The Loess Mystery

[1dave]

@edmost roused my interest in this unusual material though the book "The Lost Cities of Cibola".

en.wikipedia.org/wiki/Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt,[3] typically in the 20–50 micrometer size range, twenty percent or less clay and the balance equal parts sand and silt[4] that are loosely cemented by calcium carbonate. It is usually homogeneous and highly porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs.

The word loess, with connotations of origin by wind-deposited accumulation, came into English from German Löss ([lœs]), which can be traced back to Swiss German and is cognate with the English word loose and the German word los.[5] It was first applied to Rhine River valley loess about 1821.[6][7]

6 Large areas of loess deposits and soils
6.1 Argentina
6.2 China
6.3 New Zealand
6.4 Europe
6.5 United States

Petersen pointed out serious errors in the "Wind Blown" interpretation for their creation.
1. Loess is a geologically recent deposit that buries pre-existing landscape like a blanket from up to 300 feet thick as a massive uniform deposit.
2. "Loess" means loose, used because the material is extremely porous - exceeding 50%. In other words More than half it's bulk is vacant!
3. It has a tendency to cleave along vertical planes to form bluffs. This is because undisturbed loess is perforated with countless mainly vertical capillary tubes. How were they formed? Not by wind blown silt!

4. The loess of Iowa is filled from top to bottom with snail shells and angular (not wind nor water worn!) pebbles!

5. Odd dried clay nodules throughout that could not be wind blown..



So if not by wind or water, nor compacted by gravity, how were they formed?

[1dave]

Apr 19, 2017 18:20:42 GMT -7 @melhill1659 said:

The bluffs may not have been deposited there by wind but could they look like that from wind erosion??? Your thoughts?

Post by 1dave on Apr 19, 2017 at 8:08pm
Wind and water. Water takes it away quickly - faster than any other kind of soil.

Petersen wracked his brains trying to figure out how they were formed - slow comet particle impact, 4th dimension, but that does not explain the snail shells. Perhaps very large tornado or water spout?

[1dave]

Apr 19, 2017 19:25:19 GMT -7 @edmost said:

That Aeolian theory is really mind boggling
A more sober account (but no loess boggling)
is from Walter Alvarez book
“A Mostly Improbable Journey”
What a great title !

It explains where the sand that formed the sandstone in the
Colorado Plateau came from.
This sandstone is in area called Ruby Canyon
It is a beautiful pink hue.
Good rock’n heart material.
(Whoever ever goes hounding there )
Scientist figure the sand was deposited some 200 mya.

And where did this sand come from?
The Appalachian Mountains!
That's pretty loess !


They can tell by the dating of zircon crystals in this sandstone.
The only source rock to account for the age of these sands
Comes from eastern United States.


Well that my interpretation of the facts.
I’d get the book to get a different interpretation.

What the wind can carry may surprise.
What can fall from the sky,
in mass quantity,
in short period of time,
into a small area ?
The sky is falling?
Well maybe the Papago’s are an example ?

A Most Improbable Journey


Mostly

Apr 19, 2017 19:08:05 GMT -7 1dave said:

- slow comet particle impact, 4th dimension,

Yep the 4th Dimension, loess caliches, and Yellow soil
and not from where the huskies go

Apr 19, 2017 21:44:22 GMT -7 @edmost said:

The book does talk about clay (or lack thereof) in the loess
but I think in China its mostly caliche

Apr 19, 2017 19:08:05 GMT -7 1dave said:

Petersen wracked his brains trying to figure out how they were formed - slow comet particle impact, 4th dimension, but that does not explain the snail shells. Perhaps very large tornado or water spout?

You ask a good question Dave
One I was wondering about
I thought Petersen theory was going off swimmingly
until the snails came up


some of his reasoning seemed forced to conform to his theory
the only time he used Occam razor was when
he was slicing those snails up with his observations. Pretty intense !
but the theory certainly wasn't the simplest explanation...

mostly

[1dave]

Apr 20, 2017 1:02:09 GMT -7 @metalsmith said:

In the first image in the thread, there is a landscape with erosional features such as 'gulley' and these are subject to 'valley fill'. From an aeolian formation mechanism perspective, the loess would effectively be raining fine soil dust down onto a landscape. The setting for this would have to be generally dry for the dust to form, but there is no requirement for the entire landscape to be devoid of water. I suggest the snail shells and larger clasts are the result of alternative erosional mechanisms: glacial / fluvial and their environments (see Fig 3 here). The angularity of clasts is reduced with exposure to the erosional agents. At or close to the source of the material and the point of entry even fluvial sediment can be highly angular.

