal parts, the high-velocity part may not be present (a
proposal), and the ripples may stack? A final posibillity is, that the
provenance of each turbedite may be different and influence the individual
thicknesses of lamination and x-stratification.
At #P8 3a and 3b could constitute one unit, and another one between 3b and
3c. At #P5 it's very tempting to consider 4 and 5 one big event. The bottom
of 4 shows antidunes (high velocity), the body massive or indistinct
lamination. Thinking back on 2, the small one was a side event inside the
big one?
At #P9 the next 6a has scoured the top of 5. I've never seen high velocity
parallel-lamination lap onto the substrate, but I figure that it may be some
oddity tied up to an extreme velocity? The Bouma Td, laminated mud that
could cap the x-stratification,, naa.
Between 6 and 7 there is the very wavy layer. 5 and 6 has a broader wavy
dark band at the same position. It's the position between high and low
velocity, in a surface position called a hydrolic jump.
I'll leave it at that for now.
Carsten
B.C. Reed
> Hello,
>
> I just came from the library - they had it, I made a copy and I'm now
> reading the paper!
>
> Thanks,
>
> - Alex
>
> P.S. I'm still amazed how complete and well organized are the public
> libraries here (San Jose, CA). Every time I see their slogan -
> "Your tax dollars at work", I'm really glad that I pay taxes! :-)
Hi again:
Glad you found it. I found another general reference that might interest you
titled Turbidites and Associated Coarse Clastic Deposits, by Roger D. Walker
in Facies Models 2nd Ed. (1986) pg. 171-188. Geoscience Canada Reprint
Series 1.
The book also has numerous other sedimentary facies models that you might
find interesting.
Brian Reed
Alexander Avtanski
>>Hello,
>>
>>I just came from the library - they had it, I made a copy and I'm now
>>reading the paper!
>>
>
> Hi again:
>
> Glad you found it. I found another general reference that might
interest you
> titled Turbidites and Associated Coarse Clastic Deposits, by Roger D.
Walker
> in Facies Models 2nd Ed. (1986) pg. 171-188. Geoscience Canada Reprint
> Series 1.
>
> The book also has numerous other sedimentary facies models that you
might
> find interesting.
>
> Brian Reed
>
>
Hi Brian,
Thanks for the help. I have a small request, if this is not a problem
for you: When copying the Nilsen & Abbott paper at the library, now I
see that I've cut off the text under two of the images on p. 1274 -
presumably these are Fig. 16-G and Fig. 16-H. Those are referred from
the text, but the text under other messages often contains additional
info. If you have the paper handy, I'll be grateful if you can post
just the description of those two images. You will save me another trip
to the library. But this is _only_ if you have it handy - on your desk,
or something; don't bother otherwise.
Thanks,
- Alex
B.C. Reed
>
> Thanks for the help. I have a small request, if this is not a problem
> for you: When copying the Nilsen & Abbott paper at the library, now I
> see that I've cut off the text under two of the images on p. 1274 -
> presumably these are Fig. 16-G and Fig. 16-H. Those are referred from
> the text, but the text under other messages often contains additional
> info. If you have the paper handy, I'll be grateful if you can post
> just the description of those two images. You will save me another trip
> to the library. But this is _only_ if you have it handy - on your desk,
> or something; don't bother otherwise.
>
> Thanks,
>
> - Alex
>
16G. large clast of diorite in chaotic conglomerate south of Bird Rock.
16H. Large clast of andesite from Santiago Peak Volcanics, roadcut on
Cabrillo Memorial Drive south of Cabrillo Road intersection.
Anything else just ask.
Brian
Alexander Avtanski
>
> OK, No problem.
>
> 16G. large clast of diorite in chaotic conglomerate south of Bird Rock.
>
> 16H. Large clast of andesite from Santiago Peak Volcanics, roadcut on
> Cabrillo Memorial Drive south of Cabrillo Road intersection.
>
> Anything else just ask.
