Karen G. Jensen wrote (in part):
> Dear Jonathan,
>
> Thank you for your interesting observations on those Australian reefs.
> I've sent part of it to a friend who knows much more about reefs (live and
> fossil) than I do. Waiting for his comments.
Yes, they are interesting, aren't they? We will see what he comes up with.
> >Thin halite layers are very common in salt lakes. In most it redissolves
> >when the lakes flood, but not always. Lake McLeod has several m of permanant
> halite,
> >deposited by sewater seepage from the Indian Ocean.
>
> Is the structure of that halite layer like that of the giant layers in the
> geological record? Do you believe that giant layers formed in coastal
> areas where salt water seeped in and was concentrated by surface
> evaporation?
I had hoped that I would be able to look up the standard work on Lake MacLeod in
the library today, but alas! alack! it was out on loan. If you are really keen to
find out more about Lake MacLeod let me know next week when I can get the book.
Australia, just north of Shark Bay. It is a fault bound depression about 30 km
wide and 100 km long, slightly below sea level and separated from the Indian Ocean
by a porous barrier of Pleistocene sands and limestones. Sea water seeping though
the barrier discharges via springs into the lake and evaporates forming the salt
deposits. There are some12 m of halite in the lake, plus gypsum and limestone. I
can't remember whether the halite is depositing now but it certainly has done so in
the recent past.
> > Halite is also forming at >the bottom of the Dead Sea.
>
> How do the characteristics of this halite compare with those under Lake
> McLeod, and those in the geo record?
Halite in the Dead Sea forms into three main environments. The first is in the
shallow water at the southern end, the second is the deep water of the main basin,
and the third is interstitially in shoreline sediments of other compositions. The
halite in the first environment resembles that formed in other shallow water
environments today (Lake MacLeod, etc.). The deep water halite is fine-grained,
well laminated, and associated with laminated carbonates and gypsum. I understand
that parallels have been drawn with laminites in ancient units such as the Castile
Formation in the Permian of the US. The interstitial halite forms large cubes in
mud, and thus resemble some impure halite deposits such as those of the Cambrian
of South Australia.
> >Thicker deposits are more difficult to get, but to occur today. There is over
> 900 m of very pure halite in Salar de Atacama in northern Chile. The halite is
> still forming, albeit slowly. Certainly
> >there is nothing like the saline giants being deposited today, but the
> mechanisms are
> >there and understood, it is just the scale that is different.
> >
>
> What reasons are suggested to explain the past/present difference in scale?
It takes an unusual coincidence of geographic, climatic, and tectonic
circumstances to get a saline giant. That is why they are rare. Details vary from
basin to basin, but you need an excess of evaporation over precipitation, a deep
or actively subsiding basin, slow rates of sediment input and a water inflow with
little or no outflow. The most recent time this happened (for us conventional
geologists) was in the Miocene when the Mediterranean dried out repeatedly. There
was others in the Jurassic (South Atlantic), Permian (Europe) Devonian (Canada/US)
and other intervals I am sure I have forgotten. The overall processes of evaporite
formation are scale independent.
> And now, to get back to the GVS:
>
> It is available from University Microfilms International (Ann Arbor, Michigan).
> The dissertation is on fossil pollen and spores of the GVS (1987). A
> summary of the geological characteristics is in Appendix A (pp.110-128) --
> just a side issue (I went to the library one day to look up the geological
> history of the sequence, and spent several months discovering the history
> of geologists' radically changing views of the geological history of the
> sequence!)
Thank you. Its always interesting to follow through the history of ideas about a
particular formation.
> >Individual turbidites are of course deposited rapidly, but the lime between
> >depositional events is highly variable. If there are long periods between
> >individual events one would expect things like burrowing (providing the
> >bottom or pore waters were not anoxic), erosion surfaces, even hardgrounds.
> >Certainly the turbidite successions I have seen contain these features. They can
> be very
> >subtle though,
>
> You might expect some deposition, too. Hard to imagine much time with none.
You can get thousands of years with no deposition quite easily. There areas of
relict sediments on many continental shelves today. In clastic marine sediments,
long periods of non deposition will be marked by horizons enriched in iron,
manganese, phosphate, glauconite, etc. They are often well burrowed. With
carbonates these horizons are generally lithified crusts, called hardgrounds,
which tend to be bored, rather than burrowed. Whether this is applicable to your
section, I don't know.
> >However you can't use the fact that the limestone beds were folded
> >and not brecciated as evidence of rapid deposition. All this is evidence
> >of is lack of induration as your correctly point out. Deep water limestones,
> >especially fine-grained ones, lithify very slowly. We know this from deep sea
> >cores. Of course, even indurated limestones can fold very nicely if the
> >conditions are right.
> >
> The Great Valley Sequence is 10 to 20 km thick. The lower part is
> considered to be Tithonian (Upper Jurassic) and the higher parts Turonian
> and on up to Maastrictian (Upper Cretaceous) and even Danian (Lower
> Paleocene). This is usually described as representing about 80 million
> years. The folded areas I saw were in a locality in the lower portion, but
> the folding probably happened in conjunction with the tilting of the entire
> sequence (after it had finished being laid down horizontally). Do you
> believe induration could wait many millions of years?
Several components here to your question. As I understand you, your argument runs
like this: The thin limestone beds are not brecciated. Thus they must have been
soft when deformed. Yes, it is possible for sediments to remain uncemented for
many millions of years, especially if only shallowly buried. Obviously not the
case with your sediments though (12 km is not shallow burial to me!). You say that
the sequence was completely deposited before deformation began. With 12 km of
burial I imagine things would be quite lithified but at those sort of depths
brittle beds start deforming in a ductile manner. Your rocks should have been
metamorphosed to lower greenschist metamorphic facies. Is this the case?
> To me it looks like mega-deposition and mega-tilting, but not mega-time.
> Consistent with Genesis 7-8.
No Karen, not consistent with Genesis 7-8, but with only what with your reading is
of it. My Bible does not say anything about mega deposition or mega tilting.
>
>
> Your sister in Christ,
>
> Karen
In Christ
Jonathan