ETA

Apr 19, 2017 13:15:26 GMT -7 1dave said:

Evidence for a windblown origin of loess
In order to explore the Neogene evolution of East Asian winter monsoon circulation, grain-size analysis was conducted on the Neogene Sikouzi section, Guyuan, Ningxia, China, a relatively monsoon-sensitive region. The results show that most of the particles are fine, the sand-bearing samples (median grain size [Md] >63 μm) are less than 6.4%, and that all the grain-size distribution curves of representative samples have a similar pattern to those of eolian dust sediments on the Chinese Loess Plateau. Rare earth element (REE) patterns of eight samples from the Sikouzi section are all characterized by light (L) REE enrichments, relatively flat heavy (H) REE patterns, and slight negative Eu anomalies, similar to those of loess and the average upper continental crust. These lines of evidence point to a windblown origin of the Sikouzi fine sediments.
source

Fluvial role in loess deposition
Rivers are essential for loess deposit formation. River systems which relate directly to loess deposit formation include the Danube and the Rhine; the Mississippi and the Missouri; the Thames and the Medway; the Indus, Ganges and Brahmaputra; the Dnepr, Don and Volga; the Clutha, Mataura and Rakaia—New Zealand rivers; the Yellow River; rivers in South America; Siberian rivers (flowing north); the Odra and the Wisla (between the mountains and the ice); the Syr-Darya and Amu-Darya (into the desert)
source

Role of fluvial transport agents for loess in England
A complex series of events in the Weald
loess trap causes a concentration of loess material by rivers flowing through various gaps in
the Downs. The Thames provides loessic estuarine deposits, and the Pegwell Bay loess was
a feature of the Stour. South coast accumulations are related to rivers flowing south through
Downs gaps, such as the Adur and the Arun.
source

Loess is not necessarily simply aeolian
The ‘known’ distribution of loess in the UK cannot be accounted for under simple aeolian transportation and dumping of silt sized material, its distribution is too sporadic. Therefore the distribution is controlled by the geomorphic surroundings, processes and even in some cases several phases of reworking and redistribution
source


Periglacial setting of loess
On the Yorkshire Wolds the loess was deposited before the ice reached its extreme limit, as it is incorporated in a chalky, flinty head beneath the till.
source

The role of freezing / freeze-thaw / cryoturbation
New sedimentological evidence is presented for phases of accretion with two distinct units, a calcareous brickearth exhibiting periglacial cryoturbation
source

Role of reworking of loess
Most loess has been modified to some degree by local reworking, bioturbation and syn-depositional weathering, but a process of ‘loessification’ is not necessary for a dust deposit to qualify as loess. The terms ‘primary loess’ and ‘secondary loess’ have previously been used to described wind-deposited loess and redeposited loess, respectively, but it is more accurate to describe material reworked and redeposited by non-aeolian processes as ‘loess-derived colluvium’ or ‘loess-derived alluvium’.
source

Influences on Chinese loess
the Chinese Loess Plateau has evolved as a more dynamic landform than previous thought, where wind deflation, fluvial input, lateral transport, and accumulation of sediment are equally important
source

Lacustrine Himalayan loess
In the Kashmir valley the Quaternary geological record is preserved in the form of exposed Karewas (ancient lake beds), capped by loess.
source

Simplicity ... ahhh!

[1dave]

@metalsmith, Great information! - But it does not account for the long capillary tubes, empty spaces, odd clay masses, nor the snails.

What I like about the waterspout idea is water between the particles that later dried out accounts for the spaces. Air bubbles rising accounts for the tubes, mud balls slammed together in the wind accounts for the odd clay found. Oh, and also the snails.

It does not account for the huge volume.

[1dave]

Apr 20, 2017 10:26:55 GMT -7 @metalsmith said:

Apr 20, 2017 8:26:02 GMT -7 1dave said:

@metalsmith, Great information! - But it does not account for the long capillary tubes, empty spaces, odd clay masses, nor the snails.

What I like about the waterspout idea is water between the particles that later dried out accounts for the spaces. Air bubbles rising accounts for the tubes, mud balls slammed together in the wind accounts for the odd clay found. Oh, and also the snails.