>
> Brian
Thanks. But... I think I gave you wrong figure numbers. :-(
The images I cropped are 15-G and 15-H (still a [reasonable] guess,
because I can't see the numbers) on p. 1274. My bad, I told you
16-something in my previous message.
The 15-G and 15-H look exactly like the rocks I was looking at and,
of course, I clipped exactly those images at the copy machine :-)
BTW, I just finished reading the paper. It seems that the rocks on
my photos are channel margin and interchannel middle and outer slope
turbidites. This explains all the features that were puzzling with
the initial shallow water/lagunal interpretation. I'll go through the
paper once again and will post a summary.
Thanks,
- Alex
B.C. Reed
on my copy.
15G Starved ripples in levee sequenceof inner-fan channelized conglomerate
of Cabrillo Fm., False Pt. Pacific Beach.
15H. bioturbated (Spirophycos and Zoophycos) thin-bedded interchannel
turbidites, Point Loma Fm., Sunset Cliffs Boulevard and Osprey Street.
Alexander Avtanski
> OK , Fig. 13 is La Jolla Cove, also Fig 14, and 14E. Fig. 15C&D are cut off
> on my copy.
>
> 15G Starved ripples in levee sequenceof inner-fan channelized conglomerate
> of Cabrillo Fm., False Pt. Pacific Beach.
>
> 15H. bioturbated (Spirophycos and Zoophycos) thin-bedded interchannel
> turbidites, Point Loma Fm., Sunset Cliffs Boulevard and Osprey Street.
Thanks! That's it! :-)
- Alex
Alexander Avtanski
I promised to post a short review of what I learned from the
paper that Brian Reed recommended. I had a really hectic
week at work and couldn't find time until now (I'm programmer
and sedimentology is just a hobby).
The pictures ( http://www.avtanski.com/images/geo/lajolla ) are
are thin-bedded turbidites, most probably middle fan channel
and interchannel deposits. What I thought was a mudcracked
surface were probably cracks that happened long after
deposition, and maybe even after the outcrop was exposed. The
interesting structures on Pic 7, #9a as well as the sand body
#9 on Pic 5, are evidently previously deposited thin layers that
"were lifted off the bottom and incorporated in the thicker
bedded sandstone that initiated the next cycle of channel
cutting and channel fill". In the turbidite context all the
other features make perfect sense too.
Many thanks to all who responded.
Regards,
- Alex
Carsten Troelsgaard
> Hello again,
>
> I promised to post a short review of what I learned from the
> paper that Brian Reed recommended. I had a really hectic
> week at work and couldn't find time until now (I'm programmer
> and sedimentology is just a hobby).
>
> The pictures ( http://www.avtanski.com/images/geo/lajolla ) are
> are thin-bedded turbidites, most probably middle fan channel
> and interchannel deposits. What I thought was a mudcracked
> surface were probably cracks that happened long after
> deposition, and maybe even after the outcrop was exposed.
> The
> interesting structures on Pic 7, #9a as well as the sand body
> #9 on Pic 5, are evidently previously deposited thin layers that
> "were lifted off the bottom and incorporated in the thicker
> bedded sandstone that initiated the next cycle of channel
> cutting and channel fill".
..the sand body are evidently previously deposited thin layers that "were
lifted off the bottom...
?? I don't understand what you mean.
Alexander Avtanski
Carsten Troelsgaard wrote:
> [ ... ]
>
> ..the sand body are evidently previously deposited thin layers that "were
> lifted off the bottom...
>
> ?? I don't understand what you mean.
>
> [ ... ]
I read the paper again and I think you were right to be
suspicious of this being lift-off deposit. I assumed that
lift-off is some kind of violent (I mean high-speed, caused
by a sliding of a layer excited by the turbidity current, or
something like this) deformation process, but a net search
showed that what is meant by "lift-off deposit" is the deposit
by the slowly setting particles that were suspended in the
water by the turbidity current. That shows the dangers when
a non-professional like me tries to read science papers, :-)
So, #8 then should be caused by the deformation caused by
the movement of one layer over the other.
