>
>Dear Kevin,
>
>Your thoughts about rate are most interesting:
>
>
>>>What do you see as sure evidence for slow rates?
>>>
>>
>>The sheer thickness of the deposits (knowing that evaporation is a slow
>>process), the presence of alternating seasonal layers like tree rings, the
>>presence of different kinds of impurities deposited at different times of
>>the year forming descreet layers, the lack of significant amounts of
>>sediment mixed with the salt (which would have occurred if deposition had
>>been rapid), etc.
>>
>
>
>The great thickness --
>
>Knowing that evaporation is a slow process, and would presumably have to
>occur at about sea level, would you postulate a large coastal salt flat
>that gradually sank (or a sea level rise) at just about exactly the rate of
>the buildup of the salt?
>
Salt deposition doesn't have to occur at sea level, as the prehistoric
Mediterranean basin or the modern day Salt Lake and Dead Sea basins testify.
All you would need is a basin into which water flows but from which water
leaves only by evaporation. As long as evaporation is faster than inflow,
the basin will not fill up with water but will fill up with salt.
The scenario you outline, however, was used a couple of decades ago to
describe how the halite deposits in the Gulf of Mexico formed. I may not
have all the details right, but as I remember it this is how it went.
As the North American plate moved west the area that would become the Gulf
of Mexico and the southern United States stretched and weakened, developing
many small faults, most of which still exist east of the Mississippi valley.
The weakened crust sank until it dropped below sea level, but formed a
shallow shelf instead of a basin. Eventually it was covered by water, but
over the millions of years that followed tectonic and climatic changes
caused the shelf to be periodically exposed then resubmerged. With each
cycle (which did not remain constant but changed gradually as the conditions
changed) fresh salt would evaporate out on the exposed shelf, then be
covered with sediment when the shelf was inundated. In this way alternating
layers of halite and sediment could form that would show seasonal, climatic
and tectonic variation.
In this scenario both land level and sea level would rise and fall
independently of one another, to create the cycle of exposure and inundation
necessary to form the salt flats. Your description of this process would
imply a certain ad hoc quality to this uniformitarian explanation, but in
fact the interplay between land level and sea level is built into the system
and would occur as a natural consequence of the process. Besides, it wasn't
"exact". There were times when land rise coupled with sea fall left the
salt flats high and dry for a time, with no periodic inundation; there were
other times when land fall coupled with sea rise left the salt flats fully
submerged with no periodic exposure. Also, as more salt accumulated the
strain on the land increased, causing it to be further weakened and to be
pushed down by the weight of salt and sediment.
Though this process probably hasn't ceased completely, three events probably
changed it drastically. The first was when the weight of the accumulating
salt and sediment finally overcame the underlying crust and the area
subsided to form the Gulf of Mexico. This alone probably significantly
reduced both the number and extent of the cycles. Much of the southern
United States sank as well, but the second event was probably the
stabilization of this area by some of the same forces that were uplifting
the Rocky Mountains. The third event was the formation of river deltas that
gradually laid down enough deposits to fill in the sunken land. However,
these deposits are causing the underlying crust to sink further, such that
when a river shifts course to form a new delta, the old one gradually sinks.
>
>Why would the salt layers that had crystalized
>not re-dissolve when they went below sea level?
>
The salt layers would be in the form of dessicated, rock-hard halite. Only
the top-most layers composed of loose granular crystals would dissolve
before the halite deposits were covered by sediment.
>
>Or if it were inland salt flats, like we find in deserts today but larger,
>are you postulating that there would be so much salt in the surrounding
>hills, that the layers of crystalized salt would eventually reach 100 ft.
>thick, in a long period of time without any tectonic disturbances?
>
You seem to be suggesting that these salt flats were formed by salt being
eroded from out the hills and deposited by seasonal rains and run-off.
That's not how they formed. The vast majority were formed when large bodies
of water could no longer replace loss due to evaporation and so dried up,
leaving their salt behind. That's what happened in the Mediterranean basin
and at Lake Bonneville, and what is happening now at the Caspian Sea and the
Dead Sea.
>
>Alternating layers --
>
>Are you saying that yearly seasonal variation is the only possible
>interpretation of the observation of alternating abundances of the
>impurities?
>
No, I am saying it is the best explanation, especially since alternatives
based on flood models such as yours require so many ad hoc explanations to
make everything turn out just right.
>
>Lack of significant amounts of sediment --
>
>Do you really think _less_ sediment in the salt indicates _more_ time?
>
Compared with your rapid sedimentation flood model, yes. If I understand
right, your model would have supersaturated brines "precipitating" salt out
into sediment-laden water, so that it would be well mixed with the sediment.
When the sediment rapid deposited, so would the salt, producing not descreet
layers of "pure" halite between layers of salt-free sediment, but
salt-saturated sediments similar to sahbka. The uniformitarian model,
however, permits enough time for sediment-laden water to loose its sediment
(becoming "pure" water), then concentrate its salt by evaporation until the
water becomes saturated, at which point the salt starts to crystallize out.
As the water evaporates the salt dessicates, forming "pure" salt that
contains no coarse sediments, but can contain small amounts of plankton
and/or fine sediment that hadn't settled out yet. However, exposed salt
flats can be covered with fine wind-blown organic and inorganic debris, that
can in turn be covered with more salt. This tends to form various types of
alternating bands, the most obvious being the broad white bands formed
during dry seasons and the thin dark bands formed during wet seasons. In
any event, each layer of halite is covered either by more halite or by
sediment, thus you get layers of sediment-free halite between layers of
salt-free sediment, even after millions of years.
>
>What rate would be optimal for the least sediment (wind blown or
water-borne)?
>
Frankly, I don't know; equally frankly, I don't see that it matters. What
do you have in mind?
Kevin L. O'Brien