It does not account for the huge volume.

1dave you have a great mind for information. Consider then the volumes of sand in the Taklamakan, Mongolian Steppe and Tengger deserts. It is the winnowing of these sediments that produces loess in China that exceed 300m thick in places. Now that is volume indeed!

Without source or observation I would suggest that the tubes are more likely to be accounted for by differential cementation by percolating soil-water than water-spouts. By what mechanism would the water-spouts defy gravity?

In the UK, I'm aware that the brickearths are accompanied by calcite crystals; these would dissolve in carbonic acid (rain water+CO2) although what stimulates the precipitation, I'm not sure: there is an inverse temperature relationship with Calcite in solution (assuming temperature decreases with depth); perhaps saturation and ion-exchange with the clays is sufficient.

I don't know if you have clarity on whether the snails are terrestrial or aquatic? As per my previous post, though, I suggest the snail shells are the result of alternative landscapes such as gully-fill or lacustrine (if aquatic) loess depositional areas or washed in by fluvial mechanisms.

Do 'mud balls slammed together in the wind' have a modern equivalent / analogous environment? I wonder how pervasive these are or whether they are a local phenomenon? I would look to the local environment to explain the creation of the clay-balls in a local context before extrapolating it across the global loess deposit terrains.


[/quote

[1dave]

May 8, 2017 18:52:41 GMT -7 @thunder69 said:

A few links for the area around here ..enjoy

for-natchez.org/loess.html
www.exploresouthernhistory.com/natchezloess.html
www.natcheztracetravel.com/natchez-trace-mississippi/lorman-natchez-ms/192-loess-bluff.html

Good Info!

Image 2 of 2Where the Old Natchez Trace passed over Loess it formed sunken roads, in places 20 feet deep.

[1dave]

Mystery Solved.

www.sciencedirect.com/topics/earth-and-planetary-sciences/loess



Distribution

Loess covers approximately 10% of the earth's land surface (Figure 1) and is generally associated with semiarid to semihumid climate regimes. The thickest deposits are found in China, where approximately 500 000 km2 is blanketed by loess that is up to 300 m thick. The Great Plains has the largest area of loess in North America. Loess there can be more than 60 m thick. Other loess deposits include the Pampas of Argentina, parts of Europe, the Ukraine, large areas in central Asia, the Palouse and Snake River Plain of the US Pacific Northwest (PNW US), parts of Alaska, and parts of northern Africa. Thinner and less extensive deposits are found worldwide.



Loess in the Quaternary

Loess occupies large areas of China, Central Asia, Russia, Europe, Argentina, New Zealand, and North America. Most loess is concentrated in mid-latitude regions (Figure 5), and its texture and mineralogy make it one of the most favorable parent materials for agricultural soils. Indeed, loess-derived soils produce crops that support literally millions of people in Asia, South America, North America, and Europe. Loess is dominated by silt-sized particles (50–2 μm diameter), although most loess contains smaller amounts of sand and clay. This particle size distribution, along with slight carbonate cementation, produces interparticle binding that makes loess stand in vertical bluffs in exposures



The importance of loess in Quaternary studies is that it is one of the few terrestrial deposits that may contain nearly complete records of glacial–interglacial cycles. A classical model of loess is that it forms primarily during glacial periods, when continental ice sheets or valley glaciers produce silt-sized rock ‘flour’ by grinding. This silt is in turn transported and sorted by fluvial processes as outwash, deposited on floodplains, and ultimately entrained by the wind. It is now also known that nonglacial processes also produce silt-sized particles. Nevertheless, even in areas known to contain nonglacial loess, it appears that the most favorable times for loess entrainment, transportation, and deposition are during glacial periods. Mahowald et al. (1999) have pointed out several factors, including generally drier climates, decreased vegetation cover, stronger winds, and a decreased intensity of the hydrological cycle, that enhance loess and dust transport during glacial periods. The latter factor can account for loess or finer grained dust staying in suspension longer than is the case under nonglacial climates. Thus, Quaternary stratigraphic studies have shown that loess is characteristic of glacial periods in most regions



It is laid down over many years.

[1dave]

So glaciers grind rocks to silt particles. Spring thaw washes huge amounts into the lowlands where it settles and dries.
Bubbles drift up, snails proliferate, winter comes.
Next spring a new layer slithers over everything.