Regards,
- Alex
Carsten Troelsgaard
> Hi Carsten,
>
> Carsten Troelsgaard wrote:
>> [ ... ]
>>
>> ..the sand body are evidently previously deposited thin layers that "were
>> lifted off the bottom...
>>
>> ?? I don't understand what you mean.
>>
>> [ ... ]
>
> I read the paper again and I think you were right to be
> suspicious of this being lift-off deposit. I assumed that
> lift-off is some kind of violent (I mean high-speed, caused
> by a sliding of a layer excited by the turbidity current, or
> something like this) deformation process, but a net search
> showed that what is meant by "lift-off deposit" is the deposit
> by the slowly setting particles that were suspended in the
> water by the turbidity current.
I googled the exact term "lift-off deposit" and got 1, one, hit.
> That shows the dangers when
> a non-professional like me tries to read science papers, :-)
hm, did I do that much better? I copied a header for an article "Ten myths
of turbidites" by G. Shanmugam (see below) to point out the confusion of the
details.
> So, #8 then should be caused by the deformation caused by
> the movement of one layer over the other.
On pic 7, the 8 and 8a (Bouma a) looks like some kind of mass flow. The
particular character is, that larger clasts are 'resting', not on the base
of the unit, but at a higher position. This is the kind of inverse bedding
that I have sometimes referred to, though it's in a different scale in 8
than in 'normal' thin turbidites (as referred to below). The laminated
strata above 8a (Bouma b) 'could be' turbidites or sediments settling from
fast currents triggered by the massflow (it grades into longwave current
ripples at top?(Bouma c)). The only rationale I can make of the funny
escape-structure grading into 9a is by letting 9a be a separate later event
that excerts pressure upon loose unconsolidated sediments of 8 to release
some of it's water. The problem of seeing both 8 and 9 as different fases of
the same event is, that 9 looks as if it has a larger sediment-caliber than
the lamina below - that wouldn't make sense. If 9 is finer grained and
belong to the same event, it would be the 'Bouma d' sequence.
Carsten
G. Shanmugam, Department of Geology, The University of Texas at Arlington,
P.O. Box 19049, Arlington, TX 76019-0049, phone: 817-272-2684,
shansh...@aol.com
During the past 50 years the turbidite paradigm has promoted many myths
related to deep-water turbidite deposition. This paper provides a reality
check in undoing ten of these turbidite myths. Myth 1: Turbidity currents
are non-turbulent flows with multiple sediment-support mechanisms. Reality:
Turbidity currents are turbulent flows in which turbulence is the principal
sediment-support mechanism. Myth 2: Turbidites are deposits of debris flows,
grain flows, fluidized flows and turbidity currents. Reality: Turbidites are
the exclusive deposits of turbidity currents. Myth 3: Turbidity currents are
high-velocity flows. Reality: Turbidity currents operate under a wide range
of velocity conditions. Myth 4: High-density turbidity currents are true
turbidity currents. High-density turbidity currents are sandy debris flows.
Myth 5: Slurry flows are high-density turbidity currents. Reality: Slurry
flows are debris flows. Myth 6: Flute structures are indicative of turbidite
deposition. Reality: Flute structures are indicative only of flow erosion,
not deposition. Myth 7: Normal grading is a product of multiple depositional
events. Reality: Normal grading is the product of a single depositional
event. Myth 8: Cross bedding is a product of turbidity currents. Reality:
Cross bedding is a product of traction deposition from bottom currents. Myth
9: Turbidite facies models are useful tools in interpreting deposits of
turbidity currents. Reality: Turbidite facies models are based mostly on
deposits of plastic flows and bottom currents. Myth 10: Turbidite facies can
be interpreted using seismic facies and geometries. Reality: Individual
turbidity-current depositional events, commonly centimeter to decimeter in
thickness, cannot be resolved in seismic data.
Daryl Krupa
rip-up deposit.